Influence of free and sorbed zinc on cement hydration

Abstract

Zinc, similarly to many other metals, is known for its negative influence on the rate of hydration in cementitious systems. The present paper deals with a specific form of zinc—Zn2+ ions sorbed on a solid sorbent. The sorbent under study is a waste ceramic powder, exhibiting very good sorption capacity for (not only) zinc. Such ceramic sorbent is intended to be used for removal of Zn2+ from industrial wastewaters. The spent sorbent has to be stabilized in order to prevent leaching of Zn to the environment; the most common stabilization method is incorporation to cement matrix, where the sorbed Zn may harm the hydration course and thus retarded the setting and hardening of stabilizate. Zn is sorbed on the ceramic sorbent in form of insoluble basic zinc sulfate ZnSO4·3Zn(OH)2·4H2O. As a reference system, relatively well-described ordinary Portland cement (OPC)\u2009+\u2009ZnO (also insoluble Zn form) system was tested. The used Zn contents were 0.08, 0.16 and 0.64% by mass of OPC. Isothermal calorimetry demonstrated large delay in systems with 0.64% Zn, while 0.08% did not have any significant influence on heat evolution, regardless the Zn form. At the medium 0.16% Zn content, the sorbed Zn was less retarding than free ZnO. The influence of Zn on strength, portlandite and C–S–H formation was monitored as well in time. To conclude, the differences between ZnO and sorbed Zn were just moderate; both forms retarded the hydration progress and strength in similar way. Presence of Zn caused formation of C–S–H hydrates with higher Ca/Si ratio than it was detected in OPC control.