Abstract: Geopolymers were fabricated from some Japanese volcanic ashes. 30 g of volcanic ash with 200μm in diameter was mixed with 10 ml of sodium hydroxide solution with various concentrations to form slurry which became geopolymer after curing. When 8.5~11.5 mol/L of sodium hydroxide solution was used, the compressive strength of the resultant geopolymers reached to 25-35MPa. However, when the volcanic ash with high silica content was used, the compressive strength of the geopolymer was under 20 MPa. Furthermore, the addition of sodium silicate hydrate into starting slurry which was consisted of volcanic ash and sodium silicate solution had not effected on the compressive strength of geopolymer. In contrast, the compressive strength of the geopolymer decreased to 30 % of compressive strength compared to that of original geopolymer after water immersion for 3 days. However, crushing treatment of the volcanic ash contributed to retain the compressive strength. Actually, when 10μm of volcanic ash was used to fabricate geopolymer, the compressive strength improved to 70% compared to that of original geopolymer.
Abstract: Geopolymers are inorganic materials obtained by the alkaline activation of aluminosilicate sources. The ammonium molybdate could be used as a complexant for silica in order to complex the siliceous species in the alkaline solution. According to this, the aim of this work is to control the siliceous species and to understand the role of ammonium molybdate as a complexing agent acting on the formation of the different networks. To do this, additions of ammonium molybdate (up to 0.32% molar) in the silicate solution were realized along the formulation of geopolymer using two metakaolins. The results highlight that the addition of ammonium molybdate in geopolymer results in a decrease of the shrinkage at high temperature. Moreover, X-ray diffraction data and SEM after calcination show that geopolymers without ammonium molybdate form two phases (KAlSi2O6 and KAlSiO4) while with additions of molybdate, there were only the phase KAlSi2O6 associated with Al2O3 doped Mo and K2Mo2O7. Finally, SEM observations show that additions of ammonium molybdate seem to favor crystallization. The results allow to evidence the role of molybdate in the control of the polycondensation reaction in order to influence the formation of specific network
Abstract: Inorganic foams offer several unique properties such as low thermal conductivity, fire resistance, or UV stability. Inorganic foam specimens were synthesized from fly ash and aluminium powder through an alkali-activation process. Depending on mix proportions, bulk densities ranged between 400 and 800 kg/m3. Thermal treatment at 80°C for 12 hours accelerated curing process. Compressive strength was found in the range 4.5-9.0 MPa, flexural strength 0.6-1.7 MPa, Young's modulus 0.6-1.1 GPa, thermal conductivity 0.14-0.16 W/m/K and thermal capacity around 1100 J/kg/K. Exposing the foams to temperature 800°C led to a small decrease of compressive strength while exposure to 1100°C sintered the foam to higher strength of 13 MPa. Volumetric shrinkage 20% occurred at 1100°C without further disintegration. Residual compressive strength was determined after exposure to NaCl, HCl, Na2SO4, MgSO4, H2SO4. The highest reduction to 20% occured in both acids with pH=2 after one year of exposition. Digitized microstructures entered finite element analysis to validate a stress-strain diagram.
Abstract: The choice of precursors is a key parameter in the geopolymerization mechanism since it governs the kinetic of the reaction as well as the working properties of the final materials. This study focuses on the effect of the alkaline solution reactivity and metakaolin properties on the geopolymer formation. For this purpose, several geopolymer samples were synthesized from two alkaline solutions and three metakaolins. The structural evolution of formed geopolymers was investigated using FTIR spectroscopy and the following of the pH value during the formation. The measurement of mechanical strength was tested by compression. The results allow to evidence that for a less reactive alkaline solution, the reactivity of metakaolin governs the geopolymerization reaction. However, the alkaline solution is the steering reaction when it is highly reactive. Therefore, the extent of depolymerization of the alkaline solution and the reactivity of the metakaolin seem to control the rate of polycondensation and the mechanical properties of the geopolymer materials.
Abstract: Geopolymers, and more in general alkali activated materials (AAM), are a new class of materials obtained by alumino-silicates precursors activated by means of alkaline solutions. Indeed, the term geopolymers is usually strictly referred to pure alumino-silicates such as metakaolin as starting material, whereas when the precursors also contain calcium oxide the resulting products are usually defined AAM. Geopolymerization technology can be more easily considered a sustainable process when industrial waste is used as precursors and the consolidation process occurs at room temperature. With these premises, alkali activation may be a very promising technology for the ceramic sector as well as construction industry. In this work, waste coming from bricks production has been used to obtain, at room temperature, geopolymers with different porosity tuning the sodium silicate content in the feed. Microstructure analysis carried out by means of mercury intrusion porosimeter and scanning electron microscopy is reported and discussed.
