Production Of Alkali-activated Materials Research Articles

Sodium carbonate-enhanced calcium carbide slag activation of sewage sludge for the preparation of green cementitious materials

The utilization of calcium carbide residue (CCR) to activate solid wastes can effectively promote sustainable development. This paper explores adding sodium carbonate (Na2CO3) to CCR activators to improve the early reactivity of alkali-activated materials (AAMs) made from sewage sludge ash (SSA). The research focuses on the effects of Na2CO3 on setting, hardening, mechanical strength, and environmental benefits. The findings indicate that Na2CO3 can shorten the paste’ hardening time and significantly enhance the early mechanical properties. The introduction of Na2CO3 accelerates the consumption of CCR during the early phases and facilitates the formation of C-(N)-A-S-H gels. Meanwhile, it can significantly reduce the porosity and refine the pore structure. When the dosage reaches 8%, the total intruded pore volume of the sample decreases to 0.15 mL/g. This activation method reduces energy consumption, CO2 emissions, and costs by 69%, 50%, and over 90%, respectively, making it an efficient, eco-friendly approach to AAM production.

WITHDRAWN: From Debris to Innovation: Unveiling a New Frontier for Alkali-Activated Materials

Alkali-activated materials (AAMs) play a crucial role in addressing the environmental challenges of traditional construction binders by offering a lower carbon footprint alternative. Industrial by-products are utilized, reducing waste and energy consumption associated with lime and cement production. AAMs represent a sustainable innovation in the construction industry, promising significant reductions in CO2 emissions and resource utilization. This review aims to meticulously analyze the existing body of research on the potential of Construction and Demolition Waste (CDW) as an alternative precursor for producing AAMs. This review study introduces a novel angle by critically evaluating advancements in CDW processing techniques for AAM production, alongside assessing the environmental and mechanical outcomes. It pioneers bridging the gap between innovative recycling methods and sustainable construction practices. The ultimate goal is to synthesize findings into a comprehensive framework to guide future research toward optimizing CDW utilization in AAM production, thereby contributing to sustainable construction practices and the circular economy. This study stands out for its in-depth examination of CDW's role in mitigating the environmental impact of conventional construction materials, offering novel insights into the path toward sustainability in the construction industry. By critically assessing the latest advancements in CDW processing techniques and their environmental and mechanical implications, the study not only underscores the potential of CDW in revolutionizing the construction industry but also sets a new precedent in sustainability research. It bridges the crucial gap between theoretical innovation and practical application, offering actionable insights for optimizing CDW utilization in AAM production. This comprehensive analysis signifies a pivotal step towards integrating sustainable practices within the construction sector, advocating for a paradigm shift towards a circular economy that prioritizes resource efficiency and environmental stewardship.

Influences of silicate modulus and alkali content on macroscopic properties and microstructure of alkali-activated blast furnace slag-copper slag

The stacking of copper slag (CS) has led to the resource wastage and environmental issues. Utilizing CS as a green precursor in the production of alkali-activated materials (AAMs) offers a promising avenue for sustainable resource usage. To obtain a deeper understanding and enhance its practical application, the effects of silicate modulus and alkali content of water glass (WG) on the performances of alkali-activated blast furnace slag-copper slag (AAS-CS) are systematically investigated though the fluidity, setting time, hydration heat, compressive strength and drying shrinkage rate. Additionally, the microstructure is characterized using nuclear magnetic resonance (NMR), X-ray diffraction (XRD), Fourier transform infrared spectrum (FTIR) and scanning electron microscope (SEM), respectively. When the alkali content is a constant of 6.0 wt%, the compressive strength and drying shrinkage rate initially increases with increasing silicate modulus, but eventually decrease beyond a certain threshold. At a given silicate modulus of 1.5, the compressive strength and drying shrinkage rate firstly increases and then decrease as increasing the alkali content from 4.0 wt% to 8.0 wt%. The microstructure analyses reveal that the suitable silicate modulus and alkali content promote the dissolution of precursors to form C-A-S-H gels. The M1.5N6.0 sample has a relatively tight microstructure, which benefits to the development of compressive strength, but increases the capillary stress. Therefore, the M1.5N6.0 sample has the highest 28-day compressive strength of 71.7 MPa and the largest 182-day drying shrinkage rate of 0.1532 %.

