Managing trace elements in Portland cement – Part II: Comparison of two methods to incorporate Zn in a cement

This study compares two methods to immobilise the same amount of Zn in cement paste. In the first method, Zn is introduced into the raw material before clinkerisation, whereas in the second process, Zn is introduced in the liquid phase used to make the cement paste, which is often referred to as solidification/stabilisation. In laboratory-made clinker, Zn was mainly fixed within a new compound (Ca6Zn3Al4O15). Also, Zn was observed in tricalcium silicate (C3S) and in the interstitial phase (C3A and C4AF) with the exception of dicalcium silicate (C2S). In the present study, the threshold limit value corresponds to the maximum amount of Zn that could be incorporated without the stability of the clinker phases being modified. This threshold was reached when a decrease in C3A content and/or the presence of Ca6Zn3Al4O15 were observed by X-ray diffraction. The threshold limit of Zn in the studied clinker was established at 0.7% by weight. The reactivity of synthesised cement doped with Zn was studied by isothermal calorimetry measurements. These tests revealed that the incorporation of Zn had no effect on calcium silicate hydration (C3S and C2S), even if the Zn content exceeded the threshold limit value, i.e., 1%. These results were compared with those obtained through the solidification/stabilisation treatment of Zn in a cement paste. In these experiments, a Zn content of 0.7% induced a significant delay in the cement hydration. This difference in behaviour is due to the slower flow of zinc ions released in solution when it is incorporated in the clinker phases. Thus, the immobilisation during clinkerisation is a good alternative to treat waste with a high Zn content in Ordinary Portland Cement (OPC) without causing the well-known, deleterious effect on cement setting time.