3.1. Visual Aspect and Colour Change of the Heated Foams
The visual aspect of the sample’s surface is presented in Figure 2
. Two foams were obtained with two levels of powdered aluminium addition, Al-1 and Al-2. The difference in structure and porosity is represented in Figure 1
, where the specimens’ surface after demoulding and porosity visible on the cross-section was depicted. The double aluminium powder content did not significantly increase the number of pores in the material but induced the porosity type, size and pore distribution.
The sample surface of Al-1 was significantly different from Al-2, so we clearly distinguished large pores for Al-1. The surface of the Al-2 sample presented a characteristic wall effect—the geopolymer paste concentrated near the mould wall. On the cross-section, we observed that Al-1 had a significant number of big pores. The pore sizes varied; we had large pores, up to 5 mm, as well as large amounts of fine pores. In the case of Al-2, the distribution of pores and their sizes were more uniform (see Figure 2
The visual aspect of the sample’s surface after heating is shown in Figure 3
and Figure 4
, where the colour change is represented. As a result of heating, a progressive colour change was observed. A pinkish colouration was noted after heating to 600 °C, in all the samples. The red colourations were more pronounced after 1000 °C where the effect of iron compound oxidation was most pronounced. For the samples heated to 1000 °C, their bending was due to more intense shrinkage resulting from sintering (Figure 3
and Figure 4
). The contraction was more intense in the bottom part of samples, where more dense and less porous material was placed, and the shrinkage was more pronounced. After heating, the samples were inspected to observe any defects or chipping. None of the samples were damaged during heating. The samples’ integrity after heating was maintained for both compositions Al-1 and Al-2.
3.2. Apparent Density, Density and Porosity of Heated Foams
The apparent densities of unheated material were 615 ± 60 kg/m3
Al-1 and 835 ± 50 kg/m3
for Al-2, and their true densities evaluated with a helium pycnometer were 3021.3 kg/m3
and 2840.2 kg/m3
). Samples containing a double portion of powdered aluminium (Al-2) had a slightly higher apparent density, despite a higher amount of aluminium powder. The expected porosity increase was not observed. The evaluated total porosities reflected this observation. The porosity of Al-1 was 80% and that of Al-2 was 70% (Figure 6
). The lower-than-expected porosity of Al-2 may be due to the too low content of the compounds reactive with aluminium oxide in the precursor, releasing hydrogen, which forms porosity in the geopolymer structure. The lower density of Al-1 was attributed to a more significant number of pores in the material and their larger size, as shown in the cross-section presented in Figure 1
When heated, the apparent density of foams slightly decreased due to the matrix and aggregate drying. The moisture contained in the material was gradually removed by heat. This resulted in a slight density decrease at 200 °C. Further heating did not significantly affect the apparent density change. At higher temperatures, the physically bound water evaporated, and the hydroxyl groups were removed. Some sources point out that the process of dehydroxylation starts at 250 °C and continues up to 600 °C [31
], which induces shrinkage of the geopolymer binder. An increase in density after exposure to 800 °C was due to shrinkage and matrix densification related to the sintering process. The relatively high amount of alkalis led to a glass transition that started at approximately 700 °C and recrystallisation of melting phases.
The density (ρ) and apparent density (ρo
) values enabled the total porosity determination (P). Porosity was calculated from the equation P = 1 − (ρo
/ρ). The evolution of total porosity P with temperature is represented in Figure 6
and shows the stable behaviour of geopolymer foams from coal gangue with temperature increase, positive aspect in case of fire-resistant material. The constant level of porosity, 83% and 78% at 200 °C, remained unchanged up to 800 °C. The stable porosity level in the whole range of tested temperatures ensured constant thermal insulation parameters with increasing temperature, which is extremely important for fire protection.
