Improved durability of cement mortars exposed to external sulfate attack: The role of nano & micro additives

• Resistance against sulfate attack was remarkably enhanced with mineral additives.
• Addition of mineral additives significantly reduced the expansions.
• Disintegration of the samples under sulfate attack was caused by gypsum formation.
• Effectiveness of nano silica for limiting the expansions was highly significant.

External sulfate attack (ESA) is a critical issue which may turn into a big concern about the durability and service life of structural elements get into contact with high sulfate environments. Especially when improper concrete mixture materials and design parameters are selected, structural elements exposed to sulfate attack might be significantly deteriorated. In this experimental work, the effects of various micro and nano cementitious materials on the long-term durability of mortars against ESA were studied. For this purpose, nano (colloidal) silica (NS), micro silica (MS), fly ash (FA) and ground granulated blast-furnace slag (GGBS) were selected with various replacement ratios. All the samples were kept in 5% sodium sulfate solution for approximately 3 years and the expansions caused by sulfate attack were monitored. Additionally, at the end of 3 years of exposure, residual flexural strength, compression strength and ultrasonic pulse velocity (UPV) of mortars were determined and their results were compared with the reference samples which do not contain mineral additives and cured in water. For the micro-structural evaluations, XRD analysis was performed. Results have shown that significant improvement against sulfate attack was achieved by using the additives mentioned above by properly selecting its dosage. Within the limits of this work, mineralogical analysis via XRD tests have proved that decomposition of the sulfate exposed reference samples which do not contain any mineral additives was not caused by ettringite formation, but gypsum formation.