Compressive strength and gel structure improvement in the high-volume calcium carbide residue activated-blast furnace slag system combined with sodium carbonate and silica fume

Calcium carbide residue (CCR) with high alkalinity favorably corresponds to the alkaline activator in the alkali-activated materials (AAMs) production. Its utilization could reduce the carbon footprint and facilitate cast-in-situ applications compared to traditional liquid alkaline activators. A high-volumed CCR from 10 % to 25 % as alkaline activators in an alkali activated-blast furnace slag system was investigated, in which sodium carbonate (Na2CO3) and silica fume were used to improve the mechanical properties of CCR-activated blast furnace slag (CCR activated-BFS) pastes. Isothermal Calorimetry, X-ray diffraction (XRD), 29Si nuclear magnetic resonance (NMR), and scanning electron microscopy (SEM) were used to characterize the reaction process, phase transformation, and gel structure. The results show that the highest compressive strength of CCR activated-BFS pastes was obtained when the CCR proportion was 20 %. Na2CO3 addition significantly improved the compressive strength, especially at the CCR to Na2CO3 mass ratio at 1:1.5. However, the compressive strength deterioration appeared at 28 d because of the gel structure decomposition in CCR activated-BFS pastes. The silica fume addition inhibited the structure decomposition, improved the connectivity of gel structure, favored the formation of gel, and thus suppressed the compressive strength deterioration. The study established a sustainable waste recycling from acetylene gas production by using CCR as feedstock to mitigate carbon emissions from AAMs production.

Synthesis of solid sodium silicate from waste glass and utilization on one-part alkali-activated materials based on spent oil filtering earth.

Alkali activated materials (AAMs) commonly known as geopolymers are considered ecofriendly substitutes for Portland cement. However, these materials still have a significant environmental impact, owing mainly to the use of activators based on commercial chemical products. In this sense, this research focuses on the production and use of waste glass-derived activators AAMs as an alternative to commercial activators. Using a thermochemical synthesis method, activator compositions were systematically designed to achieve predefined activator modulus (Ms = SiO2/Na2O = 0.5; 1.0 and 1.5). These alternative activators were studied by XRD, FTIR and SEM techniques. Additionally, one-part AAMs were manufactured using spent oil filtration earth (SOFE) as precursor and activator with optimum modulus Ms = 1.0. The influence of the Na2O dosage was studied (10; 20 and 30g of Na2O per every 100g of SOFE) as well as the influence of the activator modulus maintaining the optimum dosage of 20g Na2O per 100g of SOFE. As a control, two-part AAMs were also synthetized with the optimum dosage and modulus employing commercial activators (NaOH + Na2SiO3 solution). Results indicate that the modulus of the alternative activator and especially the Na2O dosage have a significant influence on the technological properties of AAMs based in SOFE, with an optimum compressive strength (35.8MPa) for the addition of 20g of Na2O per every 100g of SOFE using activator with modulus Ms = 1.0. This research embodies a sustainable approach to AAM production and suggests waste glass as a valuable raw material for sodium silicate synthesis intended for the one-part activation of spent filtering earth from the agri-food industry, aligning with the principles of circular economy and sustainable development goals.

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.

Reaction mechanisms in geopolymers produced from sugarcane bagasse ash

The production of Portland cement exhibits significant importance for social and economic development of a country, however also generates environmental impacts from the extraction of raw materials to the production of clinker. Despite the impacts, the demand for Portland cement is still growing in developing countries such as India, China, and Brazil. Faced with this impasse, research for the development of alternative materials to Portland cement is growing around the world. Among these researches, those related to alkali-activated materials (AAMs) stand out. AAMs are considered more sustainable than Portland cement because emit less CO2 during their production process. In addition, the raw materials used in its development are generally wastes from other industrial processes. Sugarcane bagasse ash (SCBA) is an example of residue generated on a large scale in Brazil and with great potential for use in the production of AAMs. Therefore, the present work seeks to develop AAMs using SCBA as an exclusive precursor. For this, the material's viability was verified through physical, mechanical, morphological, and calorimetric tests. As a result, concluded that SCBA is a viable precursor for AAMs, promoting sustainability and innovation in the field of construction materials. In addition, the main mechanism of the reaction of SCBA pastes is the diffusion process, independent of paste formulation and the silicate is crucial for the degree of reaction at early age. At the same time, the compressive strength and microstructure are strongly dependent on alkalis and silicate content and SiO2/Na2O proportions around 1.0 are the optimum point for SCBA pastes.

