Internal sulfate attack caused by gypsum contamination of recycled aggregates: development of a swelling test protocol

Sulfate attack is a deteriorating process for concrete, where the formation of expansive minerals such as ettringite causes the swelling of a material. The sulfates necessary for this reaction can come from an external (such as sea- or groundwater) or an internal (such as cement or aggregates) source. A specific situation within this problematic is the contamination of recycled aggregates with gypsum from the demolition site. To assess the extent of the damage that this gypsum contamination can cause, the length of cementitious mixes that incorporated recycled aggregates needs to be monitored. The challenge in this research exists in finding the right testing protocol to follow the development of sulfate attack. Being a slow reaction which can take several months, accelerating procedures could be necessary. Different methods are described, but do not seem to be applicable in this specific context of gypsum contamination. In this paper, fine recycled aggregates contaminated with different levels of gypsum are subjected to a set of various storage conditions and/or aging cycles. The results show that existing protocols, set up specifically for internal sulfate attack, do not provide the acceleration or aggravation of the reaction as might have been shown before. A definitive testing protocol, to be used in this specific situation, is eventually proposed.