<p>Hydration of magnesium potassium phosphate (MKP) cements is a dissolution–precipitation process. Compared to magnesia, the dissolution effect of monopotassium phosphate (KH₂PO₄) has been gained less attention and remains unclear. In this study influence of two KH₂PO₄ materials with different fineness on properties and micro-characteristics of the MKP cement pastes at Mg/PO₄ molar ratio of 4 and 8 was investigated. Compared with the pastes without boric acid, the effect of KH₂PO₄ fineness was more obvious with the presence of boric acid due to the slowed-down reactions. The combined use of boric acid and the coarse KH₂PO₄ better slowed down the reactions, thus led to longer setting time but to reduced flowability, higher shrinkages and great strength losses. In contrast, the combined use of boric acid and the fine KH₂PO₄ facilitated a better dissolution and consumption of KH₂PO₄ and formations of robust K-struvite hydrates, resulting in less detrimental capillary pores, lower shrinkages and higher strengths, although the reaction could be faster. To guarantee a full consumption of KH₂PO₄ and mitigate strength loss over long-term, Mg/PO₄ molar ratio &gt; 4 and fine KH₂PO₄ are commended for those pure MKP cements.</p>