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Effects-of-nano-TiO2-on-strength-shrinkage-and-microstructure-of-alkali-activated-slag-pastes

Effects of nano-TiO 2on strength,shrinkage and microstructure of alkali activated slagpastesL.Y.Yang a ,Z.J.Jia a ,Y.M.Zhang a ,⇑,J.G.Dai ba School of Materials Science and Engineering,Jiangsu Key Laboratory of Construction Materials,Southeast University,Nanjing 211189,China bThe Department of Civil and Environmental Engineering,The Hongkong Polytechnic University,Hong Kong,Chinaa r t i c l e i n f o Article history:Received 21March 2014Received in revised form 19October 2014Accepted 27November 2014Available online 4December 2014Keywords:Alkali activated slag TiO 2Strength ShrinkageMicrostructurea b s t r a c tFor alkali-activated slag (AAS),high drying shrinkage is an obstacle which impedes its application as a construction material.In this investigation,nano-TiO 2was added to AAS,and its mechanical properties and shrinkage were tested to examine its effect on hardened alkali-activated slag paste (AASP).To under-stand the impact of nano-TiO 2on AASP at micro scale,FTIR,MIP and SEM were carried out.Experimental results indicate that the addition of nano-TiO 2to AAS enhances the mechanical strength,and decreases the shrinkage of AASP.FTIR and SEM results demonstrated that the addition of nano-TiO 2into the AASP accelerates its hydration process,resulting in more hydration products and denser structure.MIP results showed that the addition of nano-TiO 2reduces the total porosity of AASP and changes the pore structure.The porosity of 1.25–25nm mesopores,which is believed to be responsible for the high shrinkage of AASP,is remarkably reduced due to the addition of nano-TiO 2.Ó2014Elsevier Ltd.All rights reserved.1.IntroductionGround granulated blast furnace slag is a kind of latent hydrau-lic material which can be activated by alkaline solutions,such as water glass,NaOH,Na 2CO 3,and Na 2SO 4,to form solid mass pos-sessing high strength and good performance.Alkali-activated slag (AAS)has been found to have good resistance to sulfate [1],freeze–thaw cycles [2],acid attack [3],high temperature [4],chlo-ride attack [5,6],etc.However,the application of AAS so far is very limited because of its high drying shrinkage [7,8]and high rate of carbonation [9,10].Collins et al.[8]found that drying shrinkage of AAS concrete was about 3.33times higher than OPC concrete at 365d when exposed to environment of 23°C and 50%relative humidity (RH).Results from literature show that,the high drying shrinkage of AAS is due to the speciﬁc pore size distribution,especially mesopores [8,11,12]which are responsible for the micro-strain formed in hardened AASP.In addition,the hydration products in AAS are mainly amorphous C-S-H of low Ca/Si ratio [13,14].The absence of crystal phases like CH,one of the main hydration products of Portland cement,is thought to be responsi-ble for high drying shrinkage of AASP as well.Shrinkage when restrained may cause the cracking of concrete,and it is therefore of great importance to work out good solutions to control the shrinkage evolvement of AASP.Bakharev et al.[15]found that heat treatment could improve the early strength and reduce the drying shrinkage of AAS concrete.Collins et al.[16]used saturated blast furnace slag (BFS)to replace normal coarse aggre-gate and found the drying shrinkage reduced signiﬁcantly and the compressive strength was improved in drying condition.Fang et al.[17]revealed that the use of magnesia remarkably reduces the shrinkage of AAS concrete when its dosage does not exceed 8%.Palacios et al.[18]showed that the use of shrinkage-reducing agent (SRA)reduces the shrinkage of AASP by up to 85%and 50%when exposed to 99%and 50%RH,respectively.Nano materials are new emerging materials in the ﬁeld of civil engineering and have been utilized by some researchers to enhance the properties,such as mechanical strength,abrasion resistance,and impermeability,of Portland cement concrete [19].The commonly used nano materials in cement-based materials are nano-TiO 2[20–24],nano-Al 2O 3[19,25],and nano-SiO 2[26–29].Feng et al.[22]showed that with the addition of 0.9%nano-TiO 2,the ﬂexural strength and compressive strength of cement pastes at 28d increases by 16.12%and 14.15%,respec-tively.The investigation into the microstructure by Feng et al.[24]demonstrated that the incorporation of TiO 2decreases the quantity of inner micro cracks in Portland cement paste.Nazari et al.[30]investigated the effect of nano-TiO 2on physical,thermal/10.1016/j.cemconcomp.2014.11.0090958-9465/Ó2014Elsevier Ltd.All rights reserved.⇑Corresponding author.E-mail addresses:yanglingyan1010@ (L.Y.Yang),ymzhang@ (Y.M.Zhang).and mechanical properties of concrete using blast furnace slag replacing OPC,showingthe formation of C-S-H gel and improves the Motivated by the possibilities of achieving with AAS,this investigation aims to usethe mechanical strength and shrinkage property slag paste(AASP).2.Experimental2.1.MaterialsS95slag with speciﬁc surface area of436m2 2.90g/cm3was used.Slag particles observed ular with sharp clear edges(Fig.1).The particle the slag,tested with laser granulometry,100l m with average particle size of11.86l chemical composition of slag is shown in TableA mixture of solid NaOH and liquid water alkali activator.NaOH was analytically pure Ms.(molar ratio of Na2O to SiO2)of liquid and the mass content of Na2O was9.7%.Nano-TiO2with particle size ranging from20 used in this study(Fig.3).To avoid thenano particles,all the nano-TiO2was dispersed by28kHz ultra-sonic wave in water(half of the total mixing water of AASP)for 10mins.After dispersion,theﬂocculation of nano-TiO2particles was mitigated,as shown in Fig.3(b)and(c).The dispersed nano-TiO2was then mixed into AASP in10min.2.2.Mix proportionsThe water/binder ratio of the AASP in this investigation was0.4, which was determined after comprehensively considering the workability and strength of AASP according to previous investiga-tions.Alkali activator with a Na2O concentration of4.0%(by mass of slag)and Ms.of1.2was used.The water in the activator and 20°C.Specimens were then cured in a room of20±3°C and 90%±5%RH.The compressive strength and theﬂexural strength of the AASPs were measured according to Chinese standard GB/T 17671-1999at the age of3d,7d and28d.3.2.ShrinkageShrinkage test was conducted according to Chinese standard JGJ 70-2009.AASPs for shrinkage test were cast in40Â40Â160mm steel moulds with two small copper pieces at both ends,serving as shrinkage detectors.Specimens were sealed with plasticﬁlm after casting,and demolded after24h room curing.Then the initial length along the longitudinal axis was immediately measured with micrometer caliper.Both reference group and TiO2group were then cured under two regimes,one at20±3°C and90±5%RH, the other at20±3°C and55±5%RH.The length change was mea-sured at1d,3d,7d,14d,28d and90d,respectively.3.3.MicrostructureThe samples for microstructure analysis were taken from the specimens cured in a room with20±3°C and90%±5%RH at dif-ferent ages.These samples were then immersed immediately in ethanol for5days and then dried for48h at60°C in order to stop the hydration of AASP.3.3.1.Fourier transform infrared spectroscopy(FTIR)FTIR was carried out to determine the chemical groups of hydration products,at the frequency range of4000–400cmÀ1.All the samples for this analysis were ground into powders smaller than75l m.3.3.2.Scanning electron microscopy(SEM)The morphology of hydration products was observed with Sirion Field emission scanning electron microscopy.The samples were coated with gold to enhance the conductivity.3.3.3.Mercury Intrusion Porosimetry(MIP)MIP was used to measure the cumulative porosity and pore size distribution of AASP.Samples for this measurement were cut into size of1–2cm.Fig.1.Morphology of slag under SEM.Fig.2.Particle size distribution of slag.2L.Y.4.Results4.1.Mechanical strengthThe mechanical strength of reference group and TiO2group AASP are given in Table3.It can be seen that the addition of TiO2enhanced both the compressive and theﬂexural strength of AASP.The compressive strength of TiO2group was approximately 10%,15%and9%higher than those of reference group at3d,7d and28d,respectively.Theﬂexural strengths of TiO2group were 25%,25%and38%higher than reference group at3d,7d and 28d,respectively,which thereby resulted in higher ratio ofﬂexural to compressive strength of TiO2group.4.2.ShrinkageThe inﬂuence of curing regime on the shrinkage of reference group and TiO2group AASP are shown in Fig.4.It can be seen that when cured at a RH of90±5%,the shrinkage of reference group and TiO2group at90d reached approximately1650micro strain and1400micro strain,respectively.When the RH for curing was 55±5%,both group AASPs suffered signiﬁcant drying shrinkage from the beginning of curing till90d.The drying shrinkage of ref-erence group and TiO2group at90d reached6400micro strain and 5080micro strain,respectively.It is noticed that the addition of TiO2in AASP decreased the shrinkage at both curing conditions. At90d,the reduction in shrinkage of the TiO2group was measured to be18%and27%,when the curing RH was selected to be90±5% and55±5%,respectively,in comparison with that of the reference group.Table4shows the relative shrinkage referring to the shrinkage value at90d.It can be found that when the curing RH was90±5%, 83–84%of the shrinkage took place within7days and95–98% within28days.After28days,the curves of both groups of AASPs tend to level off.When the curing RH was55±5%,only67–70% of the shrinkage happened within7days and89–93%within 28days.After28days,the curves keep on going up and do not level off even at90d.It is obvious that the drying process of AASP under55±5%RH lasts much longer than under90±5%RH,no matter nano-TiO2is utilized or not.These results demonstrate that the addition of0.5%nano-TiO2 enhanced the mechanical strength of AASPs,and decreased the shrinkage of AASP under20±3°C and90±5%or55±5%RH. Literature data[8,11–14]indicate that pore size distribution and characteristics of