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Alkali-activated slag is a promising alternative to ordinary Portland cement. It is characterized by low carbon emission and superior mechanical properties compared to conventional OPC mixtures. However, the rapid setting time and short fresh properties life of AAS are major drawbacks and barriers halting its wide acceptance in the construction sector. The hydration for AAS is initiated as slag particles start to dissolve in the alkaline environment created by the used activator. Therefore, this study investigated the effects of powder activator (i.e., sodium meta-silicate) dissolving rate and consequently the increase in the alkalinity on the initiation of the hydration process. The effects of various adding sequences for ingredients (i.e., water and powder activator) on the workability and rheology behavior of AAS at constant water/binder ratio were explored. Results emphasized the role of powder activator concentration in accelerating the slag particle dissolution and losing workability. Adding the water to the slag showed slower hydration progress than adding the water to a mixture of slag and powder activator. The findings will narrow the gap related to selecting the convenient sequence of adding ingredients when conducting alkali-activated materials to assist in situ applications.

Lightweight alkali-activated concrete (LAAC) is a type of highly environmentally friendly concrete, which can provide the benefits of both alkali-activated material and lightweight concrete. The study aimed to investigate the influence of different water/solid (W/S) ratios on the properties of normal-weight/lightweight fly ash–slag alkali-activated concrete manufactured at ambient temperature. The relative performance of the alkali-activated concrete (AAC) mixes with limestone and sintered fly ash lightweight aggregates as the coarse aggregates was also compared to the conventional ordinary Portland cement (OPC) concrete mix in terms of their compressive stress–strain relationship, splitting tensile strength and fracture parameters. The morphologies and microstructure of the four types of interfacial transition zones (ITZs) were characterized by scanning electron microscopy (SEM). Results indicated that the AAC had a higher tensile strength, stress intensity factor, brittleness and lower elastic modulus than its cement counterpart. With the decrease in the W/S ratio, the density, compressive and tensile strength, ultrasonic pulse velocity, fracture energy, brittleness and elastic modulus of the AAC increase. However, the influence of the W/S ratio on the mechanical properties of the LAAC with lightweight porous aggregates was less than that of the normal-weight AAC. Predictive models of the splitting tensile strength, fracture energy and elastic modulus of the AAC were also suggested, which were similar to those of the OPC concrete. Furthermore, the microstructure investigation showed that no wall effect occurred in the ITZ of the AAC. The ITZ structure of the hardened AAC was also more compact and uniform than that of the OPC concrete.

Multi-technique characterisation of sodium carbonate-activated blast furnace slag binders was conducted in order to determine the influence of the carbonate groups on the structural and chemical evolution of these materials. At early age (<4 days) there is a preferential reaction of Ca 2+ with the CO 3 2− from the activator, forming calcium carbonates and gaylussite, while the aluminosilicate component of the slag reacts separately with the sodium from the activator to form zeolite NaA. These phases do not give the high degree of cohesion necessary for development of high early mechanical strength, and the reaction is relatively gradual due to the slow dissolution of the slag under the moderate pH conditions introduced by the Na 2 CO 3 as activator. Once the CO 3 2− is exhausted, the activation reaction proceeds in similar way to an NaOH-activated slag binder, forming the typical binder phases calcium aluminium silicate hydrate and hydrotalcite, along with Ca-heulandite as a further (Ca,Al)-rich product. This is consistent with the significant gain in compressive strength and reduced porosity observed after 3 days of curing. The high mechanical strength and reduced permeability developed in these materials beyond 4 days of curing elucidate that Na 2 CO 3 -activated slag can develop desirable properties for use as a building material, although the slow early strength development is likely to be an issue in some applications. These results suggest that the inclusion of additions which could control the preferential consumption of Ca 2+ by the CO 3 2− might accelerate the reaction kinetics of Na 2 CO 3 -activated slag at early times of curing, enhancing the use of these materials in engineering applications.

