Mechanical and biological characterization of geopolymers for potential application as biomaterials

In this study three different geopolymer compositions have been investigated and characterized as potential biomaterials. Two geopolymer formulations are mainly composed of metakaolin, with some silica additions so that to reach Si/Al = 2.10 molar ratio, the third one contains a reduced amount of metakaolin and it is constitutes mainly of silica gel with composition: H24AlK7Si31O79 with Si/Al = 31. While in the first two formulations the powders were added of NaOH and sodium silicate in different percentages as activator and ligand, respectively, in the third one two different KOH addition methods (separately or jointly with potassium silicate solution) were performed. Room temperature consolidation was followed by thermal activation of composition Si/Al=31 at 60°C for 150 min and at 500°C for 180 min. The work presents exhaustive microstructural characterization (FT-IR, SEM/EDS, XRD) jointly with compression resistance tests and bioactivity studies on pressed powders of the two geopolymers. The materials were composed of amorphous aluminosilicates and a limited amount of zeolitic phases, found on the top surface. The compressive strength of the first two compositions was higher than 15 MPa and flexural strength around 2 MPa after 2 days of curing at room temperature. Compressive strength tests were carried out on Si/Al = 31 geopolymer on both activated sample series and demonstrated that when added separately the activator leads to more fragile specimens (0.90 MPa vs 1.95 MPa). To authors knowledge the effect of geopolymer preparation on mechanical properties of thermally activated Si/Al = 31 formulation has never been proved before. The bioactivity was successfully tested with the soaking of the samples in a simulated body fluid (SBF) for 3 weeks. The formation of a layer of hydroxyapatite on the surface of the materials was shown both by SEM micrograph and EDS analysis

Design, obtainment and properties of glasses and glass-ceramics from coal fly ash

Glasses and glass-ceramics were obtained by mixing up to 50 wt% of Italian or Spanish coal fly ash with other wastes (glass cullet and float dolomite). The behaviour of 10 compositions was investigated by thermal (DTA) and mineralogical (XRD) analysis, microstructural (SEM) characterization, mechanical and chemical measurements. It was verified that the contribution of the alkaline-earth elements in the original composition is fundamental to easily obtain glass-ceramics with a fine microstructure which improves the mechanical properties. Otherwise, with a small addition of Ay ash and without dolomite, very stable glassy materials were obtained that did not exhibit any visible etching either in water or in acid media. Therefore, the combined vitrification/devitrification technique is a suitable methodology for the recycling and exploitation of coal fly ash. (C) 1998 Elsevier Science Ltd. All rights reserved

“Reuse of incinerator bottom and fly ashes to obtain glassy materials”,

Bottom and fly ashes coming from the urban wastes incineration represent a by-product nowadays landfilled. By mixing different amount of these residues with others inert materials, such as glass cullet and feldspar waste, two vitrifiable mixtures are tailored. Glasses, obtained by means of vitrification process, are chemically stable with low leachability of contaminants and show comparable properties to those of commercial soda lime glasses. Moreover, from the thermal and mechanical characterization the tendency of these glasses to crystallise, for their transformation into glass-ceramic materials, has been evidenced

CRYSTALLISATION AND MICROSTRUCTURE OF NEPHELINE FORSTERITE GLASS-CERAMICS

This work presents the results of a study focused on the development of forsterite-nepheline glass-ceramic with the use of rice husk ash (RHA) as a silica source. The glass-ceramics were produced by a sintering process of a glassy frit formulated in the MgO-Al2O3-SiO2 base system with the addition of B2O3 and Na2O to facilitate the melting and pouring processes. The crystallisation study was carried out by depicting the TTT curve (Time-Temperature-Transformation). The mineralogical characterisation of the glass-ceramic materials was carried out using X-ray diffraction (XRD). The crystallisation activation energies were calculated by the Kissinger method. The results obtained show that nepheline (Na2O•Al2O3•SiO2) is the major crystalline phase in the temperature interval 700-950ºC and forsterite (2MgO•SiO2) predominates at temperatures above 950ºC. A study of the microstructure by scanning electron microscopy (SEM) allowed to establish the morphological evolution in both shape and spatial arrangement of the nepheline and forsterite crystals on heating

Alkali activation as new option for gold mine tailings inertization

Abstract The mining industry produces a huge quantity of sulphidic mine tailings, which cause several short- and long-term environmental problems when disposed by landfilling in impounding lakes. The possibility of immobilizing several heavy metals from gold mine tailings by reactive geopolymerization technique has been investigated in the present study. The chemical stability of geopolymers synthetized by the alkali activation of metakaolin and blast furnace slag and the addition of 40–50 wt% gold mine tailings is demonstrated. The geopolymers were cured at room temperature, and the effects of different Si/Al and Na/Al molar ratios and curing times were investigated. The inertization effectiveness was evaluated by means of leaching tests carried out according to standard EN 12457 after 7 and 28 days and after 18 months. The samples were immersed into the water for 1 day, and the leachable metals in the test solution were determined by ICP-OES. The results show that various elements (Cr, Cu, Ni, Zn and Mn) from gold mine tailings are able to immobilize almost completely by alkali activation with proper co-binder material. The immobilization efficiency were highly improved with longer curing period also for the problematic elements As, V, Sb and B