hydration products are the critical factors affect-ing the shrinkage in OPC and AASP.In the following text,FTIR,SEM and MIP were utilized to investigate the hydration products and(a) SEM image: without dispersion (b) SEM image: after dispersion(c) TEM image:after dispersionFig.3.Images of nano-TiO2particles.Table2Mix proportions of AASP(in mass).Sample W/B Ms Na2O(%)TiO2(%)Reference group0.4 1.2 4.00TiO2group0.4 1.2 4.00.5Table3Mechanical strength of AASP with and without TiO2.Reference group TiO2group3d7d28d3d7d28dCompressive strength,MPa23.4833.9157.5625.7639.1562.96 Flexural strength,MPa 6.1710.0012.587.7112.4617.32 Ratio ofﬂexural-compressive strength0.2620.2950.2190.2990.3180.275L.Y.Yang et al./Cement&Concrete Composites57(2015)1–73the structure of AASP to reveal the inﬂuence of nano-TiO2from micro scale.4.3.FTIR resultsSince the hydration products of AASP are mainly amorphousS-H[13],FTIR was carried out to determine the hydration products and their relative quantity by differentiating the typical wave numbers and their transmittance.Fig.5shows the infrared spectra reference group and TiO2group AASPs cured under90±5%RH. The infrared spectra of reference group and TiO2group in corre-sponding curing time are very similar,except that the transmit-tance at particular wave numbers is different.The bands 3448cmÀ1and1654cmÀ1are related to O–H stretching and molecular water,respectively[13,31].Bands between1410cm and1490cmÀ1in sharp shape,and small band at876cmÀ1are associated to anti-symmetric stretching(m3)and out-of-plane bending(m2)modes of CO32Àions[32],which is the product result-ing from carbonation in air during sample preparation.Bands at 964cmÀ1and at457cmÀ1are due to anti-symmetric Si–O(Al) stretching vibrations(m3)and to in-plane Si–O bending vibrations (m2)in SiO4tetrahedra,respectively[31–33].Bands at700cmÀ1 are the result of silicon substitution by aluminum in the silicon-oxygen tetrahedron structure.These bands are attributed to C-S-H and/or C-A-S-H gel.Compared to the reference group,the transmittance of TiO2 group at1420cmÀ1,946cmÀ1and457cmÀ1is enhanced due to the addition of nano-TiO2.The higher transmittance of TiO2group at1420cmÀ1shows that more hydration products were carbon-ated than in reference group.From the higher transmittance at 946cmÀ1and457cmÀ1in TiO2group,it could be deduced that more hydration products like C-S-H and C-A-S-H were produced when nano-TiO2was added.C-S-H and C-A-S-H are known as rigid gel that contributes to the strength of cement-based materials.Therefore more C-S-H and C-A-S-H produced in TiO2group AASP account for its higher mechanical strength in comparison with the reference group.4.4.SEM observation resultsFTIR results revealed that there was no new type of hydration products produced when nano-TiO2was added to AASP,though the amount of hydration products increased.SEM was applied to further investigate the differences in micro structure between ref-erence group and TiO2group at3d,7d and28d in this investiga-tion.The typical pictures taken under SEM are shown in Fig.6.At3d,two typical types of morphology of hydration productsFig.4.Shrinkage of the AASPs with and without TiO2.Table4Relative shrinkage referring to90d shrinkage.90±5%RH55±5%RH7d/90d28d/90d7d/90d28d/90dReference group,%84957093TiO2group,%83986789Fig.5.Infrared spectra of AASP with and without TiO2(90±5%RH).Composites57(2015)1–7L.Y.Yang et al./Cement&Concrete Composites57(2015)1–75(a) Morphology of reference group at 3d: cracks in matrix (a1); reticular outerproducts (a2); rod like inner products (a3)(b) Morphology of TiO2group at 3d: less cracks in matrix (b1); reticular outerproducts (b2); rod like inner products (b3)(c) Morphology of reference group at 7d: cracks in matrix (c1); reticular outerproducts and cracks (c2); rod like inner products (c3)(d) Morphology of TiO2 group at 7d: matrix (d1); loose outer products (d2); rod likeinner products(d3)(e) Morphology of reference group at 28d: cracks in matrix (e1); loose outer productsand cracks (e2)(f) Morphology of TiO2 group at 28d: matrix (f1); dense and massive hydrationproducts (f2)SEM images of reference group and TiO2group at3d,7d and28d(90considerably reduced the width and number of micro cracks in AASP matrix.At 7d,the microstructures of both groups are much denser.For reference group,reticular C-S-H gel is still found,but the pore size is smaller than that at 3d,and micro cracks are formed across the porous C-S-H.For TiO 2group,reticular C-S-H seems to have disap-peared,granular C-S-H is however observed in originally water ﬁlled space.The structure of inner hydration products (originally rod like C-S-H area,as observed at 3d)of TiO 2group is more homogeneous than that in reference group as well.At 28d,AASPs for both groups have developed into solid mass at micro scale.Here again,the structure of TiO 2group is much den-ser than reference group,and much less cracks exist in TiO 2group.As far as the results from FTIR and SEM are concerned,the addi-tion of nano-TiO 2into AAS accelerated the hydration process,resulting in more hydration products like C-S-H and C-S-A-H,and more densiﬁed microstructure.This result is consistent with the enhanced mechanical strength of TiO 2group samples (cf.Section 4.1).4.5.MIP resultsMore quantitative results on the microstructure,particularly the pore structure of the two groups of AASPs were obtained from the MIP measurements.Fig.7shows the cumulative porosity of both reference group and TiO 2group from 3d to 28d tested with pared to the reference group,the TiO 2group had rela-tively lower total porosity,i.e.27.5%,24.5%and 19.8%at 3d,7d and 28d respectively,while the porosity in reference group was 31.6%,29.6%and 28.4%,respectively.According to Collins et al.[8],the total high porosity of pores within the mesopore region (1.25–25nm)in AASPs could explain their high magnitude of drying shrinkage.In Fig.7,the total poros-ity of AASPs is consisted of two parts,i.e.mesopores of 1.25–25nm and pores larger than 25nm.It can be seen that the addition of nano-TiO 2not only reduced the total porosity of AASP,but also remarkably changed the pore distribution.For the reference group,the porosity of 1.25–25nm pores was 28%,27%and 25.6%at 3d,7d and 28d,respectively.The corresponding value of TiO 2group was however much lower,being 14.9%,13.8%and 15.2%,respec-tively.When referring to the total porosity,the volume percentageof mesopores in reference AASPs was 91.1%,91.2%and 89.9%at 3d,7d and 28d,respectively.However,the corresponding value of TiO 2group was 54.3%,56.5%and 77.1%,respectively.When our MIP results are incorporated with the results of the shrinkage test in Section 4.2,consistence is obvious with the view of Collins et al.[8]and Tarek Aly et al.[11]that capillary tensile forces set up during drying is a very signiﬁcant factor for the drying shrinkage of AAS.The relatively lower shrinkage of TiO 2group AASP could be explained by the much less amount of 1.25–25nm mesopores in the paste,when compared with the AASP without nano-TiO 2.It should be pointed out that the enhanced stiffness of AASP with nano-TiO 2,caused by the improvement of the mechanical strength,contributes to the decreased shrinkage to some extent as well,though the impact is not valuated in this paper.5.ConclusionsIn this investigation,0.5%(in mass)nano-TiO 2was added to alkali activated slag paste (AASP),and the mechanical properties,shrinkage,hydration products and microstructure of AASP were examined and compared with that of AASP without nano-TiO 2.The following conclusions are drawn.(1)The addition of nano-TiO 2into the AASP enhances the com-pressive and the ﬂexural strength of the paste,and improves the ﬂexural to compressive strength ratio as well.(2)The addition of nano-TiO 2into the AASP reduces the shrink-age of the paste cured under 20±3°C and 90±5%or 55±5%RH.Under 90±5%RH,the shrinkage curves tend to level off after 28days,but go up even till 90days under 55±5%RH.It should however be acknowledged that the shrinkage strain is still unacceptably large and further remedies are needed.(3)FTIR and SEM results demonstrated that the addition ofnano-TiO 2into the AASP accelerates its hydration process,resulting in more hydration products and denser structure.(4)MIP results showed that the addition of nano-TiO 2reducesthe total porosity of AASP and changes the pore structure.The porosity of 1.25–25nm mesopores,which is believed by previous researchers to be responsible for the high shrinkage of AASP,is remarkably reduced due to the addi-tion of nano-TiO 2.AcknowledgementsThe funding from the National Natural Science Foundation of China (project No.51378115)and 973project (2015CB655104)is greatly appreciated.The support from the Collaborative Innovation Center for Advanced Civil Engineering Materials is also acknowledged.References[1]Bakharev T,Sanjayan JG,Cheng YB.Sulfate attack on alkali-activated slagconcrete.Cem Concr Res 2002;32(2):211–6.[2]Fu Y,Cai L,Yonggen W.Freeze–thaw cycle test and damage mechanics modelsof alkali-activated slag concrete.Constr Build Mater 2011;25(7):3144–8.[3]Bakharev T,Sanjayan JG,Cheng YB.Resistance of alkali-activated slag concreteto acid attack.Cem Concr Res 2003;33(10):1607–11.[4]Pawlasova S,Skavara F.High-temperature properties of geopolymer materials.Alkali activated materials-research,production and utilization 3rd conference,Prague;2007.p.523–4.[5]El-Didamony H,Amer AA,Abd Ela-ziz H.Properties and durability of alkali-activated slag pastes immersed in sea water.Ceram Int 2012;38(5):3773–80.[6]Shi C.Corrosion resistance of alkali-activated slag cement.Adv Cem Res2003;15(2):77–81.[7]Chi M,Chang J,Huang R.Strength and drying shrinkage of alkali-activated slagpaste and mortar.Adv Civil Eng 2012;2012:7.Fig.7.Cumulative porosity of the AASPs (90±5%RH).Composites 57(2015)1–7[8]Collins F,Sanjayan JG.Effect of pore size distribution on drying shrinking ofalkali-activated slag concrete.Cem Concr Res2000;30(9):1401–6.[9]Palacios M,Puertas F.Effect of carbonation on alkali-activated slag paste.J AmCeram 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环境的重要性英语作文

