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Geopolymers and Fiber-Reinforced Concrete Composites (Second Edition)

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A special issue of Materials (ISSN 1996-1944). This special issue belongs to the section "Construction and Building Materials".

Deadline for manuscript submissions: 20 March 2026 | Viewed by 5630

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Special Issue Editors

Dr. Mohamed K. Ismail

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Guest Editor

Department of Civil Engineering, McMaster University, Hamilton, ON, Canada
Interests: concrete structures; sustainable materials; high-performance composites; green construction; strengthening and rehabilitation; artificial intelligence
Special Issues, Collections and Topics in MDPI journals

Dr. Ahmed A. Elansary

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Guest Editor

Department of Civil and Environmental Engineering, The University of Western Ontario, London, ON, Canada
Interests: structural analysis; optimization; design; seismic analysis; reinforced concrete; recycled aggregate
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Dr. Eslam Gomaa

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Guest Editor

Department of Civil, Architectural. and Environmental Engineering, Missouri University of Science and Technology, Rolla, MO, USA
Interests: concrete structure; sustainable materials; geopolymer concrete; 3D-printing concrete; machine learning
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

The Special Issue “Geopolymers and Fiber-Reinforced Concrete Composites” will address advances in the characterization, processing, scaling-up, testing, and commercialization of various types of geopolymers and alkali-activated materials, as well as fiber-reinforced concrete composites. In this Special Issue, we welcome research articles, case studies, and reviews that aim to enrich the available knowledge regarding such high-performance construction materials and highlight the latest findings at both the material and structural levels. Topics of interest include, but are not limited to, the following:

Applications of steel, carbon, and polymeric fibers in concrete;
Fiber-reinforced concrete and high-performance cement-based composites;
Fiber hybridization;
Fire resistance;
Fresh, mechanical, and durability properties;
Impact strength and bond, shear, flexural, cyclic, and cracking behaviors;
Geopolymers and alkali-activated materials (i.e., concrete, mortar, adhesives) for different market applications;
Natural and recycled fibers;
Numerical modeling;
Repair applications;
Proposing new classes of geopolymers and fiber-reinforced concrete;
Small- and large-scale testing.

Dr. Mohamed K. Ismail
Dr. Ahmed A. Elansary
Dr. Eslam Gomaa
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 250 words) can be sent to the Editorial Office for assessment.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Materials is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

alkali-activated materials
concrete
fibers
geopolymers
green construction materials
strength and durability
structural capabilities
sustainability

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Published Papers (7 papers)

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Research

23 pages, 7506 KB

Open AccessArticle

Enhancing Tensile Performance of Lithium Slag Geopolymers Using Hybrid Fibers and Modified Multi-Walled Carbon Nanotubes

by Qing Li, Chong Deng, Yali Hu, Mingxing Luo, Daopei Zhu and Cai Wu

Materials 2026, 19(1), 213; https://doi.org/10.3390/ma19010213 - 5 Jan 2026

Viewed by 232

Abstract

This study investigates the synergistic effects of hybrid fibers and functionalized multi-walled carbon nanotubes (MWCNTs) on the mechanical and microstructural properties of lithium slag–based geopolymers (FL-EGC). Unlike conventional studies that focus on single reinforcement strategies, this work combines nanoscale modification with macroscale fiber reinforcement to overcome the inherent brittleness of geopolymers. Results show that while hybrid fibers and MWCNTs reduce flowability, the incorporation of 2.5% PVA, 1.0% steel fibers, and 0.15% MWCNTs yielded the best balance of performance, improving ultimate tensile stress by 12.7%, strain by 69.2%, and specific fracture energy by 78.2%. Microstructural analysis confirmed that MWCNTs enhanced crack-bridging and matrix densification, while hybrid fibers improved strength and ductility. These findings demonstrate a novel reinforcement pathway for developing sustainable, high-performance geopolymers from industrial by-products, providing both theoretical insights and practical guidance for green construction materials. Full article

