Aluminum Silicate Hydrate Research Articles

Hydration process and fluoride solidification mechanism of multi-source solid waste-based phosphogypsum cemented paste backfill under CaO modification

The large-scale, environmentally friendly utilization of phosphogypsum (PG) remains a global challenge. PG cemented paste backfill (PCPB) is a promising method to manage PG, but using ordinary Portland cement as the binder has drawbacks such as high cost, low mechanical strength, and high fluoride leaching risk. This paper presents a multi-source solid waste-based PCPB (MPCPB) material that enhances mechanical properties and reduces fluoride leaching risks. In MPCPB, industrial waste residues like steel slag (SS) and ground granulated blast furnace slag (GBFS) are used as precursors (SS: GBFS = 1:2). Additionally, 4–8 wt% CaO (relative to the dry weight of PG) is used as a neutralizing modifier and alkaline activator. The results indicate that an optimal amount of CaO can neutralize the residual acidity of PG, provide sufficient Ca(OH)2 for MPCPB hydration, and react with PG to produce significant amounts of AFt. Furthermore, CaO promotes the geopolymerization reaction between SS and GBFS, generating more calcium silicate hydrate (C-S-H) and calcium aluminate silicate hydrate (C-A-S-H) gels. Fluoride stabilization in MPCPB results from synergistic effects involving hydration reactions, complexation, ionic mobility, rearrangement, and physical adsorption. Notably, CaO enhances the conversion of free fluoride ions into stable compounds like fluorapatite, fluorite (CaF2), [AlF6]3-, and [FeF6]3- complexes. This approach offers a cost-effective, environmentally friendly, and efficient solution to the PG stockpiling challenge.

The Influence of Particle Size and Calcium Content on Performance Characteristics of Metakaolin- and Fly-Ash-Based Geopolymer Gels.

This research systematically investigates the influence of raw material particle size and calcium content on the geopolymerization process to gain insight into the physical and mechanical properties of geopolymer gels, including setting time, fluidity, pore structure, compressive strength, and leaching characteristics of encapsulated Cr3+ heavy metal ions. Utilizing a diverse range of particle sizes of metakaolin (MK; 3.75, 7.5, and 12 µm) and fly ash (FA; 18, 45, and 75 µm), along with varied calcium levels, this study assesses the dual impact of these factors on the final properties of both metakaolin- and fly-ash-based geopolymers. Employing sophisticated analytical techniques such as Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), and Nuclear Magnetic Resonance (NMR), the research meticulously documents alterations in chemical bonding, micro-morphology, and pore structures. Key findings reveal that reducing the size of MK and FA particles to 3.75 and 18 µm, respectively, enhances the compressive strength of their matrices by 128.37 and 297.58%, respectively, compared to their original values (63.59 and 33.87 MPa, respectively) at larger particle sizes. While smaller particle sizes significantly bolster compressive strength, they adversely affect slurry flow and reduce the leaching rates of Cr3+ from MK- and FA-based matrices, reaching 0.42 and 0.75 mg/L at 3.75 and 18 µm, respectively. Conversely, increased calcium content markedly enhances setting times and contributes to the formation of dense microstructures through the production of calcium aluminate silicate hydrate (C-A-S-H) gels, thus improving the overall curing performance and durability of the materials. These insights underline the importance of fine-tuning particle size and calcium content to optimize geopolymer formulations, offering substantial benefits for varied engineering applications and promoting more sustainable construction practices.

