Search Results (21)

Alkali-activated binders (AAB) are a suitable and sustainable alternative to ordinary Portland cement (OPC), with reductions in natural resource usage and environmental emissions in regions where the necessary industrial residues are available. Despite its potential, the lack of mix design methods still limits its applications. This paper proposes a systematic parametric validation for AAB mix design applied to pastes and concretes, valorizing steel slag as precursors. The composed binders are based on coal fly ash (FA) and Basic Oxygen Furnace (BOF) steel slag. These precursors were activated with sodium silicate (Na₂SiO₃) and sodium hydroxide (NaOH) alkaline solutions. A parametric investigation was performed on the mix design parameters, sweeping the (i) alkali content from 6% to 10%, (ii) silica modulus (SiO₂/Na₂O) from 0.75 to 1.75, and (iii) ash-to-slag ratios in the proportions of 75:25 and 50:50, using parametric intervals retrieved from the literature. These variations were analyzed using response surface methodology (RSM) to develop a mechanical model of the compressive strength of the hardened paste. Flowability, yield stress, and setting time were evaluated. Statistical analyses, ANOVA and the Duncan test, validated the model and identified interactions between variables. The concrete formulation design was based on aggregates packing analysis with different paste contents (from 32% up to 38.4%), aiming at self-compacting concrete (SCC) with slump flow class 1 (SF1). The influence of the curing condition was evaluated, varying with ambient and thermal conditions, at 25 °C and 65 °C, respectively, for the initial 24 h. The results showed that lower silica modulus (0.75) achieved the highest compressive strength at 80.1 MPa (28 d) for pastes compressive strength, densifying the composite matrix. The concrete application of the binder achieved SF1 fluidity, with 575 mm spread, 64.1 MPa of compressive strength, and 26.2 GPa of Young’s modulus in thermal cure conditions. These findings demonstrate the potential for developing sustainable high-performance materials based on parametric design of AAB formulations and mix design. Full article

An alternative to conventional methods for mine tailings disposal is stabilization with alkali-activated binders (AABs), developed from agro-industrial waste. Despite increasing interest in this topic, there is still a lack of studies focusing on the stabilization of iron ore tailings (IOTs) using AABs, particularly those that combine the characterization of cementitious gels with an evaluation of leaching behavior. This study assessed the strength, mineralogy, and leaching performance of IOTs stabilized with AABs formulated from rice husk ash (RHA) and hydrated eggshell lime (HEL), using sodium hydroxide as the alkaline activator. Tests included unconfined compressive strength (UCS), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and metal leaching analyses. The IOT–AAB mixture with the highest AAB content and dry unit weight achieved an average UCS of 2.14 MPa after 28 days of curing. UCS increased with AAB content, followed by dry unit weight and curing time, the latter showing a non-linear influence. The formation of C–S–H gel was confirmed after 28 days, while N–A–S–H gel was detected as early as 7 days of curing. The cemented IOT–AAB mixtures showed no metal toxicity and effectively encapsulated barium originating from the RHA. Full article

Black cotton soil (BCS) is unsuitable for construction due to its high plasticity, low shear strength, and significant volume changes upon wetting and drying. The present study investigates the effectiveness of an alkali-activated coconut husk ash (CHA) binder in improving the geotechnical properties of BCS. CHA is derived from coconut husk and serves as a sustainable binder. Microstructural characterization of untreated and CHA-treated BCS was carried out using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Fourier transform infrared spectroscopy (FTIR). The specific surface area (SSA) and porosity were evaluated using nitrogen gas adsorption methods based on the Brunauer–Emmett–Teller (BET) and Langmuir techniques. The Barrett–Joyner–Halenda (BJH) method demonstrated a decrease in mean pore diameter from 6.7 nm to 6.2 nm following CHA treatment. The SSA diminished from 40.94 m²/g to 25.59 m²/g, signifying the development of calcium silicate hydrate (C-S-H) gels that occupied the pore spaces. The formation of pozzolanic reaction products enhanced the microstructural integrity of the treated soil. Unconfined compressive strength (UCS) test results at 24 h and 28 days of curing for CHA-treated soil have been incorporated to analyze the optimum binder content. The UCS values enhanced significantly from 182 kPa to 305 kPa and 1030 kPa, respectively, at 9% binder content after 24 h and 28 days of curing. The microstructural and mechanical strength test analysis results indicated that CHA is a feasible and environmentally sustainable substitute for BCS stabilization. CHA-based AAB will be an eco-friendly alternative to cement and lime, reducing CO₂ emissions and construction costs. Full article