(This article belongs to the Special Issue Geopolymers and Fiber-Reinforced Concrete Composites (Second Edition))

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19 pages, 6950 KB

Open AccessArticle

Synergistic Effects of Glass and Flax Fibers Reinforced in Fly Ash Geopolymer Matrix

by Kacper Oliwa, Semanur Efe, Beata Figiela and Kinga Korniejenko

Materials 2026, 19(1), 102; https://doi.org/10.3390/ma19010102 - 27 Dec 2025

Viewed by 363

Abstract

This study compares fly-ash-based geopolymers reinforced with short glass fibers (GF) or flax fibers (FF). Four mixes were produced: reference (FA), 1 wt% GF, 1 wt% FF, and a hybrid (0.5 wt% GF + 0.5 wt% FF). These compositions were cast into prism and cube molds, cured at 75 °C for 24 h, and tested after 28 days. Mechanical testing included compressive strength and three-point bending, phase composition by XRD, and microstructure by optical and SEM microscopy. The GF composite showed the highest compressive strength (mean up to ~28–34 MPa versus ~17 MPa for the reference), while FF gave intermediate values (~11–22 MPa). During bending, the reference achieved the highest flexural strength (~5.5 MPa); fiber-reinforced mixes ranged from ~2.9 to 4.4 MPa. XRD indicated a typical amorphous aluminosilicate gel over crystalline remnants; SEM/optical observations revealed a denser, more compact matrix with fewer voids for GF systems, whereas FF and hybrid mixes exhibited localized porosity and fiber pull-out imprints affecting crack initiation/propagation. Overall, 1 wt% GF effectively enhances compressive performance and matrix densification, while fiber addition at the tested dosages does not improve flexural strength; optimizing fiber content/dispersion and interfacial treatment is recommended. Full article

(This article belongs to the Special Issue Geopolymers and Fiber-Reinforced Concrete Composites (Second Edition))

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11 pages, 2603 KB

Open AccessArticle

Effect of Different Sand Types and Fractions on Ballistic Resistance of High-Performance Steel Fibre-Reinforced Concrete

by Michal Mára, Jindřich Fornůsek, Tomáš Hrabě and Radoslav Sovják

Materials 2025, 18(21), 5020; https://doi.org/10.3390/ma18215020 - 4 Nov 2025

Cited by 1 | Viewed by 434

Abstract

High-Performance Fibre-Reinforced Concrete (HPFRC) is an advanced composite material known for its exceptional energy absorption and dissipation capabilities. To improve its ballistic resistance, HPFRC samples were prepared using 1.5% steel fibre content and varying levels of silica. Ballistic trials employed standard 7.62 × 39 cartridges, each comprising a full metal casing around a mild steel core. Resulting damage and failure mechanisms were mapped using a 3D photogrammetry system. Six different concrete mixtures were tested, each incorporating aggregate fractions of 0/2 mm along with three types of micro sand, the largest of which measured up to 1.2 mm in grain size. The results suggested that increasing the proportion of 0/2 mm silica sand was relatively minor. Full article

(This article belongs to the Special Issue Geopolymers and Fiber-Reinforced Concrete Composites (Second Edition))

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13 pages, 8025 KB

Open AccessArticle

Geopolymer Materials for Additive Manufacturing: Chemical Stability, Leaching Behaviour, and Radiological Safety

by Bahar Gharehpapagh, Meike Denker, Szymon Gadek, Richard Gruhn, Thomas Grab, Kinga Korniejenko and Henning Zeidler

