Consequences of omitting some important factors in the environmental analyses of commercial sodium silicate/sodium hydroxide use for alkaline activation in the light of comparison with cement-based composites

Alkali-activated materials (AAMs) based on various waste precursors were considered mostly as a sustainable alternative to Portland cement-based composites to date. However, a narrow focus on carbon dioxide savings in the environmental assessment of AAMs may not be sufficient to achieve a truly sustainable solution. Therefore, this paper provides a detailed insight into midpoint impact categories related to the production of AAMs based on waste precursors and conventional activators, as compared with common cement-based materials. The obtained results point to a higher environmental load of AAMs in several categories, such as ozone layer depletion, primary resource consumption, and terrestrial and aquatic ecotoxicity. In a hypothetical scenario, it is demonstrated that 10 % replacement of global concrete production by AAMs may result in notably increased emissions of ozone depletion substances (+35 %) and damage to the aquatic environment (+ 40 %). The risk for human health can then be higher. As for the aquatic environment, eutrophication can also lead to a significant increase in indirect emissions of CH4 and N2O having a high impact on the greenhouse effect. Hence, the importance of robust interdisciplinary research in the environmental assessment of AAMs should be emphasized, together with the need to use alternative alkaline substances, which would be more environment-friendly than conventional activators.

Zn(II) removal from wastewater by an alkali-activated material prepared from steel industry slags: optimization and modelling of a fixed-bed process

Removal of dissolved zinc (Zn) from water by a novel alkali-activated material (AAM) prepared from steel industry slags in a fixed-bed column was investigated. Design of experiments was used to find the optimum operation parameters [flow rate ( Q ) , adsorbent mass, (m ads ), and initial Zn concentration (C 0 )] for the removal of Zn2+ from a ZnCl2 solution. Regression models for the breakthrough (q b ), and saturation (q sat ) capacities of the bed and three other response parameters as functions of Q , m ads and C 0 were fitted with coefficients of determination (R 2 ) ranging from 0.48 to 0.99. Experimental values of q b and q sat varied within 1.42-7.03 mg Zn/g and 10.57-17.25 mg Zn/g, respectively. The optimum operation parameters were determined to be Q = 1.64 ml/min and m ads = 4.5 g, whereas C 0 had negligible effect on the response parameters in the range 73-107 mg Zn/l. Finally, three empirical breakthrough curve (BTC) models were employed to describe the individual BTCs of which the modified dose - response model was found to give the best fit (0.960 ≤ R 2 ≤ 0.998). The results of the present work demonstrate that the novel AAM has considerable potential to be utilized in water purification applications.

Reliability prediction of alkali-activated mortar during flexural loading using Weibull analysis

This study uses the Weibull analysis to predict the robustness of various mortars based on a fracture process analysis through a flexural test recorded by an acoustic emission sensor. Alkali-activated materials (AAMs) are an alternative to Portland cement that can decrease the amount of emitted CO2. This study aimed to characterise and compare the properties of AAM cement mortars to those of the commonly used ordinary Portland cement (OPC) mortars using the Weibull distribution to clarify the reliability and robustness of the prepared AAM cements; four different AAM cement mortar compositions-with fly ash (F), ground-granulated blast-furnace slag (G), and microsilica (M) alkali activation (sodium hydroxide (NaOH) and sodium silicate (Na2SiO3))-were considered in this study. The fracture process under a flexural loading of AAMs was based on four combinations of F/G/M activated by the alkaline solution-AAM-IV, AAM-V, AAM-VI, and AAM-VII, with OPC as control. The Weibull analysis showed that AAMs were more robust than the OPC mortar and possessed minor fractures compared to the OPC mortar.