Performance Evaluation of Hybrid One-Part Alkali Activated Materials (AAMs) for Concrete Structural Repair

Alkali-activated materials (AAMs) have been widely used as an alternative to Portland cement. This production of AAMs emits lesser carbon dioxide by utilizing industrial waste products to make this cement binder technology greener and more sustainable. The conventional two-part system comprises solid aluminosilicate precursors with an alkali solution to activate the AAMs. However, higher alkalinity of the liquid activator is required to complete the geopolymerazation process, making the cementitious materials costly and sticky, and thus not convenient to handle on the construction site, affecting the worker’s safety. A one-part AAMs system was introduced to overcome the two-part system’s shortcomings. The alkali solution is now replaced with a solid alkali activator which is easier and more practical to apply at construction sites. This study was carried out to evaluate the mechanical performance of one-part alkali AAMs in the form of mortar by conducting compressive and flexural strength, modulus of elasticity, and tensile strength tests at 28 days of curing age under laboratory experiments in the tropical climate of Malaysia. A drying shrinkage test was also performed to detect its durability. Three types of solid admixtures were added to complete the composition of the novel mix design formulation. According to the results obtained, the mechanical strength of one-part alkali-activated mortar achieved the minimum requirement for Class R3 structural concrete repair materials as per EN1504-3 specifications. This eco-friendly cement binder has excellent potential for further engineering development, particularly to become a new concrete repair product in the future.

A new approach to the reuse of waste glass in the production of alkali-activated materials

The massive carbon footprint of cement manufacturing has provided great opportunities for research and development of eco-friendly alternative binders, geopolymers for instance. This research aims to maximise the waste uptake in the production of geopolymers by reusing glass waste as the auxiliary activator and fly ash/blast furnace slag blend as the aluminosilicate precursor. A filtration technique is utilised to assess the dissolution efficiency of glass waste powders in alkaline medium. The advantage of filtration compared with other methods includes its simplicity, accuracy, and availability to the research team. Compressive and flexural strength, X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared spectroscopy are conducted. The binders illustrated up to 28 MPa compressive strength, 3.4 MPa flexural strength, and 2.9 GPa Elastic modulus after 7 days implying competitive properties to those synthesised by silicate-contained activators. The weight loss after dissolution confirmed glass waste participation in the hardening mechanism and geopolymerisation reactions. The microstructure is also an analogy to the conventional binders resulting in the benefits of durability.

Deformation of Alkali-Activated Materials at an Early Age Under Different Curing Conditions.

The production of alkali-activated materials (AAMs) is known for its environmentally friendly processing method, where several amorphous-rich aluminosilicate material sources combine with an alkali media solution to form solid, ceramic-like materials. In terms of the Si:Al, Na(K):Al, and Na(K):H2O ratios, the theory of AAM formation is quite well developed, but some open questions in the technology process remain, especially with regards to the means of curing, where the generation of defects can be persistent. Knowing that deformation is extremely high in the early ages, this study investigates the effects of temperature and moisture on shrinkage behavior within the first 72 h of AA pastes made from ladle (LS) and electric arc furnace (EAF) slag and activated by sodium silicate (Na2SiO3). The method to determine the deformation of alkali-activated slag-based materials, in terms of both autogenous and drying shrinkage, was based on the modified ASTM C1698-19 standard for the measurement of autogenous shrinkage in cement pastes. Autogenous deformation and strain were measured in four samples, using the standard procedure at room temperature, 40 and 60°C. Furthermore, using an adjusted method, nine samples were characterized for strain and partial surface pressure, while drying at room temperature, 40, or 60°C at a relative humidity of 30 or 90%. The results show that the highest rate of autogenous shrinkage occurred at a temperature of 60°C, followed by drying shrinkage at 60°C and 30% relative humidity, owing to the fact that the rate of evaporation was highest at this moisture content. The study aimed to provide guidance regarding selection of the optimal curing set in order to minimize deformations in slag-based alkali-activated materials. In the present case, curing at a temperature of around 40°C under lower moisture conditions for the first 24 h provided optimal mechanical properties for the slags investigated. The methodology might also be of use for other aluminosilicate sources such as metakaolin, fly ash, and mineral wool–based alkali-activated materials.

