Chemical and mechanical activation of hybrid fly ash cement

Abstract

Hybrid 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) as well as mechanical (milling) activation of fly ash was studied. This type of hybrid product falls outside of the scope of the accepted national standard, SANS 50197, which is adopted from the European standard EN 197, making it very important to understand as much as possible about the behaviour of this type of cement in the expectation of having it accepted by the standards bodies as well as the construction industry. The literature tends to discuss the compressive strength of fly ash-lime systems (calcium hydroxide or calcium oxide) rather than fly ash-cement systems, even though a few studies have been published on hybrid cements. More emphasis is also placed on early age strength development (2 days up to 28 days) as opposed to the evolution of strength over a protracted time of up to a year. This study therefore aims to fill the gap by presenting and discussing compressive strength and characterisation results of hydrated fly ash hybrid cements over an extended curing period of up to a year. This will provide much needed and valuable information required for the production of cementitious products with a low carbon footprint. It has been proven before that chemical activation in the form of sodium sulfate addition and mechanical activation via milling can both be used as effective activation methods for high fly ash containing hybrid cements. It is however not clear what effect the two activation techniques will have on compressive strength development over an extended curing period. Since fly ash chemistry (and to a certain extent cement chemistry) varies globally and even locally, it was imperative to test the effect of these activation techniques and the possible advantages they might present on local materials from South Africa. The results obtained from this study showed that a fly ash hybrid cement containing 70% of a siliceous South African fly ash and 30% ordinary Portland cement, can reach mortar compressive strengths that comply with the national standard prescriptions i.e. a 32.5R (rapid early strength gain) when a combination of chemical (sodium sulfate) and mechanical activation is applied. Characterisation and analytical techniques such as X-ray fluorescence (XRF), X-ray powder diffraction (XRD), Particle size distribution (PSD) analysis, Field emission scanning electron microscopy (FESEM), Thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR) and microcalorimetry (heat of hydration) of the raw and hydrated materials proved invaluable to some of the major findings regarding ettringite and pozzolanic reactivity of this study. Not only did the above mentioned activation techniques (especially the combination of chemical and mechanical activation) provide stable ettringite formation, but it also accelerated the pozzolanic reaction between fly ash and cement, which led to an improvement in early age strengths and strength development, resulting in hybrid cements that comply with the EN 197 cement strength requirements.