Optimizing the concentration of Na2O in alkaline activators to improve mechanical properties and reduce costs and CO2 emissions in alkali-activated mixtures
“…When sodium silicate is used as the sole activator, the alkali equivalent is crucial to the strength development of the AAS mortar. A higher alkali equivalent facilitated the dissolution of precursor materials such as GGBS more effectively than a lower one 54 – 56 . However, exceeding a certain alkali equivalent range, the activator would rapidly dissolve GGBS in the early stage, accelerate the polymerization reaction of [SiO 4 ], [AlO 4 ] and Ca 2+ , causing excessive gelation.…”
Alkali-activated materials (AAMs) possess several advantages, such as high strengths and low carbon emissions. However, their application is hindered due to their significant shrinkage. This study explored the effect of borax-modified sodium silicate activator and metakaolin (MK) on the mechanical properties and drying shrinkage (DS) of alkali-activated slag (AAS) and AAS/MK (AASM) mortars. X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared spectroscopy were used to characterize the hydration products. The results showed that the DS reduction of the AAS mortar was related to decreased Na2O content, a reduction in the proportion of mesopores, and the formation of moisture-retaining borate compounds. The DS reduction of the AASM mortar was attributed to the ultra-fine differential effect induced by MK, reducing the connected pores. The modified activator combined with MK increased the chemically bound water content in the matrix. Additionally, the B–O bond and highly active MK improved compactness of the AASM mortar. The use of borax-modified activators and MK provides a new solution to address the significant shrinkage issue in AAMs. This sets the stage for AAMs to potentially replace OPC, contributing to low-carbon emissions and promoting environmental protection.
“…When sodium silicate is used as the sole activator, the alkali equivalent is crucial to the strength development of the AAS mortar. A higher alkali equivalent facilitated the dissolution of precursor materials such as GGBS more effectively than a lower one 54 – 56 . However, exceeding a certain alkali equivalent range, the activator would rapidly dissolve GGBS in the early stage, accelerate the polymerization reaction of [SiO 4 ], [AlO 4 ] and Ca 2+ , causing excessive gelation.…”
Alkali-activated materials (AAMs) possess several advantages, such as high strengths and low carbon emissions. However, their application is hindered due to their significant shrinkage. This study explored the effect of borax-modified sodium silicate activator and metakaolin (MK) on the mechanical properties and drying shrinkage (DS) of alkali-activated slag (AAS) and AAS/MK (AASM) mortars. X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared spectroscopy were used to characterize the hydration products. The results showed that the DS reduction of the AAS mortar was related to decreased Na2O content, a reduction in the proportion of mesopores, and the formation of moisture-retaining borate compounds. The DS reduction of the AASM mortar was attributed to the ultra-fine differential effect induced by MK, reducing the connected pores. The modified activator combined with MK increased the chemically bound water content in the matrix. Additionally, the B–O bond and highly active MK improved compactness of the AASM mortar. The use of borax-modified activators and MK provides a new solution to address the significant shrinkage issue in AAMs. This sets the stage for AAMs to potentially replace OPC, contributing to low-carbon emissions and promoting environmental protection.
“…Similarly, as shown in Figure 4, with the increase in Na2O%, the AAS concretes at room temperature had a lower porosity as a result of sufficient reaction [25][26][27][28][29][30], therefore, )…”
Section: Influence Of Na 2 O% and Msmentioning
confidence: 85%
“…Similarly, as shown in Figure 4, with the increase in Na 2 O%, the AAS concretes at room temperature had a lower porosity as a result of sufficient reaction [25][26][27][28][29][30], therefore, indicates that these samples should have lower chloride permeability. However, the AAS concretes with more Na 2 O% yielded higher charge passed, indicating higher chloride permeability, as shown in Figure 5.…”
The number of fires in buildings and on bridges has increased worldwide in recent years. As a structural material, the strength of alkali-activated slag (AAS) concrete after exposure to high temperatures has been given much attention. However, research of its durability is still lacking, which limits the application of this type of concrete on a larger scale. In this context, as one of the most important aspects of durability, the chloride permeability of AAS concretes after exposure to high temperatures was examined in this study. The influence of the alkali concentration (Na2O%) and the modulus (Ms) of the activator, as well as the influence of heating regimes, including the heating rate, duration of exposure to the target temperature, and cooling method, was also discussed. The results show that the chloride permeability of the AAS concretes increased with temperature elevation. Due to the interference of pore solution conductivity, the influence of the Na2O% and the Ms of the activator on the chloride permeability of the AAS concretes was not made clear by using the ASTM C 1202 charge passed method; however, after exposure to high temperatures, AAS with a lower Na2O% and lower Ms has lower porosity and may have lower chloride permeability, which needs further investigation. Faster heating for a longer duration at the target temperature and water cooling reduced the resistance of the AAS concretes to chloride permeability as a result of their increased porosity.
“…These micro-cracks could result from internal stresses induced by the high amount of heat that evolved, in the early age, during the dissolution of NaOH powder in water, as reported by Xiang et al 54 , and Shen et al 55 . Also, Lima et al 117 , and Collins et al 118 reported that the micro-cracks are due to autogenous shrinkage and the creation of voids by the dissolution of metasilicate, which consequently exert internal stresses and micro-cracks occurred. Figure 10 SEM for TP mix.…”
Sodium hydroxide (NaOH) as an alkaline activator presents a vital limitation in the mass production of alkali-activated binders due to its severe effect on users’ safety. In this study, safe and sustainable one-part alkali-activated slag mixes (OP-AAS) were prepared through an efficient microwave sintering for a mixture of active amorphous ground granulated blast furnace slag (GGBFS) and sodium hydroxide powder (NaOH). Different microwave-sintered powders were prepared using microwave energy of power 900 W for the mixture at different treatment periods (10, 20, and 30 min). Fresh and hardened properties of different OP-AAS mixes were studied. Moreover, the phase composition and microstructure were investigated using X-ray diffraction (XRD) analysis and scanning electron microscope (SEM). Cytotoxicity/viability testing was performed to evaluate the cell death induced by the developed materials to measure their safety for the user. According to compressive strength, cytotoxicity/viability analysis, environmental impact and cost calculation of developed OP-AAS, it is concluded that employing microwave sintering for a short duration is sufficient to produce safe binding materials with adequate mechanical properties suitable for commercial applications in the construction sector.
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