Use of gypsum/brucite mixed precipitate instead of gypsum in Portland cement

Abstract

The possibility of replacing the natural gypsum used in cement production by a chemical precipitate consisting of gypsum (CaSO4.2H2O) and brucite (Mg(OH)2), was investigated. This precipitate is a by-product of a new hydrometallurgical process, which was developed in order to treat economically low-grade nickel oxide ores. More specifically, it is obtained by hydrolytic precipitation of magnesium at temperatures not exceeding 80 °C, from sulfate solutions which result from heap leaching of nickel oxide ores with dilute sulfuric acid at ambient temperature, using calcium hydroxide as a neutralizing agent. The mixture generally consists of 20-30% non-fibrous magnesium hydroxide, 60-75% gypsum and any excess of calcium hydroxide, depending on the precipitation conditions. In the present work, a mixture was produced by hydrolytic precipitation at 25 °C, using 1.1 times the stoichiometric quantity of Ca(OH)2 required to precipitate all of the magnesium. The possibility of using the above precipitate as a substitute for gypsum in cement was examined by testing four different cement mixtures, one reference sample, containing 4.5% gypsum and 0.5% anhydrite ((PC)Ref) and another three with 4.1%, 5.2% and 6.3% of gypsum/brucite mixed precipitate ((PC)B/G), in the place of gypsum. All samples were tested by determining the grindability, setting time, expansion and compressive strength. The results of the physico-mechanical tests showed that the replacement of natural gypsum by the above precipitate did not affect negatively the quality of the produced cements. © 2005 Society of Chemical Industry.