A comparative study on retarders for magnesium phosphate cements

Magnesium potassium phosphate cements (MKPC) are chemically bonded ceramics with potential biomedical, construction and nuclear waste management applications. These are formed from acid-base reaction between magnesium oxide and hydrogen phosphate source, leading to struvite (-K) as the main reaction product. The dissolution and precipitation reactions occur rapidly, generating significant heat and causing fast setting. Routinely, boric acid, a reproductive toxin is added to control the reaction of MKPC. This study investigated the activity of citric acid as alternative retarder. MKPCs with MgO/PO4 molar ratio of 4 and 0.5 w/s ratio with or without 2.5% boric or citric acid by weight of solids were investigated by isothermal calorimetry, XRD, and SEM. The results demonstrate effectiveness of citric acid in modifying reaction kinetics and MKPC microstructure. This is significant for safe production of MKPC.