Evaluating the Flexural Performance of Sintered Sludge Ash-Modified Cement Paste Using Surface Cracks and Fracture Toughness

Sintered sludge ash (SSA) represents a promising alternative to traditional cement, offering a potential pathway for reducing carbon emissions. This study examined the flexural performance of SSA-modified cement paste (SSC paste) at varying SSA proportions (S0 (0%), S1 (5%), S2 (15%), and S3 (25%)) and employed innovative digital image correlation (DIC) technology to track the evolution of surface cracks during flexural strength testing. Furthermore, Griffith’s theory of fracture toughness was employed to evaluate the fracture performance of SSC paste. The observations of flexural strength at 3, 7, and 28 days indicated that the content of SSA had an adverse effect on flexural performance. Furthermore, the monitoring of cracks confirmed the practicality of DIC in evaluating flexural properties. The analysis of maximum strain and crack propagation via DIC revealed a distinct trend: the presence of 5% SSA inhibited crack propagation and enhanced flexural ductility, whereas the presence of 25% SSA produced the opposite effect. This was corroborated by fracture toughness calculations based on Griffith’s theory. It is noteworthy that 15% SSA represented a critical threshold that delineated variations in flexural strength, ductility, and fracture toughness, which may be linked to the Ca/Si and Ca/Al ratios in the composite matrix. This study demonstrates the innovative application of digital image correlation (DIC) in the monitoring of crack behavior and offers new insights into the crucial proportion of SSA that affects the mechanical properties of SSC paste.