Geopolymerization Reaction Research Articles

Mechanical, Microstructural, and Durability Properties of Soil Stabilized with Alkali-Activated Jarofix for Road Applications

In this study, we investigated the feasibility of jarofix [a zinc (Zn) smelter by-product] as a potential geopolymer binder in soil stabilization for road applications. Different combinations of soil–jarofix (SJ) mixtures were prepared using sodium hydroxide (NaOH), sodium silicate (Na2SiO3), and a combination of NaOH + Na2SiO3 as alkali activators. Unconfined compressive strength (UCS) tests were performed to examine the mechanical performance of SJ mixtures, and the effects of the jarofix content, curing conditions, and curing time were investigated. Changes that occurred during the geopolymerization process were further corroborated using field-emission scanning electron microscope (FESEM) images and energy-dispersive X-ray spectroscopy (EDS) analysis. Durability tests were also performed to examine the weathering resistance of selected SJ mixtures against 12 cycles of alternating wetting and drying. Based on the experimental results, the compressive strength of bare soil was increased 8.8 times, from 0.31 to 2.75 MPa, under ambient curing (AC, at 27 ± 2°C) conditions and 6 times, from 1.1 to 6.55 MPa, under dry curing (DC, at 60°C) conditions. This increase in compressive strength was attributed to the formation of sodium alumino-silicate hydrate (N–A–S–H) gel structures during the geopolymerization process, which led to a compact soil matrix, as confirmed by the FESEM images. The specimens cured under DC conditions showed greater strength improvement than those cured under AC conditions owing to the faster rate of the geopolymerization reaction at elevated temperatures. In addition, the Na2SiO3 and NaOH + Na2SiO3 solutions were found to be the most efficient alkali activators for the SJ mixtures cured under AC and DC conditions, respectively. This durability study revealed that the alkali-activated SJ mixtures exhibit a significantly smaller loss in mass than bare soil when exposed to 12 cycles of alternating wetting and drying. Moreover, the specimens cured under DC conditions were found to be more resistant to weathering than those cured under AC conditions. Overall, soil amended with 15% jarofix (through alkali activation) satisfies the minimum strength and durability criteria recommended by Indian standards for its use as a subbase and subgrade material. The current study shows that jarofix has the potential to be used as a geopolymer binder for soil stabilization, which can help the mining sector in minimizing the volume to be stored in tailings storage facilities.

Strength Investigation of Slag-Based Geopolymer Composites Incorporating Different Amounts of Colemanite Waste and Silica Fume Under Different Exposure Conditions

In this study, it is aimed to investigate the strength performance of slag-based geopolymer mortar with different percentages of silica fume and colemanite waste by mixing Na₂SiO₃ and NaOH as the alkaline activator for the geopolymerization reaction, and cured at room temperature were prepared, in terms of compressive strength, flexural strength, ultrasonic pulse velocity and freeze-thaw resistance parameters. Five different mixtures were prepared by using different amounts of silica fume and colemanite waste by using the same amount of ground granulated blast furnace slag, sand and 8M sodium hydroxide for these five mixtures. The mixture, including a paste proportion of 20% slag, 40% colemanite waste, and 40% silica fume, was used as a control mix. The maximum compressive strength (21.24 MPa, 38.32 MPa) flexural strength (5.86 MPa, 6.98), weight loss caused by freeze-thaw effect (0.56%) and ultrasonic pulse wave test (3082 m/s) results were noted as for 7th and 28th day, respectively. After -60 cycles [1 cycle consists of (-18 oC) for 90 minutes and (+4 oC) for 30 minutes], the maximum compressive and flexural strength was observed as (40.18 MPa and 4.92 MPa, respectively). The results indicated that the strength results were consistently increased as silica fume increased. The addition of a certain amount of silica fume gave promising results both in terms of the strength and durability aspects. Overall, according to this experimental study, the utilization of 30% colemanite waste and 50% silica fume can be recommended so as to balance both sustainability and engineering aspects.