Critical parameters affecting the thermal resistance of alkali-activated aluminosilicate wastes: Current understanding and future directions

Many research articles and reviews have recognized alkali-activated materials (AAMs) as eco-friendly alternative binders to ordinary Portland cement (OPC) due to their economic andenvironmental advantages. However, few literature surveys reported the physical, mechanical and microstructural changes that occur after the exposure of AAMs to elevated temperatures. Owing to the wide diversity in the properties of aluminosilicates, alkali-activation conditions, and additives, a deep survey is needed to understand how different factors can affect the performance of AAMs under elevated temperatures. Therefore, this review extensively discusses the impact of recent critical parameters, including aluminosilicate compositions, aggregate type and mineral, micro, and nano additives, on the behavior of AAMs under thermal load. It can be concluded that regardless of alkali-activator type and concentration, alkali-activated fly ash shows higher thermal resistance than alkali-activated metakaolin and slag. Moreover, the presence of an adequate amount of calcium can increase the thermal stability of AAMs, while the iron has a varying effect on the thermal resistance of AAMs, either positively or negatively. Compared with all additives and aggregates, using waste glass and lightweight aggregates enhanced the thermal resistance of AAMs. Howerver, some types of aggregate having a binding ability which increase the residual strength after heat exposure. Considering the fineness of materials, evaluating the role of nano and micro materials on the properties of AAMs at high temperatures is reviewed. Based on this survey, several promising topics for future work are suggested.

Freeze-thaw cycle and abrasion resistance of alkali-activated FA and POFA-based mortars: Role of high volume GBFS incorporation

Alkali-activated binders made from various waste products can appreciably reduce the emission of CO2 and enhance the waste recycling efficiency, thus making them viable substitutes to ordinary Portland cement (OPC)-based binders. Waste materials including fly ash (FA), palm oil fuel ash (POFA), and granulated blast furnace slag (GBFS) reveal favorable effects when applied to alkali-activated mortars (AAMs) that are mainly related to the high contents of silica, alumina, and calcium. Therefore, fifteen AAM mixes enclosing FA, POFA with high volume of GBFS were designed. The obtained GBFS/FA/POFA-based AAMs were subjected wet/dry and freeze/thaw cycles. The impact of various GBFS contents on the microstructures, freeze-thaw cycle, abrasion resistance, mechanical and durability features of the proposed AAMs were evaluated. The results showed that presence of Ca can significantly affect the AAMs durability features and long-term performance. The abrasion resistance of the AAMs was decreased with the decrease of CaO contents. Furthermore, the abrasion depth of 70% AAMs (0.8 mm) was lower in comparison to the mix made by replacing 50 wt% of FA with GBFS (1.4 mm). Generally, increase in the GBFS contents from 50 to 70% could largely impact the AAMs properties under aggressive environmental exposure. The expansion and physical impacts during the freezing-thawing cycles was argued to destroy the bonds in C-S-H and paste-aggregates, causing the formation of large cracks. It is asserted that the AAM mixes made from FA, POFA and high volume of GBFS may offer definitive mechanical, durable, and environmental benefits with their enhanced performance under aggressive environments.

Efficient sequestration of malachite green in aqueous solution by laterite-rice husk ash-based alkali-activated materials: parameters and mechanism

In this work, laterite (LA) and rice husk ash (RHA)-based alkali-activated materials (AAMs) with varying %RHA contents (0, 5, 10, 15, and 20%) were prepared for the removal of malachite green (MG) dye from water. The precursors and AAMs were characterized by standard methods (XRF, XRD, TG/DTA SEM, and FTIR). The SEM micrographs and iodine index values showed that the incorporation of RHA improves the microporosity of laterite-based geopolymers. The incorporation of RHA did not result in any new mineral phases after alkalinization. Geopolymerization increased both the adsorption rate and capacity of the geopolymers relative to LA by approximately 5 times. The maximum adsorption capacity was 112.7 mg/g, corresponding to the GP_95-5 (5% RHA) geopolymer. The adsorption capacity was therefore not solely controlled by the RHA fraction. The adsorption kinetics data was best predicted by the pseudo-second-order (PSO) model. The adsorption mechanism entails electrostatic interactions and ion exchange. These results show the suitability of laterite-rice husk ash (LA-RHA)-based alkali-activated materials as adsorbents for the efficient sequestration of malachite green in aqueous solution.

