Zinc acetate as a set retarder for magnesium potassium phosphate cements

The development of not-toxic retarders with a high retardation efficiency is crucial for a sustainable development of high-performance magnesium potassium phosphate (MKP) cement-based materials. Zinc acetate is such a promising new set retarder for MKP cements. The impact of zinc acetate on the properties and hydration of the MKP cements at a Mg/PO₄ molar ratio of 8 was investigated up to 5 years. Zinc acetate had a much stronger retardation effect than borax at the same dosage, but also decreased flowability, increased drying shrinkage and led to a slower strength gain. The combined effect of zinc and acetate ions led to a lowered pH and the delayed formation of K-struvite at very early hydration times. K-struvite, amorphous magnesium (and/or zinc)-containing phosphates, an acetate-containing phase and some brucite were formed in the hardened matrix at late ages. The zinc ions were immobilized in the hydrated paste matrix, ensuring the safe use of zinc acetate as a retarder for MKP cements.