Carbonation of alkali-activated Binders and Concretes - Mechanisms and Implications (CaaB)
Final Report Abstract
The carbonation resistance of alkali-activated materials (AAMs) is essential for their use in concrete constructions; however, the parameters influencing this property are not yet fully understood. The research presented here assessed the carbonation resistance of alkali-activated concretes with varying proportions of ground granulated blast furnace slag (GGBFS) and calcined clay (CC), and thus with high, intermediate, and low Ca content. The main goals were to determine how CO2 concentrations higher than natural levels (as used in accelerated carbonation testing) influence the progress of carbonation in AAMs, to investigate the critical relative humidity for carbonation of these products, and to assess how leaching affects the rate and consequences of carbonation in AAMs. Hardened AAM pastes were characterised using X- ray diffraction (XRD), thermogravimetry (TG/DTG), Raman microscopy, mercury intrusion porosimetry (MIP), µ-X-ray-fluorescence (µ-XRF) and SEM-EDS on selected samples. The results showed that the carbonation resistance of concretes under natural CO2 concentrations was mainly influenced by their water/(CaO + MgOeq + Na2Oeq + K2Oeq) ratio, with variations in pore structure further affecting the performance. When favourable water/binder ratios and pore structures were achieved, the carbonation resistance was comparable to that of Portland cement concrete. Differences in carbonation coefficients between accelerated and natural conditions were observed for concretes, strongly suggesting that accelerated carbonation tests may be unsuitable for low-Ca AAMs. Compared to Portland cement concrete, all AAM concretes showed a strong dependence of carbonation resistance on relative humidity, with carbonation resistance increasing continuously from 45% to 85% relative humidity. To further understand the influence of leaching, three different procedures were applied: leaching for two and five weeks prior to carbonation exposure and five cycles of leaching followed by carbonation. Despite maintaining the same water/(CaO + MgOeq + Na2Oeq + K2Oeq) ratio, leaching generally decreased carbonation rates, except for the 0 % CaO sample, which exhibited similar carbonation behaviour to samples exposed to 4 vol.-% CO2. The poorer performance of this mixture was linked to its low degree of reaction at 28 days, leading to higher sodium leaching and a reduction in calcined clay activation capacity. In conclusion, the study provides insights into how a varying Ca content, carbonation conditions, and leaching impact the carbonation resistance of alkali-activated concretes, highlighting the importance of optimising both mix proportions and testing protocols for a reliable assessment of AAM carbonation behaviour.
Publications
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Carbonation resistance of alkali-activated GGBFS/calcined clay concrete under natural and accelerated conditions. Construction and Building Materials, 449, 138351.
Tambara, Luís U.D.; Hirsch, Astrid; Dehn, Frank & Gluth, Gregor J.G.
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Carbonation resistance of GGFBS/calcined clay-based alkali-activated concretes, in: Proceedings of the 78th RILEM Annual Week & RILEM Conference on Sustainable Materials and Structures, Toulouse, France, 2024.
L. Tambara, F. Dehn & G.J.G. Gluth
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Effect of alkali-activated concrete composition on carbonation rate under accelerated and natural conditions, in: Proceedings of the RILEM Spring Convention and Conference on Advanced Construction Materials and Processes for a Carbon-neutral Society, Italy, Milan, 2024.
L. Tambara, F. Dehn & G. Gluth