Innovative approaches in the Portland cement industry, aligned with circular economy principles, offer a promising solution to reduce the environmental impacts. These methods can initially target the architectural elements with lower structural demands, such as urban furniture and paving, before being applied to areas with higher cement usage. Alkali-activated binders (AABs) made from secondary resources present a sustainable alternative to Portland cement (PC), promoting resource recovery, conservation, and a low-carbon economy. Incinerator bottom ash (IBA), traditionally landfilled, has shown potential as a precursor for AABs due to its aluminosilicate content. Repurposing IBA for urban furniture and paving transforms it into a valuable secondary resource. Accordingly, this is the first study to utilize IBA as the sole precursor for urban furniture or paving applications. Research, including state-of-the-art studies and proof of concept developed in this work, demonstrates that IBA-based AABs can produce cast concrete suitable for non-structural urban elements, meeting the technical, environmental, and ecotoxicological standards. Using IBA in AAB formulations not only reduces the reliance on primary raw materials but also contributes to significant energy savings in binder production and lowers greenhouse gas (GHG) emissions, resulting in a reduced carbon footprint. Furthermore, producing concrete from local residual resources, such as IBA, facilitates the reintegration of municipal waste into the production cycle at its point of origin, fostering a sustainable approach to urban development and supporting the circular economy. Full article

There is a growing demand for sustainable solutions in civil engineering concerning the carbon footprint of cementitious composites. Alkali-Activated Binders (AAB) are materials with great potential to replace ordinary Portland cement (OPC), with similar strength levels and lower environmental impact. Despite their improved environmental performance, their durability remains a gap in the literature, influenced by aspects of mechanical behavior, physical properties, and microstructure. This paper aims to assess the impact of steel slag aggregates and curing temperature of a proposed AAB based concrete formulation by characterizing fresh state, mechanical behavior, and microstructure. The proposed AAB is composed of fly ash (FA) and basic oxygen furnace (BOF) steel slag (SS) as precursors, sodium silicate and sodium hydroxide solution as activators, in total replacement of OPC, using baosteel slag short flow (BSSF) SS as aggregate in comparison with natural aggregate. The concrete formulation was designed to achieve a high-performance concrete (HPC) and a self-compacting concrete (SCC) behavior. Mechanical characterization encompassed hardened (compressive strength and Young’s modulus), fresh state (J-ring, slump flow, and T50), and durability tests (scanning electronic microscopy, water penetration under pressure, and chloride ion penetration). The compressive strength (64.1 ± 3.6 MPa) achieves the requirements of HPC, while the fresh state results fulfill the SCC requirements as well, with a spread diameter from 550 mm to 650 mm (SF-1 class). However, the flow time ranges from 3.5 s to 13.8 s. There was evidence of high chloride penetrability, affected by the lower electrical resistance inherent to the material. Otherwise, there was a low water penetration under pressure (3.5 cm), which indicates a well-consolidated microstructure with low connected porosity. Therefore, the durability assessment demonstrated a divergence in the results. These results indicate that the current durability tests of cementitious materials are not feasible for AAB, requiring adapted procedures for AAB composite characterization. Full article

Due to the demand for decarbonization of the construction sector, research on alkali-activated binders and material-minimized carbon-reinforced structures has gained momentum in recent years. Most of the research into alkali-activated binders is focused on developing market-ready alternatives, mainly using a trial-and-error approach. In this study, an attempt is made to identify and quantify the factors influencing compressive strength development. Due to their worldwide availability, investigations are being carried out into binders based on calcined clays and natural pozzolans. The goal is to develop a method to produce tailor-made AAB for continuous manufacturing methods to combine carbon reinforcement and alkali-activated materials. For this purpose, an experimental matrix with 20 variation parameters was set up, in which the activator solution and the precursor composition varied. The design of the experiments was used to minimize the number of experiments. It was shown that no single factor is responsible for the development of compressive strength but instead involves several interacting factors. It was possible to find empirical formulas for predicting the compressive strength after 2, 7, and 28 days. Full article