环境的重要性英语作文英文回答：The importance of the environment cannot be overstated. Our environment provides us with the air we breathe, the water we drink, and the food we eat. It also offers us beautiful landscapes, diverse ecosystems, and a home for countless species of plants and animals. Without a healthy environment, our very existence would be threatened.One of the most important aspects of the environment is its role in regulating the Earth's climate. The natural processes within the environment help to maintain a stable climate, which is essential for life on Earth. However, human activities such as deforestation, industrialization, and the burning of fossil fuels have led to an increase in greenhouse gas emissions, resulting in climate change and global warming. This has led to more frequent and severe natural disasters such as hurricanes, floods, and droughts, which have devastating effects on communities andecosystems.Furthermore, a healthy environment is crucial for human health. Air and water pollution, as well as the degradation of natural habitats, can have serious consequences for human well-being. For example, air pollution from vehicle emissions and industrial activities can lead to respiratory diseases and other health issues. Contaminated water sources can cause waterborne diseases and other health problems. Protecting the environment is therefore essential for safeguarding public health.In addition, the environment also plays a vital role in supporting economic activities. Ecosystem services such as pollination, water purification, and soil fertility are essential for agriculture and food production. Natural resources such as timber, minerals, and energy sources are also derived from the environment and are crucial for economic development. Therefore, the health of the environment directly impacts the prosperity of societies.In conclusion, the environment is of utmost importancefor the well-being of both humans and the planet. It is our responsibility to protect and preserve the environment for future generations. By making sustainable choices in our daily lives, supporting conservation efforts, andadvocating for environmental policies, we can ensure a healthy and thriving environment for all.中文回答：环境的重要性不言而喻。

提高人们的环保意识英语作文六级

提高人们的环保意识英语作文六级全文共3篇示例，供读者参考篇1Raising Environmental AwarenessWe are currently facing an unprecedented environmental crisis that demands our immediate attention and action. The Earth, our home, is suffering from the detrimental effects of human activities, threatening the very foundation of life as we know it. As students and future leaders, it is our responsibility to raise environmental awareness and promote sustainable practices to safeguard our planet for generations to come.The environmental challenges we face today are multifaceted and deeply rooted in our way of life. Climate change, driven by the excessive emission of greenhouse gases, is causing rising temperatures, sea-level rise, and extreme weather events. Deforestation, driven by the insatiable demand for land and resources, is destroying irreplaceable habitats and biodiversity. Pollution, in the form of plastic waste, toxic chemicals, and air contaminants, is poisoning our air, water, andsoil, putting both human health and the health of ecosystems at risk.Addressing these issues requires a fundamental shift in our mindset and behavior. We must move away from the prevailing mindset of overconsumption and exploitation of natural resources towards a more sustainable and eco-friendly approach. This transformation begins with raising environmental awareness among individuals, communities, and societies.Education is the cornerstone of environmental awareness. From an early age, children should be taught about the intricate workings of our planet, the importance of preserving its natural resources, and the consequences of our actions on the environment. Schools and universities must integrate environmental studies into their curricula, fostering an understanding of ecological principles and sustainable practices. By equipping students with knowledge and critical thinking skills, we can empower them to make informed decisions and take responsible actions towards environmental protection.Furthermore, public awareness campaigns play a crucial role in disseminating information and inspiring behavioral change. Through various media channels, such as social media, television, and public events, we can raise awareness about pressingenvironmental issues and encourage individuals to adopteco-friendly habits. Simple actions like reducing plastic waste, conserving energy, and supporting sustainable businesses can have a profound impact when collectively embraced by society.Governments and policymakers also have a vital role to play in promoting environmental awareness. By implementing comprehensive environmental education programs, incentivizing sustainable practices, and enacting stricter regulations on polluters, governments can create an enabling environment that encourages individuals and businesses to prioritize environmental protection.Moreover, environmental awareness should extend beyond individual actions and encompass a broader understanding of the interconnectedness of socio-economic and environmental factors. We must recognize that environmental degradation often disproportionately affects marginalized communities and exacerbates social inequalities. By fostering environmental awareness, we can promote social justice and address the root causes of environmental injustice.As students, we have a unique opportunity to be agents of change and shape the future of our planet. We can participate in environmental clubs, organize awareness campaigns, andintegrate sustainable practices into our daily lives. By setting an example and inspiring others around us, we can create a ripple effect that encourages more individuals to embrace environmental responsibility.Furthermore, we can leverage our academic pursuits to contribute to environmental solutions. Through research and innovation, we can develop new technologies, sustainable materials, and eco-friendly practices that can mitigate the impact of human activities on the environment. Interdisciplinary collaboration among students from various fields, such as engineering, biology, and economics, can yield innovative solutions that address environmental challenges from multiple perspectives.Environmental awareness is not merely a passing trend; it is a fundamental necessity for the survival of our planet and the well-being of future generations. By raising awareness and promoting sustainable practices, we can collectively work towards a greener, cleaner, and more sustainable future.In conclusion, the environmental challenges we face are daunting, but they are not insurmountable. By embracing environmental awareness and taking collective action, we can inspire positive change and pave the way for a more sustainableworld. As students, we have the power to shape the future and leave a lasting legacy of environmental stewardship. Let us rise to the challenge and become the catalysts for a greener, healthier, and more harmonious coexistence with our planet.篇2Raising Environmental AwarenessAs a student, I am increasingly concerned about the state of our environment and the need to raise awareness about the critical issues we are facing. Environmental degradation poses a severe threat to our planet and its inhabitants, and it is our collective responsibility to take action and address this pressing challenge. In this essay, I will explore the importance of raising environmental awareness, discuss the key environmental problems we face, and propose strategies to promote greater understanding and encourage positive change.The impact of human activities on the environment has been profound and far-reaching. Climate change, driven by the emission of greenhouse gases, is causing rising temperatures, sea level rise, and extreme weather events. Deforestation, pollution, and the overexploitation of natural resources are also contributing to the deterioration of our planet's ecosystems.These issues not only endanger wildlife and biodiversity but also threaten human health, food security, and economic stability.One of the most pressing environmental concerns is climate change. The burning of fossil fuels for energy production and transportation has released vast quantities of carbon dioxide and other greenhouse gases into the atmosphere, trapping heat and causing global temperatures to rise. The consequences of this phenomenon are already being felt around the world, including more frequent and intense heatwaves, droughts, wildfires, and storms. Rising sea levels threaten coastal communities, and the melting of glaciers and polar ice caps could disrupt global weather patterns and water cycles.Another significant environmental issue is the loss of biodiversity. Human activities, such as deforestation, habitat destruction, and pollution, have driven countless species to the brink of extinction. The disappearance of these species can have cascading effects on entire ecosystems, disrupting food chains and diminishing the planet's ability to sustain life. Preserving biodiversity is not only important for maintaining a healthy and balanced environment but also for safeguarding potential sources of food, medicine, and other valuable resources.Pollution, in its various forms, also poses a severe threat to the environment and human health. Air pollution, caused by the burning of fossil fuels and industrial emissions, can lead to respiratory diseases and contribute to climate change. Water pollution, from industrial effluents, agricultural runoff, and improper waste disposal, contaminates our rivers, lakes, and oceans, endangering aquatic life and making water unsafe for human consumption. Plastic pollution, in particular, has become a global crisis, with vast amounts of plastic waste polluting our oceans and harming marine life.To address these environmental challenges, it is crucial to raise awareness and promote sustainable practices among individuals, communities, and governments. Education plays a vital role in this effort, as it empowers people with the knowledge and understanding necessary to make informed decisions and adopt environmentally responsible behaviors.Schools and universities should incorporate environmental education into their curricula, teaching students about the impacts of human activities on the environment and the importance of conservation and sustainability. Hands-on activities, such as school gardens, recycling programs, and field trips to natural areas, can help students develop a deeperconnection with the natural world and cultivate a sense of environmental stewardship.Beyond篇3Raising Environmental AwarenessWe live on a planet that's becoming sicker by the day. Human activities like pollution, deforestation, and overconsumption of natural resources are taking a massive toll on the environment. If we don't act now, future generations will inherit an uninhabitable world plagued by climate change, lack of clean air and water, and depleted natural resources. That's a future no one wants, which is why raising environmental awareness is so crucial.The impacts of environmental degradation are already evident all around us. Just look at the smog blanketing major cities, the shocking amounts of plastic polluting our oceans, or the rising global temperatures causing more frequent and intense natural disasters. These aren't just temporary issues - they're symptoms of humanity's careless treatment of our planet over decades. We simply cannot continue down this destructive path.Protecting the environment seems like an overwhelming challenge, but it becomes more manageable when every citizen does their part. After all, we all share the responsibility of being custodians of the Earth. Individuals can start by educating themselves on eco-friendly practices and being more conscious consumers. Simple things like reducing energy usage at home, minimizing plastic waste, and eating less meat can collectively make a big difference.Schools also play a pivotal role in nurturing environmental stewardship from a young age. More educational institutions should incorporate sustainability into their curriculums, teaching the next generation about conservation, renewable energy sources, and living in harmony with nature. Hands-on learning through school gardens, recycling initiatives, and field trips can go a long way in shaping eco-conscious mindsets early on.However, individual action alone isn't enough - governments and corporations must also prioritize environmental protection through stronger policies and regulations. Oil and gas companies need to transition towards renewable energy sources. Manufacturers should explore greener production methods and sustainable product designs. Agricultural practices have to shift from industrial farming thatrelies heavily on pesticides towards more eco-friendly techniques.Protecting forests, a critical carbon sink that slows climate change, should be another top priority. Deforestation, mainly caused by illegal logging and clearing land for agriculture, has had devastating consequences on biodiversity and indigenous communities who rely on forest resources for survival. Developed nations should provide more funding and resources towards conservation and reforestation efforts globally.In our consumer-driven society, the private sector's involvement is essential for raising environmental awareness as well. Brands have immense power to influence consumer behavior through their messaging and products. Companies should opt for eco-friendly packaging, sustainable sourcing of materials, and marketing that encourages shoppers to adopt greener lifestyles. Corporate social responsibility initiatives can fund environmental education programs and restoration projects.The media also needs to give more prominence to environmental reporting. Instead of burying climate news or giving undue airtime to skeptics, they should make this an urgent, front-page issue. Using their platforms to amplify thevoices of environmental activists, indigenous communities affected by eco-degradation, and scientific experts would go a long way in galvanizing public support for conservation efforts.On an individual level, each of us can be powerful agents of change through voting and activism. We should elect leaders who have robust, science-backed policies for environmental protection rather than those backed by corporate fossil fuel interests. Actively participating in protests, petitions, community clean-ups and other grassroots initiatives raises awareness and shows there is strong public demand for immediate climate action.Raising environmental awareness is the first step, but awareness without action is meaningless. As temporary inhabitants of this planet, it's our moral obligation to be responsible stewards and pass on a healthy, thriving Earth to future generations. We cannot afford inertia or complacency any longer. The time to protect our planet is now, before it's too late. Through collective action driven by a conscious society, we can absolutely build a more sustainable future for all.。

