Setting times of ordinary Portland cement pastes of standard consistency were investigated using mixing waters containing dissolved potassium and boron at a constant total concentration of 0.8 mol/kg water. The K:B molar ratio was adjusted between 10:2 and 2:20 in small increments resulting in 31 different compositions. Setting times were measured using an automatic Vicat apparatus as an average of five parallel pastes for each sample. The effect of concentration of equimolar K:B mixing water on setting was also assessed for seven different total concentrations ranging from 0 to 0.3125 mol K + B/300 g cement. Phase equilibria of mixing waters calculated by PHREEQC showed a strong relationship between setting times and stability fields of coexisting phases. Each marked change in setting times corresponded to the replacement of the hitherto stable (calcium)-potassium borate hydrate by another. Correlation was observed between the water demand of the precipitating potassium borates and the constituent phases of mature hydrated cement stone.