Lucas Benício Rodrigues Araújo

Alkali-activated binders (AABs), or geopolymers, have emerged as a sustainable alternative for producing cementitious materials from industrial byproducts. Although several aluminosilicate precursors have already been extensively studied, other potential sources remain unexplored. In this context, Baosteel Short Slag Flow (BSSF), a steel-industry byproduct, presents itself as a novel and practically unexplored precursor. However, since the precursor composition strongly influences the rheological behavior and structural build-up of AABs, systematic studies are essential to optimize the workability and performance of these materials for construction applications. Accordingly, the present study addresses this research gap by investigating how varying BSSF contents influence the structural build-up and rheological behavior of AABs. Binary AABs were formulated with varying proportions of BSSF slag and fly ash (FA) ranging from 0% to 100% BSSF in 25% increments. A Portland cement binder (PCB) with equivalent l/b ratio was also evaluated for comparison. Rheological characterization, isothermal calorimetry, compressive strength testing, and SEM analysis were used to assess how precursor composition governs workability, hardening kinetics, mechanical performance, and microstructure. Increasing BSSF slag content influenced viscosity, yield stress, and structural build-up, improving initial workability while accelerating hardening, although this effect may be partly influenced by the higher activator solution fraction at increasing BSSF levels. Compressive strength generally decreased with increasing BSSF content, except for B75, which achieved the highest strength (46.7 MPa). SEM analysis indicated that higher BSSF slag contents modified the binder structure. The findings underscored the potential of BSSF slag as an effective precursor for the development of AABs.

Vine shoots are potential bio-based aggregates, but they contain soluble compounds that may compromise binders’ hardening, while such compounds may find economically attractive use in other industries after removal. This study aimed to investigate the impact of different aqueous pretreatment approaches on the chemical, microstructural, thermal and mechanical performance of bio-based concretes. Vine shoot aggregates were subjected to three pretreatment protocols: hot water cycles (80 °C, 1 h per cycle), ambient temperature water cycles (20 °C, 24 h per cycle), and alkaline water cycles (20 °C, pH≈14, 24 h per cycle). The effects of these pretreatments were evaluated through VNS mass loss due to pretreatment, TGA/DTG, FTIR, and SEM/EDS analyses of the aggregates before and after pretreatment. The pretreatment water was also monitored for pH and electrical conductivity. The bio-base concretes were tested for compressive strength, bulk density, and thermal conductivity, while the concrete binder itself was further characterized via TGA/DTG, FTIR, SEM/EDS, and XRD. Results showed that untreated vine shoot aggregates inhibited binder hydration and affected both the quantity and type of carbonation products. These issues were mitigated after an optimal number of pretreatment cycles (ranging from 2 to 4), with alkaline pretreatment proving most effective, due to removal of soluble compounds and induced surface modifications. Compressive strength increased from 0.2 MPa for concretes with untreated aggregates to 1.5 MPa after two cycles of alkaline pretreatment. These findings highlight the potential of vine shoot pretreatment to enable use as bio-based aggregate, supporting the development of more sustainable construction materials.

Alkali-activated binders (AAB) are a suitable and sustainable alternative to ordinary Portland cement (OPC), with reductions in natural resource usage and environmental emissions in regions where the necessary industrial residues are available. Despite its potential, the lack of mix design methods still limits its applications. This paper proposes a systematic parametric validation for AAB mix design applied to pastes and concretes, valorizing steel slag as precursors. The composed binders are based on coal fly ash (FA) and Basic Oxygen Furnace (BOF) steel slag. These precursors were activated with sodium silicate (Na₂SiO₃) and sodium hydroxide (NaOH) alkaline solutions. A parametric investigation was performed on the mix design parameters, sweeping the (i) alkali content from 6% to 10%, (ii) silica modulus (SiO₂/Na₂O) from 0.75 to 1.75, and (iii) ash-to-slag ratios in the proportions of 75:25 and 50:50, using parametric intervals retrieved from the literature. These variations were analyzed using response surface methodology (RSM) to develop a mechanical model of the compressive strength of the hardened paste. Flowability, yield stress, and setting time were evaluated. Statistical analyses, ANOVA and the Duncan test, validated the model and identified interactions between variables. The concrete formulation design was based on aggregates packing analysis with different paste contents (from 32% up to 38.4%), aiming at self-compacting concrete (SCC) with slump flow class 1 (SF1). The influence of the curing condition was evaluated, varying with ambient and thermal conditions, at 25 °C and 65 °C, respectively, for the initial 24 h. The results showed that lower silica modulus (0.75) achieved the highest compressive strength at 80.1 MPa (28 d) for pastes compressive strength, densifying the composite matrix. The concrete application of the binder achieved SF1 fluidity, with 575 mm spread, 64.1 MPa of compressive strength, and 26.2 GPa of Young’s modulus in thermal cure conditions. These findings demonstrate the potential for developing sustainable high-performance materials based on parametric design of AAB formulations and mix design.

