Magnesium potassium phosphate cements (MKPCs) are prepared using calcined magnesia (MgO) and an acidic solution of potassium dihydrogen phosphate (KH 2 PO 4 ). Their fast setting and high heat of hydration can be problematic when large volumes of materials are produced. Boric acid (B(OH) 3 ) is thus commonly added as a set retarder. This work investigates MKPC hydration in paste (water-to-cement ratio w/c = 1) and its retardation by B(OH) 3 . The precipitation of K-struvite (MgKPO 4 ·6H 2 O) is preceded by that of phosphorösslerite (MgHPO 4 ·7H 2 O) and Mg 2 KH(PO 4 ) 2 ·15H 2 O. Cattiite (Mg 3 (PO 4 ) 2 ·22H 2 O), an end-product in diluted suspension (w/c = 100), is not observed. B(OH) 3 slows down the formation of hydrates in two ways: ( i ) by stabilizing in solution the cations that outbalance the negative charges of the polyborates formed at pH above 6, and ( ii ) through the precipitation of an amorphous mineral containing borate and orthophosphate. The first process occurs both in diluted suspensions and pastes, the second is specific to pastes. [ABSTRACT FROM AUTHOR]