3.3. Mechanical Properties Evolution with Temperature
The mineral foams with apparent densities similar to the tested ones (600 and 800 kg/m3
), for example, aerated concrete, present relatively high mechanical performance. The data presented by the Foamed Concrete Composition and Properties, British Cement Association give the range of values 1.0–1.5 and 1.5–2.0 MPa in compression for densities of 600 and 800 kg/m3
. In Figure 7
, absolute values of compressive strength in MPa and relative values of compressive strength fcT
[-] were presented. Relative value refers to the compressive strength of the material (fcT
) heated to temperature T related to the reference strength of unheated material (fc20
). As presented in Figure 7
, foams Al-1 and Al-2 presented mean values of compressive strength of 1.30 MPa and 3.30 MPa, respectively, at room temperature. This was higher than the levels reached by the aerated concretes.
For Al-1 geopolymer foam, the compressive strength remained almost unchangeable up to 400 °C when compressive strength started to increase. The Al-2 foam, after exposition at 200 °C and 400 °C, a decrease of mechanical performances in compression was observed. The first change is usually connected to dehydration [6
]. This process causes a decrease in mechanical properties [6
]. The tests of mechanical properties for Al-2 heated to 200 °C and 400 °C presented approximately 20% and 30% decrease in strength. To investigate the possible reasons for this reduction, SEM observations were undertaken. The SEM observation confirmed a more significant number of cracks in the material heated to 200 °C and 400 °C.
Nevertheless, similar to Al-1, progressive strength regain was observed with higher exposure temperatures. The essential strength growth due to sintering was noted for Al-1. The mean values of compressive strength at 800 °C and 1000 °C were 2.4 MPa and 5.4 MPa, which represents a relative increase of almost 2- and 4-fold. The microstructure of foams’ matrices presented a dense, sintered and vitreous structure.
At temperature range 550–850 °C, for many geopolymers and their composites, significant shrinkage is observed [19
]. It can be observed under a microscope as a structure with fewer voids and a smoother texture [19
]. At this temperature, densification by vitreous sintering of the geopolymer matrix causes changes in microstructure [19
]. Additionally, this kind of structure leads to crack healing [40
]. In the case of investigated materials, the obtained results showed the possibility of occurrence of this mechanism at a temperature of approximately 800 °C with a slight increase of mechanical properties and caused the first change in microstructure. At 600 °C, these changes were not observed, and the microstructure was similar to that at lower temperatures.
Material that does not present a sharp strength decrease when heated can be considered a suitable characteristic for fire-resistant material used as a thermal barrier.
The flexural strength values evolution with temperature for the two tested geopolymer foams is presented in Figure 8
. The reference values (20 °C) of tensile strength tested in three-point bending of Al-1 and Al-2 were 0.58 MPa and 1.71. Figure 8
presents the evolution of absolute values of flexural tensile strength in MPa and relative values of ftT
[-]. Relative value refers to the tensile strength of the material (ftT
) heated to temperature T related to the reference strength of unheated material (ft20
). The evolution of tensile strength with temperature follows a similar pattern for both tested foams.
It was observed that mechanical behaviour in tension remains quasi-identical for the temperatures 200–800 °C. This was a positive aspect of the developed materials. Their mechanical behaviour was favourable for thermal barriers.
3.4. Microstructure Investigation with the Use of a Scanning Electron Microscope (SEM) and Chemical Composition
To explain the observed changes in mechanical and physical properties, microstructure was investigated using a scanning electron microscope (SEM). Unheated, as well as heated, samples were investigated to target temperature (Figure 9
and Figure 10
). The pictures were taken at different magnifications to illustrate changes in the material structure and phenomena connected with exposure to high temperature.
