The Effect of Crystalline Waterproofing Admixtures on the Self-Healing and Permeability of Concrete

This paper investigates the effectiveness of a specific crystalline waterproofing admixture (CWA) in concrete as a function of a water-binder ratio. Four concrete mixes with and without CWA were prepared; two of them with a water-binder ratio of 0.45 and two of them with a water-binder ratio of 0.55. Water permeability and compressive strength were tested on hardened concrete specimens and self-healing of cracks over time was observed. Cement paste and CWA paste were prepared to clarify the results obtained on the concrete specimens. SEM and EDS and XRD and FTIR were performed on the hardened pastes to explain the mechanism of CWA working. The results show that the addition of CWA had no significant effect on the compressive strength of the concrete, but reduced the water penetration depth in the concrete, and the reduction was more effective for mixes with lower water-binder ratio. Regarding the self-healing effect, it can be concluded that the addition of CWA improves the crack healing in concrete, but the efficiency of self-healing is highly dependent on the initial crack width. The mechanisms involved in the reduction of water penetration depth and crack healing in concrete can be explained by different mechanisms; one is creation of the CSH gel from unreacted clinker grains, then formation carbonate, and additional mechanism is gel formation (highly expansive Mg-rich hydro-carbonate) from magnesium based additives. The presence of sodium silicate, which would transform into carbonate/bicarbonate, also cannot be excluded.

Automated summary

The study showed that adding CWA had minimal impact on the compressive strength of concrete, but it did reduce water penetration depth, especially in mixes with lower water-binder ratio. Additionally, CWA improved crack healing in concrete, although the efficiency of self-healing depended on the initial crack width. Mechanisms involving CSH gel formation, carbonate formation, and gel formation from magnesium based additives were identified as contributing to the reduction in water penetration depth and crack healing in concrete.