六级预测翻译

面对日益严重的环境问题，低碳经济越来越引起世界各国的关注。

对于低碳经济的界定虽各有不同，但有一点是没有争议的。

人们普遍承认，低碳经济是以低能耗、低污染、低排放为基础的经济模式，是人类社会继农业文明、工业文明之后的又一次重大进步。

低碳经济实质上是能源高效利用、清洁能源开发、追求绿色GDP的问题，核心是能源技术和减排技术创新、产业结构和制度创新以及人类生存发展观念的根本性转变。

在我国经济发展的关键时期，更加协调低碳经济与发展的关系，保护地球的生态环境，事关中国人民乃至全世界人民的福祉。

【低碳经济】Facing the increasingly serious environmental problems, low-carbon economy attracts more and more attention of the countries all over the world. For the definition of low-carbon economy is different, but one thing is not controversial. It is widely acknowledged that low-carbon economy is a economical model on the basis of low energy consumption, low pollution and low emission, a major progress of human society following agriculture civilization, industrial civilization. In essence, low-carbon economy is efficiently use of energy, development of clean energy and pursuit of green GDP, with the core of energy technology and emission reduction technology innovation, industrial structure and system innovation, and the fundamental shift of human’s survival and development ideas. In the critical period of economic development in our country, to further coordinate the relationship between low-carbon economy and the development, protect the ecological environment of the earth, is about the well-being of the Chinese people and the people all over the world as well.庐山瀑布群是中国最秀丽的十大瀑布之一，坐落于江西省九江市庐山。

2023年12月英语六级阅读原文

2023年12月英语六级阅读原文原文标题：The Importance of Environmental Protection随着工业化和城市化的进程不断加快，环境问题已经成为全球性的焦点。

