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Studying the metakaolin content, fiber type, and high-temperature effects on the physico-mechanical properties of fly ash-based geopolymer composites

Bayrak Bariş, Alcan Haluk Görkem, Durmaz Özge Çiğdem Özelmacı, ipek Süleyman, Kaplan Gökhan, Güneyisi Erhan, Aydın Abdulkadir Cüneyt

Archives of Civil and Mechanical Engineering

2025

Vol. 25, no. 1

art. no. e2, 2025

The study investigated the physicasl characteristics and mechanical performance of fly ash-based geopolymer composites when exposed to high temperatures. Geopolymer composites were produced using fly ash as an aluminosilicate-rich raw material and a combination of sodium silicate and sodium hydroxide as an alkaline activator. In this context, the study also examined the impact of partially replacing metakaolin (7.5% and 15% by weight). Furthermore, the study aims to examine the impact of adding fiber (basalt and carbon types) on the physical, mechanical, and high-temperature properties of geopolymer composites. The physical properties investigated were unit weight, apparent porosity, water absorption, and capillary water absorption, while the strength performances investigated were flexural and compressive strengths. To monitor the effect of high temperatures on the strength characteristics of the geopolymer composites, the mixtures were exposed to temperatures of 200 °C, 400 °C, and 600 °C. Besides, SEM images were provided to illustrate the degree of geopolimerization. The results indicated that metakaolin replacement yielded mixtures having higher unit weight, but lower apparent porosity and water absorption. The results indicated that metakaolin replacement yielded mixtures having a higher unit weight, reaching an increase of about 5%, but lower apparent porosity and water absorption, with decreases reaching 18.3% and 20%, respectively. The metakaolin-blended geopolymer composites resulted in better strength performance and resistance to high temperatures. Raising the metakaolin replacement level from 0 to 15% led to an increase of 17.3% in flexural strength. The compressive strength of the composites subjected to a temperature of 200 °C exhibited an increase of over 10%. Notably, this rate of increment was observed to be nearly 20% higher in nonfibrous composites. Fiber addition decreased the compressive strength up to about 21%, while increasing the flexural strength up to 65%. Strength performance improved at 200 °C, but decreased at higher temperatures up to 600 °C. The geopolymer composites experienced significant mass loss when exposed to high temperatures.

Engineering and microstructural properties of environmentally friendly alkali - activated composites containing clinker aggregate: heat-curing regime and elevated-temperature effect

Bayrak Bariş, Ipek Süleyman, Alkan Haluk Görkem, Kaplan Gökhan, Aydın Abdulkadir Cüneyt, Güneyisi Erhan

Archives of Civil and Mechanical Engineering

2024

Vol. 24, no. 4

art. 200, 1--20

The common wisdom in the literature about clinker aggregate (CA) is that it improves the performance properties of mor tar or concrete to some extent. The current study, in this context, investigated the physical characteristics and mechanical performances of alkali-activated composites (AACs) made entirely with CA. The CA was used in three particle sizes of 0-2 mm (named No.10), 2-4 mm (named No.5), and 4-8 mm (named medium). To examine the impact of the fine CA-size fraction on the characteristics of AAC, No.10 CA was partially replaced by No.5 CA up to 50%, while the content of the medium CA was kept constant in all AAC mixtures. Moreover, to evaluate the influence of the 8-h curing temperature on the performance of the AACs, different temperature-based curing strategies (ambient, 45, and 75 C) were applied to the AACs. In the production of AACs, granulated blast furnace slag was employed as an aluminosilicaterich raw material, and a sodium silicate and sodium hydroxide combination was used as an alkaline activator. Physical properties (flowability, water absorption, capillary water absorption, and dry unit weight), and 8-h strength performances (flexural and compressive) were determined. Furthermore, to monitor the influence of high temperatures on the characteristics of the AAC, the mixtures were exposed to elevated temperatures (200, 500, and 800°C). In SEM image analysis, it was determined that spherical CSH gels were formed in the heat-cured AACs. It has been observed that the geopolymerization products decompose in AACs exposed to 800°C. To evaluate statistically the experimental results, a multi-factor analysis of variance (ANOVA) was also applied. The results revealed that increasing the coarser fine aggregate fraction led to higher water absorption and apparent porosity capacities and lower unit weight. Besides, strength performance was improved by applying a heat-curing strategy to the AAC, whereas a decrease was observed by increasing the No.5 CA fraction. There was a remarkable reduction in compressive strength and considerable loss of mass when the AAC mixes were exposed to high temperatures.