Materials 2025, 18(21), 4886; https://doi.org/10.3390/ma18214886 - 24 Oct 2025

Viewed by 672

Abstract

Geopolymers are inorganic aluminosilicate binders formed by alkali activation of reactive powders, offering a sustainable, low-carbon alternative to Portland cement. Their rapid setting and chemical durability make them well-suited for additive manufacturing (AM) in demanding environments, including underwater construction, where chemical stability is essential for both structural integrity and environmental safety. This study evaluates two metakaolin-based formulations designed for underwater extrusion, differing in activator chemistry and rheology control. Standardized leaching tests revealed alkaline but stable leachates with strong immobilization of most ions; major anions and total dissolved solids remained within regulatory thresholds. Limited exceedances were observed—soluble organic carbon in the NaOH-activated mix and arsenic/selenium in the waterglass–sand system—highlighting specific areas for mix improvement rather than fundamental limitations of the material. Complementary radioactivity screening confirmed activity concentration indices well below the regulatory limit, with measured radionuclide activities falling comfortably within exemption ranges. Together, the leaching and radioactivity results demonstrate that both formulations provide robust matrix integrity and environmental compatibility, while highlighting clear opportunities for mix design improvements to further minimize ecological risks. Full article

(This article belongs to the Special Issue Geopolymers and Fiber-Reinforced Concrete Composites (Second Edition))

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19 pages, 8035 KB

Open AccessArticle

Research on Shrinkage in Lithium Slag Geopolymer Mortar: Effects of Mix Proportions and a Shrinkage Prediction Model

by Lei Wang, Gao Pan, Cai Wu, Sidong Xu and Daopei Zhu

Materials 2025, 18(20), 4766; https://doi.org/10.3390/ma18204766 - 17 Oct 2025

Viewed by 534

Abstract

Lithium slag (LS), a solid waste generated during lithium smelting, exhibits significant potential for geopolymer preparation. However, the high shrinkage of lithium slag geopolymer mortar (LSGM) severely restricts its engineering application. Currently, research on the effects of mix proportions (GBFS-LS mass ratio, water–binder ratio, and binder–sand ratio) on LSGM’s shrinkage, and the correlation between shrinkage behavior and microstructures (pore structure and thermal behavior), remains insufficient. Additionally, there is a lack of targeted shrinkage prediction models for LSGM. To address these research gaps, this study systematically investigates the shrinkage characteristics of LSGM and develops a modified prediction model. Thermogravimetric analysis–differential thermal gravimetric analysis (TG-DTG) results show that a lower GBFS-LS ratio promotes the formation of dense sodium-alumino-silicate hydrate (N-A-S-H) gels. Meanwhile, mercury intrusion porosimetry (MIP) tests demonstrate that optimizing the water–binder ratio and binder–sand ratio refines the pore structure of LSGM, where the average pore size is reduced from 53.5 nm at a GBFS-LS ratio of 3 to 28.75 nm at a GBFS-LS ratio of 1.5.Quantitatively; compared with the group with a GBFS-LS ratio of 3, the 90-day shrinkage strain of the group with a GBFS-LS ratio of 1.5 decreases by 25.8%. When the water–binder ratio decreases from 0.57 to 0.27, the 90-day shrinkage strain reduces by 36.7%; in contrast, increasing the binder–sand ratio from 0.21 to 0.39 leads to a 39.8% increase in 90-day shrinkage strain. Based on the experimental data and the fundamental framework of the American Concrete Institute (ACI) model, this study introduces mix proportion influence coefficients and constructs a novel shrinkage prediction model tailored to LSGM. The coefficient of determination (R²) of the proposed model exceeds 0.98. This model provides a reliable quantitative tool for the mix proportion optimization and engineering application of LSGM. Full article

(This article belongs to the Special Issue Geopolymers and Fiber-Reinforced Concrete Composites (Second Edition))

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28 pages, 5852 KB

Open AccessArticle

Interaction of PCE and Chemically Modified Starch Admixtures with Metakaolin-Based Geopolymers—The Role of Activator Type and Concentration

