24 research outputs found
Exploring Mechanisms of Hydration and Carbonation of MgO and Mg(OH)2 in Reactive Magnesium Oxide-based Cements
Reactive magnesium oxide (MgO)-based cement (RMC) can play a key role in
carbon capture processes. However, knowledge on the driving forces that control
the degree of carbonation and hydration and rate of reactions in this system
remains limited. In this work, density functional theory-based simulations are
used to investigate the physical nature of the reactions taking place during
the fabrication of RMCs under ambient conditions. Parametric indicators such as
adsorption energies, charge transfer, electron localization function,
adsorption/dissociation energy barriers and the mechanisms of interaction of
H2O and CO2 molecules with MgO and brucite (Mg(OH)2) clusters are considered.
The following hydration and carbonation interactions relevant to RMCs are
evaluated i) carbonation of MgO, ii) hydration of MgO, carbonation of hydrated
MgO, iii) carbonation of Mg(OH)2, iv) hydration of Mg(OH)2 and v) hydration of
carbonated Mg(OH)2. A comparison of the energy barriers and reaction pathways
of these mechanisms shows that the carbonation of MgO is hindered by presence
of H2O molecules, while the carbonation of Mg(OH)2 is hindered by the formation
of initial carbonate and hydrate layers as well as presence of excessed H2O
molecules. To compare these finding to bulk mineral surfaces, the interactions
of the CO2 and H2O molecules with the MgO(001) and Mg(OH)2 (001) surfaces are
studied. Therefore, this work presents deep insights into the physical nature
of the reactions and the mechanisms involved in hydrated magnesium carbonates
production that can be beneficial for its development
Alternative alkali-activator from steel-making waste for one-part alkali-activated slag
Abstract
In this study, the use of desulphurization dust (DeS-dust) generated as a waste material during steel-making process, as an alternative activator to commercial sodium hydroxide in ground granulated blast furnace slag (GGBFS) alkali activation is proposed. The main objective was to decrease the environmental footprint of alkali-activated materials through the reuse of industrial residues. Microsilica was added to increase the amount of soluble silica and enhance the properties of the investigated binders. The results indicate that binders from alternative activator performed better, achieving a 28 days maximum strength of 33 MPa compared to 25 MPa for the sodium hydroxide activated slag. Microsilica addition to the optimum mixes reduces the rate of efflorescence and increases the setting time. Also, microstructural studies using scanning electron microscopy (SEM), x-ray diffraction (XRD) and thermogravimetric analysis (TGA) show both samples having comparable gel formation and structure. Life cycle impact assessment shows significant savings that can be made using alternative activators over sodium hydroxide activated slag. Through the use of this waste material as alternative activator in alkali-activated binders, an environmentally friendly, and cleaner production of alkali-activated binders can be achieved having comparable or superior performance as the reference binder activated with commercial sodium hydroxide
Fiber reinforced alkali-activated stone wool composites fabricated by hot-pressing technique
Abstract
Cementitious composite that has short molding time and high mechanical performance is favorable in pre-cast concrete industry. In this context, this study reports the use of hot-pressing technique to fabricate PVA fiber reinforced composites using alkali-activated stone wool (a waste from building insulation). Eight different mixtures were developed by varying the pressing time and temperature in comparison to the conventional oven-cured alkali-activated material. The mechanical performance of all compositions was evaluated under bending and compressive loadings. Life cycle assessment was used to investigate the greenhouse gas emission and embodied energy of the developed composites. The results reveal that PVA fibers greatly enhanced the mechanical performance of all reinforced mixtures with deflection hardening behavior and improvement in compressive strength. The hot-pressing technique lowered CO₂ emission and saved energy. Finally, a multi-criteria ranking method suggests hot-pressed PVA fiber reinforced cementitious composite, manufactured at 120 °C for 2 h, is the best composition attaining balance among energy spent, mechanical properties, and CO₂ footprint
Production of lightweight alkali activated mortars using mineral wools
Abstract