Magnesium sulphate resistance of fly ash one-part geopolymers: Influence of solid alkali activators on physical, mechanical and chemical performance

The commitment to promoting circular economy and durable construction materials has led to the advocacy for one-part geopolymer (OPG), as an eco-friendly substitute for traditional ordinary Portland cement (OPC). Solid alkali activator is the fundamental component of OPG and significantly influences the sulphate resistance. To identify the types of solid alkali activators best suited for the development of sulphate-resistant OPGs, OPGs activated with Na2SiO3 (M-OPG), Na2SiO3+Na2CO3 (MC-OPG), Na2SiO3+NaOH (MH-OPG) and Na2SiO3+NaAlO2 (MA-OPG) were exposed to 5 % and 10 % of MgSO4 solutions for 28 days. The fine pore structures of MH-OPG and MC-OPG mitigated deterioration from the more concentrated MgSO4 solution, resulting in higher residual compressive strengths after exposure to the 10 % MgSO4 compared to the 5 %. Exposure to the 5 % MgSO4 solution resulted in strength deterioration for M-OPG, MH-OPG and MC-OPG due to the discontinuity of the aluminosilicate structure, as evidenced by the diminished Q4(3Al) sites. Conversely, the MA-OPG exhibited a 24.1 % increase in compressive strength, accompanied by the emergence of sodium magnesium aluminium silicate hydrate ((N,M)-A-S-H) gel. Hence, Na2SiO3+NaAlO2 should be acknowledged as the primary candidate for solid alkali activators in the development of sulphate-resistant OPGs.

Improvement of strength and microstructure of low-carbon cementitious materials with high volume of calcined coal gangue

The accumulation of coal gangue poses significant environmental challenges due to its massive volume and low reactivity. This study investigates the influence of calcium hydroxide (Ca(OH)₂) and gypsum on the mechanical properties, hydration, and pore structure of low-carbon cementitious materials incorporating high volumes of calcined coal gangue (CCG) and limestone. The results showed that the addition of Ca(OH)₂ significantly increased compressive strength, with a 19 % improvement at 28 days in LCC-40-P mix, which contained 40 % cement and 2.73 % Ca(OH)₂ by weight. The incorporation of 10 % gypsum in the LCC-40-P mix and 20 % gypsum in the LCC-70 mix (containing 70 % cement without Ca(OH)₂) effectively refined the pore structure, reduced overall porosity, and enhanced the material's density and mechanical properties, resulting in compressive strengths of 38.4 MPa (72 % of the strength of Portland cement) and 50.2 MPa (94 % of the strength of Portland cement) at 28 days, respectively. Microstructure and hydration heat tests showed that the addition of Ca(OH)₂ accelerates the hydration reaction and significantly enhanced compressive strength by ensuring sufficient Ca(OH)₂ for pozzolanic reactions, leading to the formation of additional calcium silicate hydrate, calcium aluminate silicate hydrate and carboaluminate phases. Gypsum refined the pore structure by shifting the pore size distribution towards smaller sizes, reducing overall porosity, and increasing the proportion of gel and small capillary pores. These findings highlight the potential of using CCG with optimized additives to develop sustainable, low-carbon construction materials with enhanced performance characteristics.

Utilization of industrial waste sodium sulfate for calcined clay-based sustainable binder

This study explored the utilization of industrial waste Na2SO4 to develop a calcined clay-based sustainable cementitious composite inspired by ancient Roman concrete. Na2SO4 with or without the addition of NaCl was utilized, keeping the total Na2O quantity constant, and their performances at the macro and micro scale were compared. A separate set of samples was prepared using seawater. A medium-grade calcined clay, along with lime, was utilized as the binder. The salts were first dissolved in water and then used as the mixing water for sample preparation. A diverse array of characterization methods was utilized, encompassing thermogravimetric analysis, chemical extraction, X-ray diffraction (XRD), and Backscattered Scanning electron microscopy (SEM). It was observed that the salt-containing batches achieved compressive strength faster than the seawater samples. However, the strength remained in a comparable range (∼30 MPa) for all the samples after 28 days of curing. The initial strength of salt-containing samples was attributed to the formation of gypsum, ettringite, and hydrocalumite. Additionally, the geopolymer gel and calcium aluminum silicate hydrate (C-A-S-H) gel phases were present in all samples. Finally, this study also showed that lime-clay mortars can achieve a reduction in carbon footprint of up to 50 % compared to ordinary portland cement (OPC) mortars.

Preparation and Immobilization Mechanism of Red Mud/Steel Slag-Based Geopolymers for Solidifying/Stabilizing Pb-Contaminated Soil.

