# Bricks Using Clay Mixed with Powder and Ashes from Lignocellulosic Biomass: A Review
## 5 The Effect of Lignocellulosic Biomass Ash By-Products on Fired Bricks
### What Effect on Mechanical Properties? The most important property of construction materials is compressive strength. The compressive strength of various prepared bricks combined with lignocellulosic ashes is displayed in Figure 6. It can be observed that a reduction of compressive strength occurred when the ash content in the clay matrix increased. This result is coherent with bulk density and water absorption behavior. Bricks with higher porosity, lower compressive strength, and higher water absorption were produced at higher doses of lignocellulosic biomass ash. Due to their irregular shape and microscopic flaws, open pores can concentrate pressure and reduce the compressive strength of bricks [73]. It was noted that adding 10% and 20% by weight of olive bottom ash (OPBA), respectively, resulted in compressive strength values decreasing by 29.4% (33.9 MPa) and 71.52% (14.2 MPa) [60]. However, significant additions of 30 to 50 weight percent reduced the strength by about 78% when compared to the control bricks (47.96 MPa). For olive pomace ash (CVLL) a maximum value was reached in composite bricks by adding 5% fly ash which generates a compressive strength of 50.1 MPa [71]. Moreover, it was reported that the compressive strength results are consistent with the bulk density, bulk porosity, and water absorption data, the highest value obtained corresponds to the control bricks (53.3 MPa) [61]. Similarly, Ref. [67] noted that the compressive strength of fired clay bricks was also decreased with the addition of lignocellulosic ash. Additionally, Ref. [69] observed that as the amount of palm oil fuel ash (POFA) increased to 20%, the compressive strength decreased by 74% in comparison to the witness bricks. This fact could be related to the disintegration of organic materials during the firing process. It is to be highlighted that according to ASTM62-10 the compressive strength varies from 10 MPa for weathering bricks to 20 MPa for high-weathering bricks [74]. As depicted in Figure 7, the values of the resistance to compression for all bricks made with olive waste ash were somewhat similar, and these bricks were distinguished by a strong resistance to weathering. On the other hand, for rice husk ash, sugar cane bagasse ash, and palm oil ash, studies showed low resistance to weathering. The compressive strength values were somewhat similar for all these ashes due to their similar chemical composition rich in fluxing oxides and silica (Table 2), except for the rice husk ash in the study by [32] which is characterized by important contents of auxiliary fluxing oxides (CaO and MgO) which increased the mechanical resistance. As previously mentioned, a variety of factors, including the source of the ashes, their chemical makeup, the type of clay used, the size of the particles, etc., determine the mechanical strength of the bricks. For instance, the ash from olive waste contains significant amounts of auxiliary melting oxides, particularly calcium carbonate (CaCO\({}_{3}\)) and sodium oxide (Na\({}_{2}\)O), which are crucial for the mechanical strength of brick (Table 2). In fact, during firing, CaCO\({}_{3}\) is what causes the release of CO\({}_{2}\) and CaO. When water is present, the released CaO can combine with the SiO\({}_{2}\) in the clay to form calcium silicate, improving the mechanical performance of fired bricks. Due to the excessive CO\({}_{2}\) release, which results in pore dilation, the excess of these  
Figure 6: Thermal conductivity for samples made with lignocellulosic biomass ash: 10% OPBA [60], 6.5% OMW [63], 10% WA [65], 10% RHA [65], 5% RHA [68], 5% SBA [68]. ashes, however, favors the appearance of cracks and expansions. Na\({}_{2}\)O, on the other hand, acts as a melting promoter, generating a significant amount of liquid phase that contributes to the formation of the glassy phase and the densification of the fired products while filling the pores and voids.