Martin Cyr

This study investigates alkali-silica reaction (ASR) in alkali-activated slag (AAS) concretes with reactive and non-reactive aggregates, and the mitigating effect of aluminum additions. Non-reactive aggregates caused minimal expansion, whereas reactive siliceous limestone and flint exceeded the 0.02% limit due to ASR gel formation confirmed by SEM-EDS. Flint induced greater expansion than siliceous limestone in AAS, consistent with higher silica dissolution. Compared to ordinary Portland cement (CEM I), AAS showed lower expansions with siliceous limestone, likely due to higher pore-solution aluminum concentrations reducing silica dissolution. In contrast, AAS with flint aggregate displayed significantly greater expansions than CEM I, possibly due to the absence of a pessimum effect observed in CEM I.,Aluminum addition using a slow-dissolving Al(OH)₃ source effectively reduced expansions and enhanced early compressive strength by limiting silica dissolution without hindering slag hydration. Its ASR-mitigating mechanism was further assessed through batch dissolution and mixed-flow reactor tests. In particular, 18 mmol aluminum in NaOH solution at pH 13 reduced the silica dissolution rate by a factor of 2.4. These findings highlight aluminum’s dual role in controlling ASR expansions and improving mechanical properties in AAS concrete.

Since 2024, most electricity production on Réunion Island has been achieved through combustion of biomass, using either imported wood pellets or locally produced bagasse. Their combustion generates two types of ash, depending on the biomass source: Wood Biomass Fly Ash (WBFA) and SugarCane Bagasse Ash (SCBA). Their chemical compositions differ significantly, leading to different potential applications. The composition of SCBA is similar to that of Coal Fly Ash (CFA), with low variability between batches. Therefore, SCBA could be used as an alternative to CFA, as a Supplementary Cementitious Material (SCM) or in composite cements. SCBA also meets most of the requirements of the NF EN 450-1 standard. However, grinding of SCBA appears necessary to achieve mechanical performance required by the standard. In contrast, WBFA exhibits variable chemical composition, mainly due to differences in pellet origin prior to combustion. Nevertheless, WBFA contains significant levels of chloride ions and sulfate, which may act as activators for materials such as GGBS or metakaolin (MK). Although the high unburned carbon content of WBFA increases water demand, their incorporation into GGBS-based binders (SSC or CEM III) or metakaolin-based systems shows promising potential, particularly for improving early strength.

This study investigates the alkali-silica reaction (ASR) in alkali-activated slag (AAS) concretes with reactive and non-reactive aggregates, and the mitigating effect of aluminum additions. Non-reactive aggregates caused minimal expansion due to water uptake, while reactive aggregates (siliceous limestone and flint) induced expansions exceeding the 0.02% limit, linked to ASR gel formation confirmed by SEM-EDS. AAS concretes exhibited higher expansions with flint aggregates than with siliceous limestone, attributed to greater silica dissolution. Compared to ordinary Portland cement (CEM I), AAS showed lower expansions with siliceous limestone, likely due to a probable higher aluminum concentration in the pore solution that reduces silica dissolution. In contrast, AAS with flint aggregate displayed significantly greater expansions than CEM I, possibly due to the absence of a pessimum effect, as observed in CEM I.,Addition of Aluminum using a slow-dissolving Al(OH)₃ effectively reduced expansions and enhanced early compressive strength by limiting silica dissolution without hindering slag hydration. The mechanisms of ASR reduction capacity by Al were assessed by studying the effect of the aluminum on the silica dissolution by two different tests (batch dissolution and mixed-flow reactor tests). It was shown that the presence of Al at a concentration of 18 mmol in a NaOH solution at pH = 13 caused a reduction by a factor 2.4 of the silica dissolution rate These findings highlight Al’s dual role in controlling ASR expansions and improving mechanical properties in AAS concrete.

