Crystalline admixture effects on crystal formation phenomena during cement pastes’ hydration

Crystalline admixtures (CAs), categorized as permeability-reducing admixtures, are commercial products employed in low contents in cement-based materials either to improve concrete durability or to stimulate autogenous healing of cracks. This study investigated the effects on crystal formation promoted by CA presence in cementitious pastes by thermal analysis. X-ray diffraction was used as a complementary technique to identify the formed phases. The results indicated that the increment in CA content increased calcium hydroxide (CH) and monocarboaluminate contents. However, it did not change the total combined water and calcium carbonate (CaCO3) contents. In fact, the increment in CA content prolonged the retarding of hydration reactions and decreased the dehydroxylation temperature of CH, suggesting the decrease in crystallinity and purity degree. The primary action mechanisms of CA confirmed were the long-term ability for further hydration to occur, the CH recrystallization, and the conversion of lower to higher thermal stability phases. Chemical and physical mechanisms of CAs are related to their surface area increase and their chemical nature, mainly composed of limestone. These characteristics confer with CA, the ability to act as a nucleation and precipitation stimulator to form new products.