Search Results (805)

Geopolymer concrete contains inorganic aluminosilicate polymer gels formed through the activation of industrial solid wastes. This study investigated the effects of fly ash (FA) and silica fume (SF) on the durability and microstructure of geopolymer concrete exposed to freeze–thaw cycles and single-side salt erosion. Five mixtures were prepared using Baioheng geopolymer cement, with FA replacement levels of 15% and 25% and SF replacement levels of 3% and 5%. Mechanical tests, freeze–thaw tests, single-side salt-freezing tests, SEM-EDS, XRD, and CT analysis were conducted to evaluate the relationship between macroscopic performance and inorganic polymer gel structure. The results showed that 25% FA reduced compressive strength and freeze–thaw resistance, mainly due to insufficient reaction products and increased defect connectivity. In contrast, 3% SF improved the 56 d compressive strength by 13.24%, maintained the relative dynamic elastic modulus at 86.64% after 100 freeze–thaw cycles, and limited the mass loss to 0.72%. SEM-EDS and XRD results indicated that appropriate SF addition increased the Si/Al ratio and promoted the formation of C-(A)-S-H/N-A-S-H-related gel products, leading to a denser inorganic polymer matrix. However, excessive SF weakened the improvement effect, possibly due to local heterogeneity and dispersion difficulty. These results indicate that controlling the composition and spatial distribution of inorganic aluminosilicate polymer gels is essential for improving the salt-frost durability of geopolymer concrete. Full article

During the continuous casting of high-titanium steel, traditional fluorine-containing mold fluxes are prone to causing fluoride contamination, equipment corrosion, and intensified slag–metal interface reactions. There is an urgent need to develop highly adaptable fluorine-free mold flux systems. In this study, titanium-containing blast furnace slag was used as the primary base material, while borax, soda ash, and witherite were selected as fluoride-substituting mineral raw materials. The effects of these mineral raw materials on the melting properties, crystallization behavior, crystalline phases, and microstructure of fluorine-free mold fluxes were systematically investigated, and an optimized mold flux design suitable for continuous casting of high-titanium steel was further developed. The results indicate that borax significantly reduces the melting temperature and viscosity and markedly suppresses the growth of crystalline phases such as calcium borosilicate, nepheline, and perovskite by weakening the polymerization degree of the silicate network, thereby substantially decreasing the crystallization ability of the mold flux. Soda ash primarily acts as a strong fluxing and network-depolymerizing agent, promoting the formation of low-polymerized structural units. It also enhances the tendency toward ordered atomic arrangement, thereby markedly increasing nepheline precipitation and the overall crystallization ratio. Witherite exerts a relatively mild effect on slag structure and phase evolution; its moderate addition helps synergistically reduce the melting point, viscosity, and crystallization ratio, thereby supporting performance stability. The optimized fluorine-free mold flux, designed on the basis of these findings, maintains a suitable initial crystallization temperature and critical crystallization cooling rate while exhibiting lower melting temperature, viscosity, and crystallization ratio than conventional fluorine-bearing flux. The findings establish a theoretical basis for designing eco-friendly mold fluxes suitable for high-titanium steel and for enhancing billet quality. Full article

In this study, three types of industrial solid waste—granulated blast furnace slag (GBFS), fly ash, and desulfurization gypsum (DG)—are utilized to collaboratively prepare low-carbon cementitious materials. The effects of steam curing temperature, constant temperature time, and fly ash content on the mechanical properties of multi-source solid waste cementitious materials are systematically investigated, and the optimal mix proportion ratio for low-carbon cementitious materials is determined. The results indicate that as steam curing temperature and constant temperature time increase, the compressive strength of the ternary cementitious material generally shows an upward trend, while the fly ash content exhibits a negative correlation. When the steam curing temperature is 70 °C, the constant temperature time is 10 h, the fly ash content is 20%, and the strength can reach 24 MPa, with both its engineering performance and economic benefits meeting the requirements of practical applications. Meanwhile, the steam curing temperature shows a tendency of first decreasing and then increasing shrinkage rate after 28 d, with the lowest shrinkage rate at 70 °C. Extending the constant temperature time can slightly reduce shrinkage, and the addition of 20–30% fly ash can optimize shrinkage performance. Moreover, the TG/DTG and SEM-EDS microscopic testing demonstrates that the ternary system achieves synergistic activation by accelerated mineral dissolution, ion release and enhanced alkalinity under steam curing, which jointly promotes the formation of AFt and C-A-S-H gel to refine microstructure and improve compactness. This study can not only reduce the consumption of cement, but also facilitate the recycling of industrial waste, providing theoretical support for the application of multi-source solid waste low-carbon materials in practical engineering. Full article

