Hydration mechanics and microstructure evolution of carbide slag-based low clinker limestone calcined clay cements

Abstract

Limestone calcined clay cement (LC3) with 50% clinker content represents a significant advancement in low-carbon binders but further clinker reduction is limited by insufficient portlandite (CH) availability to sustain pozzolanic reactions. Commercial CH addition defeats the low-carbon purpose due to its carbon emission higher than clinker. This study addresses this dual challenge by valorizing industrial carbide slag (CS), a CH-rich (80%) waste by-product – as a circular calcium source to enable low-clinker LC3 formulations with 30% and 40% clinker factors. Six paste mixes were systematically evaluated, comparing raw CS and calcined CS against conventional LC3-50 and OPC controls. A multi-technique characterization framework was employed to establish the hydration kinetics, phase assemblage, C-(A)-S-H gel chemistry, and pore structure evolution. The results reveal a synergistic trade-off mechanism: although the addition of both CS forms inhibits the hydration of clinker silicates (C3S/C2S), this inhibition is effectively compensated by sustained pozzolanic reactions driven by the supplemental calcium and the formation of Al rich C–(A)–S–H gel with shorter silicate chains, alongside stable carboaluminate phases. Consequently, LC3-40-CaO achieves a 28-day compressive strength of 52.5 MPa, equivalent to LC3-50, while utilizing 10% less clinker. All CS-modified pastes exhibit refined pore structures (critical pore entry radii of 14–17 nm), comparable to LC3-50 (11 nm) and substantially finer than OPC (77 nm). Environmental performance indicators demonstrated 16% reduction in embodied CO2 per MPa compared to LC3-50. This work establishes waste carbide slag as a viable, low-carbon CH source enabling mechanically robust LC3 binders with clinker factors below 50%, advancing circular economy principles in sustainable cement production.