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    Alkali-activated basalt powder/slag systems: compressive strength and microstructural characterization
    (Springer, 2023) Akturk, Busra; Ayhan, Bilal Umut
    This study investigates the potential use of basalt powder as a sole precursor or blended in high amounts in slag-based alkali-activated systems. Eight alkali-activated mixes are prepared and comprehensively analyzed to determine the compressive strength development and microstructural characterization of basalt powder-based and basalt powder/slag blends activated by sodium hydroxide and a mixture of sodium hydroxide and sodium silicate. The mixes are characterized from a microstructural viewpoint via X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric and scanning electron microscopy analyses. The results show that while basalt powder-based mixes have low compressive strength values, ranging between 2 and 9 MPa, basalt powder/slag blend mixes exhibit a moderate compressive strength, i.e., 20 MPa at 28 days. Furthermore, sodium-silicate-activated basalt powder/slag blend mixes achieve high compressive strengths at early and further ages. The low strength values of the basalt powder-based mixes are attributed to the low concentration of reactive species and lack of Ca2+ ions in the medium, while the high compressive strength of the blended mixes is mainly associated with the formation of calcium aluminosilicate hydrate [C-(A)-S-H] or Na-enriched calcium sodium aluminosilicate hydrate [C-(N)-A-S-H] gel phases along with the calcic-plagioclase, which afford a denser microstructure. The obtained results show that basalt powder can be used in high concentrations, i.e., 50%, in alkali-activated systems, and basalt powder/slag blends can be a feasible, alternative binder system for use as a structural material.
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    Fracture behavior of alkali-activated basalt powder/slag systems reinforced with basalt and hybrid fibers
    (Springer, 2023) Akturk, Busra
    Alkali-activated materials are gaining attention as a sustainable alternative to cement-based materials over the past years and studies on their fracture response are scarce. This paper presents the fracture behavior of alkali-activated fiber-reinforced basalt powder/slag blend mortars. Fracture characteristics such as the fracture toughness-K-IC(ini) and K-IC(un)-of the alkali-activated basalt powder/slag mortar mixes were determined using the double-K fracture model. Basalt fibers and a combination of basalt + polypropylene fibers (hybrid) were used to enhance the fracture properties of the mixes, and it was found that the basalt powder/slag binary mortars achieved performance comparable with that of cement-based systems in terms of compressive strength, fracture toughness, and fracture energy. The alkali-activated basalt powder/slag mixes exhibited good mechanical performance, which as established previously was due to the beneficial silicium and calcium-based resources and the synergy between the basalt powder and the slag. Incorporating basalt fibers was effective in improving flexural strength and fracture toughness significantly, and they contributed much more than the hybrid fibers did. On the other hand, incorporating hybrid fibers improved the fracture energy significantly. The fiber-matrix interface and the fiber behavior under flexure were visualized using scanning electron microscopy. The fiber-matrix bonding mechanism showed that the polypropylene fibers tended to cluster, resulting in less-improved fracture toughness compared to that with basalt fibers. The outcomes of this study show that basalt powder/slag blends can be used as structural materials: they have fracture characteristics similar to those of cement-based systems and incorporating basalt and hybrid fibers improve their fracture characteristics.
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    Hydration kinetics and performance of sodium carbonate-activated slag-based systems containing reactive MgO and metakaolin under carbonation
    (Elsevier Sci Ltd, 2022) Akturk, Busra; Abolfathi, Mehrnosh; Ulukaya, Serhan; Kizilkanat, Ahmet B.; Hooper, Thomas J. N.; Gu, Lei; Yang, En-Hua
    The hydration mechanism and strength development of sodium carbonate-activated slag-based systems mainly depend on the additives used. Although the effects of mineral additives in such systems have been extensively investigated, the effects of Mg2+, Al3+, and Si4+ ions increasing with the addition of reactive MgO (Mg) and metakaolin (Mk) on the hydration mechanism of such systems have not been established yet. This study investigated the hydration kinetics and performance of sodium carbonate-activated ternary blended slag-based binder systems. The hydration mechanism was revealed by isothermal calorimetry and mechanical performance was evaluated with the measurement of compressive strength at different ages up to 56 days. The reaction mechanisms were investigated through X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis and 29Si and 27Al solid-state nuclear magnetic resonance (NMR). C-(A)-S-H, Na and Al-enriched C-(N,A)-S-H and hydrotalcite were the main reaction products responsible for the strength development of the samples, accompanied by the minor contribution of other carbonate-containing phases. Partial replacement of slag with Mg and Mk led to high early-age strengths compared to plain samples when Mk was used at 5%. Samples incorporating Mg and Mk achieved similar or higher strengths than ordinary Portland cement-based samples. However, an increase in replacement ratio of Mk beyond 5% led to a significant decrease in compressive strength. Furthermore, the performance of samples under accelerated carbonation was studied. The use of Mg and Mk enhanced carbonation resistance due to enhanced hydrotalcite and C-(N,A)-S-H gel formation, highlighting the potential of using slag-Mg-Mk blends as an alternative binder system.