Pb-contaminated soil poses serious hazards to humans and ecosystems and is in urgent need of remediation. However, the extensive use of traditional curing materials such as ordinary Portland cement (OPC) has negatively impacted global ecology and the climate, so there is a need to explore low-carbon and efficient green cementitious materials for the immobilization of Pb-contaminated soils. A red mud/steel slag-based (RM/SS) geopolymer was designed and the potential use of solidifying/stabilizing heavy metal Pb pollution was studied. The Box-Behnken design (BBD) model was used to design the response surface, and the optimal preparation conditions of RM/SS geopolymer (RSGP) were predicted by software of Design-Expert 8.0.6.1. The microstructure and phase composition of RSGP were studied by X-ray diffractometer, Fourier transform infrared spectrometer, scanning electron microscopy and X-ray photoelectron spectroscopy, and the immobilization mechanism of RSGP to Pb was revealed. The results showed that when the liquid-solid ratio is 0.76, the mass fraction of RM is 79.82% and the modulus of alkali activator is 1.21, the maximum unconfined compressive strength (UCS) of the solidified soil sample is 3.42 MPa and the immobilization efficiency of Pb is 71.95%. The main hydration products of RSGP are calcium aluminum silicate hydrate, calcium silicate hydrate and nekoite, which can fill the cracks in the soil, form dense structures and enhance the UCS of the solidified soil. Pb is mainly removed by lattice immobilization, that is, Pb participates in geopolymerization by replacing Na and Ca to form Si-O-Pb or Al-O-Pb. The remaining part of Pb is physically wrapped in geopolymer and forms Pb(OH)2 precipitate in a high-alkali environment.

Influence of Silica Modulus on the Activation of Amorphous Wollastonitic Hydraulic Binders with Different Alumina Content: Study of Hydration Reaction and Paste Performance.

This study investigates how different sodium silicate SiO2/Na2O MS ratios (0.75, 0.9, and 1.2) affect the hydration behavior of amorphous wollastonitic hydraulic (AWH) binders containing various amounts of Al2O3 content (4, 7, 10, and 12%wt). The effects of and interaction between the MS ratio of the activator and the Al2O3 content of the sample on the hydration reaction and paste performance were investigated. The reaction was followed by calorimetry, and the pastes' compressive strength performances were tested at different curing times (2, 7, and 28 days). The hydrated pastes were characterized by FTIR, thermogravimetry analysis, and X-ray diffraction. The calorimetric results show that a higher Al2O3 cContent and a higher MS ratio result in a longer induction period. In terms of paste performance, an increase of the Al2O3 coupled with an activation with a 1.2 MS ratio results in a lower compressive strength after 28 days of hydration; the results range from 76 to 52 MPa. A decrease of the MS ratio to 0.9 allowed the obtention of a narrower range of results, from 76 to 69 MPa. Even though a decrease of the MS ratio to 0.75 led to higher hydration kinetics and high compressive strength results at early ages, at 28 days of curing, a decrease in compressive strength was observed. This may be a consequence of the fast kinetic of the mixture, since the rapid growth of hydration products may inhibit the dissolution at later ages and increase the porosity of the paste. Moreover, the high Al intake in the hydration product, facilitated by the high sodium content of the activator, promotes the formation of a higher number of calcium aluminate silicate hydrate structures (C-A-S-H) to the detriment of calcium silicate hydrate structures (C-S-H), decreasing the compressive strength of the samples. The TGA results indicate that the samples hydrated with the MS075 solution resulted in a higher number of hydrated products at early ages, while the samples hydrated with the MS09 and MS1.2 solutions exhibit a steady increase with curing time. Hence, an equilibrium in the hydration kinetic promoted by Si saturation-undersaturation appears to be fundamental in this system, which is influenced by both the MS ratio and the Al(OH)4- content in solution. The results of this study suggest that for this type of binder, optimal performance can be achieved by decreasing the MS ratio to 0.9. This composition allows for a controlled kinetic and overall higher compressive strength results in pastes produced with this AWH precursor.

