Effect of water content on microstructure and properties of magnesium potassium phosphate cement pastes with different magnesia-to-phosphate ratios

Magnesium potassium phosphate cement (MKPC) is a class of chemically bonded ceramic for special engineering applications. The selection of mass ratios of MgO-to-KH2PO4 (M/P) and water-to-cement (W/C) is an important aspect for formulating MKPC paste. This paper analyzed the stoichiometry of the primary reaction of MKPC and investigated the effect of W/C (0.14-0.40) on paste microstructure and properties at different M/P (1.5-4). At high M/P, low W/C was sufficient to complete the hydration of KH2PO4 and higher W/C yielded looser microstructure, dictating that properties of high M/P pastes were monotonic functions of W/C. However, a clear W/C threshold effect was found in medium M/P pastes at both micro- and macro-levels. Low W/C below stoichiometric value suppressed the formation of K-struvite and favored the presence of an amorphous phase, leading to intense early shrinkage, low early compressive strength, and poor long-term water resistance. The increase in W/C above stoichiometric value resulted in the segregation of excess water and thus to porous structure, with detrimental effects on dimensional stability and strength. The micro-macro analysis highlighted the importance of stoichiometric water amount in the optimal design of MKPC.

Automated summary

The study found that at high M/P ratios, lower W/C was enough for KH2PO4 hydration, while higher W/C led to looser microstructure. However, a clear W/C threshold effect was observed in medium M/P ratios, where low W/C below stoichiometric value suppressed the formation of K-struvite and favored the presence of an amorphous phase, resulting in intense early shrinkage, low early compressive strength, and poor long-term water resistance.