Study On The Preparation And Application Of Low Grade Calcined Clay-limestone Composite Cement

In recent years,as a major cement producing and consuming country,China has maintained its production as the first in the world,but the problem of high carbon emissions and energy consumption of the cement industry is an important factor limiting its sustainable green development.Calcined clay-limestone cement,consisting of ordinary silicate cement clinker,calcined clay,limestone powder and gypsum,is considered to be one of the most effective ways to achieve low carbon development in the cement and concrete industry.At the same time,China is rich in clay resources and it is feasible and economical to prepare LC³cement based on indigenous low-grade clay to achieve the promotion and application of low carbon cement.In this paper,based on a low-grade clay from a mine in Guangxi,its mineral characteristics were analysed,and the clay volcanic ash activity was stimulated by roasting activation through comprehensive thermal analysis,and then the best calcination regime was established through different activity evaluation methods.The clay has a relatively low grade with a kaolinite content of approximately 44%and an Al₂O₃ content of 26.9%.The volcanic ash flammability of the calcined clay was derived from the conversion of kaolinite to metakaolinite under roasting,with a heating rate of 10°C/min,calcination temperature of800°C,holding time of 2h and rapid cooling,when the maximum activity of the calcined clay could be stimulated,and the calcium hydroxide absorption value and activity index reached their peak values of:1196.2 mg and 1.058,respectively.The mechanism of action of calcined clay and calcined clay/limestone binary systems in composite cement systems was further analysed.The amount of calcined clay affects the compressive strength and hydration process of the composite cement system,but there is a limit to the amount of admixture,and the strength of the composite cement system decreases sharply after exceeding 30%,and the C-（A）-S-H gel content shows a gradual decrease.The calcined clay/limestone（LC²）binary mixture has a synergistic coupling effect,and due to the dilution effect,the compressive strength of the ternary composite cement system is negatively correlated with its admixture level,and at less than 50%admixture level,the compressive strength decreases to a small extent,and the C-（A）-S-H gel content of the hydration products decreases,while the Hc+Mc content increases and a new calcite mineral phase appears.The mechanical properties,microscopic analysis,durability and carbon emission,energy consumption and economic efficiency analysis of the LC³ low carbon cementitious system,its hydration mechanism and the mechanism of anti-ion erosion action were investigated.Compared with ordinary silicate cement system,the initial strength of LC³ cementitious system is relatively low at 3 d and 7 d.In the later stage of hydration,the strength development rate increases rapidly,and the flexural and compressive strengths at 28 d are close to those of LC³ cementitious system;the mechanical properties of LC³ cementitious system are enhanced with the decrease of water-cement ratio.The LC³ cementitious system is more economical and green and sustainable.Finally,LC³ cements are explored in the preparation of ultra-high performance concrete.The results show that LC³ UHPC has lower compressive strength and easier workability than normal UHPC,LC³ UHPC has a sharp calcite peak and a weak monocarbon aluminate peak,and produces a large number of internal hydrated C-S-H gels with a relatively dense microstructure.