Abstract
The influence of starting materials and synthesis route on the properties and the structure of cementitious sodium aluminosilicate gels is not fully understood, partly due their amorphous nature and the fact that they often contain residual reactants, which can make the results of single-pulse NMR spectroscopy applied to these materials difficult to interpret or ambiguous. To overcome some of these limitations, 29Si{27Al} TRAPDOR NMR as well as 27Al{29Si} and 27Al{1H} REDOR NMR spectroscopy were applied to materials synthesized by the one-part alkali-activation route from three different amorphous silica starting materials, including rice husk ash. The latter led to formation of a fully amorphous sodium aluminosilicate gel (geopolymer), while the materials produced from the other silicas contained amorphous phase and crystalline zeolites. Application of the double-resonance NMR methods allowed to identify hydrous alumina gel domains in the rice husk ash-based material as well as significantly differing amounts of residual silica in the three cured materials. Four-coordinated Al existed not only in the aluminosilicate gel framework but also in a water-rich chemical environment with only a small amount of Si in proximity, likely in the alumina gel or possibly present as extra-framework Al in the aluminosilicate gel. The results demonstrate how the employment of different silica starting materials determines the phase assemblage of one-part alkali-activated materials, which in turn influences their engineering properties such as the resistance against chemically/biologically aggressive media.
Highlights
Cementitious sodium aluminosilicate gels can be produced by activation of sufficiently reactive low-calciumsilicate precursors, such as metakaolin, y ash or volcanic rock, with alkaline solutions, such as alkali hydroxide or alkali silicate solutions.[1]
The negative framework charge caused by the Al3+– Si4+ substitution is thought to be balanced by Na+ ions and possibly by extra-framework Al (EFAL); the results of different groups differ regarding the locations and the state of hydration of the charge balancing Na+ ions
As for the materials studied here, 29Si{27Al} TRAPDOR NMR experiments have con rmed that the composites based on silica MS and silica CR contained substantial amounts of unreacted silica with a leached surface layer. 27Al{29Si} and 27Al{1H} REDOR NMR experiments revealed that the amorphous material based on silica RHA was not a phase-pure sodium aluminosilicate gel but contained coprecipitated hydrous alumina gel
Summary
Cementitious sodium aluminosilicate gels (sometimes referred to as aluminosilicate inorganic polymers or geopolymers) can be produced by activation of sufficiently reactive low-calcium (alumino)silicate precursors, such as metakaolin, y ash or volcanic rock, with alkaline solutions, such as alkali hydroxide or alkali silicate solutions.[1]. The current knowledge of the nanostructure of these sodium aluminosilicate gels and the in uence of their chemical composition on the former is based mainly on 29Si single-pulse MAS NMR studies, complemented by 27Al and 23Na single-pulse MAS NMR data.[18,19,20,21,22,23] Additional information, in particular on the state of water in these gels, has been obtained by means of 1H MAS NMR, 1H–29Si CPMAS NMR as well as two-dimensional and multiple-resonance NMR methods.[23,24,25,26] The structure that emerges from these studies is an amorphous framework silicate with partial substitution of Al3+ for Si4+ in tetrahedral sites. The obtained results highlight that the knowledge about conventional alkali-activated materials cannot be transferred to one-part alkali-activated cements without modi cations, and in addition point to a convenient way to tune the properties of the latter materials
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