Effect of Curing Temperature on Mechanical Properties of Sanitary Ware Porcelain based Geopolymer Mortar

The objective of this study was to investigate the effect of curing temperature on the mechanical properties of sanitary ware porcelain powder-based geopolymer paste and mortar under various curing temperatures. The setting time, porosity, water absorption, and compressive strength of specimens mixed with alkaline concentrations of 8M, 10M, 12M, and 14M were compared. All mortar cube (50×50×50 mm) specimens were placed into drying ovens for 24 hours at 60°C, 75°C, 90°C, and 105°C, respectively. The specimens were then air-cured for 1, 3, 7, 14, and 28 days. The results showed that the elevated curing temperature accelerated the polymerization process of the porcelain geopolymerization reaction. The setting time varied between 89 mins and 380 mins. It showed variability depending on alkaline concentration and initial curing temperature. The setting time of pastes decreased when alkaline concentrations increased. An increasing temperature in the drying oven decreased the initial and final setting times. Similar to this, the rate of water absorption and permeability of porcelain-based geopolymer mortar specimens decreased with drying oven temperatures and increments in alkaline concentration. The lowest water absorption and porosity of the specimen were 2.1% and 15.7%, respectively. The compressive strength increased as drying oven temperatures and alkaline concentrations increased. The highest 28 day compressive strength was found in 14M specimens with 105°C curing temperatures. The ultimate compressive strength was 64.45 N/mm2. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were investigated to study the microstructural properties of the geopolymers. Doi: 10.28991/CEJ-2023-09-08-01 Full Text: PDF

Metallurgical slag wastes into pervious geopolymer concrete stabilized with CO2 capture

The sustainable development agenda in the construction industry requires the development of green materials. In contrast to ordinary Portland cement (OPC), geopolymer concrete (GPC) is considered an eco-friendly construction material since it consumes less energy for production and uses industrial by-products as raw materials. Pervious concrete, a special type of concrete made of binder and coarse aggregate with a small content of fine aggregates, is also considered environmentally friendly because it protects trees on an impervious surface by providing air and water touching the roots of trees. The previous concrete (PC) can also be made of alternative aggregates such as recycled concrete aggregate and steelmaking by-products. Therefore, this research developed pervious geopolymer concrete (PGC) using ground granulate blast furnace slag (GGBFS) as a binder and gap-graded basic oxygen furnace (BOF) slag aggregate of coarse and medium size with the addition of fine aggregate under the controlled water-to-binder and AAS/binder ratio. The properties of PGC were evaluated in terms of compressive strength, splitting tensile strength, and permeability. Mix design and testing variables include an aggregate combination of BOF slag aggregate and granite and CO2 curing time. Test results indicate that the obtained PGC mixtures tend to have lower tensile and compressive strength than OPC. However, the water permeability of the mixtures is high. An increase in the CO2 curing time of PGC also tends to decrease the strength and increase the permeability of the PGC. Further research that carefully considers the chemistry of geopolymerization reaction is required to obtain the PGC with desired properties such as the increase in both tensile and compressive strengths.

Recycling of waste glass in lightweight geopolymer using incineration bottom ash as a foaming agent: Towards energy conservation

A large amount of waste glass and municipal solid waste incineration bottom ash (IBA) generated in municipalities is a serious environmental issue. For energy conservation purpose, this study aimed to develop a novel thermal insulating lightweight self-foaming geopolymer (SFG) material using waste glass as the main precursor and IBA as a foaming agent. The microstructure was investigated by XRD, FTIR, NMR, MIP and SEM-BSE techniques to reveal the mechanisms of the foaming process and geopolymerization reaction. The results showed that the use of 100% waste glass powder (WGP) as the precursor led to a connecting-cracked matrix due to silica gel shrinkage. However, the cracking was significantly alleviated when 50% IBA was incorporated as IBA contributed to more stable C-(N)-A-S-H (calcium silicate hydrate substituted with Al and Na) production as well as spherical pores formation to mitigate crack propagation. The SFG prepared with 50% IBA exhibited a geopolymer matrix with more regular and independent pores than that prepared with 100% IBA. This was because the high metallic Al content in IBA generated an excessive amount of hydrogen gas, resulting in the development of irregular and interconnected pores. Given the stability of the reaction products with fewer cracks and a uniform pore structure, the SFG prepared with 50% IBA and 50% WGP exhibited satisfactory functional properties (low density and thermal conductivity as well as desirable compressive strength), indicating a great potential for energy saving and environmental protection.

