Hydration and hydration products of two-phase Portland cement clinker doped with Na₂O

Alkalis are important minor constituents of ordinary Portland cement clinker. Na₂O, in particular, is incorporated into the aluminate phase and alters its crystal structure. For this study, well defined Portland cement clinkers consisting of alite and aluminate doped with different amounts of Na₂O were used. Isothermal conduction calorimetry of cements made from this clinker revealed that doping with Na₂O activates the initial heat evolution of the clinker during the initial hours. It also influences the main reaction. In situ X-ray diffraction for monitoring cement hydration demonstrates that the dissolution of C₃A and gypsum is accelerated by the incorporation of Na₂O. The formation of crystalline ettringite and monosulphate during the first few hours of hydration is delayed. After 90 days of hydration, most of the C₃A has reacted to monosulphate, detectable by differential scanning calorimetry, scanning electron microscopy and ²⁷Al nuclear magnetic resonance. An increase of the Na₂O content brought about less ettringite in the hydrated samples and changed the morphology of monosulphate to thinner and smaller hexagonal plates.