Citric-, tartaric- and sucrose-based retarders in clay–gypsum and air lime–gypsum mortars: effects on setting kinetics and long-term mechanical performance

Calcium sulfate hemihydrate (hereafter referred to as gypsum) is a fast-setting material that can be incorporated into mortars with non-hydraulic binders to enhance early mechanical strength. However, its rapid hardening may reduce workability and limit the applicability of these mortars. This study investigates the effect of incorporating three organic retarders, namely citric acid, tartaric acid and sucrose, on clay–gypsum and air lime–gypsum mortars. Citric and tartaric acids were tested at dosages of 0.05% and 0.1% by gypsum mass, while sucrose was tested at 1% and 5%. The results evidenced that citric acid was most effective in delaying the initial setting time of clay-based mortars, whereas tartaric acid exhibited better performance in air lime-based mortars. Sucrose was the least effective in extending the initial setting time. Despite its strong retarding effect on clay–gypsum mortars, citric acid negatively influenced the microstructure by reducing crystal interlocking and gypsum growth, consequently decreasing the mechanical strength of the mortars at 90 days. However, this effect became less pronounced with extended curing periods. Conversely, tartaric acid substantially delayed setting in air lime–gypsum mortars without hindering gypsum crystal growth or their long-term mechanical strength.