Unidirectional sulfate ingress in limestone calcined clay cement (LC3) pastes under cyclic exposure

Abstract

If sulfate attack on Portland cement systems has been largely investigated in the last decades, mechanisms of sulfate resistance for systems with new SCMs are still in dim, especially for the emerging materials that are limestone calcined clay cements (LC³). Using a new semi-immersion approach, we forced the penetration of sulfate ions in just one direction into LC³ pastes under the capillary rise effect. To enhance the degradation process, highly concentrated sodium sulfate solution of 50 g/L and a cyclic exposure regime (20˚C & 78% RH followed by 40˚C & 33% RH) were employed in this paper. During exposure, lateral expansion was measured over time, showing almost negligible expansion for the LC³ cement paste even after 56 days of very harsh exposure conditions. Simultaneously, microanalytical studies on the composition of hydration products were carried out by SEM-mapping to explain the expansion mechanisms. The results showed that the novel approach was adequate for cyclic exposure experiments, to investigate the full depth of degradation along the penetrating direction. Based on the analysis of BSE micrographs, phase distribution maps and expansion profiles, the gypsum and the ettringite were found to coexist in the voids resulting in a densified layer which could be associated with the most expansive zone. Overall, this study highlights the potential of the unidirectional semi-immersed method to link the sulfate attack expansion of cement pastes with the underlying mechanisms.