West Africa has a huge potential for raw material resources that could be used to formulate thermal energy storage material, and this was explicitly investigated in [28] and in our previous work [36]. In the present work sand, clay and coal bottom ash (CBA) from a different region in West Africa (Burkina and Niger) are mined and used to formulate ceramic for sensible heat storage. The main mineral composition of sands studied in Burkina Faso and Niger is up to 80% of quartz (SiO₂), 9% of Alumina (Al₂O₃) and some other oxide [37] [38]. Clay is an argillaceous material usually composed of complex natural mixtures of minerals presenting very variable grain size and physicochemical properties. Clay is an argillaceous material usually composed of complex natural mixtures of minerals presenting very variable granulometry and physicochemical properties [39]. The different clays studied in Burkina Faso contain mainly kaolinite (>40 wt%), illite (>13 wt%), quartz (>18 wt%) and rutile (1 wt%) [39]. The chemical composition of the CBA used is reported in Table 1 and size distribution of dune and mining sand is reported in [26] [40].

Figures 7(a)-(c) and Figures 8(a)-(c), present the results (illustrated cases) of the bulk density and axial tensile strength for each formulation respectively at 1000˚C and 1060˚C.

The bulk density values range from 1730 kg/m³ to 2050 kg/m³ (±6%) and are similar to the one developed in the literature [52]. The overall bulk density of the pellet decreased with the increase of CBA ratio and the decrease of sand ration except the one obtained with the NS-N and C1 at 1060˚C. For example, it decreases by 9.71%, 6.21%, 1.12%, and 7.82% respectively for dune and mining sand from Burkina and dune sand and natural sand from Niger for the pellet fired at 1000˚C. The volatile elements, which influence the LOI in the different sand can explain the difference in decrease ratio especially for MS-B, DS-N and NS-N. This decrease could also be explained by the reduction of sand ratio because clay-sand ceramic density increase with fine sand ratio [52].

The porosity and LOI results during the formulation of the different ceramic balls range respectively from 20.8% to 28.57% (±2.94%) and 2.08 wt% to 5.71 wt% (±4.91%) according to the formulas.

The results show a slight decrease of the bulk density with increasing CBA wt% as illustrated in Figure 7. The increase of the porosity with CBA ratio could justify the decrease of the density due to the increase of mass loss, which increases the void volume. The trend of bulk density as function of porosity is illustrated in Figure 9. As the loss on ignition of CBA is high (15%), increasing its mass will drive to the increase of the specimens’ loss on ignition during firing which will increase the porosity of the specimen as illustrated in Figure 10.

For the axial tensile strength their values ranged from 350 kPa to 3500 kPa with standard error of ±10% according to the formulation. The results are presented in Figures 8(a)-(c). An improvement with the CBA ratio in all the formulation (ceramic ball) has been observed. The results are similar to the flexure strength obtained by [52] with sand-clay ceramic. The objective of this study being to combine sand particles together to improve it thermal properties of TESM, the tensile strength of the gotten ceramics is acceptable for all balls ceramic which contain ash waste, excepted N58A12C30-N and N58A12C1.30-N. The increase of CBA ratio in sand and clay ceramic shows an improvement of the axial tensile strength while the porosity increases. This shows that the ash plays a rule of binder like clay. Despite the fact that, the porosity of the specimen increase with CBA wt%, an increase of the tensile strength is observed (Figure 11). This can be explained by the chemical composition of the CBA given in [28] and the other raw material ( clay and sand). Generally, raw materials for ceramic pastes containing CaCO₃ are liable to form crystalline phases at relatively low temperatures from 950˚C to 1000˚C [53]. In addition, alkaline contributes to the formation of liquid phases at low temperatures [54]. As the ratio of alumina (Al₂O₃) is enough in the CBA, it could contribute to an increase the quantity of kaolinite. Therefore, during heat treatment, the major component of kaolinite is transformed into metakaolinite between 550˚C to 600˚C, with a dehydrated metastable amorphous structure. Between 950˚C to 1000˚C, it is very progressively transformed into an alumina spinel phase and amorphous silica. Just above 1000˚C, feldspar grains react with the amorphous silica and metakaolinite to form a viscous liquid. This process involves the progressive dissolution of the finest quartz grains into the liquid phase [39] [55]. The last process might be limited during the firing of the present ceramic (more limited for natural sand with 0.93 mm mean grain size), because the maximum firing temperature was 1060˚C while larger quartz (main component of the raw materials) grains dissolution begins at temperature up to 1100˚C [39] [55]. That can explain the high porosity of the specimens because the dissolution of quartz drives the reduction of ceramic pores. These phenomena could be the difference from one clay to another because the highest tensile strength ceramics have abstained with pottery clays (CI). In the end, we can conclude that CBA was successfully integrated with a ceramic formulation, potentially providing an end-use for this waste at a temperature less than 1100˚C.

However, to select the best TESM base on the tensile strength, it is better to take into account the compressive strength dropped during heat treatment. This drop of compression strength can reach 30% to 60% for rocks like granite depending on the working temperature [56]. To avoid any ball crushes in the storage system, a minimum compression strength of 1MPa is needed. However, this drop of mechanical performance could be low for ceramic or porcelain specimens due to their refractory character defined as the collapse of the material under its own weight at high temperatures [57]. Therefore the ceramic balls formulated with sand, clay and CBA excepted N58A12C30-N and N58A12C130-N have been considered in the following work for thermal analysis.

