GENERALLY ABOUT GEOPOLYMER FOAMS
Roland Szabó1, Gábor Mucsi2 PhD student, Associate Professor
Institute of Raw Material Preparation and Environmental Processing, University of Miskolc
ABSTRACT
Present paper deals with the brief summary of literature in the topic of geopolymer foams using various processes and materials. Geopolymers are amorphous three- dimensional aluminosilicate materials which can be produced at ambient or elevated temperature by alkaline activation of aluminosilicates. Alkali activation of these materials together with chemical foaming agent (aluminum powder, hydrogen peroxide or sodium perborate) led to the synthesis of inorganic foam. Reaction between aluminum powder and alkali activator or decomposition of hydrogen- peroxide in geopolymers cause porous structures. Geopolymer foams are fire- resistant materials and hence, fabricating geopolymer foams with enhanced thermal resistivity can be considered as an effective way of their usage.
INTRODUCTION
Geopolymers are inorganic polymers which can be produced by reaction between alumino silicate oxides (obtained from natural minerals, industrial wastes, calcined clays or mixture of these materials) and alkali silicates in alkaline medium. This reaction yielding amorphous to semi-crystalline three-dimensional polymeric structures, which consist of Si-O-Al bonds [1, 2, 3].
Every material is suitable for geopolymerisation which contains alumina and silica barrier phases, like natural rocks or secondary raw materials (fly ash, slag and red mud) [2, 4].
Cellular structure of geopolymer foam can be prepared by chemical foaming agent.
Reaction between aluminum powder and alkali activator or decomposition of hydrogen- peroxide in geopolymers cause porous structures [5, 6, 7, 8, 9].
Geopolymer foams may be produced by gelcasting too, using the geopolymerization reaction to stabilize the gas bubbles introduced in the liquid slurry by rotational mixing [11].
Geopolymer foams possess excellent physico-chemical and mechanical properties, including low density, high strength, thermal stability, good fire and chemical resistance [5, 6, 7].
MATERIALS AND FOAMING PROCESS
Different raw materials suitable for the synthesis of geopolymer foams like fly ash [5, 6, 7, 8], perlite [9] or metakaolin [10, 11]. For the foaming of inorganic polymers can be used hydrogen-peroxide [8, 9], aluminum powder [5, 6, 7] or sodium-perborate [8] as chemical foaming agent.
DOI: 10.26649/musci.2015.014
The reaction between the Al metal powder and alkaline activator proceeds quickly, during the reaction releases hydrogen (reaction (1)) [5].
2Al + 2NaOH +2H2O 2NaAlO2 + 3H2 (1)
The hydrogen-peroxide is thermodynamically unstable and therefore can be easily decomposed to water and oxygen gas (reaction (2)) with the latter playing the role of the geopolymeric paste blowing agent [9]:
2H2O2 2H2O + O2 (2) CELLULAR STRUCTURE OF GEOPOLYMER FOAMS Effect of the amount of liquid activator and foaming agent
The fly ash based foam (FAF) nucleation and stability of the pore structure in the fresh geopolymer paste strongly depends on the viscosity of the initial mix [5]. The viscosity can be easily changed by the amount of added liquid activator, i.e. by the liquid/solid ratio (L/S ratio). In case of the viscosity of the paste is low (high L/S ratio), the foam-ability is higher than the L/S ratio is low (Figure 1) [5, 7, 10].
Figure. 1
The liquid/solid and Al/solid ratio effect on the FAF pore distribution [5]
Based on the Figure 1 can be seen that the increasing amount of Al powder resulted increasing of the pore size.
As it can be noticed in Fig. 1, the amount of Al metal powder has a less significant effect on the pore structure than the amount of liquid activator [5].
The microstructure of perlite based geopolymer foam is presented in Figure 3.
Figure 3
SEM photos of foamy materials prepared with different H2O2 content in the geopolymeric paste [9]
The cells are normally closed and almost spherical when the content of H2O2 in the paste is low. If the content of H2O2 increase coalescence occurs among cells and the cells’ geometrical shape change from spherical to oval [9].
The effect of H2O2 content in the perlite based geopolymeric paste on the mean cell size shown in Figure 3.
Based on the experience observed in Figure 3 it can be stated that higher content of H2O2 resulted higher mean cell size [9].
Figure 2.
Mean cell size as a function of H2O2 content in the geopolymeric paste [9]
PROPERTIES OF GEOPOLYMER FOAM Mechanical properties
The geopolymer foam possess relatively high compressive strength (5.5-10.9 MPa) [5, 6, 8], but increasing the liquid/solid ratio and amount of foaming agent (Al powder or hydrogen-peroxide) in the geopolymer paste results in certain decrease in compressive strength [7, 8]. Higher content of foaming agent in the paste results in lower specimen density (Figure 4.) [5, 7, 8, 9]
Figure 4.
Apparent density and cell volume of foamy materials as a function of % w/w H2O2
content in the geopolymeric paste [9]
Thermal conductivity and fire resistant
Geopolymers foams are fire-resistant materials [5, 6, 7] and hence, fabricating lightweight geopolymers with enhanced thermal resistivity can be considered as an effective way of their usage [7]. The perlite based geopolymer foams possess low thermal conductivity (0.03 W/K m) [9], but the fly ash based geopolymer foam dispose of relatively low thermal conductivity (in the range of 0.11-0.39 W/m K) [5, 6, 8].
CONCLUSIONS
Based on the above brief review the following conclusions can be drawn:
- The initial mix viscosity is affected significantly the foam-ability.
- In case of the low viscosity of the paste (high L/S ratio) the foam-ability is higher.
- When the amount of foaming agent is low, the spherical bubbles have low population density in the geopolymeric paste.
- Specimens with lower liquid content result in higher strengths and higher density.
- Increasing content of foaming agent results in lower specimen density.
- Our future work is to carry out a systematic research work using fly ash for geopolymer foam and monitor the effect of mechanical preparation (grinding, classification, mixing) of fly ash on the resulted foam structure.
ACKNOWLEDGEMENTS
Authors greatly appreciate the financial support for the project “Market orientated research and development of an innovative, eco-friendly thermal insulator material using polistyrene waste” (PIAC 13, 2014-16).
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