Field-scale evaluation of recycled steel slag as supplementary cementitious material in concrete blends

Thesis Abstract

This study investigates the field-scale performance and environmental implications of incorporating recycled steel slag as a supplementary cementitious material (SCM) in concrete blends across regional construction projects, addressing concerns over resource sustainability, durability, and lifecycle costs. The problem stems from underutilization of steel slag in cementitious systems due to uncertainties about pozzolanic reactivity, long-term performance, and potential volumetric instability, which may limit circular economy benefits in the construction sector. The aim is to quantify the short- and long-term mechanical, durability, and environmental outcomes of steel slag–cement blends under real-world exposure, and to establish scalable guidelines for dosage, mixing, and quality control. Specific objectives include (1) determine optimal replacement levels of ground steel slag (GSS) as an SCM in Portland cement concrete (PCC) blends for standard structural applications and for high-performance concrete (HPC) targets; (2) evaluate early-age and 28-, 56-, and 180-day mechanical properties (compressive strength, modulus of elasticity, and fracture energy) and microstructural evolution using X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP); (3) assess durability performance under chloride exposure, freeze–thaw cycles, and sulfate attack, incorporating accelerated field-dish testing and in-situ corrosion monitoring; (4) quantify environmental benefits and trade-offs through a cradle-to-gate life cycle assessment (LCA) and a consequential environmental assessment, including embodied carbon and potential leaching assessments; (5) develop a practical, field-validated performance model using multiple regression and ANOVA to relate GSS dosage, curing conditions, and environmental exposure to concrete properties; (6) formulate dosage guidelines and quality-control procedures for ready-mixed concrete suppliers and field contractors. The research adopts a mixed-methods design combining empirical field trials with laboratory characterization and modeling. The population comprises concrete batches produced at three urban demonstration sites, each representing different exposure classes and environmental conditions. A stratified sampling approach selects 12 laboratory-control mixes and 9 field mixes per site, yielding 36 control and 108 slag-containing specimens for laboratory testing (3 mix types × 3 dosages × 4 curing regimes) and 27 field-cured concrete slabs for performance monitoring. Data collection integrates standardized testing protocols (ASTM C39 for compressive strength, ASTM C241 for fresh concrete properties, and ASTM C672 for freeze–thaw) and advanced characterization (XRD with Rietveld refinement, SEM-EDS, MIP, and TGA) at 7, 28, 90, and 180 days. Durability assessments include rapid chloride permeability tests (ASTM C1202), accelerated sulfate resistance testing, and embedded corrosion probes for reinforcing steel. The study employs regression analysis and ANOVA to identify statistically significant effects of GSS content on strength, durability indices, and porosity metrics. Life cycle impact assessment follows ISO 14044 guidelines, utilizing an attributional LCA framework with SimaPro and process-based data for slag processing, concrete production, and end-of-life scenarios; a sensitivity analysis evaluates allocation and credit boundaries. Expected findings indicate that Ground Steel Slag, up to 40% replacement of Portland cement by mass, enhances early strength development and reduces heat of hydration, with diminishing gains beyond 30–40% depending on curing regime. Microstructural analysis is anticipated to reveal improved ettringite stabilization and refined pore structure, contributing to enhanced resistance to rapid chloride ingress and sulfate attack, particularly in coastal and winter-affected environments. The field data are expected to show reduced CO2 emissions per cubic meter of concrete and favorable life-cycle credits when slag sourcing is local and energy use for activation is minimized. A predictive model is anticipated to capture dosage–performance–durability relationships, enabling practical dosage recommendations and risk-informed decision-making for contractors. The study contributes to knowledge by providing empirically validated performance data for field-scale slag use as an SCM, bridging laboratory results with real-world viability, and delivering policy-relevant guidelines for industry adoption. The main conclusion is that recycled steel slag can serve as a viable SCM in field concretes under appropriate dosage and curing conditions, offering a measurable reduction in embodied carbon and acceptable durability performance for many exposure classes. Recommendations include adopting 20–40% cement replacement with quality-assured GSS processing, implementing standardized field QC protocols, integrating slag suppliers into prequalification schemes, and conducting site-specific LCAs to inform project-specic environmental claims. Further research is suggested to explore long-term performance beyond 180 days, regional variability in slag composition, and optimization of pre-treatment and grinding to maximize pozzolanic activity.