Citation: Lim, H.-J.; Cho, C.-G.; You, J.-Y.; Jeong, J.-J. Mechanical Properties of Alkali-Activated Slag Fiber Composites Varying with Fiber Volume Fractions. Materials 2022, 15, 6444. https://doi.org/10.3390/ ma15186444 Academic Editor: Karim Benzarti Received: 17 August 2022 Accepted: 13 September 2022 Published: 16 September 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Mechanical Properties of Alkali-Activated Slag Fiber Composites Varying with Fiber Volume Fractions Hyeon-Jin Lim 1 , Chang-Geun Cho 1, * , Jang-Yeol You 2 and Jong-Jin Jeong 1 1 Department of Architectural Engineering, Chosun University, Gwangju 61452, Korea 2 Department of Architectural Engineering, Songwon University, Gwangju 61756, Korea * Correspondence: chocg@chosun.ac.kr; Tel.: +82-62-230-7023 Abstract: The mechanical properties of alkali-activated slag fiber composites (ASFC) were investi- gated with varying volume fractions of PVA (Polyvinyl alcohol) fibers. Ground granulated blast furnace slag (GGBS) and alkali-activators were used as the main binders instead of cement, which emits a large amount of carbon dioxide during the manufacturing process. The measured slump flow of ASFC showed a high fluidity at a fiber content of 1.5 vol.% or less. The tensile, flexural, and shear strength of ASFC showed higher values as the amount of fiber increased. Compared to the existing high ductility fiber composites showing strain hardening behaviors with a fiber content of 2.0 vol.%, ASFC proved that it could exhibit high ductility characteristics due to multi-microcracks even at low fiber mixing rates of 1.0% and 1.25%. ASFC could be expected to lower the manufacturing cost with a low fiber content and provide improved workability with high fluidity. In addition, when manufacturing structural components using the developed ASFC, it is expected that the amount of fiber could be selected and used according to the required performance. Keywords: alkali-activated slag; multiple microcrack; sodium sulfate; calcium hydroxide; high ductile behavior; polyvinyl alcohol fiber 1. Introduction The world is confronted by the problem of environmental destruction, global warming, and resource depletion due to continuous industrialization, and global warming caused by greenhouse gases is considered to be the most serious problem. Ordinary Portland cement emits more than 0.9 tons of carbon dioxide to manufacture one ton of cement. It accounts for more than 7% of emissions from all industries [1–4]. As an approach to reducing carbon dioxide from cement production, some methods can be adopted such as reducing carbon dioxide emissions during cement production [3], reducing the amount of cement used by using industrial by-products, and using alkali-activated slag instead of ordinary Portland cement. Similar to ordinary Portland cement, alkali-activated ground granulated blast furnace slag (GGBS) also has hydraulic reaction characteristics so that hardened alkali-activated GGBS mortars have the mechanical characteristics of high strength at early and long-term ages, and are known to exhibit advantages such as a high resistance to chemical attack and freeze-thaw as well as a low carbonation rate [5–7]. In the construction industries, ordinary concrete has many advantages as one of most widely used materials. However, its brittle characteristics due to the low strain rate and the decrease in durability due to local cracking remains a problem awaiting a solution. As a solution to this problem, high-ductile fiber composites have been designed by micromechanics and has been manufactured by mixing chopped PVA fibers to a volume fraction of less than 2.0 vol.%. The composite exhibited a high ductile deformation with crack control under uniaxial tensile stress and showed multiple microcracks of less than 100 μm. This characteristic of high-ductile behavior of the fiber composite could be attractive for its use as a construction material [8–11]. Studies Materials 2022, 15, 6444. https://doi.org/10.3390/ma15186444 https://www.mdpi.com/journal/materials