Du Toit, GrizeldaKearsley, Elsabe P.Mc Donald, James M.Kruger, Richard A.Van der Merwe, Elizabet Margaretha2019-02-282018Du Toit G, Kearsley EP, Mc Donald JM, Kruger RA and Van der Merwe EM (2018) Chemical and mechanical activation of hybrid fly ash cement. Advances in Cement Research 30(9): 399–412, https://DOI.org/ 10.1680/jadcr.17.00156.0951-7197 (print)1751-7605 (online)10.1680/jadcr.17.00156http://hdl.handle.net/2263/68537Hybrid fly ash cement is a binder with a composition between that of pozzolanic fly ash cement and alkali-activated fly ash cement. Its production requires less cement clinker than ordinary Portland cement, facilitating a much-needed reduction in the carbon dioxide footprint related to the production of high-clinker-containing cement. Research on activation methods is required to overcome the low early age strength and slow strength development in hybrid fly ash cements. In this study the activation of a South African siliceous fly ash (70%) for use along with Portland cement (30%) in a hybrid alkaline binder was investigated. Both chemical (the addition of sodium sulfate) and mechanical (milling) activation of fly ash was studied. The aim of this work was to develop a better understanding of the characteristics of compressive strength development of hybrid fly ash cement mortars over an extended curing period of up to a year. The results indicated that the combination of mechanical and chemical activation (sodium sulfate) can be considered a viable solution to improving early strength and slow strength development, so that hybrid cements are able to comply with the strength requirements of the BS EN 197 cement specification.en© 2018 Thomas Telford Ltd.Blended cementsCompressive strengthChemical activationStrength developmentMechanical activationEarly age strengthsCement specificationAlkali-activated fly ashesActivation methodSulfur compoundsSodium sulfatePortland cementFly ash (FA)Compressive strengthCarbon dioxideBindersAlkalinityEngineering, built environment and information technology articles SDG-09SDG-09: Industry, innovation and infrastructureArticle