Impact of NaOH and Na2SO4 on the kinetics and microstructural development of white cement hydration

This work clarifies the impact of NaOH and Na2SO4 on the kinetics and microstructural development of cement hydration to better understand their effect on the mechanical properties. Na2SO4 and NaOH accelerate hydration and strength development during the first hours but lower strength values later on. For the same degree of hydration of cement, the addition of Na2SO4 leads to the same compressive strength as the alkali free system while NaOH clearly lowers it. Compressive strengths closely correlate to the total capillary porosity of the cement paste, which relates to the total volume of hydrates. Na2SO4 promotes the precipitation of a higher volume of hydrates, especially ettringite. A higher volume of hydrates can better fill the space, decrease porosity and consequently increase strength. Contrary, NaOH lowers the total volume of hydrates, mainly the volume of ettringite. Strength differences are not dependent on the bulk density, chemical composition or morphology of C-S-H.