Formation of Geopolymers Using Sodium Silicate Solution and Aluminum Orthophosphate

Abstract

This paper reports the formation and structure of fast setting geopolymers activated by using three sodium silicate solutions with different modules (1.6, 2.0 and 2.4) and a berlinite-type aluminum orthophosphate. By varying the concentration of the aluminum orthophosphate, different Si/Al-ratios were established (6, 3 and 2). Reaction kinetics of binders were determined by isothermal calorimetric measurements at 20 °C. X-ray diffraction analysis as well as nuclear magnetic resonance (NMR) measurements were performed on binders to determine differences in structure by varying the alkalinity of the sodium silicate solutions and the Si/Al-ratio. The calorimetric results indicated that the higher the alkalinity of the sodium silicate solution, the higher the solubility and degree of conversion of the aluminum orthophosphate. The results of X-ray diffraction and Rietveldt analysis, as well as the NMR measurements, confirmed the assumption of the calorimetric experiments that first the aluminum orthophosphate was dissolved and then a polycondensation to an amorphous aluminosilicate network occurred. The different amounts of amorphous phases formed as a function of the alkalinity of the sodium silicate solution, indicate that tetrahydroxoaluminate species were formed during the dissolution of the aluminum orthophosphate, which reduce the pH value. This led to no further dissolution of the aluminum orthophosphate, which remained unreacted.

Effect of Microwaves on the Rapid Curing of Metakaolin- and Aluminum Orthophosphate-Based Geopolymers

“…This was proved by the fact that for both the metakaolin-based and aluminum orthophosphate-based geopolymer pastes, it could be seen that the heat rates decrease with the increasing modulus of the sodium silicate solutions at the same temperature level. This was also found by Partschefeld et al for aluminum orthophosphate activated with different sodium silicate solutions [8] and by Cai et al for metakaolin-based geopolymers [41]. The heat quantities increased with increasing the temperature to 60 • C for all samples, before again decreasing at 80 • C. This is probably related to the already-mentioned very large heat losses that occurred during the mixing of the 80 • C samples.…”

“…Figures 5 and 6 represent some of the results of the calorimetric investigations. The obtained curves for the calorimetric investigations show the typical route of those results found in the literature [8,41]. Partschefeld et al investigated geopolymers based on aluminum orthophosphate activated with sodium silicate solutions with different moduli.…”

“…For sodium silicate solutions, the viscosity usually decreases with increasing alkali content. When a certain limit concentration is exceeded, the viscosity increases again [8]. To prepare the geopolymer mortar, crushed granite sand was added (fraction 0-0.5 mm, Granitwerk Fischer GmbH & Co. KG, Wurzbach, Germany).…”

“…It was also found that there was no proportional increase in the degree of conversion by changing the Si/Al ratio from 3 to 2, which indicates an incomplete dissolving of the aluminum orthophosphate. Furthermore, the study compared the different moduli of the sodium silicate solutions, showing that with increasing alkalinity the solubility and the degree of conversation of aluminum orthophosphate increases too [8]. Cai et al synthesized geopolymers with metakaolin and a mixture of sodium silicate solution and potassium hydroxide as the activator.…”

“…If geopolymer binders are prepared with alkali silicate solutions, it is also possible to use a solid that contains only aluminum species. Therefore, a berlinite-type aluminum orthophosphate can be utilized [8]. In this paper, synthesis based on both metakaolin and aluminum orthophosphate with activation by three different sodium silicate solutions is considered.…”