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

The possibility of replacing the natural gypsum used in cement production by a chemical precipitate consisting of gypsum (CaSO4.2H(2)O) 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 80degreesC, 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 25degreesC, 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. (C) 2005 Society of Chemical Industry.