The impact of sulfate- and sulfide-bearing sand on delayed ettringite formation

With the increase of aggregate quarrying to meet the growing need for concrete construction comes a risk associated with new aggregate sources, which may not have uniform compositions and contain potentially harmful substances, such as sulfate and sulfide. To provide insights into the impact of sulfate- and sulfide-bearing sand on susceptibility to delayed ettringite formation (DEF), this study measures the evolution of expansion, acoustic nonlinearity (representing microscale damage), and dynamic elastic modulus of mortars prepared with varying cement compositions and exposed to an early-age high-temperature curing cycle. Samples containing sulfate- and sulfide-bearing sand show substantially higher initial levels of damage, with expansion starting at earlier ages than control samples. Damage in mortars using this sand can be attributed to accelerated decomposition of alkali-feldspar, microcracking due to relative thermal deformation between phases, early release of alkali that increases the amount of sulfate in the pore solution, and release of sulfate ions from aggregates.