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Description
With the cement technology moving towards low embodied CO2 cements, optimization of ternary binders composed of Portland cement (PC)-Calcium aluminate cement (CAC) and calcium sulfate (C$) that meet the sustainability need becomes relevant. Previous reports demonstrate that replacement of CAC by slag, silica fume or fly ash hinders the conversion process. However, the incorporation of SCMs in PC-CAC-C$ ternary binders, has been overlooked.
This study, examines the role of limestone and metakaolin as cement substitute in CAC-PC-calcium sulfate ternary binders. Combinations of CAC-PC-C$ with amounts of 5 up to 25% wt.% metakaolin/limestone were investigated in terms of heat evolution, setting time, phase formation, dimensional stability and strength development up to 90 days of hydration.
Within the first 24 hours of hydration both metakaolin and limestone lead to an acceleration of the heat of hydration, presumably serving as nucleation sites.
Compressive strength increased by ~ 36%, in the metakaolin based compositions over 90 days compared to the control, while shrinkage was mitigated. No major difference between samples with 5, 15 and 25 wt.% of substitution were observed, hence indicating that that even small additions are sufficient to achieve an improvement in strength. Formation of AFm-carbonate equivalent phases was favored in presence of limestone, whereas the use of metakaolin enhanced strätlingite formation. The content of AH3 decreased with the increase of substitution level.
The study suggests that mainly metakaolin has a profound influence on the hydration and resulting microstructure of CAC-rich ternary binders. This can be used as a tool to maximize the use of SCMs in such ternary binders for environmental benefits
| Keywords | Calcium aluminate cement, metakaolin, hydration, mechanical properties |
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