BACKGROUND: The influence of different hydrating agents on the pH of the hydrating solutions, rate of hydration of MgO to Mg(OH)(2) and product surface area was studied as a function of temperature of hydration. Hydrating agents used were aqueous solutions of ammonium chloride, magnesium acetate, magnesium nitrate, nitric acid, acetic acid, magnesium chloride, sodium acetate and hydrochloric acid and distilled water as control. These were chosen to determine either the effect of addition of a common ion, the effect of changing the solution pH or due to the presence of an acetate ion, found earlier to have a beneficial effect on the hydration of MgO. RESULTS: There was no significant difference in the hydration behaviour of the hydrating agents up to 50 degrees C, where less than 10% of magnesium hydroxide was formed. The amount of hydroxide increased at temperatures above 60 degrees C. When compared with the hydration in water, all the hydrating agents, with the exception of sodium acetate, showed a significant increase in the degree of hydration. Sodium acetate formed the lowest amount of magnesium hydroxide, ranging between 1.2 and 12.2% magnesium hydroxide. The largest percentage (56.7%) of magnesium hydroxide was formed from hydration in magnesium acetate. CONCLUSION: It seems that MgO hydration is a dissolution-precipitation process controlled by the dissolution of MgO. The increased degree of hydration in magnesium acetate is possibly due to the presence of acetate and Mg2+ ions. (C) 2010 Society of Chemical Industry