This experimental study aimed to develop a fiber-reinforced lightweight mineral wool-based alkali activated mortar. The lightweight mineral wool-based alkali activated mortars were produced using premade foam and reinforced by polypropylene (PP) fibers. They were assessed in terms of fresh and hardened-state properties. Fresh-state properties were investigated by mini-slump tests. Hardened-state characteristics were assessed by ultrasonic pulse velocity, dry density, compressive and flexural strengths, drying shrinkage, efflorescence, water absorption, and permeable porosity. For the first time, the resistance of the synthesized lightweight mineral wool-based alkali activated mortars against harsh conditions (carbonation, freeze and thaw, and high temperature) were evaluated. The porous structures of the developed lightweight alkali activated mortars were also analyzed using an X-ray micro-computed tomography (CT) technique. Lightweight mix compositions with densities in a range of 770–1510 kg/m3, compressive strengths of 1–9 MPa, and flexural strengths of 2.6–8 MPa were developed. Increases in both density and strength after carbonation were also recorded, while a decrease of strength was noticed after exposure to freeze/thaw and high temperatures of up to 500 °C
Thermal stability of one-part metakaolin geopolymer composites containing high volume of spodumene tailings and glass wool
Abstract
This paper deals with the synthesis and thermal stability of one-part metakaolin geopolymer composites containing high volume of spodumene tailings (Quartz Feldspar Sand; QFS) and glass wool (GW). One of the objectives of the study was to prepare materials encompassing a maximum amount of waste streams with some potential thermal stability. Several compositions were prepared with sodium metasilicate anhydrous (Na2SiO3) wt.% of 0.5, 2.5, 5, 10 and 12,5. The one-part metakaolin geopolymer composites were cured at 60 °C for 24 h and the mechanical properties were assessed at 7 days and after post-heat treatment at 500, 750, 1000, 1100 or 1200 °C. X-ray diffraction, dilatometry, scanning electron microscopy and thermogravimetry analyses were used to study the stability of the prepared geopolymer composites until 1100–1200 °C. The results showed that more than 20 MPa compressive strength could be achieved with metakaolin geopolymer composites containing only 20 wt% of metakaolin. Metakaolin-GW geopolymer composites were stable up to 500 °C. Meanwhile, their counterparts containing QFS were stable up to 1100–1200 °C; samples prepared with higher dosage of sodium (Na2SiO3 > 5 wt%) retained more than 50% of their initial strength after thermal treatment at 1100 °C. Interestingly, for dosages of Na2SiO3 ≤ 5 wt%, more than 300% increase of strength was observed after thermal treatment at 1100–1200 °C. The use of QFS limited the thermal shrinkage at mild temperatures (<1000 °C), but favoured densification and strength development at 1100–1200 °C
Using carbonated BOF slag aggregates in alkali-activated concretes
Abstract
This experimental study aimed to develop alkali-activated concretes containing carbonated basic oxygen furnace (BOF) slag aggregates. In the first stage, the impacts of replacing normal aggregates with carbonated BOF slag aggregates in different alkali-activated concretes were determined by assessing mechanical properties (compressive and flexural strengths), morphology, thermogravimetric analyses (TGA), differential thermogravimetry (DTG) and the crystalline phases using X-ray diffraction analysis. Second, the developed plain alkali-activated concrete was reinforced by different fibre types and dosages to limit the negative impacts of the drying shrinkage and to improve strength. Therefore, the effects of using different fibre contents (1% and 1.5% in Vol.) and types (Polyvinyl alcohol [PVA], Polypropylene [PP], basalt, cellulose and indented short-length steel) on hardened state properties were evaluated. These evaluations were expressed in terms of the compressive and flexural strengths, ultrasonic pulse velocity, mass changes, drying shrinkage and efflorescence. Then, the impacts of aggressive conditions on the hardened properties of fibre-reinforced alkali-activated concretes were evaluated under carbonation, high temperature and freeze/thaw tests. The results showed that using carbonated BOF slag aggregates led to obtain higher strength than using normal aggregates in alkali activated concretes. Moreover, the maximum enhancement due to reinforcing the mixtures was recorded in alkali-activated concretes with steel fibre
Hydration of blended ladle slag and calcium aluminate cement
Abstract
Partial replacement and co-hydration of calcium aluminate cement (CAC) with ladle slag was investigated in this study as a pathway in valorizing the slag and reducing the economic cost of CAC. The impact of this replacement on the physical and microstructural properties were analysed using different techniques such as mechanical strength test, freeze-thaw, sulfate attack, XRD, SEM etc. Thermodynamic modelling was used to predict the phase assemblages of the blended cement using the hydration kinetics of the system. Experimental results showed the reference CAC mortar and the substituted mortar exhibited comparable strength gain at 7 and 28 days, and durability as measured by sulfate attack, abrasion, and freeze-thaw resistance. A low water-to-binder ratio (0.35) lessened the effect of conversion on the hydrates, as XRD showed metastable CAH₁₀ and C₂AH7.5 as the hydrates at 7, 28 and 60 days. This however can convert later to the thermodynamically stable phase C₃AH₆. Thermodynamic modelling suggests these two metastable phases as major binding phases, while Si-hydrogarnet and FeOOH appeared a minor trace in the binder.
*Cement chemistry notation used, where C = CaO, A = Al₂O₃ and H = H₂