Effect of Pre-Cracks on Cement Paste Degradation under External Sulfate Attack
Cement-based materials are vulnerable to external sulfate attack, particularly when pre-existing cracks are present in the matrix. These cracks serve as pathways for aggressive ions, accelerating chemical degradation and compromising durability. The present study investigates how pre-cracks influence the microstructural and mechanical evolution of cement pastes exposed to a low concentration sodium sulfate solution. Advanced analytical techniques, including SEM-EDS, microindentation, and microtomography, were employed to understand the chemo-mechanical processes driving localized degradation and crack propagation.
Influence of Pre-Cracks on Ion Diffusion
Pre-existing cracks significantly enhance the ingress of sulfate ions, accelerating degradation near the crack zone. The V-shaped chemical attack pattern observed indicates that cracks act as preferential channels, allowing rapid penetration of aggressive agents. This enhanced diffusion mechanism highlights the critical role of crack morphology in controlling the durability of cement-based structures under sulfate exposure.
Chemo-Mechanical Degradation of Cement Matrix
Localized degradation around pre-cracks results from coupled chemical and mechanical processes. SEM-EDS analyses revealed decalcification in the degraded zones, while microindentation demonstrated reduced hardness and stiffness. These findings confirm that chemical attack weakens the matrix, making it more prone to microcracking and mechanical instability.
Crack Propagation and Microstructural Evolution
Microtomography monitoring revealed that new cracks emerge at the interface between degraded and sound zones. These cracks tend to coalesce with the pre-existing pre-crack, promoting further widening and propagation. This self-reinforcing degradation process demonstrates how microstructural changes evolve dynamically under chemical attack.
Strain Localization and Crack Densification
Differential strain between degraded and intact regions induces microcracking around the pre-crack zone. The higher density of cracks in this area indicates localized stress concentrations. Over time, these microcracks align and merge, intensifying structural deterioration. This mechanism explains the observed progressive widening of cracks under sulfate attack.
Implications for Durability and Structural Integrity
The findings underscore the importance of pre-cracks as critical factors influencing sulfate attack severity. Structural elements with pre-existing cracks are more susceptible to rapid degradation, mineral precipitation, and expansion. Preventive measures such as crack control, supplementary cementitious materials, and protective treatments must be considered to mitigate long-term durability issues in sulfate-rich environments.
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