HYDRATION CHARACTERISTICS AND EXPANSION BEHAVIOUR OF PORTLAND CEMENT WITH B-DOPED MgO

The work focuses on a study of the hydration characteristics and expansion behaviour of Portland cement with B-doped MgO at an early age. The B-doped MgO was prepared from the co-calcination of a magnesium hydroxide reagent and a mixture of magnesium hydroxide and boric acid at 800 C. The hydration process, products, and the porosity of pastes were investigated. Meanwhile, the physical and mechanical properties of the mortars were also determined. The results show that the co-calcination of B and magnesium at 800 C changes the characteristics of the MgO crystals and their activity. With an increase in the dosage of B, the dimensions of the MgO crystals become smaller, and the specific surface area increases, but the activity decreases. The mortars with B-doped MgO show a higher expansion rate. The incorporation of B-doped MgO alters the hydration process of the cement pastes; 4 % B-doped MgO incorporation leads to the most increased cumulative heat at 120 h. The content of Mg(OH) 2 formed in the sample is the highest compared to that of the others. The initial setting time of cement paste is delayed by incorporating B-doped MgO. The higher the dosage of B, the later the initial setting time. The B-doped MgO incorporation increases the compressive strength of the cement pastes at 3 days, but significantly decreases at 28 days.