Waste valorization: Sustainable geopolymer production using recycled glass and fly ash at ambient temperature

This research explores the reutilization of waste glass powder (GP) and class F fly ash (FA) in the development of an advanced geopolymer, focusing on reducing natural resource consumption to serve as a green alternative for conventional concrete material. Investigating mix ratios of GP-to-FA (0-to-1), activated with 2–12 M NaOH, followed by ambient temperature curing, this study mainly assesses strength (compressive, and flexural) and durability at different ages. Mechanical properties concerning reaction kinetics and Al/Si ratio, reveal that GP initially acts as an inert filler with slow reaction kinetics, while significant strength improvement at later ages is attributed to the reaction between GP and FA. Optimal performance is achieved with a geopolymer mixture featuring a GP-to-FA ratio of 1:3 (Al/Si ratio of 0.51) and an 8 M NaOH solution, resulting in the highest compressive strength of 40 MPa and superior durability. Microstructural analysis (SEM-EDS, and 29Si MAS NMR) identifies the reacting product as calcium/sodium aluminate silicate hydrate (C/N-(A)-S-H) gel within the geopolymer matrix. Leaching tests further underscore potential environmental concerns related to excessive alkali leaching, necessitating meticulous mix design management. Embodied energy and carbon footprint assessments confirm the environmental friendliness of the GP-FA-based geopolymer. Overall, this research demonstrates the feasibility of producing effective geopolymers from dry waste at ambient temperature, emphasizing the potential synergy between waste glass recycling and the concrete industry towards sustainable construction practices.

Effect of Geopolymerization Reaction on the Flexural Strength of Kaolin-Based Systems.

Geopolymers exhibit broad application prospects, including construction and radiation shielding, which require excellent mechanical performances. However, investigations on the nature of geopolymerization reactions and their consequential impact on mechanical performance are still vague. In this study, the effect of the major factors of Si/Al ratio and curing time on the geopolymerization reaction and flexural strength were studied based on the microstructure evolution and chemical bonding formation analyzed using the SEM, FTIR, peak deconvolution, and XRD methods. The microstructure of geopolymers was transferred from initially layered smooth particles of kaolinite to a 3D network porous structure, corresponding to sodalite. A spectrum exclusive to the geopolymer structure occurred at 973 cm-1, corresponding to the sodium aluminum silicate hydrate (N-A-S-H) links, the integral area of which represents the degree of geopolymerization reaction. Furthermore, a controllable reaction degree was achieved by adjusting the Si/Al ratio and curing time, where the maximum reaction degree of 55% was achieved at a Si/Al ratio of 1.94 when cured for 7 d. The correlation between the flexural strength and reaction degree was found to follow a proportional relationship, achieving a flexural strength of 21.11 MPa with a degree of 45%. This study provides insight into the development of mechanical strength through controlling the reaction process.

From marble waste to monocarboaluminate binder: role of sodium aluminate composition, calcined clay type, and curing temperature

This study focuses on the synthesis of a monocarboaluminate (MCA)-based binder by treating marble waste (MW) powder with sodium aluminate (SA). The aim is to investigate the influence of various parameters, including the alumina modulus (M A) of SA, different contents of calcined clays with varying alumina levels, and curing temperatures, on the performance and phase composition of the resulting binder. The results demonstrate that the combination of SA and MW leads to the formation of an MCA-based binder, which exhibits a high compressive strength of up to 28 MPa after seven days of curing. Additional phases such as gibbsite, pirssonite, and gaylussite are observed, indicating the occurrence of a cationic exchange reaction between MW and SA, as confirmed by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). The stability of the resulting binder is primarily influenced by the M A value, with higher values associated with greater stability. Regardless of the type of calcined clay used, incorporating calcined clay results in the formation of a calcium aluminate silicate hydrate (CSAH) phase alongside MCA, contributing to the improved stability of the hardened binder over time. Notably, calcined clay with a higher alumina content exhibits the highest strength at all curing ages. Furthermore, increasing the curing temperature up to 100 °C significantly enhances the early strength, which correlates with the formation of the kotoite phase at the expense of the MCA phase.

