Mechanism of expansion in hardened cement pastes with hard-burnt free lime

The expansion characteristics of hard-burnt free lime in cement pastes were determined by measurements of free linear length changes in various periods. Hydration of lime in water and in alkali solutions was investigated to evaluate the influence of OH^- ions on the migration of Ca²⁺ ions from lime, the supersaturation of solutions for Ca(OH)₂ and the size as well as the dispersion of distribution of Ca(OH)₂ crystals formed. Based on the experimental results, an expansion mechanism of hydrating lime in cement pastes was proposed. It is found that OH^- ion in pore solutions may force Ca(OH)₂ to form in situ and promote the development of expansive forces. The expansion of hydrating lime is caused by the crystallization of Ca(OH)₂ crystals formed in situ.