Rheological, microstructural, mechanical and durability properties of bssf slag alkali-activated binders and concretes for interlocking blocks production

Abstract

The civil construction and infrastructure sectors significantly impact the environment, with Portland cement being a major contributor. Alkali-activated binders (AABs) have emerged as sustainable alternatives, synthesized by activating aluminosilicate precursors, often industrial byproducts, with alkaline solutions. While fly ash, blast furnace slag, and metakaolin are wellstudied, steel slag remains underexplored. Baosteel's Slag Short Flow (BSSF) slag, produced through advanced cooling of Basic Oxygen Furnace (BOF) slag, shows promise as an AAB precursor and aggregate due to its chemical composition and availability. This research investigated the rheological, microstructural, mechanical, and durability properties of AABs and concretes incorporating BSSF as a precursor and aggregate for interlocking block production. A comprehensive experimental campaign was conducted in four phases: precursor characterization, formulation optimization, fundamental rheological and microstructural analysis, and evaluation of BSSF as an aggregate in alkali-activated concretes (AACs). In the first phase, industry coproducts were analyzed to determine their suitability as AAB precursors using XRF, XRD, and laser granulometry. BSSF and fly ash (FA) were selected for further study. The second phase optimized AAB formulations by varying alkali content (N/B), silica modulus (S/N), precursor ratios, and curing conditions. Compressive strength tests and efflorescence analyses were performed. Increasing BSSF content generally reduced strength, except at 75% BSSF, which exhibited optimal values. Efflorescence had no significant impact on strength. The third phase examined the effects of FA replacement with BSSF on rheology, structural build-up, and microstructure. Rheological tests included mini slump, flow sweep, small amplitude oscillatory shear (SAOS), and setting time measurements, followed by microstructural analyses using XRF, XRD, FTIR, TGA, SEM, and N2 sorption. Increasing BSSF content improved workability, reduced viscosity, and accelerated hardening due to its higher calcium content. SAOS tests indicated faster gel formation and increased stiffness in BSSF-rich AABs. Microstructural analysis revealed compositional changes with higher BSSF content, particularly the formation of C-A-S-H and C-S-H gels. In the fourth phase, BSSF was evaluated as an aggregate in AACs. Rheometry, slump tests, compressive strength, modulus of elasticity (via multiple methods), water penetration under pressure, chloride migration, and electrical resistivity tests were performed and compared with Portland cement concretes. Interlocking blocks were produced and tested per NBR 9781 (2013), undergoing visual inspection, compressive strength, absorption, and abrasion resistance assessments. All AACs exhibited self-compacting behavior, with higher viscosity and greater mixing energy demands. The inclusion of BSSF aggregates enhanced compressive strength and modulus of elasticity, meeting standard requirements for interlocking blocks. This study demonstrates the feasibility of using BSSF as both a precursor and aggregate in AABs, contributing to more sustainable construction materials while maintaining performance standards.