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    Effects of reactive MgO and metakaolin on compressive strength and chloride migration coefficient of sodium carbonate activated slag mortars
    (YILDIZ TECHNICAL UNIV., 2023-02) Abolfathi, Mehrnosh; Ulukaya, Serhan; Aktürk, Büşra
    This paper presents an investigation of the compressive strength and chloride migration of sodium carbonate (NC) activated slag-based mortars. In the experimental study, NC was preferred to use as an activator since it can be designated as an environment-friendly activator. To improve the performance of sodium carbonate-activated slag-based mortars (NCAS), reactive MgO (M) and metakaolin (MK) were replaced with slag up to 15% by mass and five mortar mixtures were prepared with different M and MK replacement ratios. It was found that replacing the slag with a combination of 10% M and 5% MK achieved the highest compressive strength for both early age and ultimate strength. In addition, the mix which was composed of 85% slag, 10% M and 5% MK, was the most resistant one against chloride migration. Based on the compressive strength and chloride migration test results, it was concluded that the compressive strength and chloride migration performances of NCAS could be enhanced by the combination of M and MK.
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    Fresh state properties and compressive strength development of reactive MgO-based systems
    (Elsevier, 2022) Taj, Khalilullah; Aktürk, Büşra; Ulukaya, Serhan
    In this study, flowability, setting time, and strength gain of mortars containing reactive magnesium oxide cement (RMC) and a combination of micro-silica (MS), nano-silica (NS), slag, and fly ash (FA) as precur-sors without employing accelerated carbonation have been investigated. To increase the flowability and hydration degree, sodium hexametaphosphate (SHMP) was incorporated. The series containing RMC and MS simultaneously, had the greatest 3-day compressive strength value of about 25 MPa, whereas further strength gains in all the mixtures until the 7th day was minimal, which establishes that strength gains were due to hydration degree, not carbonation. RMC and MS simultaneously in a series, a combination of extremely high fineness materials dropped the flowability; the presence of slag and FA, on the other hand, enhanced it. The inclusion of NS in the mix didn't demonstrate a perceptible change in any of the properties measured in this study; its inertness could be attributed to the unsuitable dispersion method. Setting time of reference mix was the shortest, which was in conformity with its fast initial strength gain. Replacement of RMC with any other binder extended both the initial and final setting time. It was hypothesized that a major difference in this study is the use of SHMP as a dispersant and hydration agent. The outcomes of this study confirm that RMC can be a potential binder material to be used in mor-tar productions.Copyright (c) 2022 Elsevier Ltd. All rights reserved.Selection and peer-review under responsibility of the scientific committee of the International Confer-ence on Advances in Construction Materials and Structures.
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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.