Research On Mechanism And Improvement Of Water Resistance Of Magnesium Phosphate Cement

Magnesium Phosphate Cement （MPC）,composed of proper proportion of MgO,phosphate and retarder, is a novel cementing material. Compared to common Portlandcement, MPC is characterizes by the higher bonding strength, less shrinkage, favorablewear-resistance and freezing-resistance and the ability of fast setting under lowtemperature. It has been widely used in repair work in bridge, road or airport pavement,in addition, it has a promising application on solidification of harmful and radioactivesubstances. Therefore, it plays significant role in military affairs and national economyfields. So far, researchers have done a lot of work about the hydration mechanism,retardation mechanism, microscopic structure, hydrate composition, setting time,mechanical property, bonding mechanism and repairing performance of MPC. Earlierstudy showed that the strength of MPC decreased aggressively in water conditions,which limited its usage in many areas. Hence, the research on the water resistance ofMPC has an important practical significance.Firstly, this study explored the effects of different curing conditions and variouswater states on the compressive strength of MPC. Results showed that the strength ofMPC dramatically declined under water, especially under flowing water. MPC presentspoor water resistance and much more decrease in strength. In order to find out thefactors that affecting the water resistance of MPC, the influence of fineness and activityof MgO, the proportion of phosphate and MgO, binder to water ratio, retarder,environment temperature and humidity on strength were analyzed. Secondly, based onthe dissolution-migration-crystallization theory, the water resistance mechanism ofMPC was discussed. Finally, this study investigated some admixtures for improving thewater resistance of MPC. The conclusions are as follows：The tests showed that the compressive strength of MPC deceased to25.96%and48.52%at28d and90d under water, respectively. The expansion ratio of specimenslargely increased before28d, and the volume expansibility accelerated from0.0271%for air condition to0.0965%for water condition. Especially under running waterconditions, the strength of and water resistance of MPC decreased more sharply.Based on the dissolution-migration-crystallization theory, it is found that theprimary cause of affecting the water resistance of MPC was its hydrate products, suchas MAP, residual MgO and phosphate. In water condition, unreacted phosphates, like NH₄H₂PO₄, are corroded and create an acidic environment resulted to the dissolution ofMAP and residual MgO to form Mg²⁺、NH₄⁺、PO₄^3-. These ions can migrate to thesurface and then re-crystallize to form MAP, which leads to the reduction of hydratecontent and increase of pores in MPC. Consequently，both of the water resistance andthe strength of MPC degraded. The presence of some admixtures, such as aluminum salt,ferric salt, coal ash and external waterproof agent, can improve the water resistance ofMPC.