Characterisation of the Hydration Products of a Chemically and Mechanically Activated High Coal Fly Ash Hybrid Cement

Grizelda Du Toit,Richard A Kruger,Elizabet M Van Der Merwe,Elsabé P Kearsley,James M Mcdonald

doi:10.3390/min12020157

Abstract

Cement companies are significant contributors of the planet’s anthropogenic CO2 emissions. With increased awareness of the substantial volume of CO2 emissions from cement production, a variety of mitigation strategies are being considered and pursued globally. Hybrid cements are deemed to be technologically viable materials for contemporary construction. They require less clinker than that for ordinary Portland cement, leading to a decrease in CO2 emissions per tonne of hybrid cement manufactured. The hybrids produced in this study consist of 70% siliceous coal fly ash and 30% Portland cement, and combines chemical (sodium sulphate) and mechanical (milling) activation. The aim of this work was to develop a better understanding of the hydration products formed and the resulting effect of activation on these hydration products, of hybrid coal fly ash cement pastes over an extended curing period of up to one year. The results indicated that chemical activation increases the formation of stable, well crystallised ettringite. Chemical activation as well as mechanical activation increased the rate of the pozzolanic reaction between portlandite contained in cement and coal fly ash. The application of combined chemical and mechanical activation definitely resulted in the fastest rate of portlandite consumption, hence an increased rate of the pozzolanic reaction.

Highlights

Cement companies are significant contributors of the planet’s anthropogenic CO2 emissions
The Portland cement used in this study has a mean particle size of approximately 13 μm with a significantly smaller coarse fraction compared to unclassified coal fly ash (UFA)
The consistent presence of ettringite at all curing ages, prove that chemical activation by means of Na2 SO4 addition increases the degree of ettringite formation which may lead to denser hybrid specimens with increased strength, as was presented in Figure 3 [24]

Summary

Introduction

Cement companies are significant contributors of the planet’s anthropogenic CO2 emissions. A hybrid coal fly ash cement is a binder with a composition between that of pozzolanic coal fly ash cement and alkali activated coal fly ash cement [3,4,5]. The production of hybrid cement requires less clinker than that for ordinary Portland cement, leading to a decrease in CO2 emissions per tonne of hybrid cement manufactured. Hybrid alkaline cements is often preferred over their alkali activated counterparts because its hydration occurs at ambient temperature, and its production do not require the addition of highly alkaline chemicals, but rather rely on a safe source of alkali formed in situ (sodium hydroxide) to facilitate both the dissolution of any amorphous phases present in the source materials [5,6,7,8]

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