Marble Waste Valorization through Alkali Activation

In the present study, the valorization potential of marble waste in the presence of metakaolin via alkali activation was explored. The activating solution used consisted of NaOH and sodium silicate solutions. The effects of marble waste to metakaolin ratio, particle size of raw materials, curing temperature, and Na2O/SiO2 and H2O/Na2O molar ratios present in the activating solution on the main properties and the morphology of the produced alkali-activated materials (AAMs) was evaluated. The durability and structural integrity of the AAMs after firing at temperatures between 200 and 600 °C, immersion in deionized water and 1 mol/L NaCl solution for different time periods and subjection to freeze–thaw cycles were also investigated. Characterization techniques including Fourier transform infrared spectroscopy, X-ray diffraction, mercury intrusion porosimetry and scanning electron microscopy were used in order to study the structure of the produced AAMs. Τhe highest compressive strength (~36 MPa) was achieved by the AAMs prepared with marble waste to metakaolin mass ratio of 0.3 after curing at 40 °C. The results indicated that the utilization of marble waste in the presence of metakaolin enables the production of AAMs with good physical (porosity, density and water absorption) and mechanical properties, thus contributing to the valorization of this waste type and the reduction of the environmental footprint of the marble industry.

Evaluation of different fly ash samples on the production of alkali-activated materials

abstract: In this work, different samples of fly ash (FA-A and FA-B) classified as type F as used to produce the AAM samples. The FA-A presented a higher Fe2O3 content than FA-B, 6.1 to 3.8 wt.%, and a slightly higher SiO2/Al2O3 ratio of 3.52 in comparison to 3.34 of FA-B. The average particle size (D50) of fly ash A was 19.7 µm and of fly ash B 30.8 µm, while the specific mass of the ashes A and B were 2.38 and 2.21 g/cm3, respectively. The results revealed that the mechanical strength of the AAM produced with fly ash A was higher than fly ash B, close to 80 and 44 MPa, respectively. The variation of the strength has been attributed to the different SiO2/Al2O3 ratios and different particle sizes. The mechanical strength decreased with increasing curing time, which is attributed to excess alkali in the system. Only very small differences in porosity and density have been found.

Towards designing reactive glasses for alkali activation: Understanding the origins of alkaline reactivity of Na-Mg aluminosilicate glasses.

Alkali-activated materials (AAMs), sometimes called geopolymers, are eco-friendly cementitious materials with reduced carbon emissions when compared to ordinary Portland cement. However, the availability of most precursors used for AAM production may decline in the future because of changes in industrial sectors. Thus, new precursors must be developed. Recently there has been increased interest in synthetic glass precursors. One major concern with using synthetic glasses is ensuring that they react sufficiently under alkaline conditions. Reactivity is a necessary, although not sufficient, requirement for a suitable precursor for AAMs. This work involves the synthesis, characterization, and estimation of alkaline reactivity of Na-Mg aluminosilicate glasses. Structural characterization showed that replacing Na with Mg led to more depolymerization. Alkaline reactivity studies indicated that, as Mg replaced Na, reactivity of glasses increased at first, reached an optimal value, and then declined. This trend in reactivity could not be explained by the conventional parameters used for estimating glass reactivity: the non-bridging oxygen fraction (which predicts similar reactivity for all glasses) and optical basicity (which predicts a decrease in reactivity with an increase in Mg replacement). The reactivity of the studied glasses was found to depend on two main factors: depolymerization (as indicated by structural characterization) and optical basicity. Depolymerization dominated initially, which led to an increase in reactivity, while the effect of optical basicity dominated later, leading to a decrease in reactivity. Hence, while designing reactive synthetic glasses for alkali activation, structural study of glasses should be given due consideration in addition to the conventional factors.

