Enhancing Early-Age Strength of Low-Clinker Limestone Calcined Clay Cement Using Sodium Carbonate and Chemical Accelerators

Abstract

While limestone calcined clay cement (LC3) can reduce CO2 emissions by up to 50%, low early-age strength in low-clinker formulations limits its structural adoption. This study investigates practical strategies to accelerate early strength development in low-clinker LC3 mortars (clinker fraction ~42.5%) using sodium carbonate (1–3%) and a commercial chemical accelerator (1.5–3%), evaluated individually and in combination. Eleven mortar mixes were tested for compressive and flexural strength at 3, 7, and 28 days per BS EN 196-1, alongside ATR-FTIR hydration characterization and dry versus wet mixing comparisons. Results show that 2% sodium carbonate is the optimal single dosage, increasing 7-day compressive strength by ~29% (from 6.8 to 8.8 MPa) and flexural strength by 22% (from 2.46 to 3.00 MPa). Combining 1% sodium carbonate with 1.5% accelerator produced a pronounced synergistic effect, boosting 7-day compressive strength by 38% (9.4 MPa) and flexural strength by 34% (3.30 MPa). FTIR spectra confirmed enhanced silicate polymerization and carboaluminate precipitation. These findings provide construction practitioners with a scalable, low-cost chemical activation method to facilitate early demoulding and formwork stripping in sustainable low-carbon construction.