Investigation of the effect of partial replacement of Portland Cement by Fly Ash on carbonation using TGA and SEM-EDS

The aim of this paper is to investigate the effect of partial replacement of Portland cement by fly ash on the phase changes upon carbonation and the resulting CO2-binding capacity. Two mortars with water-to-binder ratio 0.55 and two different binders, Portland cement (CEM I) and Portland fly ash cement (CEM II/B-V) were cured sealed for 14 days at 20oC and subsequently exposed to accelerated carbonation (20oC, 60% RH and 1.5% CO2) for 4 weeks. The changes in the phase assemblage upon carbonation were studied by TGA and SEM-EDS. TGA allowed the quantification of changes in Portlandite and calcium carbonate content. SEM-EDS was used to investigate changes in the C-S-H composition and the microstructure. It was observed that both binders formed similar amounts of carbonates upon accelerated carbonation, even though their CaO content differed. This could be attributed to differences in the reaction degree of the Portland clinker and a larger contribution to the uptake of CO2 by C-S-H for the fly ash containing cement. Hence care should be taken when applying the CaO content as a measure for the CO2-binding capacity.