Evaluating the role of steel mill scale in self-compacting concrete as partial fine aggregate replacement: Experimental and modelling insights

This study evaluated the potential of steel mill scale (SMS), a steel industry byproduct primarily composed of iron oxides, as a partial replacement for fine aggregates in self-compacting concrete (SCC). SMS was incorporated at replacement levels ranging from 10 % to 40 % by weight of fine aggregate. Experimental investigations have assessed rheological, mechanical, and durability properties in accordance with the EFNARC and IS codes. Rheological results confirmed that all the mixtures met the workability criteria, with slump flows ranging from 685 mm (SMS20) to 694 mm (SMS40). SMS25 presented the shortest T50 time of 5.2 s, indicating improved flowability. The mechanical performance peaked at 15 % SMS (SMS15), yielding the highest 28-day compressive strength of 45.45 MPa, split tensile strength of 3.80 MPa, and flexural strength of 4.72 MPa. Durability assessments revealed that SMS40 had the lowest mass loss (0.9 %) under acid attack and the deepest carbonation depth of 8.0 mm at 28 days. Elevated temperature tests revealed that the SMS mixtures retained up to 85 % of the compressive strength at 500°C due to hydrothermal interactions. Microstructural analysis (SEM) revealed denser matrix development at optimal replacement levels. Machine learning models such as decision trees, random forests, and multilayer perceptrons have been applied to predict compressive strength. The random forest regressor performed best, achieving an R² of 0.98, an RMSE of 1964.47, and an MAE of 1125.55. Most existing studies focus on mechanical properties or use SMS in conventional concrete. Integrated assessments covering durability and microstructure in SCC, especially with predictive modeling, are limited. This study bridges that gap through experimental testing and advanced machine learning integration. • Steel mill scale (SMS) is an eco-friendly, effective partial replacement for fine aggregate in self-compacting concrete (SCC). • SCC with 15 % SMS achieved the highest 28-day compressive strength (45.45 MPa) and superior tensile/flexural strength. • All SCC mixes met EFNARC standards, with 25 % SMS showing the best flowability (T50 = 5.2 s). • SCC with 40 % SMS showed the best acid resistance (0.9 % mass loss) and retained 85 % strength at 500°C. • Random Forest model predicted compressive strength with R² = 0.98, and SHAP identified key predictive features.