Alkali-activated slag supplemented with waste glass powder: Laboratory characterization, thermodynamic modelling and sustainability analysis

Finding new aluminosilicate wastes to make sustainable alkali-activated materials (AAMs) has gained increasing attention in cement industry. In this work, the ground granulated blast-furnace slag (SG) was incorporated by waste glass powder (GP) as the raw materials for the production of AAMs. Laboratory tests were conducted to analyze the compressive strength and phase assemblages of hardened pastes at increasing ages. Analytical modelling based on Gibbs free energy minimization method (GEM) was implemented to compare with the experimental results and assist in the analysis of relationship between phase assemblage and compressive strength. The results show that GP had a lower reaction rate compared with SG, and about 28 days of curing was required to show its delayed contribution on the mechanical properties. The incorporation of up to 50% of GP into SG did not cause the significant detrimental effect on the late-age strength of AAMs, and the final strength ranged between 33.5 and 33.1 MPa. X-ray diffraction (XRD), thermogravimetric analysis (TG) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-DES) showed the presence of calcium (sodium) aluminosilicate (C-(N-)A-S-H) gels and hydrotalcite phases in the reaction products. In addition, it was found that a higher amount of C-(N-)A-S-H and lower porosity in AAMs generally contributed to a higher compressive strength. The sustainability and cost analysis also confirmed the environmental friendliness and the economic feasibility of incorporating GP in the traditional alkali-activated slag cement, paving the path for the value-added utilization of waste glass in cement industry.

Evaluation of locally available amorphous waste materials as a source for alternative alkali activators

The production of alkali-activated materials with excellent mechanical performance requires the use of water glass, which has a significant carbon footprint. Such materials can have a lower carbon footprint if we replace water glass with alternative activators sourced from waste. In this study, we assessed the suitability of locally available amorphous waste materials (stone wool, glass wool, bottle glass and cathode-ray tube glass) as a source for the preparation of alternative alkali activators. We quantified the amount of silicon and aluminium dissolved in the activator solutions via inductively coupled plasma-optical emission spectrometry. The alternative activators were then used to produce alkali-activated fly ash and slag. The compressive strength values of alkali-activated fly ash specimens upon the addition of NaOH, water glass and the most promising alternative activator were 38.98 MPa, 31.34 MPa and 40.37 MPa, respectively. The compressive strength of slag specimens activated with alternative activators with the highest concentration of dissolved silicon (21 g/L) was, however, 70% higher than the compressive strength of slag specimens activated with only 10 M sodium hydroxide. The compressive strength of slag specimens with the addition of the most promising alternative activator was significantly lower (3.5 MPa) than the compressive strength of those that had been activated by commercial water glass (34.3 MPa).

Modeling hydration of mine tailings: Production of hydraulic binders from alkali-activated materials

The production of alkali-activated materials and geopolymers from the finer fraction of tailings is being increasingly investigated, as they could be used for building and public works, or to retain heavy metals in a consolidated solid matrix. The present study proposes a Pitzer-based geochemical model to reproduce the alkaline leaching for two mine tailing samples and the polymerization reactions. The model fits the results of the leaching tests quite well over the reaction duration and up to NaOH 10 M and 60 °C. Discrepancy for the dissolved Mg concentration indicates that the database can handle ionic strength up to 5 mol.kg−1. A geopolymer solid solution was developed for setting reactions modeling. The results compare favorably with compressive strength tests results, with C-S-H, M-S-H and geopolymers as the main hydration products. The study proposes an innovative application of Pitzer database developments with application to the mining industry.

Gender Balance in Construction Material Research: The Analysis of Alkali-Activated Materials by a Bibliometric Study Using Scopus Database

Research in alkali activated materials (AAMs) is an innovative and dynamic Material Science topic. This is due to the relevant performances in terms of physico-mechanical properties comparable to traditional construction materials (e.g. ordinary Portland cement). The sustainability of AAMs are often highlighted since they can be developed by using many natural or industrial by-products-based precursors. The leads to challenges with the optimization of AAMs production, due to their different performances, availability, and costs. However, they are flexible and locally adapted materials. The research interest on AAMs has rapidly increased in the early 2000s. In recent years, about 1000 international papers have been published each year. This study aims at assessing the gender balance of this specific research topic to investigate the relevance of women participation. Author lists of the published papers were analyzed throughout SCOPUS database, applying “alkali activated material” and “geopolymers” as keywords in the titles, abstracts, and keywords search. The last 10 years (2009 – 2019) were taken into account. The gender of the most prolific authors was analyzed and a focus of European authors on this topic was considered, as more than 25% of the research in AAMs has been carried out in Europe. The analysis of 5900 papers in the last decade shows that there is currently a shift toward men in the gender balance in this specific research and female authors only covered 22% of the most productive authors worldwide. Considering European authors, a more equal gender distribution is reached, when the first author is considered, with a concentration of women in the range of 36-56%, recorded over 1396 papers.

