Influence of ground perlite on the hydration and strength development of calcium aluminate cement mortars

Abstract

Calcium aluminate cement (CAC) concretes are desirable owing to their rapid strength development and resistance to high temperatures, sulfates and acids. They suffer two drawbacks, however, high cost and strength loss due to conversion of water-rich hydration products to more stable structures, resulting in a drop in strength. Although its rate is dependent on temperature and moisture, the conversion process is thermodynamically inevitable but can be prevented by adding sufficient quantities of supplementary cementing materials (SCMs) to form an alternate, stable hydrate, Stratlingite (C2ASH8). Based on this premise, natural pozzolans could mitigate conversion, also reducing cost. This study investigated the influence, on hydration, of replacing 25–75% of CAC with ground perlite (GP), an inexpensive and abundant natural pozzolan. Paste and mortar mixtures with water-to-binder ratios of 0.40 and 0.60 were cured in air at 23 °C or in water at 40 °C. The influence of adding Na2SO4 to promote Stratlingite formation was also investigated. A multi-method approach comprising X-ray diffraction (XRD), isothermal calorimetry, thermal analysis, and compressive strength development was used. Stratlingite formation was confirmed in many mixtures containing GP. Wet-cured mortars with ≥50% GP and 1% Na2SO4 contained Stratlingite and showed continuous strength increases up to 90 d. Rapid-, or slow-progressing drops in strength were observed in air-cured mixtures or wet-cured mixtures containing < 50% GP. Mortars with 75% GP did not experience conversion but their ultimate strengths were much lower than even the converted strength of the pure CAC mortar. Heats of hydration of pastes decreased linearly with GP replacement of CAC despite their setting times being shortened.