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    Influence of carbonation curing and nano-silica incorporation on compressive strength and micro-structural development of binary RMC-based systems
    (Elsevier, 2023) Taj, Khalilullah; Akturk, Busra; Ulukaya, Serhan
    Fresh-state properties, compressive strength, and microstructural properties of reactive MgO cement-based (RMC) systems cured under accelerated carbonation or ambient conditions were examined in the paper. RMC, which is an environmentally greener alternative to ordinary Portland cement (OPC), was incorporated in the reference series to contrast its mechanical properties with other cementitious materials. To assess the synergy of RMC with other binders such as microsilica, slag, and fly ash, as well as to find a way to incorporate more sustainable materials in the mortars, binary systems were designed. Lastly, to evaluate the influence of ultra-fine nano-silica (NS), on the composite binders, tertiary systems were formulated. In the carbonated series, a significant pH decrease, as well as a striking increase in compressive strength were observed. Accelerated carbonation induced the formation of magnesium calcite, which is supported by eclectic microstructural analyses. The highest compressive strength was measured in the carbonated reference series, binary and tertiary series also demonstrated respectable strength which was accompanied by compact microstructure. The addition of NS correlated with a decrease in the number of pores, especially large capillary pores, whereas no perceivable change in reaction products was discovered. In conclusion, this study demonstrated that reactive MgObased cement could be used as a complete or partial replacement for conventional binders in structural concrete; the promising qualities of carbonated specimens ensure its usage as highstrength concrete.
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    One-part sodium carbonate-activated slag/r-MgO based mixes: Influence of nano-silica incorporation on compressive strength and microstructural development
    (Elsevier Sci Ltd, 2024) Ozen, Omer Can; Oktay, Didem; Akturk, Busra
    In this study, environment -friendly one -part alkali -activated slag -based mixes were prepared by using solid sodium carbonate as the alkali activator and incorporating reactive MgO (r-MgO) as the additive. Substitution of rMgO in reference mixes was carried out at various levels, up to 15%, to enhance the reaction mechanism and strength development. The strength development was measured up to 56 days and analyzed microstructurally using Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetric, and scanning electron microscopy analyses. Additionally, the influence of incorporating nano -silica in the binary mixes on strength development and microstructure was investigated. It was observed that the reaction mechanism could be improved by incorporating a low amount of reactive MgO, leading to expedited setting time and significant increases at both early age and final compressive strength values. Nano -silica incorporation was found to be effective in improving compressive strength at all ages, resulting in the formation of a higher matrix phase as demonstrated by microstructure tests. The highest compressive strength was attained in the nano -silica and rMgO incorporating ternary mixes, reaching 25.9 MPa on the 3rd day and 47.2 MPa on the 28th day after production. Furthermore, the environmental impacts of the produced mixes were assessed. This study highlights the feasibility of one -part sodium carbonate -activated slag -based materials, especially with r-MgO inclusion, and underscores the role of nano -silica incorporation in enhancing strength properties.
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    Utilization of waste Cappadocia earth as a natural pozzolan in alkali activation: A parametric study
    (Elsevier Sci Ltd, 2022) Ozata, Serife; Akturk, Busra; Yuzer, Nabi
    This study presents the investigation of the possible use of rock-cut carving waste earth in Cappadocia as a precursor in alkali activation. The Cappadocia waste earth was activated by sodium hydroxide (NH) and sodium silicate (WG). In addition, slaked lime (CH) was substituted as a calcium resource. The compressive strength development was determined and the reaction mechanisms were investigated through X-ray diffraction and Fourier transform infrared spectroscopy. It was found that different strength levels can be obtained, depending on the activator type and CH-substitution ratio. While WG-activated mixes presented higher early age values, NH-activated mixes presented higher values at further ages, i.e., ~17 MPa at 28 days. The CH substitution enhanced the compressive strength for both early and further ages, attributing to the available Ca2+ ions resulting in the formation of C-A-S-H gel. The XRD results showed that silicates group minerals prominent in control mixes, while C-A-S-H-like gel was the primary phase in CH-substituted mixes. N-A-S-H and other hydrated compounds were found in all mixes, responsible for strength increment. Moreover, secondary electron microscope images revealed a denser structure in WG-activated mixes. Results of energy dispersive spectroscopy and X-ray diffractions corroborated hydration products and alkali activation. The obtained results of this study depict that alkali-activated Cappadocia earth can be a feasible and alternative sustainable binder to be used as a structural material.