COMPREHENSIVE EXAMINATION OF DEHYDROXYLATION OF KAOLINITE, DISORDERED KAOLINITE, AND DICKITE: EXPERIMENTAL STUDIES AND DENSITY FUNCTIONAL THEORY
COMPREHENSIVE EXAMINATION OF DEHYDROXYLATION OF KAOLINITE, DISORDERED KAOLINITE, AND DICKITE: EXPERIMENTAL STUDIES AND DENSITY FUNCTIONAL THEORY
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DOI：
10.1007/s42860-020-00082-w
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发表时间：
2020-08-27
影响因子：
2.2
通讯作者：
Emmerich, Katja
中科院分区：
文献类型：
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作者：
Izadifar, Mohammadreza;Thissen, Peter;Emmerich, Katja
高岭土和粘土是通过煅烧脱羟基（DHX）热活化生产辅助胶凝材料和地聚合物前驱体的重要原料。两种类型的粘土都含有不同的高岭石多型和无序结构，但对于二八面体1:1层硅酸盐的层堆积对最佳热活化条件和随后与碱性溶液的反应性的影响知之甚少。本研究的目的是通过研究脱羟基后的原子结构，提高对二八面体1:1硅酸盐层中层堆积对热活化的影响的认识。通过同步热分析（STA）进行加热实验，然后通过核磁共振波谱（NMR）和扫描电子显微镜（SEM）对脱羟基材料进行表征，并进行第一性原理计算。利用密度泛函理论（DFT）分析了几何优化结构与热力学稳定性的关系。所得到的单位细胞体积与扩张测量研究的数据进行了比较。局部结构的变化与DHX温度升高的实验结果呈以下顺序相关：高岭石无序化、高岭石无序化、迪克立特无序化，迪克立特呈现两个脱羟基步骤。发现中间结构是热力学稳定的，并且在DHX程度上高岭石为75%，无序高岭石为25%，迪克特为50%。这些热力学稳定、部分去羟基化的中间体含有Al(V)，而偏高岭石和偏镁土只含有Al(IV)，配位壳强烈扭曲。这些结果强烈表明，有必要对高岭土和粘土中硅酸盐二八面体1:1层的结构进行表征，并将其作为预测最佳煅烧条件和反应活性的关键参数。
Kaolins and clays are important raw materials for production of supplementary cementitious materials and geopolymer precursors through thermal activation by calcination beyond dehydroxylation (DHX). Both types of clay contain different polytypes and disordered structures of kaolinite but little is known about the impact of the layer stacking of dioctahedral 1:1 layer silicates on optimum thermal activation conditions and following reactivity with alkaline solutions. The objective of the present study was to improve understanding of the impact of layer stacking in dioctahedral 1:1 layer silicates on the thermal activation by investigating the atomic structure after dehydroxylation. Heating experiments by simultaneous thermal analysis (STA) followed by characterization of the dehydroxylated materials by nuclear magnetic resonance spectroscopy (NMR) and scanning electron microscopy (SEM) together with first-principles calculations were performed. Density functional theory (DFT) was utilized for correlation of geometry-optimized structures to thermodynamic stability. The resulting volumes of unit cells were compared with data from dilatometry studies. The local structure changes were correlated with experimental results of increasing DHX temperature in the following order: disordered kaolinite, kaolinite, and dickite, whereupon dickite showed two dehydroxylation steps. Intermediate structures were found that were thermodynamically stable and partially dehydroxylated to a degree of DHX of 75% for kaolinite, 25% for disordered kaolinite, and 50% for dickite. These thermodynamically stable, partially dehydroxylated intermediates contained Al(V)while metakaolinite and metadickite contained only Al(IV)with a strongly distorted coordination shell. These results indicate strongly the necessity for characterization of the structure of dioctahedral 1:1 layer silicates in kaolins and clays as a key parameter to predict optimized calcination conditions and resulting reactivity.