This study presents an investigation of the mix proportion and mechanical properties of one-part alkali-activated geopolymer concrete (GPC). The procedure for determining the mix proportion of one-part alkali-activated GPC, which uses a solid alkali activator in crystal form, is proposed. The proposed procedure was applied to a series of mixed proportions of GPC with different amounts of solid crystalline alkali activator (AA), water (W), and superplasticizer (SP), using the ratio between them to the total amount of binder (B, fly ash, and granulated blast furnace slag) by weight in order to evaluate their effect on the workability and compressive strength of the GPC. The slump, compressive and tensile strength, and elastic modulus of the one-part alkali-activated GPC were tested in various ways. The test results showed that solid crystalline alkali activators, water, and superplasticizers have significant effects on both the workability and compressive strength of GPC. The amount of one-part alkali activator should not exceed 12.0% of the total binder amount by weight (AA/B = 0.12) in order not to lose the workability of GPC. The minimum W–B ratio should be at least 0.43 to ensure the workability of the sample when no superplasticizer is added. An amount of 2.5% can be considered as the upper bound when using superplasticizer-based polysilicate for GPC. In addition, the elastic modulus and various types of tensile strength values of the one-part alkali-activated GPC were evaluated and compared with that predicted from compressive strength using equations given by two common ACI and Eurocode2 codes for ordinary Portland cement (OPC) concrete. Modifications of the equations showing the relationships between splitting tensile strength and compressive strength, as well as between elastic modulus and compressive strength and the development of compressive strength under the time provided by ACI and Eurocode2 for OPC concrete, were also made for one-part alkali-activated GPC. Full article

Expansive soil swells with water and shrinks with water loss, causing serious safety problems for construction projects. This study emphasizes alkali-activated binder (NaOH excited fly ash) stabilized expansive soil. We found that swelling decreased with an increase in the amount of NaOH in alkali-activated binder. It was found that the alkali-activated binder stabilized expansive soils (AABS) had higher shear strength than untreated expansive soils (US), manifested by increased cohesion and friction angle. In AABS, the highest cohesion and the highest shear strength were found when the NaOH mass was 6% of the fly ash mass. The strength of AABS was similar to that of US without curing. AABS had higher strength than US after 7 and 14 days of curing. The unconfined compressive strength increased with extension of curing time. Combined with XRD and SEM analysis, it was shown that the mechanism of AABS was the formation of C–S–H and (C,N)–A–S–H and the change in the internal structure of expansive soil. This investigation can solve both the expansive soil problem and provide new concepts for green development. Full article

This paper aims to raise awareness regarding the obstacles limiting alkali-activated binders’ (AABs) application as a sustainable solution in the construction industry. Such an evaluation is essential in this industry, which has been introducing a wide range of alternatives to cement binders yet achieved limited utilisation. It has been recognised that technical, environmental, and economic performance should be investigated for the broader adoption of alternative construction materials. Based on this approach, a state-of-the-art review was conducted to identify the key factors to consider when developing AABs. It was identified that AABs’ adverse performance compared to conventional cement-based materials mainly depends on the choice of which precursors and alkali activators to employ and the regionalised practices adopted (i.e., transportation, energy sources, and data on raw materials). In light of the available literature, increasing attention to incorporating alternative alkali activators and precursors by utilising agricultural and industrial by-products and/or waste seems to be a viable option for optimising the balance between AABs’ technical, environmental, and economic performance. With regard to improving the circularity practices in this sector, employing construction and demolition waste as raw materials has been acknowledged as a feasible strategy. Full article

Limiting the consumption of nonrenewable resources and minimizing waste production and associated gas emissions are the main priority of the construction sector to achieve a sustainable future. This study investigates the sustainability performance of newly developed binders known as alkali-activated binders (AABs). These AABs work satisfactorily in creating and enhancing the concept of greenhouse construction in accordance with sustainability standards. These novel binders are founded on the notion of utilizing ashes from mining and quarrying wastes as raw materials for hazardous and radioactive waste treatment. The life cycle assessment, which depicts material life from the extraction of raw materials through the destruction stage of the structure, is one of the most essential sustainability factors. A recent use for AAB has been created, such as the use of hybrid cement, which is made by combining AAB with ordinary Portland cement (OPC). These binders are a successful answer to a green building alternative if the techniques used to make them do not have an unacceptable negative impact on the environment, human health, or resource depletion. The Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) software was employed for choosing the optimal materials’ alternative depending on the available criteria. The results revealed that AAB concrete provided a more ecologically friendly alternative than OPC concrete, higher strength for comparable water/binder ratio, and better performance in terms of embodied energy, resistance to freeze–thaw cycles, high temperature resistance, and mass loss due to acid attack and abrasion. Full article

