Long-term performance and microstructural characteristics of cement mortars containing nano-SiO2 exposed to sodium sulfate attack

Abstract

This study aims to demonstrate the long-term (up to 1080 days) degradation of nano-SiO2 (NS) modified cement mortar exposed to sodium sulfate (Na2SO4) attack. The visual appearance, weight variation, length change, and strength loss of mortars were investigated in joint with the mineralogical and microstructural changes. The results show that the incorporation of NS significantly enhances the resistance to Na2SO4 attack of mortars, and the effect is more pronounced with the increase of NS dosage. Meanwhile, a lower water-binder ratio (0.45) is also more effective in reducing the sulfate attack, but the addition of NS works more noticeably in the mortars with a higher water-binder ratio (0.55). Two major mechanisms are identified for the improvements from NS: firstly, the addition of NS reduces the porosity and refines the pore structure (before the sulfate attack), which then limits the ingress of sulfate ions into the mortars during the sulfate attack; and secondly, the consumption of available portlandite from the pozzolanic reaction of NS reduces the amount of forming expansive products like gypsum and secondary ettringite. This is not the same as the MgSO4-attack, where sufficient portlandite acted as the buffer for the degradation of CSH gels. Therefore, incorporating NS can be used as an effective way for enhancing the durability of cement-based materials exposed to Na2SO4 environment.