关于这一话题，许多人有不同的看法。

有一些人认为环境污染是制约人类社会发展的主要障碍之一，应当尽快加强环境保护。

而也有一些人对此持怀疑和否定态度，认为环境问题并不严重，环境污染对人类社会的发展不构成实质性的威胁，因此不必大惊小怪。

无论如何，我们都不能忽视环境问题的存在和严重性。

环境保护的重要性首先体现在生态系统的稳定和人类生存环境的改善。

生态系统是地球上的重要基础设施，不同的生物之间通过各种复杂的生态关系相互依存，形成生态系统的稳定性。

然而，由于人类活动过度开发和环境污染，使得原有的生态系统遭到破坏，生物多样性下降，一些濒临灭绝的物种濒临灭绝。

这对于人类生存环境产生了严重的影响。

加强环境保护，保持生态系统的稳定性，保护生物多样性是十分必要的。

环境保护对于人们的身体健康和身心健康是至关重要的。

环境污染直接危害人们的身体健康，长期暴露在污染环境中会导致各种慢性疾病的发生，甚至致癌。

而且，环境污染还会对人们的心理健康造成影响，长期生活在污染环境中会使人产生消极的情绪，降低人们的生活质量。

为了保护人们的身体健康和身心健康，必须加强环境保护，减少环境污染。

环境保护与可持续发展的理念相一致，是现代社会发展的必由之路。

可持续发展是指经济、社会和环境的协调发展，即在满足当前需求的前提下，能够保证子孙后代也能满足其需要。

如果环境得不到有效的保护，将严重威胁人类社会的可持续发展，甚至会导致资源过度消耗，生态平衡被打破，给子孙后代留下巨大的环境债务。

加强环境保护，推动可持续发展已成为全球热点问题。

环境保护对于人类社会的发展至关重要。

只有加强环境保护，才能保障生态系统的稳定和人类生存环境的改善，保护人们的身体健康和身心健康，推动可持续发展。

职教高考英语试卷

考试时间：120分钟满分：150分一、听力（共20小题，每小题1.5分，满分30分）Directions: In this section, you will hear 10 short conversations. At the end of each conversation, a question will be asked about what was said. Both the conversation and the question will be spoken only once. After each question there will be a pause. During the pause, you must read the four choices marked A), B), C) and D), and decide which is the best answer. Then mark the corresponding letter on Answer Sheet 1 with a single line through the center.1. A) He has finished his homework. B) He is doing his homework. C) He is not doing his homework. D) He doesn't have homework.2. A) She is a teacher. B) She is a student. C) She is a doctor. D) She is a nurse.3. A) They are friends. B) They are classmates. C) They are brother and sister. D) They are mother and son.4. A) It is sunny. B) It is cloudy. C) It is raining. D) It is windy.5. A) He is going to the library. B) He is going to the cinema. C) He is going to the supermarket. D) He is going to the park.二、阅读理解（共20小题，每小题2分，满分40分）Directions: There are 4 passages in this section. Each passage is followed by some questions or unfinished statements. For each of them there are four choices marked A), B), C) and D). You should decide on the best choice and mark the corresponding letter on Answer Sheet 1.Passage 1In recent years, online learning has become increasingly popular. Many people prefer online learning because it offers flexibility and convenience. However, some people argue that traditional classroom learning is more effective.Question 6: What is the main topic of the passage?A) The advantages of online learning.B) The disadvantages of online learning.C) The differences between online learning and traditional classroom learning.D) The reasons why people choose online learning.Passage 2The importance of exercise cannot be overstated. Regular exercise not only helps maintain physical health but also improves mental well-being. For example, exercise can reduce stress, enhance mood, and improve sleep quality.Question 7: What is the main idea of the passage?A) The benefits of exercise.B) The causes of stress.C) The importance of sleep.D) The role of diet in health.Passage 3The environmental crisis is a global issue that requires immediate action. Many countries have implemented policies to reduce pollution and promote sustainability. However, more needs to be done to address the root causes of environmental degradation.Question 8: What is the main topic of the passage?A) The causes of environmental degradation.B) The effects of pollution on health.C) The role of government in environmental protection.D) The importance of sustainability.Passage 4Technology has revolutionized the way we communicate. From email to social media, we can now connect with people from all over the world in seconds. However, this convenience has also brought challenges, such as information overload and privacy concerns.Question 9: What is the main topic of the passage?A) The benefits of technology.B) The challenges of technology.C) The impact of social media on society.D) The future of technology.三、完形填空（共20小题，每小题1.5分，满分30分）Directions: For each blank in the following passage, choose the best answer from the four choices given below. Then mark the corresponding letter on Answer Sheet 1.I had always been a fan of reading. As a child, I spent countless hours buried in books, exploring different worlds and learning new things. However, when I reached high school, my passion for reading began to wane. (10) __________, I found myself struggling to find books that interested me. I was (11) __________ by the vast array of options available, and it seemed impossible to choose. One day, I stumbled upon a bookshop that specialized in second-hand books. (12) __________, I felt a sense of excitement and nostalgia. I remembered how much I enjoyed exploring the shelves and discovering hidden gems. (13)__________, I spent hours browsing the books, and eventually, I found a novel that captivated me. It was a story about adventure and self-discovery, and it reignited my love for reading.10. A) Unexpectedly B) Unfortunately C) Unfortunately D) Unfortunately11. A) confused B) excited C) disappointed D) interested12. A) At first B) Suddenly C) Finally D) Suddenly13. A) Finally B) Unexpectedly C) Suddenly D) Unfortunately四、短文改错（共10小题，每小题1分，满分10分）Directions: For each of the following sentences, if there is an error, correct it by changing a letter, adding a letter, or deleting a letter. If there is no error, put a tick (√) in the corresponding blank.1. The __________ (change) of technology has changed our lives in many ways.2. Many people prefer to __________ (use) public transportation to drive their own cars.3. It is important to __________ (take) care of our health and exercise regularly.4. __________ (be) a good reader requires a lot of practice and patience.5. The __________ (offer) of online courses has made learning more accessible and convenient.6. We should always __________ (pay) attention to the safety rules when we are using the internet.7. It is important to __________ (protect) the environment and reduce pollution.8. __________ (visit) the library is a great way to spend our free time.9. __________ (be) a responsible citizen means contributing to our community.10. The __________ (make) of exercise is undeniable, as it improves both physical and mental health.五、书面表达（共1题，满分20分）Directions: Write an essay of about 100-120 words on the following topic: The Importance of Learning a Second Language. You should base your essay on the outline given below:1. Introduce the importance of learning a second language.2. Discuss the benefits of learning a second language.3. Give your own opinion on why everyone should learn a second language.【答案】一、听力答案1. B2. A3. C4. A5. A二、阅读理解答案6. C7. A8. A9. B三、完形填空答案10. B 11. C 12. A 13. A四、短文改错答案1. change → changing2. use → using3. take → taking4. be → becoming5. offer → offering6. pay → paying7. protect → protecting8. visit → visiting9. be → being10. make → making五、书面表达答案The Importance of Learning a Second LanguageLearning a second language is increasingly important in today's globalized world. It not only broadens our horizons but also offersnumerous benefits. Firstly, it enhances our communication skills and enables us to connect with people from different cultures. Secondly, it improves our cognitive abilities and makes us more adaptable to new situations. Lastly, it opens up new opportunities for career growth and personal development.In my opinion, everyone should learn a second language. It is a valuable skill that can bring us closer to the world and enrich our lives.。

Deforestation Causes and Effects

Deforestation Causes and Effects Deforestation is a critical environmental issue that has far-reaching causes and effects. The primary cause of deforestation is human activity, including logging, agriculture, and urbanization. These activities result in the clearing of large areas of forests, leading to numerous negative effects on the environment, wildlife, and human populations. In this response, I will explore the causes and effects of deforestation from various perspectives, shedding light on the urgency of addressing this pressing issue. From an environmental perspective,deforestation has devastating consequences on the Earth's ecosystems. Forests play a crucial role in regulating the climate by absorbing carbon dioxide from the atmosphere. When trees are cut down, this natural carbon sink is compromised, leading to increased greenhouse gas emissions and contributing to climate change. Furthermore, deforestation disrupts the water cycle, leading to soil erosion, decreased water quality, and disrupted rainfall patterns. This, in turn, affects the biodiversity of the forests, leading to the loss of countless plant and animal species. From a wildlife perspective, deforestation poses a significant threat to numerous species that call the forests their home. As their natural habitats are destroyed, many animals are forced to migrate to new areas, leading to increased competition for resources and heightened risk of extinction. Deforestation also fragments habitats, making it difficult for wildlife to find food, shelter, and mates. This can lead to a decline in population numbers and genetic diversity, further jeopardizing the survival of many species. From a human perspective, deforestation has wide-ranging social and economic impacts. Many indigenous communities rely on forests for their livelihoods, including food, medicine, and shelter. When these forests are cleared, these communities lose not only their homes but also their cultural heritage and traditional knowledge. Additionally, deforestation can lead to soil degradation, making it challenging for farmers to grow crops and sustain their livelihoods. This can result in food insecurity and poverty, further exacerbating social inequalities. Furthermore, deforestation contributes to the loss of ecosystem services that are vital for human well-being. Forests play a crucial role in regulating the water cycle, preventing soil erosion, and providing clean air and water. When these services are compromised, humanpopulations are at greater risk of natural disasters, such as floods and landslides, and are more susceptible to air and water pollution. Additionally, the loss of forests can impact industries that rely on forest resources, such as timber and paper production, leading to economic instability and job loss in these sectors. In conclusion, deforestation is a complex issue with far-reaching causes and effects that impact the environment, wildlife, and human populations. Addressing this issue requires a multi-faceted approach that considers the social, economic, and environmental dimensions of deforestation. By raising awareness, implementing sustainable land management practices, and supporting conservation efforts, we can work towards mitigating the causes and effects of deforestation and preserving the invaluable benefits that forests provide to the planet and all its inhabitants.。

