Influence of CA/CA2 ratios on hydration properties of phosphate-modified calcium aluminate cement at 80 °C

This study investigates the influence of CA/CA 2 ratios on the hydration kinetics, microstructure evolution, and performance of phosphate-modified calcium aluminate cement (CAPC) at 80 ºC. Five cement formulations with CA/CA 2 ratios from 2.71 to 0.19 were synthesized and modified with 10% sodium hexametaphosphate. Results show that decreasing the CA/CA 2 ratio reduced initial hydration heat, delayed the main peak, and extended setting times. However, thickening times remain within 90 minutes, limiting the efficacy of ratio adjustment for enhancing workability. Early strength (20-30 MPa at 1 day) declined by 3 days, while 28-day strength increased with higher CA/CA 2 ratios (≥0.64). Amorphous content dominated (>45% at 28 days), coexisting with metastable hydroxyapatite. Low CA/CA 2 ratios accelerated C 2 AS hydration, enhancing silicon incorporation into C-A-S-P-H gels. Primary hydration phases completed >85% within 1 day, while C 2 AS hydration was slower. P-rich C-A-P-H (I)/(II) gels and a separate Si-containing C-A-S-(P)-H phase formed, and a mesoporous structure with high specific surface area and fine pores (4-5 nm) generated. The findings offer insights for optimizing CAPC hydration and workability in geotechnical and construction applications. • CA/CA 2 ratio reduction delays hydration and extends setting to 70 min at 80°C. • High early strength declines at 3 days, then rises by 28d for CA/CA 2 ≥0.64. • Low CA/CA 2 ratios accelerate C 2 AS hydration, enhancing Si into C-A-S-P-H gels. • Amorphous phase (>45%) dominates microstructure, coexisting with metastable HAP. • P-rich gels create 4-5 nm mesopores (67.8 m 2 /g), enhancing CO 2 resistance potential.