Reaction mechanisms of magnesium potassium phosphate cement and its application

Magnesium potassium phosphate cement (MKPC) is a kind of cementitious binder in which the chemical bond is formed via a heterogeneous acid-base reaction between dead burned magnesia powder and potassium phosphate solution at room temperature. Small amount of boron compounds can be incorporated in the cement as a setting retarder. The final reaction product of MgO-KH₂PO₄-H₂O ternary system is identified as magnesium potassium phosphate hexahydrate, MgKPO₄·6H₂O. However, the mechanisms and procedures through which this crystalline product is formed and the conditions under which the crystallization process would be influenced are not yet clear. Understanding of the reaction mechanism of the system is helpful for developing new methodologies to control the rapid reaction process and furthermore, to adjust the phase assemblage of the binder, and to enhance the macroscopic properties. This study is mainly focused on the examination of the reaction mechanism of MKPC. In addition, the formulation optimization, microstructure characterization and field application in rapid repair are also systematically studied. The chemical reactions between magnesia and potassium dihydrogen phosphate are essentially an acid-base reaction with strong heat release, the pH and temperature variation throughout the reaction process could provide useful information to disclose the different stages in the reaction. However, it would be very difficult to conduct such tests on the cement paste due to the limited water content and fast setting. In the current research, the reaction mechanism of MKPC is investigated on the diluted MKPC system through monitoring the pH and temperature development, identification of the solid phase formed, and measurement of the ionic concentration of the solution. The reaction process can be explained as follows: when magnesia and potassium phosphate powder are mixed with water, phosphate is readily dissolved, which is instantly followed by the dissociation of magnesia. With the increase of magnesium ions in the solution, MgHPO₄·7H₂O is the first product precipitated, and its crystallization is accompanied with the increase of both pH and temperature. Beyond pH of 7, MgHPO₄·7H₂O is transformed to Mg₂KH(PO₄)₂·15H₂O, leading to a slight decrease of pH. The following dramatic increase of pH may be due to the formation of Mg₂KH(PO₄)₂·15H₂O. Finally, Mg₂KH(PO₄)₂·15H₂O gradually transforms to MgKPO₄·6H₂O and leads to the second decrease of pH. Both increasing molar ratio of magnesium to phosphate (M/P) and decreasing the weight ratio of liquid to solid can speed up the reaction rate while addition of small amount of boron compounds can prolong the process even though the products are not changed. The retarding mechanism of boron compounds is related to their buffering effect on the pH of the solution, i.e. decreasing pH development rate, leads to delaying the formation of reaction products. The performance of MKPC based cementitious materials can be significantly influenced by M/P molar ratio, addition of setting retarder, water content, fly ash replacement of magnesia and aggregate usage. Therefore, the formulation of MKPC based materials is optimized in terms of workability, compressive strength, and cost consideration. With optimized formulation, MKPC mortars show high early compressive and flexural strength, superior bond strength to ordinary Portland cement mortar/concrete substrate, and low drying shrinkage. Undoubtedly, the mechanical properties of this cement is closely related to its inner composition and microstructure. The microstructure examination shows that the phase assemblage and the morphology characteristics of MKPC paste vary with the different formulae. In the formulation with lower M/P ratio of 2, KH₂PO₄ residues can be found in a flat, smooth, and bulky mass form. The reaction product MgKPO₄·6H₂O, can be observed as acicular crystal habit with large aspect ratio of 30. With the increase of M/P ratio, MgKPO₄·6H₂O is crystallized in a larger size and the morphology is changed from acicular to bladed and then prismatic shape. The magnesia residues can be well identified in all of the formulations.