Violeta Nikolić

Decalcification resistance of alkali-activated slag.

This paper analyses the effects of decalcification in concentrated 6M NH(4)NO(3) solution on mechanical and microstructural properties of alkali-activated slag (AAS). Portland-slag cement (CEM II/A-S 42.5 N) was used as a benchmark material. Decalcification process led to a decrease in strength, both in AAS and in CEM II, and this effect was more pronounced in CEM II. The decrease in strength was explicitly related to the decrease in Ca/Si atomic ratio of C-S-H gel. A very low ratio of Ca/Si ~0.3 in AAS was the consequence of coexistence of C-S-H(I) gel and silica gel. During decalcification of AAS almost complete leaching of sodium and tetrahedral aluminum from C-S-H(I) gel also took place. AAS showed significantly higher resistance to decalcification in relation to the benchmark CEM II due to the absence of portlandite, high level of polymerization of silicate chains, low level of aluminum for silicon substitution in the structure of C-S-H(I), and the formation of protective layer of polymerized silica gel during decalcification process. In stabilization/solidification processes alkali-activated slag represents a more promising solution than Portland-slag cement due to significantly higher resistance to decalcification.

The Influence of Mechanical Activation of Fly Ash on the Toxic Metals Immobilization by Fly Ash-Based Geopolymers

This paper investigates the influence of mechanical activation of fly ash on the toxic metals immobilization by fly ash-based geopolymers. Fly ash was firstly mechanically and then alkali-activated. Mechanical activation of fly ash was conducted in a planetary ball mill. Alkali activation of fly ash was carried out at room temperature by use of sodium silicate solution as an activator. Toxic metals (Pb and Cr) were added in the form of water soluble salts during the synthesis of geopolymers. The immobilization process was assessed via investigation of the mechanical and leaching properties of geopolymers. Structural changes of geopolymers during the toxic metals immobilization were assessed by means of gas adsorption and SEM analyses. Mechanical activation of fly ash led to a significant increase in geopolymer strength and to a reduced leaching of toxic metals from geopolymers.

The influence of Pb addition on the properties of fly ash-based geopolymers

Preventing or reducing negative effects on the environment from the waste landfilling is the main goal defined by the European Landfill Directive. Generally geopolymers can be considered as sustainable binders for immobilization of hazardous wastes containing different toxic elements. In this paper the influence of addition of high amount of lead on structure, strength, and leaching behavior (the effectiveness of Pb immobilization) of fly ash-based geopolymers depending on the geopolymer curing conditions was investigated. Lead was added during the synthesis of geopolymers in the form of highly soluble salt - lead-nitrate. Structural changes of geopolymers as a result of lead addition/immobilization were assessed by means of XRD, SEM/EDS, and 29Si MAS NMR analysis. Investigated curing conditions significantly influenced structure, strength and leaching behavior of geopolymers. High addition of lead caused a sizeable decrease in compressive strength of geopolymers and promoted formation of aluminum-deficient aluminosilicate gel (depolymerization of aluminosilicate gel), regardless of the curing conditions investigated. According to the EUWAC limitations, 4% of lead was successfully immobilized by fly ash-based geopolymers cured for 28 days in a humid chamber at room temperature.