This study investigates the nucleation mechanism of slag alkali activation at different solid-to-liquid ratios, focusing on kinetics, including growth rates. Heat evolution during activation was monitored, and calorimetric data were analyzed using the Johnson–Mehl–Avrami–Kolmogorov model. Compressive strength and phase evolution (via wide-angle X-ray scattering) were correlated with heat evolution to enhance understanding of reaction mechanisms in alkali-activated material formation. This is essential for producing alkali-activated slag that meets standard requirements for construction applications. Results showed that the highest heat evolved (–360.60 J/g) did not correlate with the best strength performance (22.69 MPa at 1 day and 25.83 MPa at 3 days), since the lowest cumulative heat (–226.15 J/g) at an S/L ratio of 1.4 yielded the best strength. This was supported by the highest growth rate (0.1172 min–1) at this ratio. JMAK analysis indicated instantaneous nucleation with one-dimensional rod-like growth, driven by increased nucleation site availability. From the results obtained, it can be concluded that an increment in S/L ratio significantly increased nucleation and polymerization of alkali-activated slag, thereby hindering heat flow, as evidenced by the lowest total cumulative heat evolved. In addition, the highest growth rate observed corresponded linearly with the compressive strength, further confirming densification by polymeric gels formed during alkali activation.

S[sup.2−]-enriched alkali-activator (SEAA) was prepared by modifying the alkali activator through Na[sub.2]S. The effects of S[sup.2−]-enriched alkali-activated slag (SEAAS) on the solidification performance of Pb and Cd in MSWI fly ash were investigated using SEAAS as the solidification material for MSWI fly ash. Combined with microscopic analysis through scanning electron microscopy (SEM), X-ray fluorescence spectroscopy (XRF), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR), the effects of SEAAS on the micro-morphology and molecular composition of MSWI fly ash were studied. The solidification mechanism of Pb and Cd in S[sup.2−]-enriched alkali-activated MSWI fly ash was discussed in detail. The results showed that the solidification performance for Pb and Cd in MSWI fly ash induced by SEAAS was significantly enhanced first and then improved gradually with the increase in dosage of ground granulated blast-furnace slag (GGBS). Under a low GGBS dosage of 25%, SEAAS could eliminate the problem of severely exceeding permitted Pb and Cd in MSWI fly ash, which compensated for the deficiency of alkali-activated slag (AAS) in terms of solidifying Cd in MSWI fly ash. The highly alkaline environment provided by SEAA promoted the massive dissolution of S[sup.2−] in the solvent, which endowed the SEAAS with a stronger ability to capture Cd. Pb and Cd in MSWI fly ash were efficiently solidified by SEAAS under the synergistic effects of sulfide precipitation and chemical bonding of polymerization products.

This study investigates the mechanical and microstructural properties of loose sandy soil stabilized with alkali-activated Ground Granulated Blast Furnace Slag (GGBFS). To examine the effects of varying GGBFS contents, curing times, and confining pressures on mechanical behavior, undrained triaxial and unconfined compressive strength (UCS) tests were conducted. Microstructural analyses using FE-SEM, EDX, and FTIR were performed to elucidate the nature and development of cementation. The results of mechanical behavior demonstrate that even with limited GGBFS content (1–6%), the treated samples exhibited significant improvements in strength, stiffness, and energy absorption, underscoring the efficiency of alkali-activated GGBFS as a soil stabilizer. Moreover, mechanical parameters from triaxial tests revealed a nearly constant internal friction angle with increasing GGBFS content and curing duration, while cohesion showed remarkable enhancement. A strong linear correlation between UCS and cohesion was also identified, enabling cost-effective estimation of shear strength parameters. These findings highlight the potential of alkali-activated GGBFS for improving granular soils, offering practical implications for sustainable geotechnical applications, particularly in road construction.

Alkali-activated slag (AAS) has emerged as a potentially sustainable alternative to ordinary Portland cement (OPC) in various applications since OPC production contributed about 12% of global CO2 emissions in 2020. AAS offers great ecological advantages over OPC at some levels such as the utilization of industrial by-products and overcoming the issue of disposal, low energy consumption, and low greenhouse gas emission. Apart from these environmental benefits, the novel binder has shown enhanced resistance to high temperatures and chemical attacks. However, many studies have mentioned the risk of its considerably higher drying shrinkage and early-age cracking compared to OPC concrete. Despite the abundant research on the self-healing mechanism of OPC, limited work has been devoted to studying the self-healing behavior of AAS. Self-healing AAS is a revolutionary product that provides the solution for these drawbacks. This study is a critical review of the self-healing ability of AAS and its effect on the mechanical properties of AAS mortars. Several self-healing approaches, applications, and challenges of each mechanism are taken into account and compared regarding their impacts.