shows the microstructure for the Al-1 samples. The microstructure observed in the temperature range between 20 °C and 400 °C was quite similar. At 400–600 °C, some cracks are visible (Figure 9
d). After heating to 800 °C, the sintering process and vitreous structure appear (Figure 9
e). The structure of the samples exposed at 1000 °C was quite different (Figure 9
f). It is characterised by large glassy areas. The particular phases in the material were partially melted, which explains the increase in the mechanical performances (fc
). To supplement the information, energy dispersive spectroscopy (EDS) analysis was provided for the whole selected areas presented in Figure 9
to compare the oxide composition for the Al-1. The results are presented in Table 4
The oxide composition presented in Table 4
is typical for geopolymer material. Taking into account the qualitative character of this type of investigation, there were no significant differences between materials exposed at different temperatures. The dominant oxides are silicon dioxide and aluminium oxide. In the composition, we observed that there was no CaO in the geopolymer structure. SiO2
build the main structure of the material. The important compound is sodium oxide. It plays an important role in the geopolymerisation process [1
]. As the temperature decreased, the amount of sodium oxide decreased. It could be the effect of the qualitative characteristics of the method (selection of the area where sodium oxide was not fully reacted) or the decomposition of this compound that has a place in the temperature of approximately 600 °C [41
]. In addition, the amount of sodium oxide decreased with increasing temperatures. The amount of iron oxide increased. The existence of iron in this composition caused the reddish colour of the material obtained. The presence of iron is usually considered an advantage in the geopolymerisation process [42
]. It is a high probability that a large amount of this oxide appeared at a temperature higher than 800 °C. The confirmation of this phenomenon is also the change in colour of the samples exposed at 800 °C and 1000 °C, as FeO has a dark colour (usually black) compared to other iron compounds, for example, Fe2
that has a red colour. Other oxides, such as potassium and magnesium, appear in low amounts and their amount is more or less stable.
The results obtained for Al-1 were compared with the results obtained for Al-2. The microstructure of these samples is presented in Figure 10
Similar to Al-1 samples, the microstructure of samples Al-2 in the temperature range between 20 °C and 600 °C is typical for geopolymer structures [43
]. In the case of the samples exposed at the temperature of 800 °C, the vitreous sintered structure was observed, which is presented on the left side in Figure 10
f. After exposition to 1000 °C, most of the structures presented a glassy phase, and the material structure was significantly different than at lower temperatures (Figure 10
). Above the temperature 850 °C, the crystallisation of geopolymers is observed [19
]. For the investigated materials, an increase in mechanical properties was noticed, and clear changes in microstructure were observed. Some previous investigation shows similar phenomena for metakaolin-based geopolymers, where loses its strength up to 800 °C due to dehydroxylation, have displayed a strength gain at over 1000 °C due to sintering [22
]. A similar phenomenon was observed on waste clays [43
] and geopolymers with additional silica fume [47
]. Other investigation shows the possibility of changing the porous structure in the material that caused structure reinforcement [48
] or, in some cases, decreasing the mechanical properties due to intensive cracking [19
]. Bai et al. [49
] reported that for porous geopolymers containing a large amount of waste glass in the temperature range 700–900 °C, an effect of secondary foaming. In our investigation, we did not notice such behaviour.
Furthermore, for the selected areas, the oxide composition of Al-2 was provided (Table 5
), corresponding to the selected areas presented in Figure 10
The results obtained for the oxide composition for the Al-2 samples are quite similar to those of the Al-1 samples. This was expected because for both compositions, the same raw material was applied. The dominant oxides were silicon dioxide and aluminium oxide. The double aluminium portion used in the preparation process did not influence the amount of aluminium in the oxide composition of the investigated samples.
In the composition, we observed that CaO was not present in the geopolymer structure. In the ambient temperature and the samples exposed at the temperature 600 °C, titanium dioxide appeared in this composition, but the amount was small, and it should be treated as impurities that come from raw material. The overall tendency was coherent with decreasing amounts of sodium oxide and increasing amounts of iron oxide with the temperature; however, there was some incoherence for 800 °C that should be treated as imperfections of the used method of analysis that has a qualitative character.
The other possible explanation for the reinforcement mechanism is transit to metakaolinite into Al–Si spinel and mullite [50
]. This phenomenon should be confirmed by other research. The temperature resistance of geopolymers is an advantage, which is indicated by numerous literature reports [3
To provide a more extensive analysis of the results, the diffractograms of crystalline phases XRD present in unheated geopolymer foam A1-1 (20 °C) and heated to 1000 °C were provided. This enabled the comparison of phases present in the geopolymer foam matrix before and after heating. In unheated material, muscovite (39.5%), quartz (34.1%), albite (22.6%) and mullite (3%) were present. After heating to 1000 °C the mullite amount increased significantly, reaching a level of 37.7%, which is in agreement with the literature observations [50
]. The other components were quartz (33.6%), albite (27.4%) and muscovite (1.4%) (Figure 11
and Figure 12