There is a growing demand for sustainable solutions in civil engineering concerning the carbon footprint of cementitious composites. Alkali-Activated Binders (AAB) are materials with great potential to replace ordinary Portland cement (OPC), with similar strength levels and lower environmental impact. Despite their improved environmental performance, their durability remains a gap in the literature, influenced by aspects of mechanical behavior, physical properties, and microstructure. This paper aims to assess the impact of steel slag aggregates and curing temperature of a proposed AAB based concrete formulation by characterizing fresh state, mechanical behavior, and microstructure. The proposed AAB is composed of fly ash (FA) and basic oxygen furnace (BOF) steel slag (SS) as precursors, sodium silicate and sodium hydroxide solution as activators, in total replacement of OPC, using baosteel slag short flow (BSSF) SS as aggregate in comparison with natural aggregate. The concrete formulation was designed to achieve a high-performance concrete (HPC) and a self-compacting concrete (SCC) behavior. Mechanical characterization encompassed hardened (compressive strength and Young’s modulus), fresh state (J-ring, slump flow, and T50), and durability tests (scanning electronic microscopy, water penetration under pressure, and chloride ion penetration). The compressive strength (64.1 ± 3.6 MPa) achieves the requirements of HPC, while the fresh state results fulfill the SCC requirements as well, with a spread diameter from 550 mm to 650 mm (SF-1 class). However, the flow time ranges from 3.5 s to 13.8 s. There was evidence of high chloride penetrability, affected by the lower electrical resistance inherent to the material. Otherwise, there was a low water penetration under pressure (3.5 cm), which indicates a well-consolidated microstructure with low connected porosity. Therefore, the durability assessment demonstrated a divergence in the results. These results indicate that the current durability tests of cementitious materials are not feasible for AAB, requiring adapted procedures for AAB composite characterization.

This study aimed to evaluate the thermal conductivity and sound absorption performance of panels based on coconut fiber and castor oil resin, with potential application in walls, roofs and floors, considering their alignment with current standards. Panels composed of a mass ratio of 85% fiber and 15% castor oil resin were produced by centrifugation mixing and pressing to achieve thickness of 5 cm. The produced panels were assessed at densities of 60 kg/m³, 80 kg/m³, and 100 kg/m³ and were evaluated by grammage, moisture absorption, longitudinal tensile strength, thermal conductivity, and sound absorption coefficient. The production process proved to be suitable. The thermal conductivity values ranged from 0.061 W/mK to 0.064 W/mK similar values to commercial products such as rock wool (0.046 W/mK) and glass wool (0.045 W/mK). Moreover, the sound absorption performance resembled that of insulating materials like melamine foam, demonstrating better performance at low emission frequencies.

Alkali-activated concretes (AAC) offer an alternative to Portland cement concretes, with high strength and low CO₂ emissions. However, common durability testing for conventional concrete may not suit AAC. This study aimed to verify the suitability as durability characterization tests for chloride migration, electrical resistivity, and water penetration under pressure, commonly applied to conventional concretes, in AAC based on fly ash and BOF steel slag. A comparison with a Portland cement-based concrete was also done. Tests of penetration of water under pressure; electrical resistivity; chlorides migration; and ion chromatography in the solutions before and after chlorides migration were performed. The findings revealed that while AAC exhibits low permeability, its high conductivity results in elevated electrical currents during chloride migration tests, potentially overestimating chloride migration. Ion chromatography indicated lower actual chloride migration, suggesting that electrical current is not only linked to chloride migration.