A Novel Approach for the Synthesis of Eco-friendly Geopolymer Ternary Blended Mortar with GGBS, Sugarcane Bagasse Ash, and Sewage Sludge Ash under Ambient Curing Conditions

The Present work emphasizes preparing ternary blended geopolymer mortar by partial replacement of Ground Granulated Blast Furnace Slag (GGBS) with a precursor such as Sugarcane Bagasse Ash (SCBA) and Sewage Sludge Ash (SSA) with two different sodium silicate to sodium hydroxide (SS/SH) ratios to prepare an alkaline activating solution. The replacement of GGBS with SCBA and SSA varied from 0–30% by weight. Binary and ternary mortar mixes were tested for compressive strength and their morphological characters, it was found that 10% of SCBA and 20% of SSA incorporation in the binary mix resulted in a drop in strength which is less significant than other replacements for SS/SH ratio of 2.5. The ternary mix consisting of GGBS, SCBA, and SSA in the ratio of 70:10:20 respectively produced a compressive strength that is comparable with the control mortar having 100% GGBS after 28 days of ambient curing. X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FE-SEM), and Fourier-transform infrared spectroscopy (FTIR) studies showed the presence of C-A-S-H gel and participation of precursor in the geopolymer reaction which resulted in enhancement of strength. The Toxicity Characteristic Leaching Procedure (TCLP) results for SSA-incorporated mortar proved the leachability of heavy metals is within the limits. The application of geopolymer mortar proved to be vital in the sustainable production of building materials such as pre-cast elements, masonry units, etc.

Physico-mechanical properties of metakaolin and diatomite-based geopolymers

Geopolymers (inorganic polymers) are sustainable materials obtained from the dissolution of an aluminosilicate precursor by a high-alkaline liquid medium. Many studies point out that the properties of such materials depend on essential parameters, such as precursor, activating solution, SiO2 source, curing temperature, and others. This work evaluated the combination of two aluminosilicates (metakaolin and calcined diatomite) with Na-based alkaline liquid reagents (containing sodium silicate or colloidal silica) and how they affected the physico-mechanical properties of the prepared geopolymers after curing at 40 °C for 24 h. The following tests were carried out to characterize the compositions: in situ Young’s modulus evolution as a function of time; cold crushing strength; apparent porosity; and structural analyses via XRD and FTIR measurements. The synthesized geopolymers showed amorphous inorganic structure containing residual crystalline phases (quartz and illite), which are in tune with the Si-O-Si and Si-O-Al bonds identified in the ATR-FTIR spectra. The samples presented cold crushing strength and Young’s modulus up to 39.4 MPa and 13.4 GPa, respectively. The geopolymerization reactions were favored when using colloidal silica suspension, which led to the development of silicate-free geopolymers with enhanced performance at low temperature.

Investigation on physical behavior, impermeability and micromechanism of engineering geopolymer composites modified with MWCNTs

As a green cementing material, geopolymer solves the problems of environment pollution and building resource waste. The working performance, including setting time and flowability, were firstly evaluated to select a basic mix proportion of geopolymer. Then, the study was aimed at assessing the effect of PVA fibers (1.50 %, 2.00 % and 2.50 % by the volume of sample) and MWCNTs contents (0.05 %, 0.10 %, 0.15 % and 0.20 % by the mass of blinders) on the impermeability and mechanical strength of engineering geopolymer composite(EGC). Meanwhile, the pore structure, polymerization products and microscopic morphologies of geopolymers incorporated with different MWCNTs contents were researched by micro-structure analysis. The experimental results show the 0.1 % functionalized MWCNTs effectively improve the compressive strength, bending strength and impermeability of modified geopolymer by 9.4 %, 23.6 % and 63.2 %, respectively, and the corresponding relative permeability coefficient of modified geopolymer is decreased by 63.24 % compared to that of control geopolymer. Importantly, the incorporation of the functionalized MWCNTs can refine the pore structure and enhance whole compactness of the internal structure, meanwhile limit the extension and expansion of micro-cracks owning to its nano-nucleation, nano-filling and nano-laying effect. Furthermore, the introduction of functionalized MWCNTs improves the geopolymerization reaction of Si-O-Si bands by means of FT-IR analysis. The MIP analysis show that the addition of functionalized MWCNTs can increase the proportion of harmless pores but decrease the quantity of harmless pores. Significantly, the effective porosity(rather than total porosity) should be adopted to evaluate the impermeability of geopolymer because of the appearance of ink bottle pores.

