Synergistic Effects Of Calcined Clay-limestone-slag Cement

The cement industry relies more and more on the use of supplementary cementitious materials（SCMs）for preparing low-carbon-footprint binders for sustainable infrastructure construction.Utilization of industrial by-products such as blast furnace slag,silica fume,and fly ash is widely accepted for lowering the content of clinker（the most CO₂-intensive constituent）during cement production.However,it is anticipated that SCMs will run out of use in the next decades due to its wide application as well as the limitation of production.Alternative SCMs of high amount and universal distribution in the world has never been so much desired.In recent years,limestone calcined clay cement（LC³）has attracted great interests among stakeholders in the cement and concrete industry.The concept has been developed for very high replacement ratio of clinker for achieving comparable or even better performances to ordinary cement.More importantly,availability of the raw materials（calcined clay and limestone）all over the world makes LC³a sustainable low clinker,low carbon alternative cementitious material.However,the application of LC³still in the infant stage after years of development,so it is important to promote the application of LC3cement..Exploring the performance development characteristics of calcined clay-limestone cement and studying the mechanism of synergy between calcined clay and limestone is a necessary way to promote its application.The research results show that the strength of the system increases with the increase of the clinker rate,and decreases with the increase of the sulfur content.When the clinker used is 55wt.%（by mass）,increasing the fineness of calcined clay and limestone can significantly improve the compressive and flexural strength of the composite cement binder mortar,which becomes comparable to the strength of pure cement mortar.This is mainly because the reactivity of calcined clay and limestone increases with fineness,and CO₃^2-reacts with ettringite to produce hemicarboaluminate（Hc）and monocarboaluminate（Mc）,thereby increasing the strength.The replacement of slag cement with limestone and calcined clay（LC²）is beneficial to expand the source of SCMs and reduce CO₂emissions in the global cement industry.In this paper,the synergistic interaction between slag cement and LC²is studied by multi-scale characterization using TGA,XRD and SEM.The results indicate that addition of LC²enhanced both the compressive strength and flexural strength of slag mortars.Characterization of the microstructures show LC²can optimize the phase assemblages of slag cement by stabilizing ettringite phase and promoting the formation of hemicarboaluminate,monocarboaluminate.It is concluded that the reactivity of calcined clay and nucleation effect of limestone（both in LC²）compensate for the slow early hydration of slag whilst slag contributes to the later performance.The synergistic hydration effect of slag and LC²could be beneficial for the property gain（mechanical strength,hydration heat and phase composition）at both early and later ages.Durability is an effective means to test whether cement is suitable for engineering.Improving the durability of calcined clay cement is also beneficial to the promotion and application of this cement.On the basis of determining the proportion of each component and the fineness of the material,exploring the durability and action mechanism of calcined clay cement,such as chloride ion corrosion resistance,sulfate corrosion resistance,carbonization resistance,and volume stability,can provide theory and mechanism for its service.Actual basis.Due to the reactivity and physical filling effect of calcined clay and limestone,the pore structure of cement-based materials can be compacted and ion transmission can be hindered,thereby increasing the resistance to chloride ion erosion by 300%.The calcined clay consumes calcium hydroxide in the system through hydration reaction.At the same time,the CO₃^2-in limestone can prevent the conversion of AFt to AFm,compacting the cement pore structure and reducing the source of sulphate erosion volume expansion.Therefore,the LC³mortar concrete has a high sulphate erosion resistance coefficient.