Effects of straw length and content on strength and deformation properties of cemented sludge

Traditional cement-solidified sludge often exhibits low strength and brittle failure. This study systematically investigates the effects of straw fiber length (0.3-9 mm) and content (0-1.0%) on the early (7-day) mechanical behavior of cement-stabilized sludge. Unconfined compressive strength (UCS), stress-strain behavior, brittleness index, and microstructure were evaluated. Results show that UCS of solidified sludge rises then falls with increasing straw length, peaking at 0.5 mm. UCS also increases then decreases as straw content grows and the optimal content is 0.1% within 0.3-9 mm lengths. In terms of failure patterns, the straw-free sample exhibits splitting failure, while those with straw show increased cracks, manifesting as varying degrees of shear failure. Additionally, the brittleness index demonstrates non-linear behavior with changes in straw content. Below 0.1% content, the brittleness index increases for most straw lengths (except 0.5 mm). Beyond 0.1% content, the brittleness index exceeds 0.62 for most straw lengths, indicating a significant increase in material brittleness. Microstructural and compositional analyses using XRD, SEM, and EDS reveal that the addition of wheat straw enhances the density and uniformity of the cement-stabilized sludge matrix by improving particle bonding and hydration product distribution. XRD confirms consistent mineral phases with modified peak intensities. SEM shows reduced porosity and improved structural compactness at optimal fiber lengths, and EDS highlights increased carbon and oxygen content due to chemical interactions between straw fibers and hydration products, optimizing mechanical properties. Based on these findings, this study recommends the use of 0.1% straw content with a straw length of 0.5 mm for solidifying high-water-content sludge, providing an effective approach for sludge resource utilization. • Optimal Straw Fiber Parameters: A straw length of 0.5 mm and content of 0.1% maximize the strength and ductility of solidified sludge, increasing UCS by 42.3% and strain by 21.05%. • Enhanced Ductility and Reduced Brittleness: Straw fibers improve plastic deformation, reduce brittleness by 6.45%, and shift failure modes from splitting to shear. • Microstructural Benefits: Optimal straw addition densifies the matrix, enhances fiber distribution, and strengthens chemical interactions, improving mechanical and structural performance.