Study on the hardening mechanism of Bayer red mud-based geopolymer engineered cementitious composites

The Bayer red mud-based geopolymer is an economical and environment-friendly substitute for cement because of its superior performance. In this paper, low field nuclear magnetic resonance, rheometer, isothermal calorimeter and other means were used to explore the geopolymerisation mechanism of Bayer red mud-based geopolymer engineered cementitious composites, and establish the geopolymerisation kinetics model under different preparation parameters. The results showed that the compressive strength of Bayer red mud-based geopolymer engineered cementitious composites increased when the modulus of the activator lower than 1.5 M, and then decreased, and the compressive strength decreases with the increase of red mud dosage. When the modulus of the activator is 1.5 M and the content of red mud is 50%, the compressive strength of the 3d reached 11.24 MPa. The variation curves of T2 relaxation time and viscosity with hydration time can be obtained that the geopolymerisation process of Bayer red mud-based geopolymer engineered cementitious composites is divided into three stages. When the modulus of activator is 1.5 M, the effect of promoting geopolymerisation reaction is most obvious. From the geopolymerization kinetics model, it can be seen that the reaction rate constants are always K1 > K2 > K3 with changes in the modulus of the activator and the red mud dosage, indicating that NG product crystallization nucleation and growth process is dominant in the geopolymerization process. The change curve of dα/dt shows that when the modulus of the activator is 1.5 M, the excitation effect is best. The research results promoted the application of Bayer red mud-based geopolymer engineered cementitious composites, and improved the resource utilization efficiency of red mud and other solid wastes.

Alkaline Activation of Ferrous Foundry Sand: A Look at the Effect of Activator Concentration, Liquid-Solid Ratio and Curing Temperature

In this study, ferrous foundry slag was used as a precursor for the synthesis of a geopolymer. The effect of NaOH concentration, liquid solid ratio (L/S), and curing temperature on the unconfined compressive strength (UCS) of the synthesized geopolymer was investigated. The optimum concentration was found to be 15 M as it yielded a geopolymer brick with the highest UCS of 8.2 MPa. It was observed that as the concentration of NaOH increases the dissolution of alumina, calcium and silica increases resulting in higher UCS. The SEM results showed zeolite formation in large quantities at 15 M which shows that the geopolymerization reaction was near completion and the calcium, alumina and silicates dissolved in large quantities leading to increased gelling of C-S-H and C-A-H over time filling more pores inside the geopolymer brick. The liquid to the solid ratio that gave the highest UCS of 8.2 MPa was 0.15. The optimum temperature was found to be 80 °C. This increased temperature favoured the dissolution of reactive species and contributed to higher UCS. The results reveal that ferrous foundry sand geopolymers have the potential to be used as a building and construction material.

Development of Geopolymer Mortars Using Air-Cooled Blast Furnace Slag and Biomass Bottom Ashes as Fine Aggregates

The aim of this study is to compare the mechanical and physical properties of different geopolymer mortars made with granulated blast furnace slag as a geopolymer source material, NaOH (8 M) as the activating solution, and three different types of fine aggregates (air-cooled blast furnace slag, biomass bottom ashes, and silica sand). The samples were made with an aggregate/geopolymer ratio of 3/1, and physical (density and mercury intrusion porosimetry), mechanical (compressive and flexural strength), and acid attack resistance were determined. When air-cooled blast furnace slag is used, the mechanical and acid attack properties are improved compared with silica sand and biomass bottom ashes because of the existence of amorphous phases in this slag, which increase the geopolymer reaction rate despite the particle size being higher than other aggregates. It can be highlighted that the use of ACBFS as a fine aggregate in geopolymer mortars produces better properties than in cement Portland mortar.

Preparation and Properties of Phosphoric Acid-Based Porous Geopolymer with High Magnesium Nickel Slag and Fly Ash

Phosphoric acid-based porous geopolymers were prepared by two different foaming agents (H2O2 and Al powder) with phosphoric acid as the activator. High-magnesium nickel slag (HMNS) and fly ash (FA) were the precursor combination. The effects of foaming agent types and contents on the properties of HMNS-FA-phosphate-based porous geopolymers were investigated in terms of dry density, pore structure, compressive strength, thermal conductivity, and water absorption. The phase was analyzed by x-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). It was found that both foaming agents could successfully prepare porous geopolymers, and the compressive strength and dry density of porous geopolymers gradually decreased and the low-thermal conductivity and water absorption gradually increased with the increase in foaming agent content. The foaming agents formed porous structures inside porous geopolymers but did not affect the phases of geopolymerization reactions. This study demonstrates that both foaming agents can be used to prepare HMNS-FA-phosphate-based porous geopolymers for the application of phosphate-activated geopolymers in the direction of refractory materials.

