Early-Age Behavior of Calcium Carbide Residue in Slag-Based Geopolymer Binders

To evaluate the influence of calcium carbide residue (CCR) as a partial slag replacement in geopolymer mortar, binary mixes containing 0–20% CCR were prepared and tested to assess their early-age fresh properties, mechanical performance, microstructural characteristics, and sustainability metrics. Increasing CCR reduced flow (from 133.6 to 83.6 mm) and shortened setting times due to Ca(OH)₂ accelerating gel formation. Compressive strength decreased from 9.94 to 6.97 ksi as reactive aluminosilicates were diluted and partial crystallization occurred. XRD of the 20% mix showed C-S-H and portlandite peaks over the amorphous C-A-S-H hump, while SEM revealed a more porous, heterogeneous matrix consistent with strength loss. Environmental analysis showed embodied carbon decreased from 574.6 to 560.6 kg CO₂ e/m³ and material cost dropped ≈6% at 20% CCR. Despite reduced workability and strength, the mixes maintained acceptable performance and improved sustainability compared to OPC mortars. Our results identified an optimal CCR level that balances strength and sustainability, supporting the development of greener, resource-efficient materials.