Abstract: The focus of the present paper is to investigate the effect of the activating solution on the structure and mechanical properties of inorganic polymers synthesised from a slag resembling the vitrified residue from a Waste-to-Energy plasma installation. The slag consists of (in wt.%) 22 CaO, 12 Al2O3, 34 SiO2 and 20 Fe2O3 and the activation solution was 50:50 mass ratio NaOH and sodium silicate, with the NaOH solution molarities varying from 2 M to 10 M. The synthesised slag was almost completely amorphous due to the rapid cooling, with only traces of magnetite and quartz. The inorganic polymers were prepared by mixing the slag, sand and activation solution. In all cases, heat was generated during sample preparation and its amount increased with the activating solution strength. After 90 days, the compressive strength of the samples activated with 6 M or higher NaOH solutions was similar, approximately 88 MPa. For NaOH activation solutions with molarities lower than 6 M, the compressive strength was lower, both at early as well as late curing times. SEM and EPMA analysis revealed-between undissolved particle remnants-a distinct binder phase, composed of (in wt.%) 18.9±2.5 CaO, 11.5±0.1 Al2O3, 40.3±2.1 SiO2, 15.8±1.2 FeO, 5.1±1.9 Na2O and 3.7±0.6 MgO. In conclusion, the present study showed that the CaO-Al2O3-FeO-SiO2 vitrified residue could be converted into a stable inorganic polymer having reasonably high mechanical strength, when activated with a mixture of sodium silicate and sodium hydroxide solution with a molarity of at least 4 M.
Abstract: The carbonation of Portland-cement-based materials involves the reaction between atmospheric CO2 and calcium ions in the pore solution. The formation of calcium carbonate is responsible for a decrease in the pH of the pore solution from 12.5 to 9, thus leading to the depassivation of steel reinforcements and their possible corrosion, and can also lead to efflorescence (white crystals formed on the surface). In metakaolin-based geopolymer activated by sodium silicate, in which calcium is almost non-existent, the presence of CO2 will lead to the formation of sodium carbonates. Since geopolymer can be carbonated, the risk of corrosion or efflorescence needs to be assessed. A pH study of the geopolymer pore solution showed a very fast decrease compared to OPC, with almost total carbonation after only 14 days. In natural atmospheric CO2 conditions, it was found that the formation of sodium carbonate did not lead to a decrease of the pH to below a value around 9, thus limiting the risk of corrosion by depassivation of reinforcement, but the large amount of carbonate suggested a significant risk of efflorescence. A study of accelerated carbonation performed under an atmosphere of 50% CO2 highlighted the formation of sodium bicarbonate resulting in a lower pH of the pore solution and a much larger amount of product formed. Finally the study of efflorescence carried out by semi-immersion tests in natural or accelerated conditions confirmed the different nature of the crystals formed (sodium carbonate or bicarbonate) but showed no significant impact on the amount of carbonated products. This study thus demonstrates that the accelerated carbonation test had very limited usefulness, given the rapidity of the natural reaction. Furthermore, it was found that this test did not reproduce reality as it led to different reaction products.
Abstract: There are several factors that affect geopolymerization, including the type and ratios of the starting materials as well as the curing conditions of the initial mixture. The effect of the synthesis parameters on the formation of inorganic polymers are usually examined by “changing one factor at a time”. In this study Taguchi experimental designing model was applied in order to study the synergetic effect of selected synthesis parameters on the compressive strength development of metakaolin based geopolymers. The experimental design involved the variation of three control factors in five levels. The selected factors and the corresponding level range were: i) the alkali to aluminum molar ratio in the starting mixture (0.5-1.5), ii) the kind of alkali ion (Na and/or K) and iii) the molar ratio of Si to alkali oxide in the activation solution (0-2.0). The compressive strength of geopolymers was measured and the final products were also examined by means of XRD, FTIR and SEM. As it is concluded, the optimal synthesis conditions for metakaolin geopolymers are R/Al=0.75, Na/(Na+K)=0.50 and [Si]/R2O=1.50, while the factor having the highest impact on the development of compressive strength is the [Si]/R2O ratio.
Abstract: The growing focus on issues related to the control of CO2 emissions, energy conservation and waste recycling pushes the construction industry to tackle the challenge of sustainable development. The production of ordinary Portland cement (OPC), main product of the sector, is one of the most polluting in terms of CO2 emissions, thus finding alternative binder is becoming an urgent matter. Geopolymers are largely investigated for this purpose, but studies concerning the durability of reinforced conglomerates prepared with alkali activated binders are only few. The present work aims at investigating the durability performances of steel reinforced geopolymer mortar samples based on carbon fly ash in comparison with OPC mortar. The effect of different Na2O/SiO2 molar ratios in the geopolymer mixes is evaluated in terms of mechanical and microstructural properties as well as corrosion resistance in a chloride rich environment. The obtained results show that under the same severe environmental conditions more limited chloride amounts penetrate in reinforced fly-ash geopolymers where a better corrosion behaviour is also detected up to a week of exposure for samples with a nominal Na2O/SiO2 molar ratio equal to 0.12 and 0.14. Instead, the corrosion resistance is quite similar to that of reinforced OPC mortar when a period of three months is considered.