Hydration of Hybrid Cements at Low Temperatures: A Study on Portland Cement-Blast Furnace Slag—Na(2)SO(4)

Replacement of Portland cement with high volumes of blast furnace slag is known to negatively affect the early-age properties of concrete, particularly at low temperatures. In this study, the effectiveness of Na(2)SO(4) on the mechanical properties, hydration kinetics and microstructure development of a commercial CEM III/B (~69% slag) is investigated at 10 and 20 °C. Na(2)SO(4) enhances compressive strength at both 10 and 20 °C, and at both early (1 and 7 days) and later ages (28 and 90 days). QXRD shows an increase in the degree of alite hydration at 1 day with Na(2)SO(4) addition, while the degree of clinker and slag hydration is similar for all the systems from 7 to 90 days. An increase in ettringite content is observed at all ages in the systems with Na(2)SO(4). Microstructure and pore structure shows densification of hydrates and reduction in porosity on addition of Na(2)SO(4).