Utilizing carbide slag to improve the hydration kinetics and microstructure of limestone calcined clay cement systems

The urgent need to reduce CO₂ emissions from ordinary Portland cement (OPC) production has driven increasing interest in low-carbon binders such as limestone-calcined clay cement (LC³), However, LC³ cements show reduced early-age strength than OPC and absence of calcium hydroxide (CH) in long term which may delay construction process and influence long term durability. To address these issues, this study investigates the effect of additional calcium hydroxide (CH), sourced from the industrial by-product carbide slag (CS), on the hydration behavior, microstructural development, mechanical performance, and durability of LC³. Additional CH was incorporated at dosages of 4 %, 8 %, and 12 % by weight. Results indicated that additional CH slightly increased the reactivity of calcined clay, as suggested by the cumulative heat curves and phase assemblage results. However, excessive CH content (12 %) could lead to under-sulfation and quick setting of LC³ pastes. The presence of additional CH initially retarded the hydration degree of C₃S, while promoting the CH C₂S hydration, especially in LC³-8CH. Thermogravimetric analysis confirmed partial consumption of additional CH through enhanced pozzolanic reactions, which was further corroborated by the chemical composition of the C-A-S-H gel (low Ca/Si and high Al/Ca). The LC³-8CH system exhibited a denser matrix with a higher volume of C-A-S-H and well-balanced carboaluminate phases, contributing to a refined pore structure. Compared to LC³-50 paste, LC³-8CH paste demonstrated superior early and late-age compressive strength, and comparable to that of OPC, thereby addressing a key limitation of low early strength of traditional LC³-50 binders. In contrast, the 12 % CH dosage led to oversaturation of Ca²⁺ ions, promoting a porous microstructure and impairing mechanical performance. These findings highlight the potential of controlled CH addition, particularly at 8 %, to optimize the performance of low clinker LC³ systems and add significant new knowledge to the available research on the LC³ systems. Synopsis Waste calcium hydroxide source from carbide slag is utilized to improve strength and durability in LC3 systems for sustainable engineering applications.