The effect of fibrous reinforcement on the polycondensation degree of slag-based alkali activated composites

Abstract Alternative cementitious binders, based on industrial side streams, characterized by a low carbon footprint, are profitably proposed to partially replace Portland cement. Among these alternatives, alkali-activated materials have attracted attention as a promising cementitious binder. In this paper, the chemical stability of the matrix, in fiber-reinforced slag-based alkali-activated composites, was studied, in order to assess any possible effect of the presence of the reinforcement on the chemistry of polycondensation. For this purpose, organic fiber, cellulose, and an inorganic fiber, basalt, were chosen, showing a different behavior in the alkaline media that was used to activate the slag fine powders. The novelty of the paper is the study of consolidation by means of chemical measurements, more than from the mechanical point of view. The evaluation of the chemical behavior of the starting slag in NaOH, indeed, was preparatory to the understanding of the consolidation degree in the alkali-activated composites. The reactivity of alkali-activated composites was studied in water (integrity test, normed leaching test, pH and ionic conductivity), and acids (leaching in acetic acid and HCl attack). The presence of fibers does not favor nor hinder the geopolymerization process, even if an increase in the ionic conductivity in samples containing fibers leads to the hypothesis that samples with fibers are less consolidated, or that fiber dissolution contributes to the conductivity values. The amorphous fraction was enriched in silicon after HCl attack, but the structure was not completely dissolved, and the presence of an amorphous phase is confirmed (C–S–H gel). Basalt fibers partly dissolved in the alkaline environment, leading to the formation of a C–N–A–S–H gel surrounding the fibers. In contrast, cellulose fiber remained stable in both acidic and alkaline condition

Sintering and crystallization behavior of CaMgSi2O6-NaFeSi2O6 based glass-ceramics

We report on the synthesis, sintering, and crystallization behaviors of a glass with a compositioncorresponding to 90 mol % CaMgSi2O6−10 mol % NaFeSi2O6. The investigated glasscomposition crystallized superficially immediately after casting of the melt and needs a high coolingrate _rapid quenching_ in order to produce an amorphous glass. Differential thermal analysis andhot-stage microscopy were employed to investigate the glass forming ability, sintering behavior,relative nucleation rate, and crystallization behavior of the glass composition. The crystalline phaseassemblage in the glass-ceramics was studied under nonisothermal heating conditions in thetemperature range of 850–950 °C in both air and N2 atmosphere. X-ray diffraction studies adjoinedwith the Rietveld–reference intensity ratio method were employed to quantify the amount ofcrystalline phases, while electron microscopy was used to shed some light on the microstructure ofthe resultant glass-ceramics. Well sintered glass-ceramics with diopside as the primary crystallinephase were obtained where the amount of diopside varied with the heating conditions

Antibacterial properties and cytotoxicity of 100% waste derived alkali activated materials:slags and stone wool-based binders

Abstract In this study we compare the leaching behavior and the antibacterial and cytotoxic properties of 100% slag or stone wool derived alkali activated materials. The antibacterial activity was measured as the inhibiting capacity against two Gram-negative bacterial strains, Escherichia coli and Pseudomonas aeruginosa and one Gram-positive bacterial strain: Enterococcus faecalis. The cytotoxicity properties were tested on mouse embryonic fibroblast NIH-3T3 cell-line. It was proved that the high quality of the 3D aluminosilicate network of the consolidated materials obtained from powders of CaO or MgO-rich slags or stone wool, opportunely activated with NaO and/or Na-silicate, was capable of stabilizing heavy metal cations. The concentrations of leachate heavy cations were lower than the European law limit when tested in water. The effect of additives in the composites, basal fibers or nanocellulose, did not reduce the chemical stability and slightly influenced the compressive strength. Weight loss in water increased by 20% with basalt fibers addition, while it remained almost constant when nanocellulose was added. All the consolidated materials, cement-like in appearance, exhibited limited antibacterial properties (viability from 50 to 80% depending on the bacterial colony and the amount of sample) and absence of cytotoxicity, envisaging good acceptance from part of the final consumer and zero ecological impact. CaO-rich formulations can replace ordinary Portland cement (showing bacterial viability at 100%) with a certain capability for preventing the reproduction of the E. coli and S. aureus bacteria with health and environmental protection results