Degree of reaction and phase content of silica-based one-part geopolymers investigated using chemical and NMR spectroscopic methods

Abstract

One-part geopolymers were synthesized from two different silica materials (a silica-rich residue from chlorosilane production and a commercial microsilica) and sodium aluminate at three different SiO 2 /Al 2 O 3 ratios and a nominal water/solids ratio of 0.5. The degree of reaction of the silica in the cured geopolymers (i.e. the fraction of silica dissolved to form aluminosilicates and minor products) was determined using two different methods: chemical attack with HCl to dissolve the reaction products and evaluation of peak areas of 29 Si MAS NMR spectra. It was found that the degree of reaction of the silica decreases with increasing the silica content of the starting mix, and that it is almost constant after 1 day of curing and almost independent from the kind of starting silica. From the results of the NMR-based method, the mean SiO 2 /Al 2 O 3 ratio of the reaction products (aluminosilicates and minor products) can be estimated to be ca. 2.0, nearly independent of the starting composition of the geopolymers. The dissolution method is biased, but of sufficient precision to be useful for following changes of the degree of reaction. Major crystalline phases in the cured geopolymers are zeolite A and/or hydrosodalite. Depending on the starting composition, the relative amounts of these zeolites vary; additionally, sodalite (only for the residue from chlorosilane production with [1 wt% Cl -), faujasite, and zeolite EMT can appear in the geopolymers. The 29 Si and 27 Al MAS NMR results indicate mainly Si(4Al) and Al(4Si) sites, in line with the presence of zeolite A, hydrosodalite, sodalite, and geopolymeric gel of comparatively low SiO 2 /Al 2 O 3 ratio.

29Si{27Al}, 27Al{29Si} and 27Al{1H} double-resonance NMR spectroscopy study of cementitious sodium aluminosilicate gels (geopolymers) and gel–zeolite composites

“…As reported previously, 26,31 MS_3.5_1d and CR_3.5_1d yielded composites containing mainly zeolite Na-A [Na 4 (AlSiO 4 ) 4 -$9H 2 O] and an amorphous sodium aluminosilicate gel with a molar SiO 2 /Al 2 O 3 ratio of $2 (Si/Al z 1). In the case of MS_3.5_1d, some hydrosodalite [Na 6 (AlSiO 4 ) 6 $4H 2 O] was present in addition, while for CR_3.5_1d very minor amounts of a faujasite-type zeolite and/or zeolite Na-EMT were observed by XRD.…”

“…À85 ppm, which has been shown to represent an amorphous sodium aluminosilicate gel with a molar SiO 2 /Al 2 O 3 ratio of $2 (Si/Al z 1). 26,31 For CR_3.5_1d, the fraction of Si in that gel has been determined to be 11% of the total Si in the system. 26 The S 0 spectrum of RHA_3.5_1d exhibited an essentially feature-less, broad hump in the range À75 ppm to À120 ppm, i.e.…”

“…A more detailed description of the starting materials has been provided elsewhere. 31,32 Pastes were obtained by rst dry-mixing the sodium aluminate with RHA, MS or CR in relative amounts to obtain a molar SiO 2 /Al 2 O 3 ratio of the mixture of $3.5 (i.e. Si/Al z 1.75) and subsequent mixing with water at a nominal water/solids ratio of 0.50.…”

“…The recycle delay was chosen to ensure a quantitative measurement (recycle delay of at least ve times the longest relaxation time, T 1 , of all components) 39 despite the comparatively long T 1 that may occur for some species in the present samples. 31 27 Al{ All 29 Si NMR spectra were referenced to kaolinite with its upeld resonance at À91.5 ppm, and all 27 Al NMR spectra were referenced to YAG with its AlO 6 resonance at 0.6 ppm. Deconvolution/tting of the spectra was performed with the SOLA module of the Bruker TopSpin soware, version 3.1.…”

Clay-Based Materials in Geopolymer Technology

“…Almost similar research was carried out by Geraldes et al [104], whereby geopolymers are used as restoration materials for tiles. In Cement Based Materials order to further enhance the usage of geopolymers in civil engineering, researchers have investigated on one-part geopolymer system [105][106][107], whereby geopolymer mixture can be prepared by just adding water. The interest of this study is caused by the limitation of geopolymer technology for in-situ application which lowers its economical value.…”