Research On Water Resistance And Steel Corrosion Of Magnesium Phosphate Cement Based Materials

Magnesium Phosphate Cement Based Materials(MPCBM), which consist ofmagnesium oxide, soluble phosphate, retarder and mineral mixture, crystallize into aninterlocking microstructure of struvite when reacting with water. This paperinvestigated the influence of fineness of magnesium oxide on the fluidity, setting time,temperature change, content of hydrated products and compressive strength ofmagnesium potassium phosphate cement. According to the particle size distribution ofMgO, it was found that fluidity and setting time of MPCBM slurry were controlled byMgO with diameter less than30μm. The results of temperature measurements anddifferential thermal analysis showed that the specific surface area of MgO had a greateffect on hydration of MPCBM. When the specific surface area was less than322m2/kg, the hydration temperature profiles of MPCBM indicated two distincttemperature peaks and could be divided into four stages including dissolution,transition, acceleration of hydration and deceleration of hydration. When the specificsurface area was greater than322m2/kg, there was only one temperature peakappeared during the whole processes. From the compressive strength results, it wasfound that the specific surface area of MgO did not have obvious effect on thecompressive strength of MPCBM. However， it could be inferred that the latercompressive strength of MPCBM was influenced by MgO in the diameter range from30to60μm. Accordingly in order to control the hydration rate of MPCBM andachieve higher later compressive strength, the specific surface area of MgO should bebetween228and322m2/kg in this experiment.The existence of large amount of soluble phosphate within MPCBM was themain reason of the weakness in water resistance. Unreacted phosphate withinMPCBM was dissolved in water which decreased the pH value of the solution whenMPCBM were cured in water, thus the main hydration product—MgKPO46H2O wasdissolved in acidic solution which result in the increase of porosity and significantdecrease of strength. The addition of limestone powder that can react with unreactedphosphate and generate anhydrous calcium hydrogen phosphate can improve the waterresistance of MPCBM. The MPCBM with addition of limestone powder had lessamount of soluble phosphate in matrix and lower porosity leading to a betterperformance in water resistance. Phosphate to MgO ratio had an significant effect on the amount of hydrationproducts and remaining amount of soluble phosphate in MPCBM. The low ratio ofphosphate to MgO led to the decrease of the amount of hydration products andremaining amount of soluble phosphate in MPCBM without wet and dry cycles.MPCBM with small ratio of phosphate to MgO had stable steel corrosion resistance.Due to the dispersing effect, there was less amount of remaining soluble phosphate inMPCBM with addition of admixture, which led to a better steel corrosion resistance.MPCBM with addition of limestone powder had better steel corrosion resistance dueto the generation of anhydrous calcium hydrogen phosphate.