Analysis of reaction degree and factors of alkali-activated fly ash/slag

The dissolution rates of alkali-activated fly ash (FA) and alkali-activated slag in hydrochloric acid (HCl) with various pH values were investigated. The geopolymerisation reaction degree of alkali-activated FA/slag (AAFS) can be determined based on the content of hydrochloric-acid-insoluble matter, so X-ray fluorescence, X-ray diffraction and Fourier-transformed infrared spectroscopy were used to analyse the hydrochloric-acid-insoluble matter of the alkali-activated FA and alkali-activated slag. Acid dissolution tests on the AAFS showed that the addition of slag increased the geopolymerisation degree of FA in most cases. For a certain slag content, the reaction degree of FA decreased with an increase in the modulus of the activator and increased with an increase in the liquid/solid ratio. An increase in activator concentration had a greater effect when the slag content was low. At low activator concentrations, the reaction degree of FA increased with an increase in slag content. The slag content corresponding to the highest reaction degree of the FA gradually reduced with an increase in activator concentration.

Development of Heavy-Weight Hematite-Based Geopolymers for Oil and Gas Well Cementing.

In the petroleum industry, ordinary Portland cement (OPC) is utilized for different cementing applications. Yet, there are some technical and environmental issues for the usage of OPC in well cementing. The technical problems include gas invasion while setting, instability at corrosive environments, cement failure while perforation and fracturing due to high stiffness and brittleness, and strength reduction and thermal instability at elevated temperatures. Moreover, OPC production consumes massive energy and generates high greenhouse gas emissions. This study introduced the first hematite-based class F fly ash geopolymer formulation that can be used in oil and gas well cementing. Different properties of the designed slurry and hardened samples such as rheology, thickening time, strength, and elastic and petrophysical properties were evaluated. Moreover, mixability and pumpability challenges of heavy-weight geopolymer slurries were investigated. Unlike most of the studies in the literature, this work used 4 M NaOH solution only as an activator that can reduce the overall cost. The results showed that increasing the hematite percentage significantly decreased the thickening time. The developed formulation fell within the recommended fluid loss ranges for some cementing applications without using a fluid loss control additive. A proposed mixture of retarder and superplasticizer was introduced to enhance the thickening time by almost 5 times. The compressive strength increased by 49% and the tensile strength was enhanced by 27.4% by increasing the curing time from 1 to 7 days. The improvement in both compressive and tensile strength with curing time indicated that the geopolymerization reaction continued for extended time but with a smaller rate. The developed slurry acted more like a power law fluid at low temperatures and more like a Bingham plastic fluid at high temperatures. The elastic properties of the developed geopolymer samples proved that they are more flexible than some cement systems.

Understanding the changes in engineering behaviors and microstructure of FA-GBFS based geopolymer paste with addition of silica fume

Geopolymers are attracting attention as a new green cementitious material. However, the severe early drying shrinkage deformation of geopolymers limits the use of geopolymer materials in construction projects. Therefore, this study investigates the effect of silica fume (SF) on the drying shrinkage behavior of geopolymer composite. Fly ash (FA) and granulated blast furnace slag (GBFS) were activated by a combination of sodium hydroxide and water glass solutions to produce geopolymer paste. The modification mechanism of SF content on the drying shrinkage and mechanical properties of FA-GBFS geopolymer paste was studied, as well as the important effect of SF on the internal geopolymerization process of geopolymer composite. The results indicate that the SF changes the binding energy and relative content of Si–O-T (T: Si or Al) structure and tetrahedral coordinated Al–OH bond in FA-GBFS geopolymer composite, delaying the early geopolymerization and calcium silicate hydration reaction in the geopolymer. It diminishes the polymerization degree of gel structure in the geopolymer, which also lowers the early strength of the geopolymer paste. In addition, when the SF content reaches 20%, it not only improves the 28d compressive strength of FA-GBFS geopolymer, but also greatly reduces the relative content of harmful pores inside the geopolymer, which reduces the early drying shrinkage strain by about 50%.