Mining tailings and alkali activation: a comprehensive bibliometric review

Significant amounts of mining tailings are generated and disposed of every year in dams, leading to potentially serious environmental and safety problems. To identify alternatives for the disposal of these wastes, research works involving their potential application as precursors in the development of alkaline-activated materials have been published in recent years. In this context, the objective of this paper is to present an overview of the main contributions already made on the subject, identified through a bibliometric review and content analysis in the Scopus and Web of Science databases. There was an exponential growth of interest in the subject in the period 2019-2021, when more than 50% of the papers were published. The most used tailings and sub-areas of research were also identified.

Valorizing (cleaned) sulfidic mine waste as a resource for construction materials

Proper management and storage of mine waste, e.g., tailings and waste rock, is one of the main issues that mining industries face. Additionally, there is already an uncountable amount of existent historical mine waste, which may, even centuries after the closure of the mine, still be leaching contaminants into the environment. One solution to minimize the risks associated with the mine waste, with also potential economic benefits, is through the valorization of the waste. This can be done by first recovering valuable metals and removing hazardous contaminants. Then, the remaining residue can be valorized into green construction materials, such as geopolymers, ceramics or cement. For some mine waste materials, such as those with only trace levels of metals that are not economically viable to extract, the "waste" can be reused directly without this additional cleaning step. In the present study, mine waste originating from three different sites was characterized and compared with the cleaned mine waste (i.e., cleaned by bioleaching or flotation methods) and with different types of green construction materials containing 13-80 wt% (cleaned and uncleaned) mine waste. Particular emphasis was given to the mobilization of metal(loid)s from the mine waste and construction materials (i.e., ceramics, alkali-activated materials and cement) under different conditions, through a series of leaching tests (i.e., EN 12457-2, US EPA's Toxicity Characteristic Leaching Procedure, and a pH-dependent leaching test). The leaching tests were applied to either mimic current 'natural' conditions at the mining site, conditions in a landfill (end of life) or extreme conditions (i.e., extremely acidic or alkaline pH). Most of the original mine waste samples contain high levels of Pb (18-3160 mg/kg), Zn (66-10500 mg/kg), and As (10-4620 mg/kg). . The cleaning methods were not always efficient in removing the metal(loid)s and sulfur. In some cases, the cleaned mine waste samples even contained higher total metal(loid) and sulfur concentrations than the original mine waste samples. Based on the leaching studies, some alkali-activated materials, ceramics, and cement effectively immobilized certain metals (e.g., <0.5 mg/kg of Pb and <4 mg/kg of Zn). Also, longer curing times of the alkali-activated materials, in most cases, improved the immobilization of metal(loid)s. Additionally, for ceramics, the temperature at which the test pieces were fired (up to 1060 °C), also played a major role in decreasing the mobility of some metal(loid)s, while increasing others (e.g., As, potentially via the structural rearrangement of As and Fe). Overall, through this detailed characterization, the environmental impact from the mine waste to the downstream products was evaluated, determining which valorization methods are the most viable to close the circular economy loop.

Manufacture of alkali-activated and geopolymer hybrid binder (AGHB) by municipal waste incineration fly ash incorporating aluminosilicate supplementary cementitious materials (ASCM)

