Use of jarosite/alunite precipitate as a substitute for gypsum in Portland cement

The possibility of replacing the natural gypsum, used in cement production, by a jarosite/alunite chemical precipitate 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. For this purpose, nine mixtures were produced by substituting gypsum, from 0% to 100%, by the jarosite/alunite precipitate. All samples were tested by determining the setting time, compressive strength, grindability and content of water soluble chromium. Furthermore, XRD analysis was used to determine the hydration products after 2, 7, 28 and 90 days. The jarosite/alunite precipitate was produced from a real laterite heap leach liquor by precipitation at atmospheric pressure, during a series of six runs. More specifically, iron and aluminum were precipitated at 95 °C. That was accomplished by gradually raising the initial pH of the leach liquor to an equilibrium pH value of 3.5, at such a rate as to keep supersaturation of iron and aluminum below a critical value.