Geopolymers were produced using an environmentally friendly alkali activator (based on Rice Husk Ash and potassium hydroxide). Aluminosilicates particles, carbon and ceramic fibres were used as reinforcement materials. The effects of reinforcement materials on the flexural strength, linear-shrinkage, thermophysical properties and microstructure of the geopolymers at room and high temperature (1200 °C) were studied. The results indicated that the toughness of the composites is increased 110.4% for geopolymer reinforced by ceramic fibres (G-AF) at room temperature. The presence of particles improved the flexural behaviour 265% for geopolymer reinforced by carbon fibres and particles after exposure to 1200 ºC. Linear-shrinkage for geopolymer reinforced by ceramic fibres and particles and the geopolymer G-AF compared with reference sample (without fibres and particles) is improved by 27.88% and 7.88% respectively at 900 °C. The geopolymer materials developed in this work are porous materials with low thermal conductivity and good mechanical properties with potential thermal insulation applications for building applications.
Geopolymers are amorphous materials synthesized by the alkaline activation of a wide variety of aluminosilicate minerals, including metakaolin (MK), and industrial by-products such as fly ash, blast furnace slag, waste glasses, and others (
Rice husk is the hard coating of the rice grain, and is composed of minerals that need to be removed for human consumption. Between 20–25% of Paddy rice is an indigestible shell, this product is generally used as a fuel in a boiler for plant electricity generation or as a fertilizer for agriculture. The combustion of this shell generates ~18% volume of ash, however, the production of 1 ton of rice result in about 45 kg (RHA), depending on the rice variety, climate, type of furnace and burning temperature. Chemical composition and crystalline content of the RHA varies depending of processing conditions (
Geopolymers exhibit brittle behaviour under flexural loads, affecting their potential use for extensive engineering applications. By bridging cracks, a wide range of polymeric, mineral and natural fibres have been used to improve geopolymer tensile and flexural strength, toughness, and energy absorption capacities (
Geopolymer composite materials reinforced with ceramic particles have also been assessed, and the inclusion of alumina, nanosilica, zirconia (
This paper presents the assessment of MK-based composite geopolymers produced using an environmentally friendly alkali activator and two different types of fibres, carbon fibre and alumina fibre, which were introduced to reduce cracking. Refractory particles were also added to the geopolymers for the improvement of volumetric contraction and the study of mechanical performance. The effectiveness of these reinforcements was assessed by the examination of flexural strength loss resulting from exposure to temperatures up to 1200 °C. Effects of the high temperature on microstructure were studied using Scanning Electron Microscopy (SEM), and X-Ray Diffraction (XRD). Additionally, physical and thermal properties were evaluated.
A commercial metakaolin (MK) MetaMax®, supplied by BASF, was used as an aluminosilicate precursor. The MK exhibited a particle size distribution between 1 and 40 μm, with a mean particle size of 7.8 μm. A potassium silicate solution, derived from the dissolution of analytical potassium hydroxide pellets (KOH) along with rice husk ash (RHA), was used as an alkaline activator. RHA with 92% amorphous SiO2 content was obtained by controlled calcination of rice husk at 600 °C for two hours, followed by ball milling for 30 minutes, resulting in an average particle size of 22.84 μm. The proportions of RHA and KOH were adjusted to produce alkali activator solutions with a K2O/SiO2 molar ratio of 0.28. The commercial potassium silicate (K2O·SiO2·H2O) used as an alkaline activator (for comparison) was supplied by Productos Químicos Panamericanos®, the mass composition is K2O = 13.06%, SiO2 = 26.38% and H2O = 60.56%.
To increase the mechanical performance of the material, aluminosilicate refractory particles (RP) with a mean size of 81.95 μm were used for reinforcement. These particles were obtained from a KT33 brick (supplied by REFRASTRABE S.A.) using jaw crusher milling followed by ball milling. The fibrous reinforcement was carried out using the two fibres types described below:
Carbon fibres (CF): Panex 35®, supplied by ZOLTEK, had an absolute density of 1810 kg/m3, an average diameter of approximately 9 μm, and a length of approximately 100 μm.
