Will the magnesium sulfate attack of cement mortars always be inhibited by incorporating nanosilica?

Abstract

This paper presents a detailed investigation on the durability of nanosilica (NS) blended cement mortars with different water-binder ratios (w/b, 0.35, 0.45, and 0.55) immersed in magnesium sulfate (MgSO4) solution up to 36 months. The visual appearance, liner expansion, mass change, and strength loss of the attacked specimens were measured along with the corresponding microstructural changes. The results showed that the incorporation of the lower content of NS (such as 1 wt%) was effective for inhibiting the MgSO4 attack, irrespective of the w/b; however, further increasing the NS replacement (5–8 wt%) aggravated the deterioration of blended mortars exposed to MgSO4 solution. At the low NS replacement ratio (1 wt%), the blended mortars with the w/b of 0.45 exhibited the lowest strength losses in comparison to the those with the w/b of 0.55 and 0.35. Nevertheless, at the high replacement ratio (3–8 wt%), lowering the w/b from 0.55 to 0.35 reduced the MgSO4 resistance of the blended mortars. Increasing the NS dosage and decreasing the w/b caused the greater degradation of C-S-H gel attacked by MgSO4. Both the lower permeability and the higher amount of CH were necessary for improving the durability of cement-based materials under the MgSO4 environment.