Influence of calcium oxide and hydroxide on non-ferrous metallurgical slag hydration in alkaline environments
Vitalii Ponomar, Juho Yliniemi, Katja Kilpimaa
Abstract
The search for new supplementary cementitious materials (SCMs) to reduce CO2 emissions has become increasingly vital in cement and concrete research. Non-ferrous metallurgical slag (NFMS), often overlooked due to their low pozzolanic and hydraulic reactivity, present a promising alternative, particularly given their enhanced reactivity in the presence of alkalis. This study explores the hydration behaviour of NFMS under varying concentrations of CaO, Ca(OH)2, and NaOH, with a focus on dissolution kinetics and phase evolution. 1 M and 6 M NaOH solutions were selected to examine the effect of calcium additives on NFMS hydration, representing scenarios without (1 M NaOH) and with (6 M NaOH) alkali activation reactions. In 6 M NaOH, the addition of CaO/Ca(OH)2 had a minimal impact on dissolution rates but significantly influenced phase assemblage, with CaO promoting strätlingite formation and 10 % Ca(OH)2/CaO leading to katoite. At higher replacement levels (20 % CaO/Ca(OH)2), phase formation was hindered, although microstructure densification persisted, accompanied by reduced strength. In 1 M NaOH, low concentrations of CaO/Ca(OH)2 enhanced Al and Si dissolution but had limited impact on phase assemblage. Higher CaO content favoured katoite formation, while Ca(OH)2 exhibited traces of hydrocalumite. Both additives improved microstructure, but CaO addition, associated with katoite formation, predominantly enhanced strength. The findings confirm that NFMS has significant potential as a cement replacement in hybrid systems utilizing high ratios of slag, Portland cement, and alkalis. By promoting the formation of Ca-Al phases, NFMS improves microstructure and strength, making it a viable and environmentally friendly alternative in construction materials.