Taís Oliveira Gonçalves Freitas

This study investigated the hydration behaviour, phase assemblage, microstructure and mechanical performance of magnesium oxysulfate (MOS) cement produced with highly reactive MgO obtained by calcining Mg(OH) ₂ (active MgO ≈ 89%). Formulations with MgO:MgSO ₄ molar ratios of 5, 6 and 8 were evaluated. MOS pastes were prepared by increasing the MgO content while maintaining a fixed amount and concentration of the MgSO ₄ solution. The pastes were characterised by isothermal calorimetry, pH measurements of diluted aqueous suspensions, X-ray diffraction with Rietveld refinement, thermogravimetric analysis and scanning electron microscopy coupled with energy-dispersive spectroscopy. Compressive strength was determined at 7 and 28 days. The main exothermic peak occurred at approximately 11–12 h, and most measurable phase transformations took place within the first 24 h. At 7 days, MOS5 contained approximately 40% of the 5 · 1 · 7 phase and 9.5% brucite, whereas MOS8 contained approximately 27% of the 5 · 1 · 7 phase and 28% brucite. A substantial amorphous fraction of approximately 40–46% persisted after the first day. At 28 days, compressive strength decreased from 59.6 MPa for MOS5 to 49.1 MPa for MOS8. The pH of the diluted aqueous suspensions remained between 9.51 and 10.19 over 91 days. The investigated Mg(OH) ₂ -derived MgO system exhibited rapid hydration and early phase stabilisation. Under the adopted formulation conditions, increasing MgO content at a fixed MgSO ₄ solution composition was associated with greater brucite formation and lower 5 · 1 · 7 content. The strength trends were consistent with this phase balance, although simultaneous changes in water-to-binder ratio and fresh-state behaviour prevent the isolated effect of the MgO:MgSO ₄ ratio from being established.

Textile-reinforced concrete (TRC) is an alternative class of mechanical reinforcement for cement composites. The biaxial braided reinforcement structure in composite materials with diverse cross-sectional shapes offers high adaptability, torsional stability, and resistance to damage. In general, 3D textile reinforcements improve the mechanical properties of composites compared to 2D reinforcements. This study aimed to verify reinforcement behavior by comparing multidimensional braided textiles, 2D (one- and two-layer) reinforcements, and 3D reinforcement in composite cementitious boards. Experimental tests were performed to evaluate the effect of textile structures on cementitious composites using four-point bending tests, porosity measurements, and crack patterns. All textiles showed sufficient space between yarns, allowing the matrix (a commercial formulation) to infiltrate and influence the composite mechanical properties. All composites presented ductility behavior. The two layers of 2D textile composites displayed thicker cracks, influenced by shear forces. Three-dimensional textiles exhibited superior values in four-point bending tests for modulus of rupture (7.4 ± 0.5 MPa) and specific energy (5.7 ± 0.3 kJ/m²). No delamination or debonding failure was observed in the boards after the bending tests. The 3D textile structure offers a larger contact area with the cementitious matrix and creates a continuous network, enabling more uniform force distribution in all directions.

The use of ground glass as a partial substitute for cement has been studied for its good performance, especially when its particles are smaller than 300 µm. Thus, this study focused on evaluating the behavior of mortar mechanical properties through the filler effect and the pozzolanic effect of glass powder. Cement substitution proportions with glass powder (0%, 10%, 20%, 30%, and 50%) having a granulometry below 75 µm were tested. To distinguish between the filler effect and the pozzolanic effect, limestone filler, having a similar particle size distribution to glass, was used. The tests evaluated axial compressive strength, capillary water absorption, tensile strength by diametral compression, and static modulus of elasticity. The results showed that 20% cement substitution with glass powder improved mechanical properties, evidencing both the filler and pozzolanic effects. The chemical activity of the glass powder was slower, but it resulted in a 24.06% increase in compressive strength at 91 days, highlighting the contribution of the pozzolanic effect.

This study investigated the effects of substituting magnesium oxide (MgO) with dolomitic limestone (DL) on the mechanical and physical properties of magnesium oxysulfate (MOS) cement. Additionally, the hydration formation phases and the influence of the molar ratio on the MOS cement’s performance were examined. The corresponding action mechanisms were identified and explored by compressive strength tests, scanning electron microscopy (SEM), X-ray diffraction (XRD), isothermal calorimetry, and a thermogravimetric analysis (TGA). The results showed that replacing MgO with DL decreased the reaction speed and heat release rate generated in the hydration process of the MOS cement. This substitution also reduced the quantity of non-hydrated MgO particles and delayed the formation of Mg(OH)₂. The diminished formation of Mg(OH)₂ contributed to an increase in the apparent porosity of pastes containing DL, thus alleviating internal stresses induced by Mg(OH)₂ formation and enhancing their mechanical strength after 28 days of curing. Conversely, the increased porosity improved the CO₂ diffusion within the structure, promoting the formation of magnesium carbonates (MgCO₃). Through the characterization of the cement matrix (XRD and TGA), it was possible to identify phases, such as the brucite, periclase, and 318 phases. The obtained results revealed the potential of incorporating mineral fillers like limestone as a promising approach to producing MOS cement with a reduced environmental impact and better properties at higher curing ages.