Electrolytic manganese residue pretreated by filter press-water washing and carbide slag: Manganese recycling, depth harmless and dispersion

Filter press-water washing and carbide slag (CS) were utilized for recycling manganese, ensuring depth harmless and dispersion of electrolytic manganese residue (EMR). Mn2+ recover profit, pH value, leaching concentration and particle sizes of treated water-washed EMR (TWEMR), as well as the microstructure and composition changes during disposal process was explored. Mn2+ recovery efficiency was 94.11 % with a profit of 10.56 $/(1 t EMR). CS primarily served as a dispersant and disposal agent, with a stepwise addition of CS (40 % + 60 %) and a change in stirring process (slow speed for 5 min and fast speed for 10 min) further enhancing the reaction process. Mn2+ and NH4+ leaching concentrations can be reduced to below the detection limits, while Mg2+ concentration reduced to 1.26 mg/L. Moreover, the dispersion of water-washed EMR (WEMR) was realized. Mn2+ and Mg2+ were solidified/stabilized in forms such as MnOOH, Mn3O4, Mn2O3, MnCO3, Mg(OH)2 and MgCO3. Furthermore, these ions could also be solidified/stabilized through the formation of ettringite, thaumasite, butlerite and calcium (aluminate) silicate hydrate (C-(A)-S-H) during the reaction process. This study provides a method for manganese recycling from EMR, ensuring depth harmless and dispersion of WEMR filter cake.

Phenomenological description of the effect of glass dissolution on cement paste evolution in an integrated glass dissolution experiment

The interaction between nuclear glass surrogate and ordinary Portland cement (OPC) was investigated through integrated static leach tests conducted at ambient temperature. Tests were performed in stainless-steel cells in which OPC hydrated cement pastes were separated from glass powders by a stainless-steel filter. This article gives a phenomenological description of the processes affecting the hydrated cement paste arising from glass dissolution. The results show that the ingress of Si from glass dissolution leads to the degradation of the cement paste over a distance of 100–200 μm within 900 days of interaction. The Hardened Cement Paste (HCP) is affected by two main processes, which are the pozzolanic reaction and the alkali-silica reaction (ASR). These processes lead to porosity changes of HCP in contact with the stainless-steel filter and significant changes in the diffusive properties of the HCP. Especially, the effect of such processes on boron and lithium retention was studied and showed that precipitation of ASR-like gel and low Ca/Si C–S–H (Calcium Silicate Hydrate) highly retain these two elements. In the presence of an Al-rich glass (SM539), ASR is unlikely due to the preferential formation of C-A-S-H (Calcium Aluminium Silicate Hydrate).

Synthesis of Eggshell Powder and GGBS Geopolymer Designed by the Taguchi Method

This research endeavors to develop a geopolymer mortar synthesized from eggshell powder and ground granulated blast-furnace slag, utilizing the Taguchi L25 optimization technique to refine the synthesis process across molarity (A), precursor ratio (B) and curing duration (C). Analyzed at five distinct levels across 25 experimental setups, denoted as A1B1C1 through A5B5C5, these variables were rigorously tested. Compressive-strength measurement was employed as a criterion to assess the performance of three samples from each setup, alongside setting-time evaluation to ascertain the geopolymer concrete's workability. Optimal results were observed with a 60% eggshell to 40% slag ratio (A5B1C5), yielding the highest average compressive strength of 49.5 MPa after a curing period of 56 days. Conversely, a mix comprising 100% eggshell (A2B5C1) manifested the lowest compressive strength of 5 MPa at a 3-day curing period. Taguchi's signal-to-noise ratio analysis pinpointed the precursor ratio as a crucial determinant of compressive strength. Additionally, setting-time investigations revealed A5B1C5 as exhibiting the most advantageous initial and final setting times of 15 minutes and 30 minutes, respectively, compared to the protracted durations of 240 minutes and 540 minutes for A2B5C1. Through comprehensive chemical, micro-structural and mineralogical characterizations via X-ray fluorescence, Scanning Electron Microscopy (SEM) and X-ray diffraction, it was found that an increased proportion of ground granulated blastfurnace slag relative to eggshell enhances surface smoothness and density, indicative of successful polymerization. This was further corroborated by X-ray diffraction, which confirmed the formation of sodium alumina-silicate hydrate, calcium aluminum silicate hydrate and calcium silicate hydrate gels. Consequently, the study substantiates the potential of employing eggshell powder and ground granulated blast-furnace slag in the sustainable fabrication of geopolymers from waste materials. Keywords: Geopolymers, GGBFS, Eggshell, Taguchi Method, Compressive strength