Compressive Strength and Rapid Chloride Permeability of

Compressive Strength and Rapid Chloride Permeability ofConcretes with Ground Fly Ash and SlagOzkan Sengul 1and Mehmet Ali Tasdemir 2Abstract:Concretes with binary and ternary blends of portland cement,ﬁnely ground ﬂy ash and ﬁnely ground granulated blast furnace slag were produced to investigate their effects on compressive strength and rapid chloride permeability.Portland cement was partially replaced by ﬁnely ground ﬂy ash ͑Blaine speciﬁc surface:604m 2/kg ͒and ﬁnely ground granulated blast furnace slag ͑Blaine speciﬁc surface:600m 2/kg ͒.Two series of concrete with water/binder ratios of 0.60and 0.38were produced and for both water/binder ratios,portland cement was replaced by:͑i ͒50%ﬂy ash;͑ii ͒50%blast furnace slag;and ͑iii ͒25%ﬂy ash+25%blast furnace slag.At the high water/binder ratio,compressive strengths of the concretes with the pozzolans are lower compared to that of the portland cement concrete.At the low water/binder ratio,however,these strength reductions are less compared to the high water/binder ratio and compressive strength of the concrete produced with 50%slag was even higher than the portland cement concrete.The test results indicate the ground ﬂy ash and ground granulated blast furnace slag greatly reduce the rapid chloride permeability of concrete.It was concluded that to reduce the chloride permeability of concrete,inclusion of pozzolans are more effective than decreasing the water/cement ratio.DOI:10.1061/͑ASCE ͒0899-1561͑2009͒21:9͑494͒CE Database subject headings:Fly ash;Slag;Compressive strength;Chlorides;Optimization;Concrete;Portland cements;Permeability .IntroductionCement production is an energy intensive process which also has an important effect on the environment.Producing one ton of portland cement releases about one ton of CO 2green house gas into atmosphere and as a result of this production 1.6billion tons of CO 2is released every year which is estimated at about 7%of the CO 2production worldwide ͑Mehta 2001;Malhotra 1999͒.The pressure of ecological constraints and environmental regulations are bound to increase in the coming years which will lead to greater use of supplementary cementitious materials such as ﬂy ash or ground granulated blast furnace slag ͑GGBS ͒͑Aitcin 2000͒.There are two major reasons to use these by-products in concrete:͑1͒decreasing cement consumption by replacing part of cement with these pozzolanic materials and ͑2͒improving fresh and hardened concrete properties.In recent years,the reduction of water/cement ratio by using superplasticizers and usage of ul-traﬁne mineral admixtures lead to high performance concrete.Be-side the advantages,pozzolanic materials have certain drawbacks.To overcome some of the disadvantages and to be able to use the pozzolan in higher amounts,quality of the pozzolan can be im-proved.Chemical composition,particle-size distribution,ﬁneness,andpozzolanic activity,and curing conditions of concrete are impor-tant factors affecting the properties of concretes with pozzolanic materials ͑ACI Committee 2321996;ACI Committee 2331995;ACI Committee 2341996͒.In recent years,it has been shown that the ﬁller effect of mineral admixtures may be as important as their pozzolanic effects;according to some researchers,however,the ﬁller effect can be more important than the pozzolanic effect ͑Goldman and Bentur 1993;Isaia et al.2003͒.Particle-size dis-tribution clearly plays a very important role in the rate of chemi-cal reactivity and in the water demand.Pozzolanic reaction takes place on the surface of the particles and increasing surface area has an important effect on pozzolanic activity.Thus,the ﬁneness of the pozzolan is very important for the improvement of cement paste-aggregate interfacial zone,which is the weakest link in con-crete.In a previous investigation done by the research group of this study ͑Demir et al.2002͒,a coarse F type ﬂy ash with a Blaine surface area ͑BSA ͒of 222m 2/kg was ground to four different ﬁnenesses such as 337,450,538,and 604m 2/kg.The purpose of the work mentioned was to study the effects of ﬂy ash grinding on physical properties and strength development of concrete.It was concluded that as the ﬁneness of ﬂy ash increases,the compres-sive strength of concrete increases signiﬁcantly.The particle size is an important factor also for the pozzolanic activity of the granulated blast furnace slag.Similar to the ﬂy ash,the strength of slag concretes also increases with slag ﬁneness ͑Tasdemir et al.1997;Niu et al.2002͒.Besides increasing the ﬁneness,another solution to overcome the disadvantages of using high amounts of pozzolan,is to use ternary or quaternary blends of portland cement and pozzolans.By using different pozzolans together,some of the shortcomings can be compensated and more environmentally friendly concretes with speciﬁc properties can be obtained.The main objective of the work presented herein is to investi-gate the effects of ground ﬂy ash and blast furnace slag on com-1Assistant Professor Doctor,Faculty of Civil Engineering,Istanbul Technical Univ.,34469Maslak,Istanbul,Turkey ͑corresponding author ͒.E-mail:sengulozk@.tr 2Professor Doctor,Faculty of Civil Engineering,Istanbul Technical Univ.,34469Maslak,Istanbul,Turkey.Note.This manuscript was submitted on December 17,2007;ap-proved on March 26,2009;published online on August 14,2009.Dis-cussion period open until February 1,2010;separate discussions must be submitted for individual papers.This paper is part of the Journal of Materials in Civil Engineering ,V ol.21,No.9,September 1,2009.©ASCE,ISSN 0899-1561/2009/9-494–501/$25.00.D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y S o u t h e a s t U n i v e r s i t y o n 12/12/13. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .pressive strength and chloride permeability of concrete at high replacement percentages.In this work,a ﬁnely ground low-lime ﬂy ash ͑Blaine ﬁneness of 604m 2/kg ͒and ﬁnely ground blast furnace slag ͑Blaine ﬁneness of 600m 2/kg ͒were used in normal and high strength concretes to partially replace ordinary portland cement with a replacement amount of 50%.Concretes with ter-nary blends were also produced containing 25%ﬂy ash and 25%pressive strength and rapid chloride ion permeability of the concretes were obtained at 28and 90days.To attain a durable concrete mixture low chloride permeability should be obtained.Achieving high compressive strength in a concrete structure is important for structural safety.The cost of concrete is also impor-tant from the applicability point of view.For this purpose a mul-tiobjective optimization method was performed in which the compressive strength was maximized but the rapid chloride per-meability and the cost were minimized.The responses in the op-timization were considered to be of equal importance.Experimental Details MaterialsSame ordinary portland cement ͑PC 42.5͒,ﬁnely ground ﬂy ash,and ﬁnely ground blast furnace slag were used in the concretes.The 7-and 28-day compressive strengths of the standard RILEM-Cembureau cement mortars were 45.8and 57.3MPa,respec-tively.The ﬂy ash used in this study was brought from Catalagzi power plant,which is located in the northwest coast region of Black Sea in Turkey.The blast furnace slag was obtained from Karabük production plant also located in the same region.Chemi-cal compositions of ordinary portland cement ͑OPC ͒,ﬂy ash and slag are shown in Table 1.The ﬂy ash and blast furnace slag were ground in a laboratory ball mill.The original Blaine ﬁneness of the ﬂy ash was 222m 2/kg and was increased to 604m 2/kg by grinding.The ground blast furnace slag used in this study also had Blaine ﬁne-ness of 600m 2/kg.Some physical properties of the ﬂy ash and slag are shown in Table 2.The average particle size of the ﬂy ash used was relatively ﬁne and characterized by a high density.In this study,the ground ﬂy ash indicated in Table 2was used.Physical properties of the ﬂy ash such as density and ﬁneness change as it is ground.The physical changes due to grinding are:͑1͒the ﬁneness of ﬂy ash increases;͑2͒there is a remarkable increase in density by reduc-ing the porosity of the ﬂy ash particles;and ͑3͒the spherical ﬂy ash particles transform into the mostly irregular shapes;somesmall ﬂy ash particles keep their original shapes.These conclu-sions were also reported in other works ͑Demir et al.2002;Sen-gul et al.2005͒.Effects of grinding on the ﬂy ash particles are shown in Fig.1.Fig.2shows that the average particle size de-creases by grinding of the coarse ﬂy ash.Table 1.Chemical Compositions of Portland Cement,Fly Ash,and Slag Oxide composition ͑%͒OPC ͑PC 42.5͒Fly ash Blast furnace slag SiO 220.060.240.5Fe 2O 3 3.6 6.7 1.2Al 2O 3 5.121.810.3CaO 63.2 2.532.2MgO 1.1 1.611.3SO 3 2.80.5 1.3K 2O 0.8 4.9 1.1Na 2O 0.30.50.35Cl -0.030.0060.0105Loss on ignition2.80.31.9Table 2.Some Physical Properties of Fly Ash and Blast Furnace SlagPropertyFly ashBlast furnace slag Before grinding After grinding Density2.00 2.51 2.86BSA,m 2/kg222604600Retained on 200␮m sieve,%12.00.00.0Retained on 90␮m sieve,%33.00.70.0Retained on 45␮m sieve,%50.03.70.2(a)Original fly ash particles (before grinding)(Blaine surface area:222m 2/kg)(b)Ground fly ash particles (Blaine surface area:450m 2/kg)(c)Ground fly ash particles (Blaine surface area:604m 2/kg)Fig.1.SEM images of ground ﬂy ash particles:͑a ͒original particles;͑b ͒ground ﬂy ash,BSA:450m 2/kg;and ͑c ͒ground ﬂy ash,BSA:604m 2/kgD o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y S o u t h e a s t U n i v e r s i t y o n 12/12/13. