Retardation of cement hydration caused by heavy metals present in ISF slag used as aggregate

A study was carried out into the potential use of slag from the Imperial Smelting Furnace method of zinc production (ISF slag) as a fine aggregate in concrete. One of the main issues associated with the use of ISF slag was that zinc and lead ions are known to cause retardation of the cement setting process. The effect of various inorganic additives to attempt to offset the retardation is reported. The previously favoured explanation for the mechanism of retardation could not fully explain observations made during this and other studies. Based on evidence gathered during this work, a detailed mechanism is proposed for the retardation of setting caused by heavy metal ions such as zinc and lead, involving the conversion of a metal hydroxide to a metal hydroxy-species. This conversion reaction consumes calcium and hydroxide ions from the surrounding solution and delays their supersaturation and hence the precipitation of CSH gel and Ca(OH)2.