|Institution:||University of Illinois – Urbana-Champaign|
|Full text PDF:||http://hdl.handle.net/2142/34257|
There were two objectives for this study. The first was to determine the effects of the Si/Al ratio and calcium hydroxide on the chemical composition and nanostructure for metakaolin geopolymers. The second was to determine how the composition and nanostructure correlate with mechanical properties of the geopolymer. For this study, the geopolymers were made using metakaolin, calcium hydroxide, sodium hydroxide, sodium silicate and water. The geopolymers contained two or three phases, depending on whether or not calcium hydroxide was used. For geopolymers with no calcium hydroxide, the samples contained two phases: unreacted metakaolin and geopolymer gel. To ensure geopolymer gel was forming and to monitor the amount of the geopolymer gel, hydrochloric (HCl) acid extractions were performed. For geopolymers with calcium hydroxide, samples contained three phases: unreacted metakaolin, geopolymer gel and calcium silicate hydrate with aluminum substitution (CASH). In conjunction with the HCl extraction, salicylic acid/methanol (SAM) extractions were performed to verify the presence and amounts for each phase. X-ray diffraction (XRD) was used to identify crystalline phases as well as monitor the changes in the amorphous peak from metakaolin to geopolymer. XRD analysis showed that the geopolymers with varying Si/Al ratios produced the same pattern. The patterns with calcium hydroxide in the geopolymer produced an amorphous peak that was narrower and centered at a higher 2?? value than the geopolymers with no calcium hydroxide. The patterns also confirmed the presence of calcium silicate hydrate in XRD patterns. Both 29Si and 27Al magic angle spinning nuclear magnetic resonance (MAS-NMR) were used to quantitatively observe the individual silicon and aluminum structures in the different phases in the geopolymer. From 29Si NMR analysis, the composition and amount of the different phases in the geopolymer could be determined. Increasing the Si/Al ratio caused a decrease in Si-O-Al bonds and an increase in Si-O-Si bonds in the geopolymer gels, which caused the compressive strength in the geopolymer to increase. The 29Si NMR analysis showed that geopolymers with calcium hydroxide produced calcium silicate hydrate that had cross-linking tetrahedra with alumina substitution in bridging tetrahedral sites. The increasing amount of calcium hydroxide increased the amount of CASH and decreased the amount of the geopolymer gel. Increasing calcium hydroxide caused the Si/Al ratio of the geopolymer gel to decrease. The combination of geopolymer gel and CASH increased the strength of the geopolymer gel.