Clays have the potential of being the most important supplementary cementitious material for the future. The quality of calcination, which has a very important influence on the suitability of clay as a partial replacement of cement, is influenced by the changes in the characteristics of clays during calcination. An improved knowledge of these changes can help to assess the suitability of clays. In this study, clays were calcined at seven temperatures ranging from 400 °C to 1000 °C and the effect of calcination temperature on physico-chemical properties like specific gravity, specific surface area, water demand, morphology and reactivity were investigated. The results showed that the transformation process of kaolinite to metakaolin, which is the main reactive phase in cements, completes by 800 °C and this conversion process governs the reactivity, without changing the physical properties. The metakaolin becomes unstable at calcination temperature of 1000 °C, and transforms into the spinel phase as confirmed by the differential scanning calorimetry. The spinel phase reduces the reactivity and also changes the physical properties of calcined clays, making the calcined clay unsuitable for use in cements. New test methods that can be employed to rapidly assess the quality of calcination are also discussed.

中文翻译：

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煅烧温度对胶凝材料用高岭土物理化学特性的影响

粘土有潜力成为未来最重要的辅助胶凝材料。煅烧质量，对粘土作为水泥的部分替代品的适用性有着非常重要的影响，它受到煅烧过程中粘土特性变化的影响。更好地了解这些变化有助于评估粘土的适用性。在这项研究中，粘土在 400 °C 至 1000 °C 的七个温度下进行煅烧，并研究了煅烧温度对比重、比表面积、需水量、形态和反应性等物理化学性质的影响。结果表明，高岭石向偏高岭土（水泥中主要反应相）的转化过程在 800 °C 时完成，并且该转化过程控制反应性，而不改变物理性能。差示扫描量热法证实，偏高岭土在 1000 °C 的煅烧温度下变得不稳定，并转变为尖晶石相。尖晶石相降低了煅烧粘土的反应性并且还改变了物理性质，使得煅烧粘土不适合在水泥中使用。还讨论了可用于快速评估煅烧质量的新测试方法。