Optimized Matching Of Cement Clinker And Supplementary Cementitious Materials

"Energy saving, emission reducing, resources conservation and environment protection"are the main ways for China to achieve sustainable development and harmonious society.However, the production of Portland cement seems unsustainable due to the consumption ofhuge natural resources and energy and significant CO2emissions. China is the largest cementproduction country, and the output of cement is beening continually increased as a largenumber of infrastructures are beening built. Meanwhile, nearly2billion tons of industrialsolid wastes are generated annually in China, leading to numerous environmental problems.Metallurgical slags together with coal combustion ashes can be used as supplementarycementitious materials (SCM) to produce blended cement, which is considered as one of thedevelopment directions of cement industry. Due to the low hydraulic (or pozzolanic) activityof SCM, blended cements usually present low early strength, especially for blended cementswith a high content of low activity SCM. Many attempts, such as chemical activation andultra-fine grinding, have been made to improve the early properties of blended cements.Although these methods, mainly focusing on the enhancement of the activity of SCM,contribute greatly or somewhat to the improvement of the properties of blended cement, theinitial microstructure and hydration process of blended cement paste have not been optimizedcompletely, thus the key factors,"low hydration efficiency" and "poor binding between SCMparticles and hydration products" leading to the low early strength of blended cement, havenot yet been solved perfectly.In this thesis, optimized matching of cement clinker and SCM were investigated.According to the hydration and hardening characteristics of cementitious material fractionswith different particle size, desired hydration process and micro-structural development ofblended cement paste, the type, addition and particle size distribution of cementitiousmaterials consisting of blended cement were redesigned. Through optimization of the initialmicrostructure and hydration process of blended cement paste, the microstructure of blendedcement paste is formed and densified gradually, at the same time hydration products bindSCM particles firmly. Consequently, the strengths and volumetric stability of hardenedblended cement paste are improved significantly, while the addition of cement clinker isreduced and the amount of SCM (especially for SCM with low activity or inert fillers) isincreased. The research significances in this thesis are described in detail as follows:The relationship among the size fraction, composition and properties of cement clinkerand SCM has been clarified. Due to grindability difference of each mineral, coarse cementitious material fractions contain a higher amount of harder grindability minerals, whilehigher proportion of easier grindability minerals is observed in fine cementitious materialfractions. The hydration and hardening characteristics of cementitious materials varysignificantly with their size and mineral composition. The water requirement and earlystrength of cementitious materials are increased dramatically with the decrease of particle size,while cementitious materials (e.g. cement clinker and BFS) only show the highest latestrength when the particle size lie in a certain range. For instance,8-24μm cement clinkerfraction has a low water requirement and high hydration rate at late ages, resulting inrelatively higher early strength and the highest late strength. Further, fine SCM fractions havean acceptable early strength index and late strength index higher than100%, indicating thatthe replacement of cement clinker by these SCM fractions will not lead to late strength loss.The adaptability of the classical particle size distribution (PSD) models generally used incement-based materials has been analyzed. Few of the existed models including S. Tsivilisdistribution and Fuller curve are adapted to blended cements which consist of cement clinkerand SCM with different hydraulic activity (perhaps even inert particles). In order to optimizethe initial microstructure of blended cement paste, a gap-graded PSD was proposed andmodified according to the distance between solid particles in actual pastes. The gap-gradedPSD leads to a reduced water requirement and an increased packing density of blendedcement pastes, and modified gap-graded PSDs have better effects.The term 'strength contribution ratio' and 'cementitious ability' of SCM fractions wereproposed, by which the contribution of SCM fractions to the properties of blended cement canbe quantitatively described. Fine BFS and steel slag fractions (<8μm) show more desirablehydration processes and higher strength contribution ratios in comparison to thecorresponding cement clinker with the same size, while middle size and coarse SCM fractionshave low early and late strength contribution ratios due to their low hydraulic activity. Theoptimized matching principle of cement clinker and SCM is as follow: SCM with highactivity, cement clinker, and SCM with low activity (or inert fillers) should be preferablyarranged in the fine (<8μm), middle size (8-32μm) and coarse (>32μm) fractions,respectively.42.5grade blended cements with only25%cement clinker by volume can be preparedusing the gap-graded PSD by mixing cementitious materials according to the optimizedmatching principle. The blended cement pastes have a low water requirement and highpacking density, a small amount of hydration products generated from the hydration of clinkerform the initial microstructure of cement paste during the first24h. The hydration products of BFS, accounting for about40%of the total hydration products, densify the microstructure ofblended cement paste gradually after30h. The Ca(OH)2content remained in blended cementpaste is very low, while the total porosity of the matrix and the amounts of C-S(A)-H gels andun-hydrated components almost equal to those in Portland cement paste, and large-size poresare reduced dramatically due to the hydration of SCM at late ages. The above mentionedeffects lead to a significant increase in both early and late strengths of blended cements.Blended cements with simplified gap-graded PSD and prepared by mixing commercialPortland cement, fine BFS fraction and coarse SCM fraction with low activity also havehigher early and late strengths than blended cement prepared by co-grinding.High performance blended cements with low cement clinker content present superiorvolumetric stability and durability. The related improvement mechanisms are summarized asfollows:(a) The main hydration products of gap-graded blended cement are outer hydrationproducts with high Al content, which has a low chemical shrinkage compared with innerhydration products. To maintain the charge balance of hydration products, a large amount ofK~+and Na~+are absorbed into the layer of C-A-S-H gel, leading to an enhanced interactionforce among gel layers (van der Waals force), thus the hydration products show a superiorresistance to volumetric deformation.(b) Stress and strain remained in gap-graded blendedcement pastes are low and uniformly distributed due to low hydration heat and morehomogenous, densified microstructure.The cement clinker content in42.5grade blended cement is usually higher than75%inindustrial practice, however42.5grade blended cement can be prepared using only25%cement clinker,36%BFS and39%low activity SCM (or inert fillers) in present study, and theblended cements also have superior volumetric stability and durability. The results providetheoretical foundation and technical support for the preparation of high performance blendedcements with larger amount of industrial wastes (especially for low activity and inertindustrial wastes). By this method, huge natural resources and energy can be saved, andsignificant CO2emissions can also be reduced, resulting in numerous economic, social andenvironmental benefits.