Thaumasite sulfate attack characteristic of seawater and sea sand concrete with limestone powder in different sulfate environment at low temperature

Abstract

The shortages of river sand and freshwater in marine engineering can be addressed by utilizing seawater and sea sand concrete (SWSSC). However, the sulfate resistance of SWSSC at low temperature is not clear. In this study, the seawater and sea sand cement mortar (SSCM) and ordinary cement mortar (OCM) with different limestone powder (LP) content were corroded by different sulfate solutions at 5 °C. The change of compressive strength and compositions of corrosion products in mortars after different times of corrosion were studied. The results show that SSCM presents 9.4∼19 % lower strength loss and better sulfate resistance than OCM. The presence of Friedel’s salt in SSCM contributes to a reduction in corrosion products like ettringite, gypsum, and thaumasite, thereby minimizing microstructural damage and mitigating the thaumasite sulfate attack (TSA). All mortars containing LP suffer from TSA and significantly damage with 17.4∼48.7 % strength loss after 360 days of MgSO₄ or Na₂SO₄ corrosion. Increasing the LP content exacerbates the TSA damage in SSCM. This is primarily due to the additional soluble carbonate from LP promoting the formation of gypsum and thaumasite, thereby worsening the microstructure deterioration of SSCM. Meanwhile, the thaumasite is not found in SSCM without LP after corrosion. In contrast to Na₂SO₄ corrosion, the thaumasite formation under MgSO4 corrosion mainly results from indirect reaction. The combined corrosion effects of Mg²⁺ and SO₄^2- lead to a more significant increase in expansive products (thaumasite and gypsum) and TSA damage. The findings of this study help to advance the sulfate resistance of SWSSC in marine environments.