Reactivity Evaluation Of Raw Materials And Composition Design For Alkali-activated Slag Cements And Concretes

Alkali-activated cement concrete is a composite material made by chemically activated alumino-silicate cementitious binders.It has excellent corrosion and high temperature resistance and performs even better than conventional Portland cement concrete in terms of the mechanical properties.Alkali-activated cement concrete can be produced solely upon the use of alumino-silicate materials that do not need any calcination of the raw materials.The production and usage of alkali-activated cement concrete can lead to a significant reduction in environmental pollution,carbon emission and energy consumption,and therefore,meets the needs of sustainable development of the national economy and construction industry.Nevertheless,there is a lack of systematic research on the theories for evaluation method of raw material reactivity,setting and hardening control technology,and composition design and production of alkali-activated cement concrete.This has heavily impeded the wide use of alkali-activated cement concrete in the construction industry.To fill in the above knowledge gap,this paper aims to conduct a systematic research program focusing on three key scientific issues:(1)development of alkali reactivity index of different slag composition;(2)the mechanisms for the setting and hardening,microstructure development and optimization of alkali-activated slag cement;and(3)composition design theory and method of alkali-activated cement concrete.As the first section of this study,the effect of different slag composition on the reaction kinetics of alkali-activated slag cement was investigated.The results showed that the reaction kinetics of alkali-activated slag cement mainly depended on the total contents of Ca O and Mg O in the slag,among which Ca O was beneficial to the generation of C-(N)-A-S-H gel,Mg O was beneficial to the generation of hydrotalcite(Mg Al-OH-LDH),while excessive Al and Ca O and Si O₂ in the slag would generate hydrated mililite(C₂ASH₈).The phase evolution of pore solution and the changes of Ca/Si and Al/Si in C-(N)-A-S-H gel are well correlated with the reaction exothermic process of alkali-activated slag cement.The chemical composition of slag has a significant effect on the reaction kinetics of alkali-activated slag cement,and the reaction kinetics of alkali-activated slag cement has a certain relationship with the ratio of(Ca O+Mg O)/Si O₂ in slag,because the dissolution rate of Ca O and Mg O contents in slag at high alkali concentration controls the development of the phase composition of early reaction products.Then the effects of different slag compositions on the compressive strength and microstructure of alkali-activated slag cement were studied.The results showed that the compressive strength of alkali-activated slag cement with different slag compositions is different due to the different reaction products.C-(N)-A-S-H gel and Mg Al-OH-LDH phase were observed in all alkali-activated slag cement,and hydrated C₂ASH₈ was usually observed in the slag composed of high aluminum content,while katoite was more easily generated in the Na OH acitation system.The content of Al₂O₃has an important effect on the phase types and composition of alkali-activated slag cement.The lower the Al₂O₃ content is,the higher the content of C-(N)-A-S-H gel is,and the lower the content of hydrated C₂ASH₈ is.When the Al₂O₃ content in the slag is less than 13%,the system will not produce hydrated C₂ASH₈.The content of Mg Al-OH-LDH phase is related to the Mg O content in the system.Generally,the higher Mg O content in the system,the higher Mg Al-OH-LDH.In addition,the intrinsic porosity of sodium silicate activated cement is lower than that of Na OH activated slag system.The thermodynamic results showed that,with a certain reaction degree,the higher Ca O content in the slag,the smaller pore filled capacity by the reaction products,and the greater the chemical shrinkage of the system.Eventually,the higher porosity was formed in the matrix,which is negative for the final compressive strength of alkali-activated slag cement.Thus,for the formula of"alkali"reactivity index of slag proposed in this paper,Ca O is considered as a negative factor for the first time,and the new reactivity index has a good correlation with the compressive strength of alkali-activated slag cements.Moreover,sodium silicate-activated slag often gives high early strength but sets too rapidly.Commercial retarders for Portland cement and partial replacement with other cementitious components in alkali-activated slag(AAS)system have been proven ineffective.This study investigated the approach of controlling setting by altering activator ion composition and silica polymer status.The roles of activators during the alkali-activation process were studied via pore solution chemistry analysis and microstructural analysis of hydration products.The addition of Na₂CO₃ does not,but Na OH does alter the polymerization of silicate ion groups in sodium silicate solution due to the increases in p H and Na₂O concentration.The specific effects of Na₂CO₃ on the setting and hardening of AAS depend on the relative contents of Na OH and Na₂O·2Si O₂ in alkaline solution.The reaction degree of AAS is dependent on activator compositions,which govern the kinetics of formation and intrinsic characteristics of the reaction products calcium aluminosilicate hydrates.The setting and strength development of sodium silicate-activated slag can be controlled by manipulating the compositions of Na₂CO₃-Na OH-Na₂O·2Si O₂.Finally,this paper reports a general mixture design procedure for alkali-activated slag concrete,which is an essential step towards industrial application.The procedure involves three steps:1)the determination of coarse and fine aggregate ratio according to close packing model;2)the determination of liquid phase(water content and activator)based on compressive strength;and 3)the determination of excess paste content by workability requirement and measurement.Effects of mixture proportional factors,including activator composition,water content,fly ash content,and binder/aggregate ratio are examined on consistency,setting time and compressive strength.The relationship between performance and precursor composition is established using simplex centroid design method.Using the mixture proportioning method,alkali-activated concretes with compressive strength grades of C40,C60,and C80 are successfully prepared with initial setting time of 1 to 3 h and slump of more than 200 mm.