Solidification/stabilisation of Zn²⁺ ions in metakaolin and homra-blended cement matrices

Ordinary type I Portland cement (OPC) and blended OPC with 30 wt% metakaolin and/or homra (crushed clay bricks) were used for stabilisation/solidification of zinc (Zn²⁺) ions. Contents of Zn²⁺ ions representing 0·5 and 1·0 wt% of the solid binder were introduced as zinc nitrate (Zn(NO₃)₂) in the water of hydration. Compressive strength, percentage of total porosity and chemically combined water content were determined at 1, 3, 7, 28 and 90 days of hydration. The phases formed upon hydration were identified using X-ray diffraction analysis (XRD). The blended OPC contained 30 wt% cement less than OPC; however, its immobilisation efficiency for Zn²⁺ ions up to 120 days did not decrease. Furthermore, blended OPC showed higher compressive strength than OPC, with metakaolin showing greater enhancement than homra. The results of XRD have indicated that in both cases, OPC and blended OPC, the fixation of Zn²⁺ ions proceeds by way of formation of calcium zincate (CaZn₂(OH)₆.2H₂O).