The impact of accelerator type and dosage on cement-GGBS mortar performance

This study investigates the comparative effects of calcium formate (CF) and calcium nitrate (CN) accelerators on the performance of cement mortar containing 70% ordinary Portland cement (OPC) and 30% ground granulated blast-furnace slag (GGBS). Mortar samples were prepared with accelerator dosages ranging from 0% to 3.75% by binder weight and tested for workability, setting time, compressive, flexural, and tensile strengths, water absorption, shrinkage, and microstructural characteristics. The results showed that both accelerators improved early age hydration and strength, with optimal performance at a 2.25% dosage. CN provided a faster setting and slightly higher early strength, whereas CF delivered better long-term performance. CF also offered superior shrinkage control, supported by quantitative reductions at both 28 and 90 days. At 2.25% dosage, CF reduced 28-day shrinkage from 450 µε (control) to 420 µε (6.7% reduction) and 90-day shrinkage from 620 µε to 580 µε (6.5% reduction). In contrast, CN showed higher shrinkage at equivalent dosages, reaching 505 µε at 28 days and 700 µε at 90 days. Water absorption decreased at moderate dosages for both accelerators, correlating with denser microstructures observed in the FE-SEM images, although overdosing led to increased porosity and reduced performance. ATR-FTIR analysis confirmed distinct chemical fingerprints of each accelerator, with CF introducing carboxylate groups and CN exhibiting nitrate-specific bands, both influencing hydration kinetics. Overall, CF and CN enhanced the mechanical and durability properties, although with different advantages, suggesting tailored applications based on project requirements.