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .Pozzolanic ActivityPozzolanic activity is one of the critical properties of the mineral admixtures and there are different methods for the determination of the pozzolanic activity of these materials.For the ﬂy ash used in this study,the pozzolanic activity test with lime was done according to ASTM C 311–85͑ASTM 1985a ͒.The test results obtained are given in Table 3together with ASTM C 618–85limits ͑ASTM 1985b ͒.An increase in the ﬁneness of the ﬂy ash leads to a substantial increase in its activity index at 7days.As seen in Table 3,increasing the BSA from 222to 604m 2/kg has resulted in an increase of approximately 80%in corresponding compressive strengths at 7days.Pozzolanic activity of the slag was obtained according to ASTM C989-06͑ASTM 2006͒which is based on the comparison of the compressive strengths of reference cement mortars and those of mortars produced with portland cement-slag blend.ASTM C989-06͑ASTM 2006͒mentions three grades for slag:Grade 80,Grade 100,Grade 120.The requirements for these grades and the results obtained for the slag used in this study are shown in Table 4.Based on these test results,the ground slag used can be classiﬁed as Grade 100.However,it should be noted that the results obtained are very close to the limits for Grade 120.Mixture ProportioningTwo series of concretes were produced with water/binder ratios of 0.60and 0.38.For both water/binder ratios;portland cement was replaced by:͑i ͒50%ﬁnely ground ﬂy ash;͑ii ͒50%ﬁnely GGBS;and ͑iii ͒25%ﬁnely ground ﬂy ash+25%ﬁnely GGBS.In all concretes,partial replacement of cement by ﬂy ash and slag wason one to one weight basis.Portland cement concretes were also produced for each water/cement ratio.Basalt type coarse aggre-gate was used in all concretes to obtain better concrete strengths ͑Sengul et al.2002͒.The aggregate had a low porosity and high speciﬁc gravity.To obtain a better aggregate-cement paste inter-face,the aggregates were washed and used in saturated surface dry state.The aggregate grading,water-binder ratio,and the maximum particle size of aggregate were kept constant in all concretes.The grading curve of concrete aggregate was chosen between ISO A16-B16and closer to B16.Natural sand,crushed basalt sand,and crushed basalt No.I was used in the concretes.A superplasticizer was used in low water/binder concretes to main-tain approximately the same slump.The concrete mixtures were designated as follows:50S-60,50F-60,25FS-60,50S-38,50F-38,and 25FS-38.The ﬁrst two digits show the partial replacement amount of cement by the ﬂy ash or slag.The letters after the digits represent the binder type,where S shows the blast furnace slag and F indicates the ﬂy ash.The last two digits indicate the water/binder ratio as %.For ex-ample 25FS-38represents the concrete with a water/binder ratio of 0.38and containing 25%ﬂy ash+25%blast furnace slag.On the other hand,the mixtures 100PC-60and 100PC-38show port-land cement concretes with water/cement ratios of 0.60and 0.38,respectively.All mixtures were prepared in a laboratory mixer with vertical rotation axis by forced mixing.Details of the mixtures are shown in Table 5.All the specimens were demolded after 24h and stored in a water tank saturated with lime at 20°C until the testing day.Test ProcedureThree 150mm cubes were used for the standard compressive strengths of concretes.The rapid chloride ion permeability of the concretes was obtained according to ASTM C 1202–05͑ASTM 2005͒;three concrete disc specimens of 100mm in diameter and 50-mm thick were used for the test.The compressive strength and the rapid chloride permeability of concretes obtained at 28-and 90-day old specimens are given in Table 6.Results and DiscussionCompressive Strengths of ConcretesCompressive strengths of the concretes are illustrated in Figs.3and 4.As seen in Fig.3,for the water/binder ratio of 0.60,the compressive strength of the concrete with 50%slag replacement is slightly lower than the portland cement concrete both for 28and 90days.The compressive strength of the concrete with 50%ﬂy ash replacement,however,is signiﬁcantly low.For the 0.60water/binder ratio,at 28days the strength of the ﬂy ash concrete is 59%of the portland cement concrete.At 90days,however,this ratio is 75%,which shows the signiﬁcant strength development rate of the ﬂy ash concrete between 28and 90days.Despite this substantial strength reduction compared to portland cement con-crete,at 0.60water/binder ratio and 28-day age,the compressive strength of the concrete with 50%ﬂy ash is still over 34MPa and this concrete can be classiﬁed as a structural concrete.The pozzolanic reaction of ﬂy ash in concrete depends on the break-down and dissolution of the glass phase which occurs when the pH of pore solution is higher than 13͑Fraay et al.1989͒.The pozzolanic reaction takes place between the ﬂy ash and the CHParticle Diameter,µmC u m u l a t i v e p e r c e n t a g e501002575Fig.2.Particle-size distributions of ﬂy ashes before and after grind-ingTable 3.Results of Pozzolanic Activity Index Test on the Fly AshCompressive strength at 7days,MPaFly ashBefore grinding,BSA:222m 2/kgAfter grinding,BSA:604m 2/kgExperiment7.914.2Class F in ASTM C618-85,min5.55.5Table 4.Results of the Slag Activity Index Test Slag activity index ASTM C989-06,minBlast furnace slag used in the studyGrade 80Grade 100Grade 1207-day index —75959128-day index7595115114D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y S o u t h e a s t U n i v e r s i t y o n 12/12/13. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .produced from the hydration of portland cement.This pozzolanic reaction is a slow process and is responsible for the low early strength of the high volume ﬂy ash concrete.At the 0.60water/binder ratio,the compressive strength in-crease between 28and 90days is about 27%for the ﬂy ash concrete,3%for the portland cement concrete,8%for the slag concrete,and 19%for the concrete with the ternary binder.This important strength increase of ﬂy ash concrete indicates that the pozzolanic reaction of the ﬂy ash continues at a higher rate for a longer period.The pozzolanic reaction of the blast furnace slag is more rapid when compared to that of the ﬂy ash ͑Bijen 1996a ͒.For the dis-solution of the glass phase of blast furnace slag,pore water pH of about 12is enough and this alkalinity level occurs in a short period after mixing the slag-portland cement blend with water.This more rapid reaction is one of the factors causing a higher early strength of slag concretes.As shown in Fig.3,for the water/binder ratio of 0.60,the compressive strength of the concrete with ternary blend binder is between those of the slag concrete and ﬂy ash concrete.The ter-nary binder containing 25%ﬂy ash and 25%slag also has a signiﬁcant strength increase between 28and 90days.The compressive strength of the concretes at 0.38water/binder ratios are shown in Fig.4.As seen in the ﬁgure,the strength of the slag concrete is higher than that of the portland cement con-crete at 28days and almost equal at 90days.The compressive strength of the concrete with 50%ﬂy ash is 72.8MPa at 28days and 78.2MPa at 90days and this high volume ﬂy ash concrete can be classiﬁed as a high strength concrete.For the water/binder ratio of 0.38concretes,the strength of the ﬂy ash concrete is about 85%of the portland cement at both ages.For normal strength concretes,however,this strength ratio is 59%at 28days and 75%at 90days.When the results for the 0.38and 0.60water/binder ratios are compared,it can be concluded that the ﬂy ash is much more effective at the low water/binder ratio for enhancing the strength of concrete.The better performance of ﬂy ash at lower water/binder ratios was also reported in other studies ͑Demir et al.2002;Poon et al.2000;Lam et al.2000;McCarthy and Dhir 1999͒.Similar conclusions can also be drawn for the slag concrete and the concrete with the ternary binder.For the high water/binder ratio,the ratio of the compressive strength of slag concrete to that of portland cement is 92%at 28days and 96%at 90days,but this strength ratio increases to 105%at 28days and 99%at 90days for 0.38water/binder ratio.Mineral admixtures have two effects in concrete:pozzolanic effect and ﬁller effect.A ﬁner pore size distribution and less cap-illary pores can be obtained by using ﬁne mineral admixtures in concrete.Fine pozzolanic materials also have an important effect on the aggregate-cement paste interface which is the weakest link in concrete and the thickness of this transition zone can be re-duced by using pozzolans ͑Bijen 1996b ͒.Better particle packing due to ﬁner materials and the pozzolanic reaction may be the reasons for this enhancement.By the densiﬁcation of the aggregate-cement paste transition zone,concrete becomes more homogeneous and higher strengths can be obtained.This im-provement may be more effective at lower water/binder ratios which can cause a better performance of the pozzolan.For the low water/binder ratio concretes,superplasticizers areTable 5.Mix Proportions and Some Properties of the Fresh Concretes Mix code100PC-6050S-6050F-6025FS-60100PC-3850S-3850F-3825FS-38Cement,kg /m 3348175176176450222221222Slag,kg /m 3017508802220111Fly ash,kg /m 3001768800221111Water,kg /m 3209210211211167168167168Water/binder0.600.600.600.600.380.380.380.38Superplasticizer,kg /m 30000 4.4 4.5 3.4 4.7Natural sand,kg /m 3503503497502500499487493Crushed basalt sand,kg /m 3380380375379377377368372Crushed basalt I,kg /m 3951952940951946945921933Slump ͑mm ͒9013014014080150110160Unit weight,kg /m 32,3892,3942,3752,3962,4362,4382,3892,415Air ͑%͒2.21.61.10.93.32.73.12.8Table 6.Mechanical Properties,Chloride Permeability,and Relative Cost of ConcretesWater/binder Mixture code Cube compressive strength of concrete͑MPa ͒ASTM C1202chloride ion penetration test͑Coulomb ͒Relative costs of concretes28days 90days 28days 90days 0.60100PC-6055.757.56,8135,500348.050S-6051.355.3703372288.850F-6034.243.3926161264.025FS-6042.951.2660376277.20.38100PC-3886.192.21,8771,780450.050S-3889.991.5395206366.350F-3872.878.2531144331.525FS-3883.186.6387217349.7D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y S o u t h e a s t U n i v e r s i t y o n 12/12/13. