Abstract
Upcycling and reclaiming of low quality or stored coal combustion fly ashes could enable to tap into a voluminous resource of supplementary cementitious materials (SCMs) for low-carbon blended cements. Low reactivity fly ashes are usually either too crystalline or too coarse. Beneficiation treatments for coarse fly ashes comprise size classification or milling processes to extract or produce fine size fractions of higher pozzolanic reactivity. This article compares the effect of size classification and milling treatments on the reactivity of a siliceous fly ash (FA). The intrinsic chemical reactivity is assessed using the R3 heat release test method. The results showed significant increases of 57 and 40% for fine classified and milled fly ash compared to the initial fly ash, respectively. In addition heat release and portlandite consumption were measured for blended cements with 30 wt.% Portland cement replacement by the fly ashes. Both test results are combined to calculate the degree of reaction of the fly ashes over time in blended cement. The results demonstrate a strong effect of particle size on fly ash reactivity and degree of reaction. It is shown that increasing the inherent reactivity of fly ashes is an effective way of both accelerating compressive strength gain and enhancing late age strength with fine classified fly ashes reaching equivalent strength as neat Portland cement by 28 days and attaining a strength activity index of 137% by 90 days.
Highlights
To meet CO2 reduction targets and keep global temperature rise below 2◦C, carbon-intensive industries such as cement production need to change their operations and processes drastically (Lehne and Preston, 2018)
2 kg of the fly ash was dry-milled in a Retsch PM 400 planetary ball mill in order to obtain a finely milled fly ash (FMFA) of similar particle size distribution as the FCFA obtained by classification
Intrinsic Pozzolanic Reactivity The intrinsic reactivity of the fly ash test materials was measured in the clinker-free R3 model system
Summary
To meet CO2 reduction targets and keep global temperature rise below 2◦C, carbon-intensive industries such as cement production need to change their operations and processes drastically (Lehne and Preston, 2018). In case of the cement industry measures need to go beyond improving process efficiency, switching to alternative fuels or electrification, mainly because the production of clinker involves significant CO2 emissions from the decarbonation of raw materials, i.e., limestone (Scrivener et al, 2016; Favier et al, 2018). Material efficiency and carbon capture and storage or use will be needed to completely mitigate emissions and meet reduction targets (CEMBUREAU, 2016; IEA, 2018). Amongst the wide variety of materials being evaluated and studied, reclaimed or low-quality fly ashes may provide large volumes of SCMs
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