Alkali-activated slag (AAS) is a promising alternative to ordinary Portland cement (OPC) as sole binder for reinforced concrete structures. OPC is reportedly responsible for over 5% of the global CO2 emission. In addition, slag is an industrial by-product that must be land-filled if not re-used. Therefore, it has been studied by many investigators as environmentally friendly replacement of OPC. In addition to recycling, AAS offers favorable properties to concrete such as rapid development of compressive strength and high resistance to sulfate attack. Some of the potential shortcomings of AAS include high shrinkage, short setting time, and high rate of carbonation. Using ground granulated blast furnace slag (GGBS) as an alternative to OPC requires its activation with high alkalinity compounds such as sodium hydroxide (NaOH), sodium sulfate (Na2SO3), sodium carbonate (Na2CO3), or combination of these compounds such as NaOH and Na2SO3. The mechanism of alkali-activation is still not fully understood and further research is required. This paper overviews the properties, advantages, and potential shortcomings of AAS concrete.

The main objective of this study is to clarify the effect of bottom ash (BA) powder replacement rate, water-cement ratio, and sand-cement ratio on alkali-activated slag mortar (AASM) by comprehensively investigating the following parameters: fluidity, consistency, setting time, pH, compressive strength, and flexural strength, as well as observing the microstructure by XRD and SEM. Finally, the relationship between the parameters was explored by means of correlation coefficient heatmaps in concert with scatter plots (including quadratic polynomial linear fitting). The results show that it is feasible to use up to 60% of BA powder to replace slag for AASM. Meanwhile, it is suggested that the AASM with a water-cement ratio of 0.44 and a sand-cement ratio of 2.6 can obtain better workability, mechanical properties, and a denser microstructure. The incorporation of BA powder produces the unique hydration product Magadiite. In addition, the mechanical models of compressive strength and flexural strength of AASM were proposed . This study provides a reference for the application of BA powder in the alkali-activated system, which is beneficial for resource recycling.

As is well established, slag precursor offers promising performance characteristics; however, its origin as an industrial byproduct leads to variability in both mineralogical and chemical composition. Furthermore, the global availability of slag is limited compared to that of Portland cement (PC), raising concerns about long-term supply stability. To address these issues, this study investigates the incorporation of natural materials—specifically Egyptian natural basalt powder (BP)—as a partial replacement for slag. The research explores BP as a supplementary component in alkali-activated slag (AAS) systems. Blends containing 2.5 wt% to 40 wt% BP were prepared, and both pure slag and slag/BP mixtures were subjected to alkali activation to produce BP-modified AAS cement. The study aimed to assess the impact of varying BP ratios on flow characteristics, setting time, compressive strength, resistance to simulated real-world environmental conditions, and transport properties of the produced cement cured in air and water. In addition, the impact of varying BP ratios on drying shrinkage was monitored. This study also involved interpreting the key results through the use of a variety of contemporary scientific tools. Notwithstanding, BP might have slightly hindered the mixture flowability (up to 10.9% reduction) and prolonged setting time (1.23-fold for initial and 1.28-fold for final setting), the results demonstrated that including 2.5–20% BP improved the overall properties of the cement. An optimal ratio of 20% yielded the highest compressive strength, with an increase of up to 17.65% at 90 days under water curing, the lowest transport properties, with a decrease of 20%, and the lowest strength loss (3.63%) due to environmental conditions exposure under water curing, alongside reduced drying shrinkage. However, including 30% BP showed only a marginal effect, whilst including 40% BP showed a detrimental effect. Additionally, water curing proved superior to air curing, exhibiting higher strength, lower transport properties, and mitigating microcrack formation, thereby enhancing durability against wetting-drying cycles.