This study investigates the effects and mechanisms of three mineral admixtures—fly ash (FA), silica fume (SF), and metakaolin (MK)—on the fresh, mechanical, and microstructural properties of Yellow River sediment (YRS)-based shotcrete. A comprehensive experimental program was conducted, including setting time determination, workability assessment, and mechanical strength evaluation, complemented by microstructural characterization using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The results indicate that the incorporation of FA prolonged initial and final setting times and improved pumpability but reduced build-up thickness and compressive strength; splitting tensile strength at later ages remained comparable to the control. SF shortened the final setting time and reduced flowability but enhanced shootability, layer build-up, and medium- to later-age compressive and tensile strengths, with an optimal dosage of 5%. MK accelerated the final setting time, slightly reduced early-age compressive strength, but improved early-age splitting tensile strength and achieved 28-day compressive strength comparable to the control. Microstructural analyses revealed that FA participates in pozzolanic reactions forming C–(A)–S–H gel, while SF and MK promote the formation of dense C–S–H and carboalumination phases, enhancing matrix densification. Based on performance evaluation, the recommended dosages are FA ≤ 20%, SF ≤ 15%, and MK ≤ 15%. These results establish clear links between macroscopic performance and microstructural evolution, providing experimental guidance for the sustainable development of YRS-based shotcrete. Full article

The growing accumulation of industrial waste and the depletion of natural mineral resources underscore the need for sustainable approaches to producing ceramic and construction materials. Among the most promising secondary raw materials are coal combustion by-products and metallurgical slags, which are suitable for ceramic applications. This review summarizes recent advances in the use of coal ash, blast furnace and steelmaking slags, together with clay-based raw materials, for the fabrication of ceramic and composite materials. Special attention is given to the physicochemical properties of technogenic raw materials and their effects on sintering, porosity, densification, mechanical strength, and thermal stability. Modern processing methods, including pressing and high-temperature firing, are also discussed. The influence of key technological parameters, such as oxide composition, particle size distribution, firing temperature, and activation conditions, is analyzed. In addition, the review examines major challenges related to raw material heterogeneity, structural instability, thermal stress development, cracking, free CaO reactivity, and environmental risks associated with heavy metal leaching. Recent studies show that incorporating industrial waste into ceramic systems reduces waste disposal, natural resource consumption, energy use, and CO₂ emissions, while promoting sustainable and resource-efficient technologies. Ash- and slag-based ceramics therefore remain highly promising materials for construction applications. Full article

This study aims to explore the preparation process and properties of unburned lightweight aggregate using red mud synergistically with fly ash, granulated blast-furnace slag, and other multi-source solid wastes. Curing regimes and alkali-activated systems were controlled. Their effects on physical properties and environmental safety of lightweight aggregate were systematically evaluated. Results show that curing temperature and alkali activator exert significant synergistic effects on physical properties of lightweight aggregates. Steam curing performs better than standard curing. Performance improves with increasing steam temperature. Sodium silicate solution with a modulus of 1.0 is determined as the optimal activator. Under 90 °C steam curing, Sample D2 achieves the best overall performance. Its cylinder compressive strength reaches 6.92 MPa. 1 h water absorption is 14.8%. Softening coefficient is 0.93. Porosity is as low as 31.07%. Microscopic analysis reveals that higher curing temperature significantly accelerates the hydration reaction of the RMLWA system. It promotes the formation of abundant cementitious products such as C-S-H gel. These products fully fill internal pores and microcracks of the aggregate. A dense three-dimensional network skeleton structure is finally formed. For environmental safety, heavy metal leaching concentrations of steam-cured samples are generally lower than those of standard-cured samples. This study realizes high-value resource utilization of industrial solid wastes. It also provides a new technical route for the development of green building lightweight aggregate. Full article

