Previous studies have primarily focused on the effects of various supplementary cementitious materials (SCMs) on the mechanical and durability properties of geopolymers under isolated conditions such as elevated temperatures or sulfate exposure. However, the combined effects of fly ash (FA), metakaolin (MK), and recycled aggregates on geopolymers subjected to the simultaneous exposure of high temperatures, sodium sulfate, and sodium chloride have yet to be thoroughly investigated. Unlike earlier research, which tends to examine mechanical or chemical stressors in isolation, our study covers this gap by exploring the synergistic effects of these multiple stressors on key performance characteristics of FA- and MK-based geopolymer composites. This approach provides a more comprehensive understanding of the material's behavior in multi-stressor environments, closely replicating the complex conditions that real-world structures encounter. The present work investigates the mechanical, durability, and microstructural properties of recycled aggregate geopolymer concrete (RAGC) made with MK and FA, particularly under the influence of sulfate and salt following high-temperature exposure. To achieve this, five MK-based RAGC mixtures with varying FA contents were prepared. The samples were then subjected to elevated temperature in the range 200–800 °C. Afterward, the samples were immersed in 5 % sodium sulfate (SS) and 5 % sodium chloride (SC) solutions. The study assessed the effects of elevated temperatures combined with SS and SC on RAGC using mechanical strength, ultrasonic pulse velocity, mass loss measurements, capillary water absorption, scanning electron microscopy (SEM), X -ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). A statistical test was also performed to analyze the improvement in the results. Results showed that compressive strength increased with MK inclusion, achieving minimal losses even at high temperatures, with FMK-20, FMK-40, and FMK-60 showing only 11.65 %, 13.65 %, and 16.58 % losses at 200 °C. Capillary water absorption decreased with MK, with the FMK-20 blend showing a minimum absorption coefficient at 200 °C + SS. However, at 800 °C, absorption increased due to matrix degradation. After exposure to high temperatures with SS, crystalline formations like nepheline, quartz, calcite, and mullite were observed. Increased crystalline phases improved strength, but reduced quartz intensity and new cristobalite and anorthoclase phases appeared after high temperatures and SC. Conclusively, MK inclusion significantly improved RAGC's performance, particularly in elevated temperature and sulfate environments, confirming its potential for durable, high-strength concrete applications.

中文翻译：

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高温、硫酸盐和氯化物对粉煤灰和偏高岭土基再生骨料地质聚合物混凝土性能的综合影响


以前的研究主要集中在各种补充胶凝材料 （SCM） 在高温或硫酸盐暴露等孤立条件下对地质聚合物的机械和耐久性能的影响。然而，粉煤灰 （FA）、偏高岭土 （MK） 和回收骨料对同时暴露在高温、硫酸钠和氯化钠下的地质聚合物的综合影响尚未得到彻底研究。与倾向于孤立地检查机械或化学应力源的早期研究不同，我们的研究通过探索这些多种应力源对基于 FA 和 MK 的地质聚合物复合材料的关键性能特征的协同效应来填补这一差距。这种方法可以更全面地了解材料在多应力源环境中的行为，紧密复制真实世界结构遇到的复杂条件。本研究研究了用 MK 和 FA 制成的再生骨料地聚合物混凝土 （RAGC） 的机械、耐久性和微观结构特性，特别是在高温暴露后硫酸盐和盐的影响下。为了实现这一目标，制备了 5 种具有不同 FA 含量的基于 MK 的 RAGC 混合物。然后将样品置于 200–800 °C 范围内的高温下。 然后，将样品浸入 5% 硫酸钠 （SS） 和 5% 氯化钠 （SC） 溶液中。该研究使用机械强度、超声脉冲速度、质量损失测量、毛细管吸水、扫描电子显微镜 （SEM）、X 射线衍射 （XRD） 和傅里叶变换红外光谱 （FTIR） 评估了高温结合 SS 和 SC 对 RAGC 的影响。 还进行了统计测试以分析结果的改进。结果表明，MK 夹杂物的抗压强度增加，即使在高温下也能实现最小的损失，FMK-20、FMK-40 和 FMK-60 在 200 °C 时仅显示出 11.65%、13.65% 和 16.58% 的损失。 毛细管吸水率随 MK 降低，FMK-20 混合物在 200 °C + SS 时显示出最小吸收系数。然而，在 800 °C 时，由于基质降解，吸收增加。在 SS 中暴露在高温下后，观察到霞石、石英、方解石和莫来石等结晶结构。结晶相的增加提高了强度，但降低了石英强度，并且在高温和 SC 之后出现了新的方石英和无正长石相。总之，MK 的加入显着提高了 RAGC 的性能，特别是在高温和硫酸盐环境中，证实了其在耐用、高强度混凝土应用中的潜力。