The treatment and disposal of municipal solid waste incineration fly ash (MSWI FA) faces many challenges, such as landfill space occupation, high costs and potential environmental threats. In this study, coal fly ash (CFA), metakaolin (MK) and silica fume (SF) were used as aluminosilicate supplementary cementitious materials (ASCM), and mixed with MSWI FA as precursors for the synthesis of alkali-activated and geopolymers hybrid binder (AGHB). The results show that this alkali-activated technology efficiently immobilized the heavy metals in MSWI FA, and the ASCM contributes to the compressive strength enhancement of the AGHB. The highest compressive strength of the synthesized products that mixed MSWI FA with CFA and MK as precursors, reached 5.34 and 9.06 MPa, respectively. The compressive strength of the ASCM synthesized by mixing MSWI FA and SF in the mass ratio of 70:30 with the alkali activator modulus of 1.6 M could reach 11.2 MPa after 28 d of curing, which met the quality standard of MU10 (NY/T 671-2003) for load-bearing brick.The leaching concentrations of Hg and Pb were reduced from 0.15 to 3.96 mg/L to less than 0.003 and 0.107 mg/L, which were below the limit established by the Chinese standard (GB 8978-1996). The research provides the technical parameters of the optimization conditions on the synthesis of MSWI FA-based AGHB, for the resource utilization of MSWI FA and reduction of the environmental risk.

---Mechanical behavior of textile reinforced alkali-activated mortar based on fly ash, metakaolin and ladle furnace slag

The need for repair and maintenance has become dominant in the European construction sector. This, combined with the urge to decrease CO 2 emissions, has resulted in the development of lower carbon footprint repair solutions such as textile reinforced mortars (TRM) based on alkali-activated materials (AAM). Life cycle studies indicate that AAM CO 2 savings, when compared to Portland cement, range from 80% to 30%. Furthermore, in this study, recycled aggregates were considered with the aim to promote a circular economy mindset. AAM mortars formulation based on fly ash, ladle furnace slag and metakaolin were tested for compressive and flexural strength. Three out of all formulations were chosen for an analysis on the potential of these mortars to be used for TRM applications. Tensile and shear bond tests, combined with a concrete substrate, were executed as indicators of the TRM effectiveness. Scanning electron microscopy and chemical analysis based on energy dispersive X-ray spectroscopy were used to interpret the results and reveal the reasons behind the different level of performance of these composites. Results indicated that TRM based on high calcium fly ash are unsuitable for structural strengthening applications due to low bond between matrix and/or substrate and fibers. Metakaolin-based TRM showed good performance both in terms of tensile strength and bond capacity, which suggests potential as a repair mortar.

Low carbon binder modified by calcined quarry dust for cemented paste backfill and the associated environmental assessments

Cemented paste backfill (CPB) favors the sustainable development of mine industry. However, as the primary cementitious binders in CPB, the high cost of ordinary Portland cement (OPC) discourages CPB utilization. In the present work, low-carbon and low-cost binders activated by Na₂CO₃ supplemented by calcined quarry dust were used in CPB. The binder was prepared using a 'one-part' method. It was found that binders prepared using 8% Na₂CO₃ and 5% CQD show the best performance. The superior properties of the binders were attributed to the promoted binder hydration and special phase assembles of the hydration products. Cost and carbon emission analysis showed that Na₂CO₃ activated binder was cheaper and greener. The cost and CO₂ emission of binder B8Q5 were lower than OPC by around 34.16% and 87.76%, respectively. Besides, leaching tests showed that all the toxic metals were stabilized, which posed no environmental risk.

Investigating the microstructure of high-calcium fly ash-based alkali-activated material for aqueous Zn sorption

The performance of adsorbents prepared by alkali activation of high calcium fly ash was investigated for removing aqueous Zn. Two formulations involving the use of NaOH and Na₂SiO₃ activating solutions were used to prepare the adsorbents that feature different microstructural characteristics. The Zn sorption data indicates a sorption process that is controlled by both chemisorption and intra-particle diffusion. The Na₂SiO₃-activated material displayed higher sorption rates compared to the NaOH-activated material. The sorption kinetics show strong dependence on the microstructures of the adsorbents, wherein the Na₂SiO₃-activated material featuring higher contents of amorphous phases (96 %_mass) in the hydrated phase assemblage, with attendant improved porosity and surface area, performed better than the NaOH-activated material (86 %_mass amorphous phases) which showed higher degree of crystallinity and coarse morphology. The Na₂SiO₃-activated material exhibited 100% Zn removal efficiency within the first 5 min in all studied initial adsorbate concentrations(corresponding to sorption capacity of up to 200 mg/g), while the NaOH-activated analogue tends to lag, reaching 99.99% Zn removal efficiency after about 240 min in most cases. The two formulations were also examined with thermodynamic modeling and the results agree with experimental data in indicating that the use of alkali-silicate activating solution is most suitable for converting high calcium fly ash into efficient adsorbent for removing aqueous heavy metals.

