The hydrophobic agents polydimethylsiloxane emulsion (BS 1042) and sodium methyl silicate were used to modify expanded perlite (EP). The effects of the hydrophobic agents and modification methods on the water absorption, thermal conductivity, and bulk density of EP were investigated. First, the orthogonal experiments of the two hydrophobic agents on the modification of EP were designed, and the effects of the drying temperature, hydrophobic concentration, soaking time, and soaking temperature on the performance of EP were analyzed. The optimum modification conditions for the two hydrophobic agents were obtained. Second, under the optimum conditions, the EP was modified by BS 1042 under vacuum conditions and then modified by the atmospheric pressure with sodium methyl silicate, so the composite-modified EP with excellent hydrophobic performance was obtained. The results showed that the water absorption of the composite modified EP was significantly reduced, from 390.1 to 16.2%; the thermal conductivity increased from 0.0532 to 0.0555 w/(m·k), with a small increase of only 4.3%; and the bulk density increased from 51.6 to 59.2 kg/m3, with an increase of 14.7%.

This study evaluates the influence of the elemental distribution in the fly ash particles and their impact on phase formation and compressive strength of five low-calcium fly ash geopolymers. The degree of geopolymerization in each geopolymer system was assessed by FT-IR and solid state 27Al MAS-NMR analysis. The corresponding pore volume changes were investigated by mercury intrusion porosimetry (MIP). The uniformity of the distribution of SiO2 and Al2O3 in the fly ash was observed to directly influence the dissolution of the amorphous surface layer in the initial geopolymerization process and control aluminosilicate gel precipitation and gel-phase creation. The results showed that the higher the uniformity of distribution (coupled with the stable conversion of aluminium from octahedral to tetrahedral coordination), the higher the aluminium amalgamation with silicates. The result of this is the production of a three-dimensional (3-D) polysialatesiloxo (Si-O-Al-O-Si) polymeric gel structure with high rigidity and stability, which in turn results in higher compressive strength. It was also observed that an increase of meso-porosity in geopolymer phase formation coupled with a cumulative pore volume below 1000 nm (3.937 × 10–5 in.) is a good indicator of the degree of geopolymerization.

In this paper, a total of nine potassium-poly(sialate-siloxo) (K-PSS) geopolymeric cement matrixes, with different molar ratios of SiO2/Al2O3, K2O/Al2O3, and H2O/K2O, is designated to investigate the influence of the three ratios on mechanical properties and microstructure in accordance with the orthogonal design principle. The experimental results show that SiO2/Al2O3 has the most significant effect on compressive strength among the three ratios. The highest compressive strength (5.04 ksi [34.8MPa]) can be achieved when SiO2/Al2O3 = 4.5, K2O/Al2O3 = 0.8 and H2O/K2O = 5.0. Comparing the infrared (IR) spectra of nine K-PSS geopolymeric cement matrixes also indicates that the geopolymeric cement matrix with the highest strength is the most fully-reacted one and possesses the largest amount of geopolymeric products. Subsequently, X-ray powder diffraction (XRD), environment-scanning electron microscope equipped with energy dispersion X-ray analysis (ESEM-EDXA), transmission electron microscopy-electron diffraction spectroscopy (TEM-EDS), and magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR) techniques are employed to further characterize the microstructure of the fully-reacted geopolymeric cement matrix. The microscopic analysis reveals that the fully-reacted K-PSS geopolymeric cement matrix possesses structural characteristics similar to glassy or gel substances in having a wide range of Si endowments, but predominantly the framework molecular chains of Si partially replaced by four-coordinated Al tetrahedral. A three-dimensional (3D) molecular structural model is also proposed based on the decomposition of MAS-NMR spectrum of the fully-reacted K-PSS geopolymeric cement matrix synthesized from the optimum mixture proportion.

This paper presents a summary of work in progress on an examination of the hydration chemistry and microstructure of a paste prepared incorporating 58 percent of a typical ASTM Class F fly ash and a portland cement from U.S. sources, and a paste with portland cement only. Thermal analysis, x-ray diffraction, pore fluid extraction, and SEM have been employed to study cement and cement-fly ash pastes cured up to 180 days. High levels of nonevaporable water and removal of alkali ions from pore solutions in pastes cured for 7 to 14 days were found. Etching of fly ash particles and extensive deposition of reaction products at ash/matrix boundaries were evident in scanning electron micrographs. Together, these observations clearly demonstrate extensive participation by the fly ash in hydration and cementation reactions. However, despite the extensive reactivity, up to 180 days, many fly ash particles remain as intact pseudomorphs embedded in the hydrate mass. A model based on siloxane and siloxane hydrolysis, alkali ion exchange, and precipitation of calcium silicates, aluminates, and aluminosilicates is proposed to explain the observed processes.