Microstructures and Properties of Modified Magnesium Phosphate Cement Mortar Prepared at Low Temperatures and Subjected to Freeze-Thaw Cycling at Early Ages

Microstructures and properties of mortar using ammonium phosphate and potassium phosphate were tested and compared in this case. Moreover, two cementitious additions and two lightweight aggregates, including fly ash, redispersible latex powder, ceramsite sand, and rubber powder, were respectively tried to be applied in magnesium ammonium phosphate cement mortar in order to modify the microstructures and properties. The experimental results show that potassium phosphate is not suitable for magnesium phosphate cement mortar for cold region construction purpose. Although fly ash can bring positive modification in the condition of normal temperature curing, it brings negative effects in the condition of sub-zero temperature curing. Either redispersible latex powder or ceramsite sand can improve the freeze-thaw cycling resistance of magnesium phosphate cement mortar in the conditions of low temperature coupled with freeze-thaw cycling, but only the ceramsite sand can improve both mechanical properties and freeze-thaw cycling resistance. The modification caused by ceramsite sand is mainly due to the exceptional bonding strength between hardened cement paste and the porous surface of ceramsite and the porous structure of ceramsite for the release of frost heave stress.