Improvement of mechanical strength of alkali-activated materials using micro low-alumina mine tailings

Low-alumina mine tailings (MTs) have shown the possibility of being a precursor in the production of alkali-activated materials (AAMs). The effects of the addition of sub-micron MTs (10 wt%) with the average size of 400 nm to improve the performance of AAMTs with alkali activator (10, 15, 20, and 30 wt% sodium silicate) were investigated by using X-ray diffractometry (XRD), Attenuated total reflection-Fourier-transform infrared spectroscopy(ATR-FTIR), scanning electron microscopy (SEM), and nitrogen adsorption technique (BET). The mechanical properties of the materials were also analyzed. The results indicate that the addition of sub-micron MTs to AAMTs plays an important role in mineral compositions and enhances the mechanical strength performance in comparison to plain AAMTs, especially after just 7 days of aging. This result is attributed to the different microstructure between AAMTs and sub-micron MTs. BET results showed that the addition of sub-micron MTs reduces the total porosity of alkali-activated products and changes the pore structure. The pores of AAMTs were refined by the filling effects of sub-micron particles and the enhancement of the hydration process due to thenucleationeffect of those sub-micron particles. This could be a significant reason for the increase in early age mechanical strength. This work introduces a novel approach to improve the performance of tailings-based alkali-activated materials using nano-sized precursors.

Factors affecting co-valorization of fayalitic and ferronickel slags for the production of alkali activated materials

The first objective of this experimental study is the assessment of the alkali activation potential of two types of fayalitic slags, an as-received one (FS) and the one obtained after plasma treatment (FSP) of the initial FS, for the production of alkali activated materials (AAMs). Furthermore, the second objective is the elucidation of the co-valorization potential of FS and FSP slags when mixed with ferronickel (FeNi) slag (LS). The alkaline activating solution used was a mixture of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3). The effect of various operating parameters, such as H2O/Na2O and SiO2/Na2O ratios present in the activating solution, curing temperature, curing period and ageing period on the compressive strength, density, water adsorption, porosity and toxicity of the produced AAMs was explored. The structural integrity of selected AAMs was investigated after firing specimens for 6 h at temperature up to 500 °C, immersion in distilled water and acidic solution or subjection to freeze-thaw cycles for a period of 7 or 30 days. The results of this study show that FS- and FSP-based AAMs acquire compressive strength of 44.8 MPa and 27.2 MPa, respectively. When FS and FSP were mixed with LS at 50:50%wt ratios the compressive strength of the produced specimens increased to 64.3 MPa and 45.8 MPa, respectively. Furthermore, selected AAMs produced after co-valorisation of slags retained sufficient compressive strength after firing at 500 °C, 45–68 MPa, and exhibited very low toxicity. These findings prove the alkali activation potential of fayalitic slags as well as their co-valorization with ferronickel slag for the production of AAMs, an approach which is in line with the principles of zero-waste and circular economy.

Autoclaving of alkali-activated materials

Alkali-activated materials are amorphous materials that look like natural stone or a concrete structure. As in the case of concrete, on the surface of alkali-activated materials, a white coating appears in the form of efflorescence. For conventional efflorescent concrete, the reaction between Ca(OH)2 with water and CO2 is generally harmless, except for the discolouration. In the case of alkali-activated materials, efflorescence can be a big problem, especially if they are exposed to moisture or in contact with water. The white efflorescence on the surface of the materials is sodium carbonate heptahydrate Na2CO3·7H2O [1]. Various chemicals to prevent primary and secondary efflorescence can be used to reduce the appearance of the efflorescence. In the case of alkali-activated materials, this is a more complex process. The following paper proposes the use of hydrothermal processing during alkali-activated materials production. Hydrothermal treatment in an autoclave significantly reduced the appearance of efflorescence on the alkali-activated materials surfaces.