New perspective to improve the sulfate attack resistance of mortar by coral sand

Sulfate attack is one of several chemical mechanisms of concrete deterioration. It can cause expansion, crack and mechanical property degradation of concrete due to the formation of ettringite and gypsum. A new perspective is proposed in this study to improve the sulfate attack resistance of mortar by considering the porosity characteristic of coral sand. The role of coral sand in the cement mortar when it is exposed to sulfate solution is explored comprehensively. Two kinds of sands, standard sand and coral sand, are used. The mortar sample is exposed to 5 wt % Na₂SO₄ solution after a hydration period of 28 days. Some supplementary cementitious materials (SCMs), such as coral sand powder, limestone powder, fly ash, are used in this study to replace cement partially. After a certain exposure time, the visual appearance, mechanical properties, length and mass change, phase composition and microstructure development of the cement mortar are investigated. The results show that the cement mortar made with coral sand has a much better resistance to sulfate attack than that made with standard sand. Although sulfate attack happens inevitably in coral mortar, the use of coral sand can largely mitigate the loss of properties of mortar, such as strength, mass and linear change, etc. In particular, after 1 year of exposure, the use of coral sand in mortar largely reduces the expansion of mortar due to sulfate attack; the linear change of coral mortar is only one-seventh in that of normal mortar. Formation of ettringite in empty spaces of coral sand and other voids explains why coral mortar has better properties than normal mortar under accelerated sulfate attack. The achievement of this study can largely improve the resistance of marine concrete to chemical attack.