Effect of Iron Ore and Copper Ore Tailings on Engineering Properties and Hydration Products of Sustainable Cement Mortar

Abstract The prohibition of river sand mining has drawn the attention of researchers in finding practicable alternatives. In the approach of finding these alternatives, it is essential to ensure minimal or zero impairment to the ecological balance, which can be mainly attained by making use of industrial waste/byproducts. The wastes from the mining industry are the major contributors in causing impairment to the environment, and their influence on the stability of mortars on using as fine aggregates needs to be systematically investigated with the view of long-term performance concerns. Thus, the present study explores the applicability of mine tailings and finding the optimum dosage in cement mortars by investigating the engineering properties and microstructure development with the aid of qualitative and quantitative analysis associated with hydration products. The studies confirm that the increased consumption of portlandite for secondary hydration reactions followed by the additional formation of calcium silicate hydrate (CSH) and calcium aluminum silicate hydrate (CASH) phases in mine tailing-based mortars helped in achieving a quality microstructure. These additional formations of CSH and CASH phases are also confirmed through Fourier transform infrared spectroscopy by identifying the shift of Si-O-Si stretching vibration bands toward a lower wavenumber. The lowering of calcium/silicate atomic ratio and increased formation of mineralogical compounds related to CSH and CASH in x-ray diffraction patterns also confirms the same. Gismondine, chabazite, and hillebrandite are the additional phases formed and found to take part in refining the pore structure. This enhanced performance of mine tailing mortars was also verified with the aid of a modified Andreasen and Andersen particle packing model. The formation of high-quality microstructure is reflected in the hardened properties of optimized cement mortar in the proportion of 20 % for iron ore tailing and 30 % for copper ore tailing.

Investigating the impact of alkaline activator on the sustainability potential of geopolymer and alternative hybrid materials

Geopolymer binders are considered highly sustainable due to their utilization of industrial wastes. However, previous studies highlight the limited industrial-scale applications of geopolymers. Therefore, this study critically investigates the strength, cost, and environmental impact of geopolymers and alternative hybrid cement mixes to provide a complete picture for the sustainability analysis. Geopolymers and hybrid cement mixes are designed with varying proportions of alkaline activator (sodium hydroxide 5–25 wt%) and ordinary Portland cement (OPC) (15–35 wt%) respectively for the comparative analysis. The strength characteristics of designed mixes have been assessed by analyzing their compressive strength and microstructure. A comparative economic assessment has been made by contemplating the manufacturing and production costs of different materials used in geopolymer and hybrid cement mixes. ReCiPe Midpoint Life cycle assessment (LCA) has been performed to examine the environmental performance by considering the manufacturing and production processes of various binder and aggregate materials used in geopolymer and hybrid cement mixes. Mechanical and microstructural analysis shows that hybrid cement mix with 35 wt% of OPC yields the highest strength due to the formation of excess calcium aluminum silicate hydrate (C-A-S-H) and calcium silicate hydrate (C–S–H) gels than geopolymer mix containing 25 wt% of NaOH. Economic and life cycle assessment reveals that the geopolymer mix with 5 wt% NaOH offers the lowest cost and environmental impact as compared to others containing higher proportions of alkaline activator and OPC. Owing to the conflicting nature of the results, multi-criteria decision making approach has been used which presents the hybrid cement mix (with 35 wt% OPC) as the optimal and sustainable solution for the construction applications. The study's findings offer a sustainable direction for the construction industry by suggesting the application of hybrid concretes.

