This study develops an ordinary Portland cement (OPC)-free, 3D-printable loess-based material through an integrated formulation–process–performance approach. Thirteen modified mixtures (M1–M13) were designed using a multi-component calcium-bearing binder, graded aggregates, biopolymer modifiers, and treated straw fibers. Fresh-state performance was assessed through flow-table measurements, comparative rotational viscometry, and direct buildability tests, while mechanical properties were evaluated through compressive testing. Complementary SEM–EDS, XRD, XRF, TG/DTG, and mercury intrusion porosimetry (MIP) analyses were conducted to characterize the traditional control (TRA) and optimized mixture M13. Among the thirteen modified formulations, M13 achieved the highest mean 14-day compressive strength of 1.064 ± 0.064 MPa. It also exhibited the highest buildability among the four directly tested formulations, sustaining 7.67 ± 0.58 stable layers. MIP revealed lower mercury-accessible porosity in M13 (32.30%) than in TRA (38.56%), indicating a refined accessible pore-throat structure. In the preliminary 28-day printing validation, the recorded compressive strength of printed M13 was approximately 3.9% lower than that of its cast counterpart, while the printed element exhibited a 2.5% height reduction relative to its nominal initial dimension. These findings demonstrate that coordinated formulation design can improve the balance between mechanical performance and layer-stacking capacity. The study provides a process-oriented experimental framework for developing OPC-free printable earthen materials, while further durability and interface-performance assessments remain necessary.
Lead-contaminated soils threaten human health, ecosystems and impair the engineering performance of soils, yet traditional cement-based treatments incur costs and substantial CO₂ emissions. Paper sludge ash (PSA) with accelerated carbonation is a potential low-carbon option, but the effects of carbonation timing on strength, reduction in Pb leachability, and CO₂ fixation remain unclear. This study evaluated PSA, with ordinary Portland cement (OPC) as a reference binder, for treating Pb-contaminated surplus clay under conventional curing and early and late carbonation curing sequences. Strength development, Toxicity Characteristic Leaching Procedure (TCLP) leachable Pb concentration, pH evolution, and acid-decomposable carbonate-derived CO₂ content were evaluated, while the underlying mechanisms were elucidated through XRD, SEM, FTIR, and TGA-DTA. Both binders improved strength and reduced Pb leachability. Carbonation increased CO₂ fixation, formed calcite and cerussite, lowered pH, and partially altered hydration products, generally reducing strength and Pb leachability relative to conventional curing; in contaminated soils, early carbonation outperformed late carbonation. TCLP-leachable Pb decreased by more than 95% relative to the untreated soil at APSA = 20% and AOPC = 10%, satisfying the USEPA limit of 5 mg/L, and the treated soils met the Japanese embankment criterion (qc ≥ 800 kN/m²) at sufficient binder dosage. Mineralogical analyses associate reduction in Pb leachability with cerussite precipitation and a dense cementitious matrix, with possible contributions from adsorption, co-precipitation, and physical encapsulation. These findings show that PSA can improve strength and reduce Pb leachability under conventional curing and early carbonation, with early carbonation additionally reducing pH and fixing CO₂, supporting resource recovery and potentially lower-carbon ex-situ treatment for reuse of Pb-contaminated surplus soil in embankment construction.
Historical masonry buildings require conservation materials compatible with the original materials to ensure durability and maintain authenticity. Nevertheless, there is limited research focused on a direct comparison of historic and new bricks, especially in the context of South Asian heritage. This research provides a comparison between burnt clay bricks of the 16th century from Lahore Fort and new bricks available in the local market. Standardized tests were performed on both historic and new bricks related to physical, mechanical, durability, chemical, and mineralogical properties. The findings not only highlight the variations in material properties and performance resulting from the traditional and modern manufacturing processes, but also indicate similarities in the basic composition. For example, the new bricks showed a higher average compressive strength (≈ 42 MPa) as compared with the historic bricks (≈ 37 MPa), as well as a higher average modulus of rupture (≈1.92 MPa versus ≈1.26 MPa), reflecting their more consistent mechanical performance. These results show that even though modern bricks can have similarities in mineralogical composition, their compatibility with historic bricks cannot be ensured without comprehensive physical, mechanical, and durability characterization. The paper highlights the significance of paying attention to the inherent heterogeneity of historic masonry when choosing repair materials. The results provide practical guidance for selecting suitable materials in the conservation of the Mughal-period buildings of South Asia and are also applicable to the heritage buildings of other areas with similar construction practices.
Maturity functions estimate concrete strength development from a recorded temperature history, with accuracy depending critically on the apparent activation energy. A recent extension of the fib Model Code maturity function allows this activation energy to vary with curing temperature and age, improving accuracy for concretes with supplementary cementitious materials, but its regression is calibrated on the mixes it predicts and its piecewise form is not portable to a different reference temperature. This paper presents a hybrid physics-based–machine-learning surrogate that retains the fib strength equation as the physics backbone and replaces the empirical activation-energy regression with an XGBoost model predicting the temperature-induced time-scaling factor log(te/t) directly. This target is physically equivalent to a variable activation energy; its advantage is not accuracy on the training domain, which a direct activation-energy surrogate matches, but that it is defined at every observation including the reference temperature and, in the test reported here, transfers from a 20 °C to a 30 °C reference temperature.The model was trained on 196 strength measurements from seven self-compacting concrete mixes covering Portland cement, silica fume, fly ash, ground granulated blast-furnace slag and rice husk ash, cured at 10–50 °C. Primary accuracy estimates use the 147 off-reference-temperature observations, excluding the 20 °C curves used for per-mix calibration. Under leave-one-mixture-out validation, the nested and fixed configurations both gave R2 = 0.943 and RMSE = 4.23 MPa; for the fixed configuration, the 95% mix-level bootstrap intervals were 0.899–0.969 for R2 and 3.28–5.21 MPa for RMSE. The modified fib gave R2 = 0.895 and RMSE = 5.74 MPa under the same validation scheme, compared with R2 = 0.922 and RMSE = 4.97 MPa for its published coefficients evaluated in-sample. Applied without retraining to independent literature data with a 30 °C reference temperature, spanning vibrated and self-compacting concretes (including one lightweight mix) and fly ash up to 45%, the model achieves R2 = 0.934 and RMSE = 5.43 MPa on the 119 off-reference observations, against 0.917 / 6.11 MPa for the original fib and 0.888 / 7.07 MPa for the modified fib; a surrogate trained on the activation-energy factor itself falls to 0.850 / 8.18 MPa on the same observations. SHAP attributions are consistent with the expected temperature and age dependence of hydration kinetics, and conformal intervals whose calibration residuals come from models fitted without the assessed mix give 94.6% pooled empirical coverage (nominal 95%; 81–100% by mix). Every mix, seen or unseen, still requires its reference-temperature strength–age curve, from which the two fib parameters are fitted; the model adds generalisation to unseen binders, demonstrated transfer from a 20 °C to a 30 °C reference temperature, and empirically calibrated uncertainty.