Alkali-activated binders (AABs) offer the opportunity to upcycle a variety of residues into products that can have added value. Although AABs are reported to have a superior high-temperature performance, their thermal behavior is heavily governed by their microstructure. The present study, therefore, evaluates the effect of varying fly ash:slag ratios, activator modulus (Ms), and high temperatures on the microstructure of AAB using X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy coupled with energy-dispersive spectroscopy. Furthermore, the mechanical properties of alkali-activated concrete (AAC) are investigated through compressive, bond, flexural, and split tensile strengths. A life cycle assessment of AAC is performed using the ReCiPe 2016 methodology. The results from microstructural experiments show the formation of new crystalline phases and decomposition of reaction products on high temperature exposure, and they correlate well with the observed mechanical performance. The 28-days compressive strength with slag content is enhanced by 151.8–339.7%. AAC with a fly ash:slag ratio of 70:30 and Ms of 1.4 is proposed as optimal from the obtained results. The results reveal that the biggest impact on climate change comes from transport (45.5–48.2%) and sodium silicate (26.7–35.6%). Full article

The challenges of handling user-hostile alkaline solutions in the conventional alkali-activated binders (AAB) have initiated the development of “just add water” or one-part solid-based AAB systems. This paper aims to present a preliminary investigation on the development of one-part ambient-cured alkali-activated binders produced by synthesising high-calcium fly ash (HCFA) and ground granulated blast furnace slag (GGBS) using sodium metasilicate anhydrous. Three test series were conducted in this study to investigate the effects of GGBS/binder, activator/binder and water/binder ratios on the fresh and hardened properties of the one-part synthesis AAB system. It was found that the SiO₂/Al₂O₃ molar ratio plays an important role in the attainment of compressive strength and limits the amounts of solid activators effective in contributing to the alkali-activation reaction process. The optimum SiO₂/Al₂O₃ molar ratio was found between 3.20 and 3.30. The test results revealed that the optimum proportion between HCFA and GGBS was discovered at a GGBS/binder ratio of 0.50. The optimum activator/binder ratio was between 0.08 and 0.12, and it is recommended that the water/binder ratio should not exceed 0.50. This study demonstrated the potential of the one-part synthesis method in the production of alkali-activated binder for practical structural applications. Full article

Alkali-activated binders (AABs) are developed using a dry mixing method under ambient curing incorporating powder-form reagents/activators and industrial waste-based supplementary cementitious materials (SCMs) as precursors. The effects of binary and ternary combinations/proportions of SCMs, two types of powder-form reagents, fundamental chemical ratios (SiO₂/Al₂O₃, Na₂O/SiO₂, CaO/SiO₂, and Na₂O/Al₂O₃), and incorporation of polyvinyl alcohol (PVA) fibers on fresh state and hardened characteristics of 16 AABs were investigated to assess their performance for finding suitable mix compositions. The mix composed of ternary SCM combination (25% fly-ash class C, 35% fly-ash class F, and 40% ground granulated blast furnace slag) with multi-component reagent combination (calcium hydroxide and sodium metasilicate = 1:2.5) was found to be the most optimum binder considering all properties with a 56 day compressive strength of 54 MPa. The addition of 2% v/v PVA fibers to binder compositions did not significantly impact the compressive strengths. However, it facilitated mitigating shrinkage/expansion strains through micro-confinement in both binary and ternary binders. This research bolsters the feasibility of producing ambient cured powder-based cement-free binders and fiber-reinforced, strain-hardening composites incorporating binary/ternary combinations of SCMs with desired fresh and hardened properties. Full article

The negative environmental impacts associated with the usage of Portland cement (PC) in concrete induced intensive research into finding sustainable alternative concrete mixes to obtain “green concrete”. Since the principal aim of developing such mixes is to reduce the environmental impact, it is imperative to conduct a comprehensive life cycle assessment (LCA). This paper examines three different types of sustainable concrete mixes, viz., alkali-activated concrete (AAC) with natural coarse aggregates, AAC with recycled coarse aggregates (RCA), and bacterial concrete (BC). A detailed environmental impact assessment of AAC with natural coarse aggregates, AAC with RCA, and BC is performed through a cradle-to-gate LCA using openLCA v.1.10.3 and compared versus PC concrete (PCC) of equivalent strength. The results show that transportation and sodium silicate in AAC mixes and PC in BC mixes contribute the most to the environmental impact. The global warming potential (GWP) of PCC is 1.4–2 times higher than other mixes. Bacterial concrete without nutrients had the lowest environmental impact of all the evaluated mixes on all damage categories, both at the midpoint (except GWP) and endpoint assessment levels. AAC and BC mixes are more expensive than PCC by 98.8–159.1% and 21.8–54.3%, respectively. Full article