Behavior of blended cement mortars exposed to sulfate solutions cycling in relative humidity

In recent years, several cases of damage to concrete structures due to sulfate exposure have occurred essentially in the above ground parts of structures. Such distress, often characterized by white efflorescence and surface scaling, is driven by salt crystallization in pores and/or repeated reconversions of certain sulfates between their anhydrous and hydrated forms under cycling temperature and relative humidity (RH). However, the effect of the water/cementitious materials ratio (w/cm), pozzolanic additions, and other parameters on the durability of cement-based materials under such exposure conditions is still misunderstood. In this study, 12 cement mortars having different w/cm (0.30, 0.45, and 0.60) and made with ordinary portland cement (OPC) or OPC incorporating 8% silica fume, 25% class F fly ash, or 25% blast furnace slag were made. Standard bars from each of these mortars were submerged in both 10% magnesium sulfate (MgSO₄) and 10% sodium sulfate (Na₂SO₄) solutions; their expansion and surface degradation was monitored for up to 9 months. In addition, cylinders made from these 12 mortars were partially submerged in 50-mm-deep 10% MgSO₄ and 10% Na₂SO₄ solutions. Half of the cylinders were maintained under constant temperature and RH, whereas the others were subjected to cycling RH. The effect of the w/cm and mineral additions on the classic chemical sulfate attack and development of efflorescence was investigated, and the results are discussed in this article.