Swell-shrink and microstructural response of expansive soil treated with nanomaterials

ABSTRACT Expansive soils like Black Cotton Soil, exhibit numerous engineering hurdles due to their high swell-shrink behaviour, resulting in structural instability. Traditional stabilization techniques often prove inadequate and available nanomaterials are expensive, creating a demand for a cost-effective and eco-friendly stabilizer like nano rice husk ash (nRHA). This study tackles the challenge of stabilizing expansive soil using nRHA, synthesized via two distinct ball milling methods. Various concentrations of nRHA (0%, 0.2%, 0.4%, 0.6%, 0.8%, and 1.0%) were tested to assess their effectiveness in reducing the swell-shrink behaviour of Black Cotton Soil.r Result indicate that a 0.4% dosage of 2 + 5 hours nRHA (n 2,0.4) significantly decreases the soil’s swell-shrink potential. This treatment triggers the formation of Calcium Aluminium Oxide (CAO) and Calcium Aluminium Silicate Hydrate (CASH) gels, validated by X-ray diffraction (XRD) and Scanning electron microscope (SEM) analysis. This study advances nRHA treatments in enhancing the swell-shrink behaviour of expansive Soil.

Effect of blast furnace slag substitution for cement in carbon-reduced and low-cost solidified/stabilized cementitious materials

The development of low-carbon cementitious materials involves the selection of the appropriate raw materials and the transformation of the hydration mechanism. In this study, low-carbon and low-cost cementitious materials were prepared using municipal solid waste incineration fly ash (MSWI FA), blast furnace slag (BFS), and desulfurization gypsum (DFG) as raw materials to reduce clinker usage. Results showed that the compressive strength of K5 (mass ratio of BFS: DFG: MSWI FA = 7:1:2) after 360 d of curing was 41.49 MPa, with a low leaching concentration of heavy metal residues that meet groundwater Class II standards, a dioxin content of only 25 ngTEG/kg, and a stable pH value ranging between 11 and 11.5. Microscopic analysis revealed a continuous decrease in the Ca/Si atomic ratios of K4 (mass ratio of BFS: DFG: MSWI FA: P·I 42.5 = 42:10:20:28) and K5, i.e., 1.18–1.54 and 1.04–1.23, respectively, with the increase in the hydration age. The highest Al/Si atomic ratio of K5, i.e., 0.26–0.31, was observed with the strongest conversion trend of calcium–silicate–hydrate gel into calcium–aluminum–silicate–hydrate gel, and the network structure of sodium–(calcium)–aluminum–silicate–hydrate gel zeolite-like phase was generated. The water-to-binder (WTB) mass ratio of 0.35 was determined to be more suitable for the K4 and K5 systems and resulted in a 56.83% and 90.82% reduction in half-life t∂ compared with the WTB ratio of 0.5, respectively. Notably, the value of the reaction velocity constant K in the induction period was 10 times that of K1, and the autocatalytic reaction controlled the value of N to < 1. The X-ray absorption near-edge structure indicated that Zn solidification produced Zn2SiO4 with a small solubility product. The production of 1 t of K5 emitted only 10.83 kg/t of CO2, which was 40 times less than that of K1. Overall, K5 provides the highest economic benefit at 40.08 USD/t, and the clinker-free cementitious system with multisolid waste synergy has significant advantages in terms of solidifying harmful substances, reducing carbon emissions, and lowering costs.