Alumina fibres (AF): Alumina-silica-zirconia fibres, denoted as CERACHEM®, were supplied by Thermal Ceramics and had an average diameter of 2.18 μm. Chemical compositions and some physical properties of the raw materials used are listed in
Geopolymers were synthetized by adjusting the quantities of the precursor (MK) and the alkali activator to obtain overall SiO2/Al2O3 and K2O/SiO2 molar ratios of 2.5 and 0.28, respectively. A water-to-solid ratio of 0.4 was used for geopolymer matrix and 0.44 for composite geopolymer, the higher water-to-solid ratio for composites geopolymer was to achieve the same workability. The alkali activators were prepared 20 h prior to usage (by mixing RHA, KOH pellets and water) were stored in sealed plastic containers with magnetic stirring to achieve the completely dissolution of RHA. For comparison purposes, commercial potassium silicate was used like alkali activator for preparation of reference paste (KS). The concentration of KOH solution was 8 M.
Chemical composition and physical properties of raw materials used
MK | RHA | RP | AF | ||
---|---|---|---|---|---|
Chemical composition (wt %) | SiO2 | 51.52 | 92.33 | 57.99 | 50.00 |
Al2O3 | 44.53 | 0.18 | 36.62 | 34.90 | |
TiO2 | 1.71 | -- | 2.03 | 0.04 | |
Fe2O3 | 0.48 | 0.17 | 1.52 | 0.05 | |
Na2O | 0.29 | 0.07 | -- | -- | |
K2O | 0.16 | 0.15 | -- | -- | |
MgO | 0.19 | 0.49 | 0.51 | 0.07 | |
CaO | 0.02 | 0.63 | 1.32 | 0.08 | |
L.O.I. (950 °C) | 1.09 | 2.57 | -- | -- | |
Physical Properties | Density (kg/m3) | 2500 | 2140 | 2863 | 2650 |
BET Specific surface (m2/kg) | 12.7 | 78.28 | -- | -- |
L.O.I.: Loss on ignition.
The geopolymers matrix were obtained by mixing of alkali activator with MK using a HOBART mixer for 7 min to achieve adequate fluidity of paste. After mixing, the fresh paste was cast into plastic moulds and vibrated to release any residual air bubbles. Subsequently, the moulded samples were sealed with plastic film to minimize loss of evaporable water and then transferred into sealed containers.
Composites geopolymer were prepared following the procedure: (i) For the fibre-composites, fibres were pre-mixed for five minutes with alkali-solution before being added to the MK, composite geopolymers were produced using 3 vol.% of fibre mixture. (ii) For fibre-RP composites, refractory particles (20 vol.%) were added to the blend (activator, fibres and MK), after five min of mixing RP was added and continue the mixing for other two min to achieve adequate homogeneity before being cast.
All the samples were cured in sealed containers at 70 °C for 20 h and 90% RH. At the end of the curing time, the specimens were removed from their moulds and stored in sealed containers at room temperature (
Nine samples were prepared for each study (3 unexposed, 3 exposed to 600 °C and 3 exposed to 1200 °C). A summary of the different geopolymer composites is listed in
Compositions of the synthesized geopolymer composites
Geopolymer composite ID | Type of Fibre | Fibre volume, % | Particles volume,% | Silica source |
---|---|---|---|---|
KS | 0 | 0 | 0 | Potassium silicate |
G | 0 | 0 | 0 | RHA |
G-CF | Carbon | 3 | 0 | RHA |
G-AF | Alumina | 3 | 0 | RHA |
G-CF/RP | Carbon | 3 | 20 | RHA |
G-AF/RP | Alumina | 3 | 20 | RHA |
After 7 days of curing, the samples were dried for eight days at room temperature and for eight more days at 60 °C, with the aim that the adsorbed water was released slowly without generating thermal contractions that then will cause cracking before exposure at high temperature. To perform flexural strength tests on specimens exposed to elevated temperatures, the specimens were first heated at a rate of 1 °C per minute to the target temperature in an electrical furnace. Once the predetermined target temperature was reached, specimens were kept at the target temperature for 120 min to attain thermal stability. Then, the furnace heat was turned off, and the specimens were allowed to cool naturally. After cooling to ambient temperature, the specimens were taken out of the furnace, and flexural strength tests were conducted.
