Expansion of concrete by secondary ettringite formation, caused by fine recycled aggregates contaminated with gypsum

Recycled aggregates, and especially the fine (0/4 mm) fraction, are often contaminated with sulfates coming from gypsum residues on the demolition site. When these aggregates are used in concrete, the sulfates can induce internal sulfate attack which causes the expansion of concrete. Standard EN206 sets the water soluble sulfate limit at 0.2 % by weight of the aggregate but other studies suggest this limit could be safely increased. In addition to the sulfate content, other parameters like the porosity and alkalinity of a mix have been seen to influence the swelling results. In this study, the different proposed sulfate limits are evaluated on concrete made with 100 % fine recycled aggregates. It is also researched whether mixing parameters could change the swelling amount regardless of sulfate content. The results showed that the incorporation of fine recycled aggregates with sulfate contents up to 0.8 mass% is safe when combined with coarse natural aggregates. If coarse recycled aggregates are used, the sulfate content of fine recycled aggregates could reach up to 0.3 %. The swelling caused by these sulfate levels was not high enough to be influenced by porosity or alkalinity.