Rheological Response, Strength Development and Conditional Age-Progressive Strength Updating of Cement Kiln Dust-Modified Cemented Tailings Backfill

Cement kiln dust (CKD) can partially replace Portland cement in cemented tailings backfill, but its application must satisfy both pipeline transportability and mechanical strength requirements. This study aims to evaluate the rheological and mechanical evolution of a low-carbon CKD-modified backfill to optimize its transport–strength trade-off. This study introduced a binder containing 10% CKD and 90% ordinary Portland cement at a binder-to-tailings ratio of 1:6. Thirteen single-factor mixtures were prepared to examine silica fume (0–7% of binder), sodium silicate (0–4%), polycarboxylate powder (0–0.40%), and solids mass concentration (72–74%). The Bingham yield stress and plastic viscosity were determined in the fresh state, and unconfined compressive strength (UCS) was determined after 3, 7, 28, and 56 d. Increasing polycarboxylate dosage strongly reduced yield stress, whereas raising solids concentration from 72% to 74% increased yield stress from 5.82 to 58.83 Pa and plastic viscosity from 0.0953 to 0.2889 Pa·s. Silica fume increased 56 d UCS from 1.39 to 2.28 MPa, while sodium silicate mainly improved early-age strength and produced non-monotonic later-age responses. At 74% solids, the 56 d UCS reached 2.54 MPa, accompanied by substantially higher rheological resistance. This study introduces a path-dependent, age-progressive framework to improve backfill strength prediction accuracy. A conditional age-progressive regression model was developed using min–max normalized mix variables and the measured UCS at the preceding age. In-sample R2 values were 0.9697–0.9805 for 7–56 d; leave-one-out validation with measured prior-age UCS gave R2 values of 0.7951–0.8198. The model should therefore be interpreted as a field-updating tool after early-age testing rather than as an independently validated design-only predictor. The results quantify the transport-strength trade-off within the investigated materials and dosage ranges.