The overall ceramic balls developed with sand clay and CBA excepted the formulations N58A12C30-N(1060) and N58A12C120-N(1060) have been selected based on the mechanical strength performance. Their thermal properties have been assessed with accuracy of ±10% and measurement error, and presented as Table 3 and Table 4 with the different standard errors.

The thermal conductivity of the different specimens ranges from 0.331 Wm⁻¹·K⁻¹ to 1.014 Wm⁻¹·K⁻¹ with standard error of less than 10% for each formulation. In general, the thermal conductivity obtained is less than the required value need for a thermal energy storage material (≥1 Wm⁻¹·K⁻¹) except the formulation M58A12C1.30-B(1000). The overall thermal conductivity is low for the pellets. The effect of CBA ratio and firing temperature is not significate on thermal conductivity. However, the minimum thermal conductivity is 0.331 W·m⁻¹·K⁻¹ showing that sand and ash improve the thermal conductivity of the different ceramic balls developed and can even reach 1.014 Wm⁻¹·K⁻¹, compared to clay ceramic thermal conductivity obtained without any additive fired at 1200˚C in [52] which is around 0.3 Wm⁻¹·K⁻¹. Below 1050˚C, the clay ceramic is not properly fired, which strongly reduced its thermal conductivity as demonstrated in [58] where a value less than 0.3 Wm⁻¹·K⁻¹ was measured for kaolin clay ceramic fired at 1050˚C. As the effect of thermal conductivity on TES performance can be negligible [33], the obtained ceramic balls could stand well in term of conduction characteristic.

The thermal diffusivity of the different pellets ranges from 0.123 mm²·s⁻¹ to 0.177 mm²·s⁻¹ with standard error less 10% for each formulation. As the thermal diffusivity is the ratio of thermal conductivity to volumetric heat capacity, it can be defined as the rate of temperature spread through a material and measured the heat transfer from the hot material to the cold. A high value of thermal diffusivity can drive to the degradation of thermocline energy storage by thermal diffusion [59]. In the literature, a required thermal diffusivity value for thermal energy storage is not available, however, a reasonable thermal diffusivity is required to avoid thermocline degradation.

The volumetric heat capacity or volumetric specific heat (VHC) of the different ceramic balls ranges from 2.4075 MJ·m⁻³·˚C⁻¹ to 3.426 MJ·m⁻³·˚C⁻¹ with standard error less 10% for each formulation as presented in Table 1 and Table 2. The results are better for the composition with 58 wt% or 55 wt% sand, 12 wt% ash and 33 wt% or 30 wt% clay because the highest VHC is obtained with M58A12C30-B(1060). This can be explained by the sand particle size effect on the specific heat capacity which is around 0.26 mm, 0.91 mm and 0.93 mm respectively for dune sand, mining sand and natural sand. Increasing and reduction respectively of the ash powder wt% and sand wt% could drive to the reduction of the mean particle size in the ceramic. As consequence, the volumetric heat capacity can drop as demonstrated by Bwayo et al., in 2014 on specific heat capacity which increases with particle size (sawdust) of fired brick developed from sawdust and clay [60]. However, all the ceramic ball developed in this study met the required VHC (up to 2 MJ·m⁻³·˚C⁻¹) to be used as sensible heat storage materials in the CSP plant according to Fernandez et al., [32]. Some ceramic balls have a volumetric heat capacity higher than 3 MJ·m⁻³·˚C⁻¹ at room temperature (Table 3 and Table 4) and can become better at high temperature. Therefore, the ceramics developed could compete with molten salt at high temperature because the favoured molten nitrate salt has a volumetric heat capacity of 3 MJ·m⁻³·K⁻¹, and limited to a temperature change at approximately 300˚C [61].

The thermal stability qualifies the sample loss of mass with temperature or under heat treatment. A material with high thermal stability shows high resistance to thermal decomposition. Forty-two (42) ceramic balls (numbered from 1 to 42 before the test) have been treated during 60 cycles as demonstrated in Figure 8 including those with low tensile strength like numbers 1, 4, 6, 7, 10, 17, 20, 21, 23, 26, 29, 32, 35, 38 made only with sand and clay. After every 20 cycles, the specimens were photographed and presented in Figure 12 as follows.

Only a colour change has been observed for all the different ceramic balls. Even the one with the lowest tensile strength did not crack after the 60 cycles. No crack has been observed after 60 thermal cycles even for the most fragile ceramic in this study. In addition, Figure 13 present the mass loss obtained with the base of the selected specimen on tensile strength after 60 cycles. The maximum mass loss registered is 0.87% compared to 0.6% obtained by [20]. This difference can result from the raw materials type, the mechanical properties, the number of cycles which are 60 in this study again 20 and the heating and cooling rate. Therefore, we can conclude that all the ceramic balls made with sand, clay and CBA could be suited for SHS from the axial tensile strength assessment, thermal properties and thermal stability analysis.