This work investigates Ferro-Rock concrete as a carbon-negative alternative to ordinary Portland cement (OPC), which accounts for 5–9% of global CO₂ emissions, and evaluates its viability as a sustainable construction material. Ferro-Rock is an iron-based binder comprising recycled iron powder, fly ash, metakaolin, limestone powder, and oxalic acid. This is enhanced by a carbonation reaction in which iron particles react with CO₂ and water to form iron (II) carbonate (FeCO₃), the main binding phase, thereby locking in atmospheric CO₂. The experimental program was divided into two groups. Group 1 studied 100% Ferro-Rock binders with different types of aggregate, specimen sizes, and CO₂ curing periods (0–6 days) with a new locally manufactured stainless steel curing chamber that provided a controlled CO₂ environment of 99.9% and 1.2–1.5 bar gauge pressure. Group 2 investigated Ferro-Rock as a partial cement replacement at 0%, 5%, 10%, 15% and 20% levels of substitution with 5% increments. The 7 and 28 days of compressive, flexural and indirect tensile strengths were determined. The results showed the Ferro-Rock with 100% iron ductile waste aggregates (Mix F4) achieved a 28-day compressive strength of 5.5 MPa, 37.5% higher than the standard Ferro-Rock reference mix. The optimum replacement range of Group 2 was 5–10% with an increase in compressive strength by 5–10%, flexural strength by 11%, and indirect tensile strength by 16% over the OPC control. When replacement exceeded 25%, the bonding was weakened, and all strength measures decreased significantly, reaching a 46% reduction in compressive strength at 50% substitution. Scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM–EDS) microstructural analysis verified the gradual formation of the iron carbonate crystalline phase and provided mechanistic insights into the observed strength trends. Fully cured Ferro-Rock specimens sequestered as much as 11% CO₂ by weight, with a verifiably carbon-negative profile that no OPC-based system can match. Full article

This study investigates the effect of mineral additives of different natures, namely blast-furnace slag, fly ash, and bentonite, on structure formation, phase composition, microstructure, and physicomechanical properties of cement matrices. The analysis included measurements of mass change and linear shrinkage during hardening, determination of density and microhardness, X-ray phase analysis, and microstructural examination by scanning electron microscopy. It was found that the introduction of mineral additives reduced linear shrinkage from 6.06 mm for the control composition to 0.25 mm for the composition with blast-furnace slag, 2.31 mm for the composition with fly ash, and 1.01 mm for the composition with bentonite. The maximum density and microhardness values were obtained for the matrix with blast-furnace slag and amounted to 1.99 ± 0.03 g/cm³ and 39.95 ± 1.12 HV1, respectively, whereas the overall range of values for the investigated compositions was 1.52–1.99 g/cm³ and 30.2–39.95 HV1. X-ray phase analysis showed that the amorphous component varied from 61 to 78%, reaching its maximum value in the composition with blast-furnace slag, which is associated with the formation of poorly crystalline C–S–H and aluminosilicate phases. According to the SEM data, the average size of visible pore-like defects was 2.4 μm for the control composition, 1.4 μm for the composition with blast-furnace slag, 1.3 μm for the composition with fly ash, and 1.7 μm for the composition with bentonite. The most favorable combination of high density, microhardness, developed amorphous component, and homogeneous microstructure was established for the composition with blast-furnace slag. The obtained results can be used as a materials-science basis for the development of cement matrices intended for further studies on the immobilization of solid radioactive waste. Full article

Red mud (RM) and fly ash (FA) were used as a 30% replacement of cement in a sodium silicate-activated system. Composite mortar specimens with RM/FA ratios of 0:30, 1:5, 1:2, 1:1, and 2:1 were prepared with polypropylene fibers (PPF) for toughness enhancement. The mechanical properties and microstructure of the fiber-reinforced mortar were systematically investigated. The results showed that RM20F10 (RM/FA = 2:1) exhibited the best overall mechanical performance among all tested proportions. At this ratio, the 28-day compressive, flexural, and splitting tensile strengths reached 32.4 MPa, 7.3 MPa, and 4.2 MPa, exceeding the control mortar by 12.5%, 15.9%, and 23.5%, respectively. The RM/FA ratio of 1:1 achieved the highest 7-day flexural-to-compressive strength ratio. At 28 days, autogenous shrinkage increased from 910 με to 1100 με as the RM/FA ratio rose from 0:30 to 2:1, and all RM-containing specimens exhibited higher water absorption than the control mortar. Microstructural analysis by SEM, XRD, and FTIR revealed a denser matrix with reduced porosity, attributed to the synergistic formation of C–S–H, C–A–S–H, and N–A–S–H gels. RM reduced early-age porosity by promoting C–A–S–H gel formation, while FA facilitated late-age densification through delayed activation. PPF effectively bridged microcracks via fiber pull-out, leading to a ductile failure mode. Full article