Advanced characterization of IGCC slag by automated SEM-EDS analysis

Integrated gasification combined cycle (IGCC) is a highly efficient method for producing electricity but discharges a byproduct in the form of a glassy slag, similar to other electricity generation operations. Several technologies for recycling IGCC slag have been developed thus far, although the results obtained are not promising or universally applicable. We quantitatively characterized an IGCC slag by using various testing methods, including an automated scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) system, to recognize its potential for recycling. The IGCC slag did not contain free CaO, and the absence of free lime would address a concern of volumetric expansion during hydration. Automated SEM-EDS analysis revealed that approximately 98% of the IGCC slag particles consisted of calcium-rich aluminosilicate materials. Obvious differences in the concentrations of Si, Al, and Ca between the amorphous phases and the average chemical bulk were recognized. The chemical composition of the amorphous Si-Al-Ca phases was similar to that of Class C fly ash, while the average bulk composition of the IGCC slag was in between that of Class C and Class F fly ashes. Considering this discrepancy, understanding the dissolution mechanism of the reactive amorphous fraction as well as an exact assessment of the reaction products based on the role of Ca in alkali-activated materials provides a new approach for the valorization of IGCC slag.

Structural evolution of binder gel in alkali-activated cements exposed to electrically accelerated leaching conditions

The structural evolution of a binder gel in alkali-activated cements exposed to accelerated leaching conditions is investigated for the first time. Samples incorporating fly ash and/or slag were synthesized and were exposed to electrically accelerated leaching by applying a current density of 5 A/m². The leaching behavior of the samples greatly depended on the binder gel formed in the samples. The N-A-S-H type gel abundant in fly ash-rich samples showed some extent of dissolution upon accelerated leaching, while slag-rich samples underwent hydration of the anhydrous slag after leaching. The obtained results are discussed in view of the degradation of the binder gel induced by accelerated leaching, and their potential performance under repository conditions where groundwater-induced leaching is the main durability concern.

Green remediation of As and Pb contaminated soil using cement-free clay-based stabilization/solidification

Stabilization/solidification (S/S) is a low-cost and high-efficiency remediation method for contaminated soils, however, conventional cement-based S/S method has environmental constraints and sustainability concerns. This study proposes a low-carbon, cement-free, clay-based approach for simultaneous S/S of As and Pb in the contaminated soil, and accordingly elucidates the chemical interactions between alkali-activated clay binders and potentially toxic elements. Quantitative X-ray diffraction and ²⁷Al nuclear magnetic resonance analyses indicated that the addition of lime effectively activated the hydration of kaolinite clay, and the presence of limestone further enhanced the polymerization of hydrates. X-ray photoelectron spectroscopy showed that approximately 19% of As^[III] was oxidized to As^[V] in the alkali-activated clay system, which reduced toxicity and facilitated immobilization of As. During the cement-free S/S process, As and Pb consumed Ca(OH)₂ and precipitated as Ca₃(AsO₄)₂·4H₂O and Pb₃(NO₃)(OH)₅, respectively, accounting for the low leachability of As (7.0%) and Pb (5.4%). However, the reduced amount of Ca(OH)₂ decreased the degree of hydration of clay minerals, and the pH buffering capacity of the contaminated soil hindered the pH increase. Sufficient dosage of lime was required for ensuring satisfactory solidification and contaminant immobilization of the clay-based S/S products. The leachability of As and Pb in high-Ca S/S treated soil samples was reduced by 96.2% and 98.8%, respectively. This is the first study developing a green and cement-free S/S of As- and Pb-contaminated soil using clay minerals as an environmentally compatible binding material.