BACKGROUND: Low grade magnesium oxide (LG-MgO) is a by-product from the calcination of natural magnesite that is currently hydrated to magnesium hydroxide by storing it in the open for up to 6 months. It is eight to ten times cheaper than pure magnesium oxide and therefore the revalorization of this by-product is very attractive for those applications requiring great quantities of magnesium hydroxide for which high purity is not required. Here the hydration of LG-MgO is studied as a function of two parameters: hydrating agent and temperature. RESULTS: Addition of acetic acid during the hydration of LG-MgO improved the effectiveness of treatment. At 50 degrees C, the maximum percentage hydration was 40% in pure water and increased to 65% and 70% using aqueous solutions of 0.5 and 1.0 mol L-1 acetic acid. Increase of temperature also had a positive effect on the final degree of hydration. When the treatment was carried out with 0.5 mol L-1 acetic acid, the hydration increased from 50 to 65 and 80% at 25, 50 and 90 degrees C respectively. Accordingly under the optimum conditions of 90 degrees C and 0.5 mol L-1 acetic acid 80% hydration was achieved within 8 h. CONCLUSIONS: The results showed that much shorter hydration times are possible and therefore an industrial alternative to the spontaneous process could satisfy an increasing demand for magnesium hydroxide. Moreover, agitation is not needed as the reaction is chemically controlled. Copyright (c) 2012 Society of Chemical Industry