The bulk density, percent absorption, and percent voids in geopolymer samples were determined following the procedure described by the standard ASTM C642-13.
Linear shrinkage measurements were performed on a Netzch DIL 402 PC horizontal pushrod dilatometer. Measurements were taken between 25 °C–1000 °C, using cylindrical samples with a 4.6 mm diameter and a 25 mm length at a constant heating rate of 10 °C/min and a constant load of 30 × 10−2 N. Sapphire was used as a reference pattern.
Flexural strength was measured using a three-point bend test following ASTM C1341-13 for prismatic samples of dimensions 30×10×180 mm (
The thermal diffusivity of the samples was measured according to ASTM E1461-07, using the laser flash technique with an Anter Flashline 4010 system. Samples of each composition were tested in an argon atmosphere (~55 Pa) at room temperature. The specimens were prepared such that they were homogeneous without fissures and holes. The surface was coated with a graphite powder spray so as to avoid any reflectance. Three shots were taken for each sample at each temperature with a 1300Wlaser, and the diffusivity was calculated by utilizing the Clark and Taylor correction method (
Here α is the thermal diffusivity, Cp is the specific heat capacity at constant pressure and ρ is the density.
Fracture surfaces of the composites tested for flexural strength were observed by scanning electron microscopy (SEM), performed using a JEOL JSM-6490LV and 20 kV of accelerating voltage. The samples were coated with Au and observed in a low vacuum mode.
X-ray diffraction (XRD) data were collected on an X’Pert MRD PANalytical diffractometer with Cu Kα1 radiation generated at 20 mA and 40 kV. Typical specimens were step scanned from 8° to 60° 2θ at 0.02° 2θ steps, integrated at the rate of 4.0 s per step.
A summary of density and open porosity for the different geopolymers is presented in
Density and open porosity in geopolymers
Material | % Open porosity | Bulk density (kg/m3) |
---|---|---|
KS | 33.4 | 1740 |
G | 35.5 | 1690 |
G-AF | 38.4 | 1620 |
G-CF | 37.7 | 1700 |
G-AFRP | 30.2 | 1790 |
G-CFRP | 35.4 | 1450 |
The bulk densities in G, G-AF and G-CF are similar regardless of the fibre used. The open porosity of the geopolymers with fibers is significantly higher than the open porosity of the geopolymer without fibers (G and KS). This is expected since the composites samples contain higher amounts of water compared to the G matrix. Comparing the geopolymers G-AFRP with the reference sample based on RHA (G), the open porosity was found to be lower for G-AFRP due to the presence of RP reduces the porosity of the sample. G-CFRP is less dense than G, there are controversial results in the bulk density, because this material must be the material with higher density due to presence of BS and the result was not expected.
The load-displacement curves for the geopolymer composites are given in
P-d curves for geopolymers composites (a) at room temperature and (b) after exposition at 1200 °C.
The results showed that geopolymer composites with alumina fibres can withstand higher loads than those with carbon fibres can. This difference could be attributed to better bonds and possibly higher interactions between the alumina fibres and the geopolymer matrix, both due to similar chemical composition.
The toughness and the loading capacity of composite materials at 25 °C increased due to the use of both fibres and particles. These results were attributed to the modified post-cracking performance of the composites (
The effect of exposure to 1200 °C was different for the fibre-reinforced samples compared to the samples that were reinforced with both fibres and particles. It has been reported that the tensile strength of carbon fibres decreased with increasing temperature and that after exposure to 500 °C, carbon fibres only retained 25% of their ambient temperature strength (
Furthermore, in this case, the carbon fibres likely acted as particles due to their short length of 100 microns. This suggests that the incorporation of low fibre volume (3%) did not modify the ductility of the material but rather contributed to the strengthening of the matrix as a consequence of the obstruction of crack propagation paths.