This article analyzes the effect of the rehydration method on the physical–mechanical properties of accelerated carbonation-cured magnesium-based fiber cement boards. The rehydration process of the boards was analyzed in conjunction with the analysis of the pre-curing time before accelerated carbonation (24, 48, and 72 h before carbonation), resulting in eight different curing parameters used in this investigation. The physical–mechanical performance and microstructural characteristics of magnesium oxysulfate boards before and after carbonation were investigated by water absorption, apparent porosity, and bulk density using the four-point bending test, X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy. According to the results, the accelerated carbonation process improved the mechanical properties of the boards. The samples that carbonated after 48 h showed a higher modulus of rupture. The rehydration process of the composites before carbonation led to enhancements in the pre-cured boards for 48 and 72 h, demonstrating that carbonation occurred more effectively after water rehydration. The mechanical improvements were associated with the formation of hydration products, which preferentially formed in the pores and voids of the fiber cement matrix. These carbonation products altered the physical properties of the composites, increasing the density of the boards and reducing the void volume. The decomposition of the formed carbonates was confirmed by thermogravimetric analysis, which indicated that the rehydration process favored the carbonation of the composites.

The alkali-silica reaction (ASR) is one of the problems that can compromise concrete durability, as the chemical reactions between the reactive aggregate and concrete pore solution, specifically sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺) and hydroxyl (OH^-) in the presence of moisture, trigger expansion and cracking of concrete elements. The use of pozzolanic materials, such as finely ground glass powder (particles smaller than 75μm), to partially replace cement has been widely studied and has demonstrated beneficial results. However, because the behavior of pozzolans towards ASR is not completely understood yet, this work evaluates the effect of incorporating ground glass powder into cementitious composites against ASR. To this end, 8% silica fume was used as a control admixture, and four levels of glass powder (10, 20, 30, and 50%) were incorporated into the cementitious mixture to partially replace cement. Expansion tests were carried out in accordance with NBR15577-4 (ABNT, 2018) on mortar bars using the accelerated test method to assess the mitigation of expansions. The results show that the expansions caused by ASR decreased with the increasing content of glass powder in the cement mixtures. Furthermore, only the 50% level reduced the expansions to a safe value according to the Brazilian standard.

During carbonation process, the presence of water is an important parameter to accelerate this phenomenon. The carbon dioxide (CO2) has been used as curing process production of fiber cement boards keeping the dimensional stability of cellulose fibers in cementitious matrix and promoting the CO2 capture of environment. In addition, magnesium oxysulfate (MOS) cement has been used as another alternative binder to reduce the amount of CO2 in the nature. The aim of this scientific paper is analysis the effect of rehydration method on the physical-mechanical properties of CO2-cured magnesium-based fiber cement boards. These boards were produced by Hatschek process simulation. The physical-mechanical performance and microstructural characteristics of MOS boards before and after carbonation were investigated by: water absorption (WA), apparent porosity (AP), and bulk density (BD); four-point bending test; X-ray diffraction (XRD); thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). According to the results, it was possible observe which the accelerated carbonation process resulted in improvements in the mechanical properties of the boards. The boards carbonated after 48 hours showed higher Modulus of Rupture values. The rehydration process of the composites before carbonation led to enhancements in the boards pre-cured for 48 and 72 hours, demonstrating that carbonation occurred more effectively after the supply of water through rehydration. The improvements in mechanical properties were associated with the formation of hydration products, which preferentially formed in the pores and voids of the fiber-cement. The presence of these carbonation products altered the physical properties of the materials, increasing the density of the boards and reducing the void volume. The decomposition of the formed carbonates was confirmed by thermogravimetric analysis, which indicated that the rehydration process favored the carbonation of the materials

Novel developments on concrete technology such as high and ultra-high-performance concrete (HPC and UHPC) are notorious by its high consumption of Portland cement. Supplementary cementitious materials have been used as partial replacement of Portland cement aiming to minimizing clinker content, optimizing the use of binders, reducing CO2 emissions, and increasing durability. Waste glass powder could be an alternative due to its silica-rich nature and wide availability. This work aims to assess the influence Portland cement substitution by finely ground waste glass powder in contents of 10%, 20%, 30% and 50% on physical and mechanical behavior of ultra-high-performance cementitious composites (UHPCC). Results indicates the use of glass powder as substitution up to 50% of Portland cement does not significantly affect the analyzed properties at 28 days.