Application of SCMs and seawater to cement-bonded calcareous sand: Macro performance, micro mechanism, and strength prediction

Cement-bonded calcareous sand (C-BCS) demonstrates promising potential for marine island construction due to its exceptional mechanical properties. However, challenges such as freshwater scarcity and the erosive effects of seawater hinder the extensive utilization of C-BCS. To address this, this study comprehensively examines the feasibility of incorporating supplementary cementitious materials (SCMs) and seawater in C-BCS, focusing on macroscopic properties, microscopic mechanisms, and strength prediction. Macroscopic tests reveal that blast furnace slag and silica fume effectively enhance the hydration process and mechanical properties of C-BCS. Moreover, the cemented specimens developed in this study partially meet the strength standards for road pavement sub-base and airport runway upper-base applications. Microscopic analyses indicate that calcareous sand serves as a plentiful source of calcium, facilitating the formation of calcium aluminate silicate hydrate (C-A-S-H) and tobermorite when combined with Al2O3 and reactive SiO2 from slag and silica fume. This enhances the mechanical properties of the samples and their resistance against seawater erosion. Additionally, a back propagation (BP) neural network model accurately predicts the unconfined compressive strength of the cemented specimens. The findings of this study support the broader implementation of SCMs and seawater in practical island engineering projects involving C-BCS.

The Performance and Reaction Mechanism of Untreated Steel Slag Used as a Microexpanding Agent in Fly Ash-Based Geopolymers

Steel slag is an industrial by-product of the steelmaking process, which is under-utilized and of low value due to its characteristics. Alkali-activated technology offers the possibility of high utilization and increased value of steel slag. A geopolymer composition was composed of steel slag, fly ash, and calcium hydroxide. Four experimental groups utilizing steel slag to substitute fly ash are established based on varying replacement levels: 35%, 40%, 45%, and 50% by mass. The final samples were characterized by compressive strength tests, and Fourier-transform infrared spectroscopy measurements, thermogravimetric measurements, scanning electron microscopy with energy dispersive spectroscopy, X-ray diffraction, and mercury intrusion porosimetry were used to investigate the chemical composition and microstructure of the final products. Higher steel slag/fly ash ratios lead to a lower bulk density and lower compressive strength. The compressive strength ranges from 3.7 MPa to 5.6 MPa, and the bulk density ranges from 0.85 g/cm3 to 1.13 g/cm3. Microstructural and energy-dispersive X-ray spectroscopy analyses show that the final geopolymer products were a type of composite consisting of both calcium aluminate silicate hydrate and sodium aluminate silicate hydrate, with the unreacted crystalline phases acting as fillers.

Utilising Phosphogypsum and Biomass Fly Ash By-Products in Alkali-Activated Materials

Significant environmental issues are raised by the phosphogypsum (PG) waste that is being produced. In Lithuania, about 1,500,000 tons of PG waste is generated yearly, and about 300 Mt is generated yearly worldwide. A by-product of burning wood biomass in thermal power plants is biomass fly ash (BFA). By 2035, compared to 2008 levels, industrial biomass incineration for combined heat and power and, as a consequence, BFA, is expected to triple. This study revealed the possibility of using these difficult-to-utilise waste products, such as BFA and PG, in efficient alkali-activated materials (AAM). As the alkaline activator solution (AAS), less alkaline Na2CO3 solution and Na2SiO3 solution were used. The study compared the physical–mechanical properties of BFA-PG specimens mixed with water and the AAS. After 28 days of curing, the compressive strength of the BFA-PG-based, water-mixed samples increased from 3.02 to 6.38 MPa when the PG content was increased from 0 to 30 wt.%. In contrast, the compressive strength of the BFA-PG-based samples with AAS increased from 8.03 to 16.67 MPa when the PG content was increased from 0 to 30 wt.%. According to XRD analysis, gypsum crystallisation increased when the PG content in the BFA-PG-based samples with water increased. The presence of AAS in the BFA-PG-based samples significantly reduced gypsum crystallisation, but increased the crystallisation of the new phases kottenheimite and sodium aluminium silicate hydrate, which, due to the sodium ions’ participation in the reactions, created denser reaction products and improved the mechanical properties. The outcome of this investigation aids in producing sustainable AAM and applying high volume of hardly usable waste materials, such as BFA and PG.