This research investigates the relationship between mineralogical composition and compressive strength in alkali-activated cement–sand mortars incorporating palm oil fuel ash (POFA) as a partial replacement of Portland cement. POFA was introduced at 5 wt.% and 10 wt.% of the binder, and activation was achieved using a NaOH–Na₂SiO₃ solution (3:1 mass ratio). Compressive strength and bulk density were evaluated at 7 and 28 days, while phase evolution was analyzed by X-ray diffraction (XRD) coupled with Rietveld refinement. The results demonstrate that POFA incorporation significantly modified the CaO–SiO₂–Al₂O₃ balance of the system, promoting the consumption of portlandite and the formation of Na- and K-rich aluminosilicate phases such as albite and muscovite. The control mixture exhibited the highest compressive strength values, whereas increasing POFA content reduced both strength and density due to calcium dilution, lower gel compactness, and increased porosity. Nevertheless, all mixtures exhibited progressive strength development over time, indicating continued hydration and geopolymerization reactions associated with the formation of hybrid C–(N,K)–A–S–H gels. These findings demonstrate that POFA can effectively participate in alkali-activated hybrid binders when applied at controlled replacement levels, highlighting its potential as a sustainable supplementary material for lower-carbon cementitious systems. Full article

The study presents an approach to the synthesis of micro- and nano-sized biosilica from rice husk ash (RHA) and describes its effective incorporation into cementitious composites for 3D concrete printing (3DCP). It is demonstrated that the calcination of rice husk at 700 °C, followed by sonochemical treatment, leads to the formation of a nanoscale silica phase with high pozzolanic reactivity. X-ray powder diffraction (XRD), infrared spectroscopy (IR), differential thermogravimetric analysis (DTG), and scanning electron microscopy (SEM) show that the incorporation of nano-biosilica (NBS) into the cementitious composites accelerates the hydration process through a nucleation effect and pozzolanic reaction. This, in turn, densifies the hardened cement microstructure and improves compressive strength significantly. Laboratory 3D concrete printing tests demonstrate that adding 1.72 wt.% NBS improves shape retention, decreases layer slump, and improves interlayer bond strength. The results indicate the viability of rice husk ash-derived biosilica as a supplementary cementitious material (SCM) in 3DCP due to its positive influence on the concrete mortar properties and parameters. Full article

Waste materials are abundant and often act as slow environmental contaminants, creating severe ecological challenges. With rapid industrialization, electricity demand has increased substantially, and in India, coal-based thermal power plants (TPPs) remain the dominant source of power generation. Coal combustion produces two major by-products: fly ash and bottom ash (BA). While fly ash is widely utilized in blended cements due to its pozzolanic nature, BA has received comparatively limited attention despite having similar chemical characteristics. Owing to its coarser particle size, BA shows strong potential as a substitute for natural river sand, the excessive extraction of which has led to severe resource depletion and sustainability concerns. Unlike previous studies that focused on single-source BA or limited performance evaluation, this study investigates the use of BA from multiple sources to develop resource-efficient bottom ash concrete (BAC). Concrete mixes containing 0%, 20%, 35%, and 50% BA as volumetric replacements of river sand were evaluated for their fresh, mechanical, durability, and microstructural properties. The results indicate that BA significantly influences concrete performance due to its porous structure. Among the investigated mixes, 35% river sand replacement with BA showed the most favorable performance for the specific materials and sources used in this study, achieving up to 17.46% higher compressive strength and up to 16.14% higher resistance to transport-related properties at 90 days. Microstructural analysis confirmed the formation of secondary C–S–H gel, which enhanced matrix densification. However, 50% replacement resulted in reduced performance. The findings demonstrate that BA can be effectively utilized in concrete at replacement levels of up to 35% as a sustainable substitute for river sand under the investigated material conditions. Full article