Estimation of strength, rheological parameters, and impact of raw constituents of alkali-activated mortar using machine learning and SHapely Additive exPlanations (SHAP)

One-part alkali-activated material (AAM) is a new eco-friendly developed low-carbon binder that utilizes alkaline activators in solid form. This study deals with the experimental synthesis of one-part alkali-activated mortar (AAM) based on the partial replacement of fly ash (FA) with hydraulic lime (LM) as a precursor, and machine learning-based gene expression modeling (GEP) modeling for the optimization of the developed AAM. The datasets were established by the experimental work performed during this current study. The chosen input parameters were fly ash, hydraulic lime, sodium silicate, sodium hydroxide, sand/binder, water/binder, curing age, and time after mixing. The experimental results showed greater compressive strength and rheological parameters for the specimens having a high quantity of hydraulic lime. The GEP model has shown a strong generalization capability and prediction capacity for the future estimation of compressive strength, plastic viscosity, and yield strength. All the models showed a strong correlation of 0.92, 0.89, and 0.96 for compressive strength, plastic viscosity, and yield stress respectively. SHapely Additive exPlanations (SHAP) were employed to explore the effect of each input parameter of AAM on the predicted outcomes. The results revealed a strong interaction and positive effect of LM on the YS and PV while a negative impact was observed for the compressive strength. While fly ash has shown a negative impact on three outcomes of PV, YS, and CS respectively. The addition of LM and SS-activator leads to earlier structural build-up due to the flocculation of particles caused by the faster geopolymerization reactions.

Development of Lightweight Geopolymer Composites by Combining Various CDW Streams

This study regards the development of lightweight geopolymer composites through the valorization of various construction and demolition wastes. Brick waste was utilized as the sole aluminosilicate precursor for the geopolymerization reactions, expanded polystyrene and polyurethane wastes were used as artificial lightweight aggregates, and short polyethylene fibers developed from CDWs reinforced the geopolymer matrix. The curing conditions of the geopolymer synthesis were optimized to deliver a robust geopolymer matrix (T = 25–80 °C, t = 24–72 h). Both raw materials and products were appropriately characterized with XRD and SEM, while the mechanical performance was tested through compressive strength, flexural strength, Poisson’s ratio and Young’s modulus measurements. Then, a comprehensive durability investigation was performed (sorptivity, wet/dry cycles, freeze/thaw cycles, and exposure to real weather conditions). In contrast to polyurethane waste, the introduction of expanded polystyrene (0.5–3.0% wt.) effectively reduced the final density of the products (from 2.1 to 1.0 g/cm3) by keeping sufficient compressive strength (6.5–22.8 MPa). The PE fibers could enhance the bending behavior of lightweight geopolymers by 24%; however, a geopolymer matrix–fiber debonding was clearly visible through SEM analysis. Finally, the durability performance of CDW-based geopolymers was significantly improved after the incorporation of expanded polystyrene aggregates and polyethylene fibers mainly concerning freeze/thaw testing. The composite containing 1.5% wt. expanded polystyrene and 2.0% v/v PE fibers held the best combination of properties: Compr. Str. 13.1 MPa, Flex. Str. 3.2 MPa, density 1.4 g/cm3, Young’s modulus 1.3 GPa, and sorptivity 0.179 mm/min0.5.

Red mud-slag blends as a sustainable road construction material

Red mud is a highly alkaline solid waste that contains significant amounts of heavy metal ions. This causes serious environmental issues if disposed of unscientifically. Thus, developing an economical and eco-friendly technique for utilization of red mud can solve the disposal problem efficiently. The present study deals with the evaluation of physical, mechanical and chemical properties of red mud treated with ground granulated blast furnace slag to assess its suitability as a construction material for highways. To accelerate the geopolymerization process, additional doses of alkali activator is used. Mineralogical and microstructural analyses have been made to identify the geopolymerization products. An optimization technique is adopted to find out the optimum slag and alkali dose to have the highest strength. The durability of stabilized red mud has been assessed under slaking fluid. Leachate analysis has been carried out on source materials and stabilized materials to examine the levels of major and trace elements. It is observed that the alkali present in red mud can initiate geopolymerization reaction upon the addition of slag. However, additional alkali is needed to accelerate the geopolymerization reaction. Stabilized red mud performed good strength and stability. The alkalinity is reduced after stabilization and the heavy metals are encapsulated by geopolymeric reaction products. Target strengths for different constructions can be achieved with appropriate combinations of slag and alkali.

