Performance Enhancement of Sustainable Cementitious Material with Ultrahigh Content Limestone and Calcined Clay

Calcined clay can be combined with limestone to partially substitute cement for developing the sustainable cementitious material. However, an increase in substitution level beyond a certain range limits the compressive strength development. This study proposes to increase the substitution level of calcined clay and limestone to an ultrahigh level (as much as 75%) by optimally adding portlandite and gypsum. First, the reactivity of calcined clays with different kaolinite contents is explored by mixing the clay with different amounts of portlandite. A linear relationship is obtained between the kaolinite content of the calcined clay and the amount of portlandite such that optimal activation effect is achieved. Based on the optimum dosage, external portlandite is mixed to enhance the performance of cementitious material with ultrahigh content limestone and calcined clay. The addition of an appropriate amount of portlandite is beneficial to the continuous reaction of the calcined clay, refining the pore structure and significantly improving the 28 d compressive strength. Then, different amounts of gypsum are added to further enhance the mechanical properties of cementitious material with ultrahigh content limestone and calcined clay. The amount of gypsum equal to 30% of the calcined clay in the system could effectively improve the compressive strength in the early stage (3 and 7 d). The performance of cementitious material with ultrahigh content limestone and calcined clay is significantly enhanced by the synergistic effect of portlandite and gypsum, as revealed by the improvement in compressive strength and microstructure.