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .used to obtain enough workability.These chemical admixtures causes better dispersion of cement particles and higher early com-pressive strengths can be achieved due to the greater hydration rate in a well dispersed system ͑Mehta and Monterio 1993͒.The superplasticizer used in the 0.38water/binder ratio concretes may have helped for a better dispersion of the pozzolans which may have caused a less strength decrease when compared to the 0.60water/binder ratio concretes.The enhanced hydration in a well dispersed binder system also depends on the size of the mineral admixture.The particle sizes of the ﬂy ash and slag used in this study are relatively ﬁne and have Blaine ﬁnenesses of 600m 2/kg.The ﬁne particle size of the pozzolans may have contributed to the better performance of concretes at the low water/binder ratio.Resistance to Chloride Ion PenetrationThe ASTM C 1202-05͑ASTM 2005͒rapid chloride ion penetra-tion test ͑RCPT ͒results of the concretes are given in Figs.5and 6.This test is based on the electrical conductivity of concrete.The concrete sample is subjected to a potential difference of 60V and the total charge passing through sample at the end of 6h is mea-sured and expressed in terms of Coulombs.A reduction in this total charge value indicates the better resistance to chloride ion penetration and lower permeability.As seen in the ﬁgures,for both water/binder ratios,pozzolan replacements caused great re-ductions in the rapid chloride permeability of the concretes.As shown in Fig.5,for the 0.60water/binder ratio and age of 28days,replacing 50%of portland cement by the ﬁne slag caused a decrease of about 90%in the rapid chloride permeability of the concrete.Similarly,the reductions for the ﬂy ash concrete and the concrete with the ternary binder are more than 86and 90%,re-spectively.For this water/binder ratio and 28-day age,the con-crete with the ternary binder has the lowest RCPT value.As seen in the ﬁgures,the rapid chloride ion permeability de-creases with age.For the water/binder ratio of 0.60,the total charge passing through the portland cement concrete decreases about 20%between 28and 90days.This decrease,however,is more substantial for the concretes containing high volume of poz-zolans.For the 0.60water/binder ratio,the reduction of the chlo-ride permeability between 28and 90days is about 47%for the slag concrete,83%for the ﬂy ash concrete,and 43%for the concrete with the ternary binder.At 90-day age and 0.60water/binder ratio,the concrete with slag and with the ternary blend have RCPT values of about 7%of the portland cement concrete and with a value of 161Coulombs the ﬂy ash concrete has the lowest value which is less than 3%of the normal concrete.If Figs.5and 6are compared,the effect of water/binder ratio on the rapid chloride permeability can be seen.For both 28and 90days,decreasing water/binder ratio from 0.60to 0.38caused a reduction of the RCPT value of about 70%for the portland ce-ment,44%for the slag concrete,and 42%for the concrete with the ternary binder.As a result of decreasing water/binder ratio,the total charge passing through the ﬂy ash concrete reduced about 43%at 28days,but only about 10%at 90days.From these results,it can be concluded that the decrease in the water/binder10203040506070100PC-6050S-6050F-6025FS-60MixtureC u b e c o m p r e s s i v e s t r e n g t h (M P a )pressive strengths of concretes with water/binder ratio of 0.6020406080100100PC-3850S-3850F-3825FS-38MixtureC u b e c o m p r e s s i v e s t r e n g t h (M P a )pressive strengths of concretes with water/binder ratio of 0.3855003721613762000400060008000100PC-6050S-6050F-6025FS-60MixtureR a p i d c h l o r i d e p e r m e a b i l i t y (C o u l o m b )2890Fig.5.Rapid chloride permeability test results of concretes with water/binder ratio of 0.60206144217500100015002000100PC-3850S-3850F-3825FS-38MixtureR a p i d c h l o r i d e p e r m e a b i l i t y (C o u l o m b )90Fig.6.Rapid chloride permeability test results of concretes with water/binder ratio of 0.38D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y S o u t h e a s t U n i v e r s i t y o n 12/12/13. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .ratio affects the RCPT value of the portland cement concrete more than those of the concretes with high amount of pozzolans.For the water/binder ratio of 0.38;at 28days,the concrete with the ternary binder and the slag concrete have the lowest RCPT values,which are about 20%of the portland cement con-crete.At 90days,however,the ﬂy ash concrete has the lowest RCPT value which is about 8%of the portland cement concrete value.At the low water/binder ratio,the chloride permeability of pozzolanic concretes also decreased substantially between 28and 90days.For the 0.38water/binder ratio,between 28and 90days the RCPT value of portland cement concrete decreased only about 6%,however,decreases of 48,73,and 44%were obtained for the slag concrete,ﬂy ash concrete,and the ternary binder concrete,respectively.In this study,the total replacement ratio of portland cement by the pozzolanic materials was 50%for all the mixtures.In the binary blended mixtures,concretes contain 50%slag or ﬂy ash,and the ternary blended mixtures contain 25%slag and 25%ﬂy ash which also correspond to 50%replacement.As seen in Figs.5and 6,for both water/binder ratios,the RCPT results of the ter-nary blended mixtures at 28days are slightly lower than those of the slag or ﬂy ash concretes.At the age of 90days,however,the RCPT results of the ternary blended concretes are almost the same as the slag concretes and higher than those of the ﬂy ash concretes.For the ternary blended mixtures,the reduction of the RCPT results between 28and 90days were also not lower com-pared to those of the binary blended concretes.In theory,for the ternary blended mixtures with equal percentage of ﬂy ash and slag replacement,greater reduction in chloride permeability may be expected compared to those of the binary blended mixtures.However,based on the test results,it seems that such an improve-ment was not obtained by the ternary blended mixtures prepared.The ﬂy ash and slag used have almost the same Blaine ﬁnenesses which indicate that they have similar particle sizes.Due to this similar particle sizes,the particle packing in the ternary blended mixtures may not be substantially different compared to those of the binary blended concretes and as a result the RCPT of the ternary blended concretes were not substantially lower than those of the binary blended concretes.In this study,all the specimens were cured in water until the testing day and for both water/binder ratios,the substantial reduc-tion of the chloride ion permeability of pozzolanic concretes be-tween 28and 90days indicates the high pozzolanic activity of the pozzolans taking places during this period.The ASTM C 1202-05͑ASTM 2005͒classiﬁes concretes in terms of chloride ion pen-etrability and based on this classiﬁcation,the concretes containing the ground ﬂy ash and slag can be considered as concretes of very low permeability.The lower chloride permeability of the concretes with high volume pozzolans is a result of a denser microstructure.The poz-zolanic reaction may cause lower amount of capillary pores and clogging of the pores,which reduces chloride ion transport in concrete ͑Li and Roy 1986͒.Improvement of the aggregate-cement paste interface by the pozzolanic reaction may also play a role in decreasing the chloride ion permeability.Better chloride ion resistance of high volume ﬂy ash concretes was also shown in other studies ͑Sengul et al.2005;Zhang et al.1999͒.The ground ﬂy ash and slag used in this study has a high ﬁneness which may have contributed to obtaining lower chloride ion permeability ͑Dhir and Jones 1999͒.Decreasing the water/cement ratio of a concrete reduces the amount of capillary pores which are mainly responsible for thepermeability of the concrete.Even though the RCPT value of the portland cement concrete is reduced about 70%by lowering the water/cement ratio,the values for the portland cement concrete at 0.38water/cement ratio are still 2times or even more higher than those of the concretes with pozzolans at 0.60water/binder ratio.This result indicates that to reduce the chloride permeability of portland cement concrete,inclusion of pozzolans are more effec-tive than reducing the water/cement ratio.Reduced capillary pores and reduction in their connectivity due to the pozzolanic reaction and better particle packing may be the reasons behind the better performance of the concretes with high amount of poz-zolans.Also,as indicated above,between the ages of 28and 90days,the chloride permeability of the concretes with pozzolans decreased substantially compared to those of the portland cement concretes.Pozzolanic reaction is relatively slow compared to the hydration of portland cement,which is the main reason of the low early age strength or high early age permeability of the concretes containing high volumes of pozzolanic materials.At later ages,however,the pozzolanic materials can be more effective in im-proving concrete properties due the pozzolanic reaction continu-ing at a higher rate for a longer period.It should also be noted that curing conditions is more important for such concretes.As seen in Table 5,the decrease in the water/binder ratio was obtained by both reducing the water amount and increasing the binder content which may have also contributed to the better per-formance of the low water/binder ratio concretes.Studies show that,for a same water/binder ratio,increasing the binder content can also help to reduce the permeability of the concrete ͑Buenfeld and Okundi 1998;Dhir et al.1996͒.OptimizationOptimization is a procedure for obtaining the best possible option and it is an important tool for decision making process.This procedure is based on deﬁning performance criteria ͑objective functions ͒,independent and dependent variables,and formulation of the parameters based on constraints ͑Brandt and Marks 1993͒.For the optimization of concrete mixtures;the performance crite-ria may be the strength,permeability properties,fracture proper-ties,durability characteristics,cost,or other concrete properties.The independent variables,for example,may be the amounts of constituent materials.The responses ͑dependent variables ͒,which are based on the independent variables,are compared to the per-formance criteria.In the optimization of concrete mixtures,con-crete properties such as the compressive strength obtained for a given mixture design may be one of the responses.Optimization usually involves considering several responses simultaneously,such as high strength and low cost.To optimize several responses,multicriteria optimization techniques were used in this study.A useful approach for the optimization of multiple responses simultaneously is to use desirability functions which reﬂect the levels of each response in terms of minimum and maxi-mum desirability ͑Sengul et al.2005͒.A desirability function ͑d j ͒varies over the range of 0Յd j Յ1.In case of maximizing and minimizing the individual responses,d i is deﬁned by Eq.͑1͒and ͑2͒,respectively ͑Myers and Montgomery 2002͒d j =ͫY j −min f jmax f j −min f jͬt͑1͒D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y S o u t h e a s t U n i v e r s i t y o n 12/12/13. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .。

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