Fracture Work of geopolymer composites at different heat treatment temperatures
Sample | Fracture Work, (J.mm) | |
---|---|---|
25 °C | 1200 °C | |
KS | 28.23 | 9.63 |
G | 22.14 | 16.28 |
G-CF | 27.76 | 53.34 |
G-AF | 46.63 | 59.66 |
G-CFRP | 49.81 | 47.13 |
G-AFRP | 47.73 | 7.089 |
Modulus of rupture for geopolymer composites before and after heat treatment at 1200 °C.
The results of MOR for fibre-reinforced composites (G-CF and G-AF) indicated that the inclusion of 3% volume had a noticeable effect on strength after exposure to elevated temperature fibre. Zhang et al. (
At 1200 °C, the MOR of the matrix was reduced, and the values obtained are similar to values reported by Zuda et al. (
The RP did not react with the matrix, and the composite material effectively acted as a large-particle composite. These particles reinforced by restraining the movement of geopolymer matrix in the vicinity of each particle; in essence, the matrix transferred some of the applied stress to the particles (
SEM line scanning images for composites with RP (a) to 25 °C and (b) to 1200 °C.
The physical appearances of the specimens heated to 1200 °C are presented in
Appearance of the MK-based geopolymer composites after exposure to 1200 °C for 2 h: (a) G, (b) G-CF and (c) G-AFRP.
Dilatometer curves of the G, G-AF and G-AFRP geopolymers are shown in
Linear shrinkage of the geopolymer samples (bulk samples were cured in sealed plastic tubes at 72 °C for 20 h).
Specific heat, thermal diffusivity and thermal conductivity were evaluated. The measured specific heat (standard deviation of 0.04) for geopolymers evaluated in this study is shown in
Thermal properties for geopolymer composite at room temperature
Sample | Specific Heat (J/kg.K) | Diffusivity (m2/s) | Thermal Conductivity (W/m.K) |
---|---|---|---|
KS | 525.8 | 2.1 x 10−7 | 0.2341 |
G | 936.91 | 1.6 x 10−7 | 0.2005 |
G-AF | 1151.95 | 1.6 x 10−7 | 0.2278 |
G-CF | 838.66 | 1.7 x 10−7 | 0.2369 |
G-AFRP | 894.69 | 2.0 x 10−7 | 0.2054 |
G-CFRP | 692.85 | 1.6 x 10−7 | 0.1598 |
The XRD patterns of the samples before and after heat treatment are shown in
XRD patterns for geopolymer composites before and after heat treatment at 1200 °C.
SEM was performed on polished geopolymer samples before (
SEM images of composite fracture surfaces before (a) G-AF, (b) G-CF, and after (c) G-AF, (d) G-CF heat treatment at 1200 °C for 2 h.
Physical properties such as density and porosity and the flexural strength, thermal conductivity in alternative geopolymers produced by RHA-based alkali activator are similar to the geopolymers produced with traditional commercial silicates making the alternative geopolymers a good ecological choice. Aditionally, flexural strength, density, porosity, thermophysical properties and microstructure of MK-based geopolymers activated with RHA and KOH and reinforced with CF, AF and RP have been presented. The high-temperature behaviour of composite geopolymers has also been described. Different types of fibres (carbon and ceramic) and RP were appropriate reinforcements for room temperature applications. Composites G-CFRP and G-AFRP achieved higher fracture work during flexural testing by (
Exposure to high temperatures promoted the structural rearrangement of the geopolymer matrix and densification of interfaces between different components of the composites. G-AFRP and G-CFRP reached peak MOR values of 18.62 MPa and 20.79 MPa, respectively. Although the fibres were not stable at high temperature, they prevented cracks, created by thermal stresses generated in the process of elimination of the OH groups, forming at lower temperatures. The thermal stability and increase in MOR after exposure to high temperatures could be attributed to matrix densification, leucite formation, and proper RP/matrix interface bond strength. The uses of RHA such a novel silicates source making the geopolymers a good ecological choice. The geopolymer materials developed in this work are porous materials with low thermal conductivity and good mechanical properties with potential thermal insulation applications for building applications.
This study was supported by the Colombian Institute for the Development of Science, Technology, and Innovation (Colciencias) (Project Hybricement, Contract N° 0638-2013), Center of Excellence for Novel Materials – CENM, and Universidad del Valle (Cali, Colombia).