Pore size analyses of cement paste exposed to external sulfate attack and delayed ettringite formation

Significance 

Concrete is defined as a composite material composed of fine and coarse aggregates bonded together with cement paste (binder) that hardens over time. The binder constituent is critical regarding durability issues, owing to the fact that it is subject to react differently in various environments and even with aggregates. For instance, constructions in sulfur rich soil deposits or marine environments increases the chances of External Sulphate Attack (ESA). On the other hand, Internal Sulphate Attack (ISA) is due to a source of sulfate being incorporated into the cement at the time of mixing: Delayed Ettringite Formation (DEF) is one form of ISA that can affect concretes exposed to elevated temperature, mainly due to precast processes or to the heat of hydration at early age in massive concrete structures.

Essentially, ettringite is a primary product of hydration of Portland cement. Its formation plays an important role in the control of setting. However, an excessive precipitation of ettringite due to sulfate attack in a mature cementitous material damages the concrete through the formation of cracked and deteriorated  zones due to an expansion of the material. Attempts have been made to document the damaging mechanisms of ESA and DEF in concrete and even differentiate their damaging mechanism. Nonetheless, with the existing similarities in majority of the reported approaches, a global expansion mechanism ought to be investigated so as to explain the behavior of specimens under different sulfate attack conditions.

To address this, a team of researchers from the Paris-Est University: Dr Yushan Gu, Dr Renaud-Pierre Martin, Dr. Othman Omikrine Metalssi, Dr. Teddy Fen-Chong and Dr Patrick Dangla; carried out an experimental study on cement paste specimens exposed to external sulfate attack (ESA), delayed ettringite formation (DEF), and the coupling effect of both reactions. Their goal was to compare the degraded state of specimens, namely the length and mass variations, and the pore size distribution (PSD) obtained by mercury intrusion porosimetry (MIP) of cement paste subjected to these different exposure conditions. Their work is currently published in the research journal, Cement and Concrete Research.

In brief, the team set up six small experimental specimens (2x2x12 cm3)  that were then subjected to three different sulfate attack conditions (ESA, DEF and the coupling effect of both reactions) and were tested at two states: initial and final states (i.e. respectively before and after the development of degradations). Technically, the initial state was the moment after the 28 days of curing, right before exposing the specimens to their respective aging conditions while as the final state for the specimens exposed to ESA, DEF and ESA + DEF was the time when the specimens were seriously degraded and showed significant expansions. One more set of big cement paste specimens (11x11x22 cm3) subjected to DEF were added to compensate a limited expansion degree of small DEF specimens, and they were measured at different states: the initial state, the latent period and the final states when expansions reach a plateau.

A comparison of the MIP obtained PSD of the specimens before and after the sulfate attacks revealed that a global expansion mechanism could be proposed, such that; ettringite first precipitates in the biggest pores without inducing obvious expansion, and then penetrates into capillary and gel pores leading to an accelerated swelling. Altogether, the authors reported that under the same experimental conditions, the greatest degree of degradation was observed with coupling specimens, while the DEF specimen showed the smallest degree.

In summary, in the study presented, specimens exposed to ESA, DEF and the coupling effect of both were monitored during the degradation process. Overall, the coupling effect of ESA and DEF was found to be the most damaging expansion, which raises a high durability problem for cement-based materials. In a statement to Advances in Engineering, Professor Patrick Dangla, the lead author, emphasized that their work represented the right impetus to steer further research aimed at better comprehending the variation of the assessed pore structure in cement mortars and concrete.

Reference

Yushan Gu, Renaud-Pierre Martin, Othman Omikrine Metalssi, Teddy Fen-Chong, Patrick Dangla. Pore size analyses of cement paste exposed to external sulfate attack and delayed ettringite formation. Cement and Concrete Research, volume 123 (2019) page 105766.

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