by Stephan Partschefeld, Jasmine Aschoff and Andrea Osburg

Materials 2025, 18(17), 4154; https://doi.org/10.3390/ma18174154 - 4 Sep 2025

Cited by 1 | Viewed by 1243

Abstract

Water-reducing admixtures are of enormous importance to adjust the workability of alkali-activated materials. Especially in geopolymers activated by highly concentrated alkaline solutions, the polycarboxylate ether (PCE) superplasticizers are less effective than in conventional cementitious systems. The aim of this study was to clarify the reasons for the lower dispersing performance of PCE and the synthesis of alternative dispersing agents based on the biopolymer starch to improve the workability of highly alkaline geopolymers. Furthermore, the focus of investigations was on the role of activator type and concentration as key parameters for geopolymer reaction and interaction of water-reducing agents. Therefore, in this study the conformation of three different types of PCE (MPEG: methacrylate ester, IPEG: isoprenol ether, and HPEG: methallyl ether) and synthesized starch admixtures in sodium and potassium hydroxide solutions (1 mol/L up to 8 mol/L) were studied. Furthermore, the dispersing performance, adsorption behavior, and influence on reaction kinetics in metakaolin-based geopolymer pastes were investigated in dependence on activator type and concentration. While the PCE superplasticizers show coiling and formation of insoluble aggregates at activator concentrations of 3 mol/L and 4 mol/L, the synthesized starch admixtures show no significant change in conformation. The cationic starch admixtures showed a higher dispersing performance in geopolymer pastes at all activator concentrations and types. The obtained adsorption isotherms depend strongly on the activator type and the charge density of the starch admixtures. The reaction kinetics of geopolymer pastes were not significantly influenced using the synthesized starch admixtures. Especially the cationic starch admixtures allow the reduction of liquid/solid ratios, which leads to higher flexural and compressive strengths. Full article

(This article belongs to the Special Issue Geopolymers and Fiber-Reinforced Concrete Composites (Second Edition))

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13 pages, 3233 KB

Open AccessArticle

The Influence of Alkali-Resistant MiniBars™ on the Mechanical Properties of Geopolymer Composites

by Gabriel Furtos, Codruta Sarosi, Marioara Moldovan, Kinga Korniejenko, Michał Łach, Viorel Ungureanu, Leonard Miller and Iveta Nováková

Materials 2025, 18(4), 778; https://doi.org/10.3390/ma18040778 - 10 Feb 2025

Cited by 4 | Viewed by 1226

Abstract

Geopolymer concrete reinforced with MiniBars™ could be an eco-friendly, innovative, durable, high-strength material substitute for common Portland cement in buildings. AR glass fiber MiniBars™ composites (AR MiniBars™) (ReforceTech AS, Royken, Norway) 60 mm in length were utilized to strengthen the geopolymer matrix for the fabrication of unidirectional geopolymer composites reinforced by AR MiniBars™ (AR MiniBars™ FRBCs). New AR MiniBars™ FRBCs were fabricated by adding different amounts of AR MiniBars™ (0, 12.5, 25, 50, 75 vol.%) into the fly ash geopolymer paste. Geopolymers were obtained by combining fly ash powder with Na₂SiO₃/NaOH in a ratio of 2.5:1, which served as an alkaline activator. AR MiniBars™ FRBCs were cured for 48 h at 70 °C and tested for different mechanical properties. Fly ash, AR MiniBars™, and AR MiniBars™ FRBC were evaluated by optical microscopy and SEM. The addition of AR MiniBars™ increased the mechanical properties of AR MiniBars™ FRBCs. The mechanical properties of AR MiniBars™ FRBCs were heightened compared to the geopolymer without AR MiniBars™; the flexural strength was 18.80–30.71 times greater, the flexural modulus 4.07–5.25 times greater, the tensile strength 3.49–8.27 times greater, the force load at upper yield tensile strength 3.6–7.72 times greater, and the compressive strength for cubic samples 2.75–3.61 times greater. The fractured surfaces and sections of AR MiniBars™ FRBCs were inspected by SEM and optical microscopy analyses, and even though there was no chemical adhesion, we achieved a good micromechanical adhesion of the geopolymer to AR MiniBars™. These results obtained encouraged us to propose AR MiniBars™ FRBCs for application in construction. Full article

(This article belongs to the Special Issue Geopolymers and Fiber-Reinforced Concrete Composites (Second Edition))

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