Cleaner production of geopolymer materials: A critical review of waste-derived activators

Geopolymer materials as a sustainable building material have received ever-growing interest from researchers worldwide. Geopolymer binders have high early strength, improved economic benefits, the ability to withstand aggressive environments for long periods, and facilitate utilization of agricultural, industrial, and municipal wastes. However, the commercially synthesized NaOH and Na2SiO3 predominantly contribute to greenhouse gas (GHG) emissions and the cost of geopolymer materials. In order to promote the industrial acceptance of geopolymers, clean and sustainable alkaline activators are essential. Additionally, these alkaline activators are toxic and caustic in nature which can cause severe health issues. Researchers have been investigating the viability of various wastes as precursors for developing geopolymers since over a decade. Therefore, there is need for identifying and assessing waste derived activators for production of sustainable geopolymers. The purpose of this paper is to provide a brief discussion about the common precursors, geopolymer reaction mechanism, and environmental impacts of conventional activators and a critical review of waste-derived alkaline activators for the development of a robust and user-friendly geopolymer system. This literature review provides a comprehensive insight into the potential applications of waste-derived alkaline activators by comparing their effects on compressive strength and setting time, enabling an appropriate selection of activators based on availability and desired performance. This study highlights waste-derived activators that exhibit similar mechanical properties, if not better than geopolymers produced with conventional activators, which encourages these wastes for structural applications.It was found that the utilization of waste-derived activators, for example, biomass ashes and waste glass can significantly reduce the environmental impacts of geopolymer materials. However, further studies on feasibility and life cycle analysis are required to validate their sustainability potential on industrial scale, succeeded by assessment of long term durability.

Valorisation of Water Potabilization Sludges as Precursors for Alkali-Activated Binders: Characterization and Feasibility Study.

Water potabilization sludges (WPS) are a heterogeneous waste generated from the coagulation-flocculation process of drinking water production, whose composition is highly dependent on the geological context of reservoirs, the composition and volume of treated water, and the types of coagulants used. For this reason, any feasible approach for reusing and valorising of such waste cannot be disregarded from the detailed investigation of its chemical and physical characteristics and they have to be evaluated at a local scale. In this study, WPS samples from two plants serving the Apulian territory (Southern Italy) were subjected for the first time to a detailed characterization with a view to evaluating their recovery and reuse at a local scale as a raw material for producing alkali activated binders. WPS samples were investigated by X-ray fluorescence (XRF), X-ray powder diffraction (XRPD) including phase quantification by the combined Rietveld and reference intensity ratio (RIR) methods, thermogravimetric and differential thermal analysis (TG-DTA), Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX). Samples showed aluminium-silicate compositions with up to 37 wt% of Al2O3 and up to 28 wt% of SiO2. Small amounts of CaO were also found (6.8 and 4 wt%, respectively). The mineralogical investigation indicates the presence of illite and kaolinite as clayey crystalline phases (up to 18 wt% and 4 wt%, respectively), in addition to quartz (up to 4 wt%) and calcite (up to 6wt%) and a large amorphous fraction (63 wt% and 76 wt%, respectively). WPS were subjected to heating from 400 °C to 900 °C and mechanical treatment by high energy vibro-milling in order to determine the best pre-treatment condition in view of their use as solid precursors to prepare alkali-activated binders. Alkali activation (8M NaOH solution; room temperature curing) was attempted on untreated WPS, on 700 °C heated and on 10-minute high-energy milled samples, which were considered the most suitable based on the preliminary characterization. Investigations of alkali-activated binders confirmed the geopolymerisation reaction occurrence. Variations in gel features and compositions depended on the amount of reactive SiO2, Al2O3 and CaO available in the precursors. WPS heated at 700 °C led to the most dense and homogeneous microstructures, due to a greater availability of reactive phases. The results of this preliminary study demonstrate the technical feasibility of preparing alternative binders from the investigated Apulian WPS, paving the way for a local reuse of these waste products, leading to economic and environmental benefits.