3.1. Oil Shale Ash Material Characterization
The BET specific surface area (SSA) and the mean particle size of the OSAs considered in this paper are summarized for APP (EPA, TA) in
Figure 2, and for EPP (EPA, TA) and EN280 (TA, CA) in
Figure 3. The variation of the physical properties is shown for different sampling dates. In case of APP OSA, EPA shows consistently finer particle sizes than TA, even though BET SSA fall within the same range. The EPP EPA and TA do not show this difference in particle size and are somewhat coarser (
Appendix A:
Table A1 and
Table A2). The particle size of EN280-TA is coarser than EN280-CA and showed a correspondingly lower SSA.
All OSAs studied mainly consist of SiO
2, CaO and Al
2O
3, however in variable proportions (
Figure 4). The main reason of such differences could be explained with several factors including characteristics of raw OS, OS processing conditions, thermochemical conversions, gaseous treatments for cleaning, boiler specifications, etc.
In general, the high content of CaO can be explained by the decomposition of CaCO
3 which originally exists in the mineral part of OS and partially decomposes during the thermal processes. The presence of SiO
2 can be attributed to the inorganic part of the OS and mainly is representing finer particles as the content of silica is higher and lime content is lower in smallest size fractionated ashes. The chemical composition of OSAs differs and each type appears to show a distinct composition (
Appendix B:
Table A3). For instance, the high LOI of the EN280 residues is related to the lower temperature processing during retorting compared to combustion of OS causing delay in decomposition of carbonates and oxidation of unburnt carbon. Specifically, in case of EN280 TA, the high LOI of the residue correlates with a low SSA and coarse PSD which are all negative factors in terms of self-cementing behavior of these types of ashes especially when it is considered that fly ashes with high LOI can negatively affect the strength of the concrete (more water absorption, air-entraining, etc.) and increase setting times [
20,
21].
The variations in chemical composition are reflected in the phase composition as well as shown in
Table 1,
Table 2 and
Table 3. Some fluctuations in phase composition of the ashes were observed throughout the year in the EPA fractions. The amount of free CaO is higher than 10% for EPP and APP ashes (EPA, TA) and mineral CO
2 content does not reach above 11%. Calcite and dolomite contents of EN280-CA and TAs are quite high, and the content of Ca/Mg silicates (C
2S, akermanite, mervinite) and aluminates are quite low compared to all other studied samples due to the low temperature processing during retorting.
The samples that were used in last two sub-studies do not include ashes from oil shale retorting and these sub-studies only focus on the ashes produced from the combustion of oil shale. Both ashes used in the sub-studies are fly ashes (70% of the total ash produced) collected from electrostatic precipitators where most of the ash is accumulated during the combustion of oil shale. By studying these ashes throughout the year, we noticed that the minerology, chemical and physical properties do not change drastically. Another important factor for selecting the ashes in the last two sub-studies, is the content of free CaO.
3.2. Self-Cementing Properties of Oil Shale Ash
Self-cementing ability and setting times of the considered OSAs are shown in
Figure 5 and
Figure 6. The self-cementing ability is affected by the chemical composition and fineness of ashes, in particular the content of free CaO, portlandite, Ca/Mg silicates and SO
3 [
22,
23]. The self-cementing ability of ashes with coarse particle sizes and lowest temperature of treatment is generally lowest. This is demonstrated for EN280-CA and TAs.
Comparing the self-cementing properties of all studied ashes, the performance of the APP-EPA and APP-TA was found to have better properties. Between these two samples of ashes, APP-EPA is shown to have optimal suitability. This is tentatively explained by a higher BET SSA and a considerable content of lime and portlandite (~14–23%), Ca-Mg silicates (C2S) and C4AF (~15–22%) that can participate in hydraulic or pozzolanic reactions. Therefore, APP-EPAs and EPP-EPAs have potential self-cementing properties.
