Effect of Alkali Fusion Methods on the Preparation of One-Part Geopolymer From Coal Gasification Slag.

This study investigated the one-part geopolymer synthesis by coal gasification slag through a calcination treatment. Two preparation methods, the traditional alkali fusion method (M-I) and the alkali fusion and modification method (M-II), were compared to illustrate the more suitable way for coal gasification slag-based one-part geopolymer (CGS-based geopolymer) synthesis. The reaction products and structure of the geopolymer were tested by XRD, FTIR, TG-DTG, and SEM. The results showed that the compressive strength of geopolymers prepared by M-I was raised as the NaOH amount increased, and the maximum compressive strength of geopolymers was achieved at 8.44 MPa. The low compressive strength suggested that M-I failed to be used for CGS-based geopolymer synthesis. When solid NaOH was selected for one-part geopolymer synthesis by M-II, the geopolymer possessed a compressive strength of 25.52 MPa and exhibited a well-combined and dense matrix. More Si and Al species in F-SH could be released for geopolymerization, and the coal gasification slag additive could be partially dissolved and act as the filler in the geopolymer matrix, which enhanced the geopolymerization degree. The M-II has been proven to be more suitable for preparing CGS-based geopolymers than the M-I.

Immobilization mechanism of cesium in geopolymer: Effects of alkaline activators and calcination temperature.

Geopolymer is always regarded as a promising material for the immobilization of radioactive waste. In the present study, the stabilization of Cs in geopolymers activated by NaOH and Na2SiO3 solutions and calcined at various temperatures was studied via toxicity characteristic leaching procedure (TCLP), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope and energy dispersive spectroscopy (SEM-EDS), solid-state nuclear magnetic resonance (SSNMR), and N2 adsorption-desorption isotherm. For both NaOH-activated and Na2SiO3-activated geopolymers, the leaching concentrations of Cs decreased with the increase of calcination temperature. Specifically, most of the amorphous substance was crystallized to nepheline at 1000 °C for NaOH-activated geopolymer, and Cs+ can be incorporated into the structure of nepheline, contributing to the reduction of Cs leaching concentration. However, the amorphous structure was still maintained for Na2SiO3-activated geopolymer even after calcination at 1000 °C. It has been deduced that the main structure of Na2SiO3-activated geopolymer after calcination at 1000 °C should be in short-range order and Cs+ can be locked in a micro "crystal" structure. In addition, the change of specific surface area was not fully consistent with the decreasing trend of Cs leaching concentration. Therefore, the inner structure and the specific surface area of geopolymer should have a combined effect on the leaching behavior of Cs. This study can provide new insights into the application of geopolymer to immobilize radionuclides.

Influence of aluminate and silicate on selenate immobilization using alkaline-earth metal oxides and ferrous salt.

Effects of aluminate and silicate species on the SeO42- immobilization using alkali-earth metal oxides and ferrous species have not been clearly elucidated. In the present study, Al and Si species were separately added into MgO/Fe(II) and CaO/Fe(II) reactions containing SeO42-, studied by toxicity characteristic leaching procedure (TCLP), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), X-ray absorption fine structure (XAFS), and PHREEQC simulation. Approximately 42 % of SeO42- was reduced to SeO32- for MgO/Fe(II) reaction in the presence of Al species, being consistent with the case without Al species. The Al species only showed slight inhibition of Se leaching for the MgO/Fe(II) reaction. Most of Se oxyanions were adsorbed onto Mg(OH)2 through outer-sphere complexation. For CaO/Fe(II) reaction, all of SeO42- was reduced to SeO32- with or without Al species. However, the Se leaching amount (3 %) of sample added with Al species (CE3) is much lower than that (12 %) of sample without Al species (CE2). This is mainly because SeO32- can be sorbed onto the iron-based minerals through binuclear bidentate corner-sharing (2C) complexation instead of monodentate mononuclear corner-sharing (1V) complexation of the case without Al species. On the other hand, SeO42- was not reduced to SeO32- in the presence of silicate, and almost all of Se was leached out for silicate-contained samples except CaO/Fe(II) reaction with the addition of Al species. This is due to the polymerization of Al and Si species under a high-alkalinity environment, thereby stabilizing SeO42- in the amorphous silicon-aluminum structure and contributing to the decrease of Se leaching.

Application of Geopolymer in Stabilization/Solidification of Hazardous Pollutants: A Review.

Geopolymers, as a kind of inorganic polymer, possess excellent properties and have been broadly studied for the stabilization/solidification (S/S) of hazardous pollutants. Even though many reviews about geopolymers have been published, the summary of geopolymer-based S/S for various contaminants has not been well conducted. Therefore, the S/S of hazardous pollutants using geopolymers are comprehensively summarized in this review. Geopolymer-based S/S of typical cations, including Pb, Zn, Cd, Cs, Cu, Sr, Ni, etc., were involved and elucidated. The S/S mechanisms for cationic heavy metals were concluded, mainly including physical encapsulation, sorption, precipitation, and bonding with a silicate structure. In addition, compared to cationic ions, geopolymers have a poor immobilization ability on anions due to the repulsive effect between them, presenting a high leaching percentage. However, some anions, such as Se or As oxyanions, have been proved to exist in geopolymers through electrostatic interaction, which provides a direction to enhance the geopolymer-based S/S for anions. Besides, few reports about geopolymer-based S/S of organic pollutants have been published. Furthermore, the adsorbents of geopolymer-based composites designed and studied for the removal of hazardous pollutants from aqueous conditions are also briefly discussed. On the whole, this review will offer insights into geopolymer-based S/S technology. Furthermore, the challenges to geopolymer-based S/S technology outlined in this work are expected to be of direct relevance to the focus of future research.

Effect of Si/Al molar ratio on the immobilization of selenium and arsenic oxyanions in geopolymer.

The effect of Si/Al molar ratio of geopolymer on the immobilization of Se and As oxyanions was studied through leaching test and solid characterizations including XRD, FTIR, TG, NMR, XAFS, and N2 adsorption-desorption isotherm. As a whole, the leaching percentages of Se and As oxyanions increased with the increase of the Si/Al molar ratio of geopolymer. Linear combination fitting confirmed that most of selenite, selenate and arsenate ions existed in geopolymers through electrostatic interaction. Thus, Al tetrahedrons in geopolymer structure control the charge stability for these oxyanions to a large extent. Differently, as for arsenate ions, they were recrystallized into an arsenate compound (Na3.25(OH)0.25(H2O)12)(AsO4) in geopolymers. The additive of these pollutants has an adverse effect on the compactness of geopolymer, then influencing the leaching performance in turn. However, the changes in leaching results did not follow the variation trend of specific surface areas and pore volumes of geopolymers with different Si/Al ratios. The number and distribution of Al tetrahedron and compactness of geopolymer have a synergistic effect on the immobilization of these oxyanions. Besides, the compressive strengths of geopolymer samples are always higher than 20 MPa, which meets the requirement of safe disposal of hazardous waste.