The effect of retarders on the microstructure and mechanical properties of magnesia-phosphate cement mortar

Magnesia-phosphate cement (MPC) mortars based on magnesia and ammonium dihydrogen orthophosphate (ADP) were prepared with additions of sodium triphosphate (STP), boric acid, and borax as set-retarders. The resulting materials were characterised in terms of their setting characteristics, mechanical properties, and microstructures. The retarders were found to increase the setting time and to reduce the intensity of the exothermic reactions during the initial setting and hardening stages. The effectiveness of STP as a retarder was limited by its solubility in the saturated ammonium phosphate solution, determined as 340 g dm(-3), which meant that the maximum setting time obtained was approximately 15 min. In contrast, successive additions of boric acid and borax led to a cumulative increase in setting time to a maximum of 1 h for the compositions investigated. The use of retarders also led to an extension of the initial hardening period, as indicated by reductions in the early values of Young's modulus and flexural strength. The microstructural characteristics of the materials produced were very dependent on the type and level of additive employed; for STP, the binding phase was relatively fine grained (< 0.5 <mu>m), while for boric acid relatively large-grained (> 10 mum) structures were observed as a result of a reduction in the supersaturation of the aqueous solution. Well-aged mortar specimens exhibited a high incidence of microcracks on the fracture surfaces, which were related to the occurrence of a fracture process zone ahead of the main crack tip during fracture. (C) 2001 Elsevier Science Ltd. All rights reserved.