This study evaluates the structural behavior of two low-carbon self-compacting concretes in comparison to an Ordinary Portland Cement (OPC) concrete, used as a reference: a binary binder S70 (OPC substituted by 70% of ground granulated blast furnace slag, GGBS,−58% of carbon emission (em-CO₂) versus OPC) and a supersulfated cement SSC (− 78% of em-CO₂ versus OPC). For each binder, a mechanical characterization through compressive strength, tensile strength and Young’s modulus tests after 28 days and 90 days, was carried out. Steel–concrete bonding at 28 days was also tested through pull-out tests. Additionally, two beams per binder (3-meter long, section 280 × 150 mm²) were cast, varying the longitudinal reinforcement section: a low steel reinforcement ratio (1.6 cm², Conf1) leading to steels rupture, and a high steel reinforcement ratio (7.1 cm², Conf2) for higher beam resistance. The design of the beams was made according to Eurocode 2. Beams were tested using 4-point bending test with the recording of strains at different locations of the beam using Digital Image Correlation (DIC). For both configurations, S70 and SSC binders resulted in an equivalent load–deflection behavior compared to OPC beam, despite lower mechanical properties for SSC concrete, while reducing the associated carbon emissions. The DIC system helped to explain the observed fracture modes (reinforcements or “shear force” ruptures) and revealed slight differences in behavior between the beams, such as variations in the position of the neutral axis, as well as number of cracks and opening. These results suggest that structures with low-carbon binders can be dimensioned according to Eurocode 2, similar to OPC.

This paper presents an experimental approach to the study of the compressive strength, isothermal calorimetry and life cycle assessment (LCA) of alkali-activated pastes based on soda–lime–silica glass, established to investigate the effect of the nature and proportion of the activator. Four different activators are compared: Portland cement, sodium silicate, sodium carbonate (at four percentages by weight: 5, 10, 15 and 25 wt% relative to glass) and sodium hydroxide (3.5 wt%). Portland cement and sodium carbonate were added in dry form (powder), while sodium hydroxide (pellets) and silicate were used in solution. At room temperature, glass exhibited slow reaction kinetics, with mechanical performance increasing significantly beyond 28 days of curing. The nature of the activator had a direct impact on the mechanical performance of the activated glass. Cement-activated pastes and those containing 25 wt% of sodium carbonate developed strength at an early age (0–7 days). The other activators showed lower strength development before 28 days of reaction. While a higher activator content improved short-term performance, it also increased the environmental impact, primarily due to the activator. The LCA, conducted on 11 indicators, revealed that the environmental impact was largely driven by the type and amount of activator used. A performance impact indicator (PII) related to global warming was introduced to compare pastes with different performance values. At an early age (0–28 days), the PII was lower when the activator level was high but decreased over time as the strength improved. In terms of long-term performance (360 days), hydroxide and sodium carbonate (10 wt%) achieved compressive strengths of 91 and 74 MPa, respectively. These systems offered a balance between high performance and a reduced environmental impact, making them of interest for sustainable applications.

This study’s aim is to fully characterize ferronickel slag from New Caledonia, considered a multiphase mineral containing amorphous material. The methodology consisted of combining chemical, mineral, and morphological characterization techniques, such as ICP-AES, TGA, Q-XRD, microscopy, spectroscopy, etc. The ferronickel slag consisted of 44 wt. % forsterite, with the inclusion of iron as a substitution for magnesium (Mg_1.8Fe_0.2SiO₄), 1.7 wt. % chromite and 54 wt. % amorphous phase containing iron, magnesium, aluminum, and silica (Mg/Si = 0.4; Fe/Si = 0.2; Al/Si = 0.1). This material was slightly reactive in a cementitious medium, thus limiting its use as an SCM in the construction sector. The ferronickel slag was then subjected to an attrition-leaching carbonation process at 180 °C and a partial pressure of CO₂ of 20 bar. The obtained product, carbonated at 80% of its capacity, was also characterized. It was composed of carbonates (37% of magnesite and 4% of siderite), remaining forsterite (7 wt. %), chromite (1 wt. %), and 50% of an amorphous phase, mainly composed of silica and aluminum. The complete characterization of those products helped in understanding the chemistry of the carbonation process and finding valorization paths for the carbonated products in the construction sector. The carbonated product may be used either as an SCM in blended cement or as a precursor of magnesium–silicate binders.