This study investigates the development of sustainable ceramic materials using industrial and agricultural waste from the Kyzylorda region of Kazakhstan. The research focuses on the combined use of local clay, ash from the Kyzylorda thermal power plant (TPP), and rice husk ash (RHA). Experimental investigations included the evaluation of chemical composition, linear and volumetric shrinkage, water absorption, bulk density, and compressive strength of ceramic samples fired at 950–1050 °C. Microstructural (SEM) and phase composition (XRD) analyses were performed to explain the observed behavior. The results showed that the optimal composition was 70% clay, 20% TPP ash, and 10% RHA, which demonstrated the highest compressive strength (15.45 MPa), reduced water absorption, and improved densification. The enhanced performance is attributed to partial vitrification and viscous-phase-assisted densification and the formation of crystalline phases such as mullite, cristobalite, and anorthite. The study confirms that the combined use of TPP ash and RHA enables effective recycling of local waste materials and improves the physical and mechanical properties of ceramic products. Full article

A quaternary solid-waste-based binder was prepared from low-titanium slag, municipal solid waste incineration (MSWI) fly ash, steel slag, and flue-gas desulfurization gypsum (FGDG) to clarify the activating effect of MSWI fly ash on low-titanium slag and its influence on hydrate evolution. Unlike conventional solid-waste-based binders in which MSWI fly ash is mainly regarded as a hazardous residue requiring stabilization, this study demonstrates its specific role as a Ca-rich alkaline activator for promoting low-titanium slag depolymerization and coordinated hydrate formation. The results showed that the compressive strength first increased and then decreased with increasing MSWI fly ash content. Considering both strength development and MSWI fly ash utilization, the optimum mixture was identified as low-titanium slag:MSWI fly ash:steel slag:FGDG = 43.0:17.2:25.8:14.0, with compressive strengths of 9.51 and 46.32 MPa at 3 and 90 d, respectively. These values corresponded to 5.66 and 1.04 times those of the reference mixture without MSWI fly ash, respectively. Ettringite and C-(A)-S-H gel were the main strength-contributing hydration products, while Friedel’s salt was identified as a chloride-bearing AFm phase. Moderate MSWI fly ash addition promoted alkaline activation and low-titanium slag depolymerization, leading to increased formation of ettringite, C-(A)-S-H gel, and Friedel’s salt, which contributed to improved compressive strength. In contrast, excessive MSWI fly ash disturbed the Ca-Si-Al balance and inhibited effective hydrate formation. These results demonstrate that MSWI fly ash can serve as an effective Ca-rich activator for low-titanium-slag-based low-carbon cementitious materials and provide a feasible route for the synergistic utilization of multiple solid wastes. Full article

To promote the synergistic utilization of phosphogypsum (PG), steel slag (SS), and fly ash (FA), a ternary all-solid-waste binder, namely PG-SS-FA cementitious material (PSA), was prepared. The effects of SS content on workability, setting behavior, mechanical properties, hydration products, pore structure, and microstructure were systematically investigated. The results showed that increasing SS content continuously reduced the fluidity of PSA, while the setting time first shortened and then increased. The fastest setting was observed at 40% SS, with initial and final setting times of 126 and 321 min, respectively. Increasing SS from 20% to 40% enhanced the hydration reaction, promoted the formation of AFt and C-(A)-S-H gel, reduced residual unreacted phases, and refined the pore structure, resulting in the highest compressive and flexural strengths for M40. However, further increasing SS to 60% and 80% reduced the fly ash proportion and limited the sustained supply of reactive Si/Al species, despite increasing Ca²⁺ availability and alkalinity, thereby restricting later-age gel accumulation and pore refinement and ultimately weakening mechanical performance. Overall, the performance evolution of PSA is governed by the coupled effects of alkali/Ca supply from SS, sulfate supply from PG, and reactive Si/Al supply from FA. The optimal performance at 40% SS is attributed to the synergistic construction of an AFt framework and continuous pore filling by C-(A)-S-H gel. Full article