In order to investigate the influence of three key molar ratios (n(SiO2)/n(Al2O3),n(K2O)/n(Al2O3) and n(H2O)/n(K2O)),a total of nine potassium poly-sialate-disiloxo (K-PSDS) geopolymeric cement matrices were designed according to orthogonal design principle. Subsequently,XRD,ESEM-EDXA and MAS-NMR techniques were employed to further characterize the microstructure of the most fully reacted geopolymeric cement matrix. The experimental results show that n(K2O)/n(Al2O3) has the most significant effect on compressive strength amongst the three ratios. The highest compressive strength (20.1 MPa) can be achieved when n(SiO2)/n(Al2O3)=6.5,n(K2O)/n(Al2O3)=0.8 and n(H2O)/n(K2O)=10.0. The FTIR spectra of nine PSDS geopolymeric cement matrices also indicate that geopolymeric cement matrix with the highest strength is the most fully reacted one and possesses the largest amount of geopolymeric cement products. The microscopic analysis reveals that PSDS geopolymeric cement matrix possesses structural characteristics similar to gel substances in having a wide range of Si endowments,but predominantly the framework molecular chains of Si partially replaced by 4-coordinated Al tetrahedral.
Two aspects of studies were carried out: 1) synthesis of geopolymer by using fly ash and metakaolin; 2) Immobilization behaviors of fly ash based geopolymer in a presence of Pb and Cu ions. As for the synthesis of fly ash based geopolymer, 4 different fly ash content (10%, 30%, 50%, 70%) and 3 types of curing regimes (standard curing, steam curing and autoclave curing) were investigated to obtain the optimum synthesis condition based on the compressive and flexural strength. The experimental results show that geopolymer, containing 30% fly ash and synthesized at steam curing (80 ℃ for 8 h), exhibits higher mechanical strengths. The compressive and flexural strengths of fly ash based geopolymer reach 32.2 MPa and 7.15 MPa, respectively. Additionally, Infrared (IR) and X-ray diffraction (XRD) techniques were used to characterize the microstructure of the fly ash geopolymer. IR spectra shows that the absorptive band at 1086 cm-1 shifts to lower wave number around 1033 cm-1, and the 6-coordinated Al transforms into 4-coordination during the syn-thesis of fly ash based geopolymer. The resulting geopolymeric products were X-ray amorphous materials. As for immobilization of heavy metals, the leaching tests were employed to investigate the immobilization behaviors of the fly ash based geopolymer synthesized under the above optimum condition. The leaching tests showed that fly ash based geopolymer can effectively immobilize Cu and Pb heavy metal ions, and the immobilization efficiency reached 90% greater when heavy metals were incorporated in the fly ash geopolymer in the range of 0.1% to 0.3%. The Pb exhibits better immobilization efficiency than the Cu, especially in the case of large dosages of heavy metals.
