Experimental investigation on the hydration reaction kinetics of CaO composite for high-temperature thermal energy storage and heat upgrading

The CaO/Ca(OH)₂ thermochemical system exhibits promising potential for heat storage and temperature upgrading through hydration reactions, particularly under high-pressure steam conditions. To overcome the critical challenge of volume expansion in pure CaO/Ca(OH)₂ systems during hydration operation, this work systematically investigates vermiculite-CaO composite materials as an alternative solution. A comprehensive experimental study was conducted to evaluate hydration performance across different steam pressures (1–5 bar) and composite formulations (50 %, 65 %, and 75 % CaO content), with particular focus on thermal behavior evolution, structural integrity maintenance, and energy storage characteristics. Key findings reveal that increasing steam pressure from 1 to 3 bar substantially enhances both exothermic peak temperature and reaction kinetics, though the enhancement effect gradually diminishes with a further increase in pressure from 3 bar to 5 bar. Vermiculite demonstrates remarkable structural stabilization capability, effectively constraining volume expansion below 15 % while maintaining morphological integrity throughout hydration, as verified by macroscopic dimensional analysis and microscopic SEM characterization. Notably, hydration conversion rates remain pressure-independent and mass ratio-insensitive, consistently achieving approximately 80 %. While vermiculite incorporation reduces mass-based energy storage density, optimal composite formulation achieves enhanced volumetric energy density through improved structural compactness. • Continuous 600 °C heat regeneration demonstrated using 5-bar steam supply. • Steam pressure affects hydration temp/kinetics, not final conversion. • Optimal volumetric energy density achieved at 65:35 CaO/vermiculite mass ratio.