Preparation,Activation And Application Of Calcined Clay In LC~3 System

Ordinary Portland Cement（OPC）is the most widely used building material in the world today,but the global cement production industry is now facing many problems such as resource shortage,excessive energy consumption,environmental pollution and excessive carbon emissions.The use of Supplementary Cementitious Materials（SCMs）to partially replace cement clinker is one of the most effective ways to solve the above problems.Limestone calcined clay cement（LC³）ternary cementitious material system,which is formed by partially replacing cement with calcined clay and limestone as supplementary cementitious materials,is a new type of green building material with broad application prospects.This paper relies on the international cooperation project of the National Key R&D Program“Demonstration Research on the Application of Green Building Materials for Post-disaster Reconstruction in Pakistan and Nepal”（Project No.:2018YFE0106300）.A new method for quickly and accurately assessing the reactivity of calcined clay by measuring the cumulative exotherm of the reaction is proposed,and the factors and mechanisms affecting the change of the reactivity of calcined clay are identified,and the activation method and related applications of calcined clay are further broadened.It provides new ideas for the sustainable development of LC³.The main research work and research results of this paper are as follows:（1）The indicative geographic distribution of natural clays from Nepal and Pakistan as supplementary cementitious materials are explored,and the clays are systematically characterized to determine their chemical composition,mineral phase composition and kaolinite content.Based on the pozzolanic reaction of calcined clay with Ca（OH）₂,Reaction thermal method is developed to rapidly determine the reactivity of calcined clay.The changes of mineral composition,micromorphology,particle size and chemical structure of clay before and after calcination are characterized,and the effect of calcination temperature and kaolinite content on the reactivity of calcined clay and its mechanism are studied.The research shows that with the gradual increase of calcination temperature,the reactivity of calcined clay first increases and then decreases,which can be divided into three stages.When calcined to 500～600℃,the clay particles are broken,the kaolinite is dehydroxylated into metakaolin,the degree of structural disorder increases,and the reactivity appears and gradually increases.When calcined to 700～800℃,the clay particles are flocculent,the degree of structural disorder is further increases,and the reactivity reaches the highest in this temperature range.When calcined to 900℃,sintering and recrystallization occur,the particle size increases,and the reaction activity drops suddenly.After the heating and heat preservation process,in order to further improve the reactivity of the calcined clay,the rapid cooling of the fan is better than the natural cooling in the furnace.There is a good linear correlation between the reactivity of the calcined clay and the kaolinite content of the clay.（2）Kaolinite content of some clays are low and mix with large amounts of the inert material quartz.The use of mechanical grinding activation technology shows that kaolinite exists in the finer part of the clay powder,and quartz exists in the coarser part.In the particle classification-enrichment technology,the laboratory centrifuge can separate and settle the clay particles according to the different particle sizes,and can select the finer parts of the mechanically ground clay,which improves the kaolinite content of the clay and its reactivity after calcination.For calcined clay with different kaolinite content,activation by adding different proportions of Ca（OH）₂,and the pozzolanic reaction characteristics of Ca（OH）₂and calcined clay are investigated.Activation study results show that the use of Ca（OH）₂can meet the needs of alkaline environment and provide Ca²⁺to generate C-A-S-H gel.When the ratio of Ca（OH）₂to calcined clay is different,the cumulative heat release is different,and the amount of Ca（OH）₂required to achieve the best activation effect increases linearly with the increase of the kaolinite content of the calcined clay.（3）The hydration of cement is simulated by GEMS,and the production and development process of Ca（OH）₂is determined.Combined with the optimal Ca（OH）₂incorporation ratio of activated calcined clay,Ca（OH）₂and gypsum are added to LC³-25（Contains 25%cement,50%calcined clay and 25%limestone）,and the effects on the mechanical properties,hydration products,pore structure and reaction kinetics of the system are also investigated.The research results show that an appropriate amount of Ca（OH）₂fills up the lack of Ca（OH）₂in the reaction of LC³-25,promotes the pozzolanic reaction in the system,and generates more C-A-S-H gels and monocarboaluminate.The pore structure is refined,and the pore size distribution evolves to smaller gel pores;the compressive strength is greatly improved.The addition of gypsum significantly increases the degree of reaction in the early stage of hydration,produces more ettringite,and the formation of additional ettringite compensates for the reduction of other reactions,effectively improving the early strength of the system.