Setting times were tested according to EN 196-3:2016 in order to evaluate how long the samples remain in a plastic state that enable placing or casting the mixes. However, most of the studied mixes demonstrated very short setting times. This is tentatively related to the quick hydration of free CaO [
24].
Because of this reason, less reactive ashes (EN280) would require post-treatments like higher temperature treatment, grinding, sieving etc. in order to increase their performance. Due to insufficient amounts of reactive phases present the EN280 ash specimens fell apart during the curing of samples as cohesion is lost after evaporation of water in the 60% RH environment (
Figure 7).
3.3. Clay Brick Production Using OSA
It can be seen from
Table 4 that EPP-EPA has more complex and different chemical composition than the clay which mostly consists of SiO
2, Al
2O
3, Fe
2O
3 and K
2O. The main oxides in EPP-EPA are SiO
2—31.94%, CaO—35.25%, Al
2O
3, MgO and the content of SO
3 is relatively high as well. The LOI is 4.8% due to the release of combined water, crystalline water, combustion of organic carbon and oxidation of sulfur.
The mineralogy of the clay shows a heterogeneous mixture of minerals (
Table 5) and can be subdivided into clay minerals (~60%) including kaolin, illite, illite-smectite, mica and chlorite, and non-clay minerals (~40%) including mainly quartz, orthoclase and minor constituents such as gypsum, pyrite and calcite. The clay used in this work is also characterized by a higher BET SSA—30.86 m
2/g and smaller mean particle size (15.1 μm).
Based on the observations made on prepared samples, as a first impression it can be mentioned that the color is one of the parameters to consider when producing bricks, as different type of ashes would lead to significant changes in color when compared with the reference bricks. It can be observed in
Figure 8, EPP-EPA does make the color of bricks lighter and give a new yellowish color.
The densities of bricks after molding, drying and sintering are presented in
Table 6. Dry densities are lower than wet densities and values are proportional to the required initial water content for molding the bricks. The reaction between free CaO and water bonds water chemically which makes the dry density not exactly proportional to initial water added to the EPP-EPA added raw mixtures. It can also be seen that the values of reference bricks show a noteworthy increase in density after sintering while the density of EPP-EPA added bricks show only a slight increase. The shrinkage of bricks after sintering is given based on the volume changes and it can be noted that the reference bricks have the highest shrinkage. With the addition of EPP-EPA, a notable reduction of shrinkage can be explained due to the different phase composition of EPP-EPA compared to sand which is mainly inert and as a result accompanies different volume reduction. The presence of Ca(OH)
2, Ca and Mg carbonates, partially Ca- sulphate (the decomposition of sulphates can proceed well below 1000 °C in presence of CO [
25]) in EPP-EPA strongly influences the brick microstructure by releasing H
2O, CO
2 and SO
2 due to the new decomposition reactions attributed to EPP-EPA (Equations (1)–(4)) in addition to clay mineral dehydroxylation, quartz inversion, crystallization and formation of vitreous phase reactions [
26].
The physical and performance properties of the bricks are affected by the increased porosity of the EPP-EPA bricks. The compressive and flexural strength constitute the main parameters of brick performance. The obtained average value for the compressive strength EPP-EPA added bricks was ~20 MPa and for reference bricks ~30 MPa. These results correlate with the increased porosity of the EPP-EPA bricks as higher water absorption is usually associated with higher porosity as well. Water absorption values (
Table 7) showed lowest water absorption for the reference bricks and EPP-EPA addition led to an increase in water absorption value (2.5 times).
Thermal properties are important in terms of heat insulation and energy performance of buildings. The thermal conductivity of a composite material is determined by the properties of its constituents. In case of clay bricks the thermal conductivity varies depending on porosity and conductivity of the solid constituents and exhibits a decrease in trend with bulk density [
27]. It can be seen from the measured thermal conductivity values of the fired bricks, given in
Table 7, that EPP-EPA bricks have 50 % better insulation properties than the reference bricks.
