Durability performance against sulphates of lightweight mortars made with supplementary cementitious materials

Abstract

This paper focuses on the durability properties of lightweight mortars (LWMs) made with supplementary cementitious materials (SCMs), such as silica fume, fly ash and their combination. The mortars were subjected to wetting-drying cycles in Na₂SO₄ solutions, consisting of 35 repetitions and 1-week per cycle, giving a total duration of 245 days. The LWMs performance was studied in terms of compressive strength, mass and longitudinal variation, ultrasonic speed, open porosity and water vapour permeability. In addition, optical and electron microscopy, and XRD and XRF analyses were performed to examine secondary compounds induced by sulphates. It was found that both additions - silica fume and fly ash - contribute to increasing the durability of LWMs. The silica fume mortars showed the highest mechanical strength after completing the cycles and negligible mass loss. However, the control mortars showed evident weathering, cracks and weight loss (6.9 % w/w). The damage was especially visible on the edges and external surface of mortars, and it was confirmed by optical microscopy and scanning electron microscopy. The final open porosity was rather similar, although water permeability was slightly reduced in mortars made with SCMs. Finally, XRD showed that silica fume may have a protective role against sulphates and some differences were found for key minerals associated with alteration processes and pozzolanic mechanisms.