An overall porosity reduction is expected when above 1000 °C the vitreous phase fills the pores and the ceramic body shrinks. From the porosity results shown in
Figure 9, it can be understood that the EPP-EPA samples show a higher volume of pore features in the range of 0–4 μm compared to the reference.
Additionally, the SEM images of the brick samples are shown in
Figure 10 and it is evident that a higher degree of particle interlocking, and more homogeneous texture can be observed for the reference brick microstructure.
The presence of carbonates strongly influences the brick microstructures and lowers the degree of shrinkage. This is partially due to the additional porosity generated during decomposition of the Ca carbonate into free lime and CO
2 gas (Equation (2)). The CaO further recombines with the clay minerals during crystallization reactions and results in changes of the brick phase composition. Similar findings were also mentioned in other studies (Junge [
28], Sütcü and Akkurt [
29]) where other waste additives with high CaCO
3 content (i.e., paper making sludge, limestone powder) were evaluated for brick production and these types of residues were identified as pore-forming additives.
The phase composition of the EPP-EPA and the reference bricks is compared in
Table 8, the main difference is the presence of a significant fraction of plagioclase (Ca-feldspar) in the EPP-EPA bricks. In addition, small amounts of anhydrite and mullite were identified as well. Clearly CaO introduced by the EPP-EPA reacted with the aluminosilicates in the clay to form plagioclase feldspar. Anhydrite did not decompose fully during the sintering.
3.4. Carbonate Bound Monolith Production from Oil Shale Ash
CaO/MgO and Ca/Mg-silicates in OSA are potential phases for CO2 sequestration via carbonation. By carbonation of compacted samples, CO2 can be permanently stored as Ca or Mg carbonates. The formation of carbonates is associated with an increase in solid volume and a decrease in porosity of the compacts. The carbonates act as cement by forming solid bridges between reacting particles and infilling of porosity.
The compressive strength test results of the compacted APP-EPAs are quite promising and compacts that are made of coarser fraction have greater compressive strength (up to 41.4 MPa) for both tested pressure levels (
Table 9). Interestingly, the compacts carbonated at 5 bar show higher strength values compared to 10 bar. This may suggest that high pressures lead to fast reactions causing pore clogging near the surface of the compacts and zonation instead of homogeneous carbonation. In fact, previous studies have also shown that excess CO
2 pressure does not necessarily lead to a higher compressive strength as slower reaction would allow for dissipation of heat and reduce stresses on the product [
30]. Further research into the microstructure of the carbonated compacts is needed to verify the mechanism controlling the carbonation reaction and to further optimize the performance of the carbonated products.
Thermogravimetric analysis measurements of the compacts made from coarse fraction of APP EPA are presented in
Figure 11. The mass loss due to the release of CO
2 that is related to decomposition of carbonates is indicated for the uncarbonated sample as well as it already includes CaCO
3 in the raw untreated ash which is measured as 6%. The samples cured at 5 and 10 bar show higher mass loss which is related to the mineral CO
2 bounded during carbonation, indicating that the CO
2 uptake during the carbonation treatment can be up to ~9% of the total mass of the sample.
The phase composition of the uncarbonated and carbonated compacts are given in
Table 10. Portlandite Ca(OH)
2 and ettringite are major phases present in the uncarbonated sample. In the carbonated samples a strong increase in the calcite (CaCO
3) fraction is notable. Calcite appears to form mainly at the expense of portlandite and ettringite. Additional to calcite, some gypsum has formed by carbonation of ettringite. Also, C
2S and merwinite appear to have partially carbonated. As noticed as well in the TGA, portlandite is not fully consumed indicating that further improvement of the process is possible. The fine ash fraction of 0–100 μm shows differences in its initial phase composition being higher in quartz, K-feldspar and mica and lower in anhydrite phases. This is reflected in the phase composition of the carbonated material.