# 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
### Effect on Physical Properties
#### 5.1.3 Water Absorption  
The compressive strength and durability of the elaborated samples are greatly impacted by water absorption, making it a crucial bricks parameter. For lignocellulosic biomass ash bricks, the water absorption depends mainly on the percentages of the added ashes. All of the aforementioned research projects showed that water absorption rises as the proportion of biomass ashes used in the clay matrix increases. The hydrophilic nature of biomass ash accounts for the increased ability of the hardened matrix to absorb water [70]. Additionally, the addition of the continuous organic matter in the lignocellulosic ash acts as a porogen material, leaving pores and voids during the curing process so that the open porosity increases the water absorption. The olive bottom ash (OPBA) used by [60] is a water-absorbing material. By adding this type of ash to the clay matrix, water absorption capacity is increased significantly. The increase becomes even more important when adding more than 20% OPBA. Results show that water absorption varies from 16.3% for the control brick and reaches up to 31% for 50% by weight of OPBA. This increase in water absorption could be attributed to a higher open porosity in the matrix structure. The addition of OPBA produced numerous open porosities, as well as large macrospores. As a result, the open porosity of the OPBA bricks increased from 5.6% for the pure clay to 9.5% for the brick  
Figure 2: Linear firing shrinkage for samples made with lignocellulosic biomass ash: olive bottom ash (OPBA) [60], combustion of olive pomace (CVLL) [61], olive stone ash (OSA) [62], oil mill wastewater (OMW) [63], Rice Husk ash (RHA) [63], palm oil fuel ash (POFA) [70]. produced by 50% by weight of OPBA. However, the closed porosity remained constant regardless of the reinforcement rate. By adding 10 wt.% OPBA, the microstructure seemed to be similar to pure clay with fewer pores, which resulted in a small increase in water absorption. According to the results of testing the olive pomace (CVLL) [61] addition to the clay bricks, higher water absorption was observed by adding 10% or 15% by weight of CVLL. In brick firing, the degradation of organic matter contained in the lignocellulosic ash is believed to be the main cause of this increase in porosity. The addition of olive stone ash (OSA) also increases water absorption [62]. Water absorption for clay bricks was 16.9%, but with 10% and 30% by weight of OSA, it increased to 19.9% and 23.1%, respectively. Regarding rice husk ash (RHA), it was found by [31; 32] that adding 2% to 4% of this ash to the clay matrix reduces water absorption when compared to clay bricks and produces a good stable, and less porous structure. This character could be mainly attributed to the presence of amorphous silicon oxide which is present in large quantities in the chemical composition of these ashes (Table 2). However, percentages greater than 4% may result in a possible reduction in the surface's adhesion to the clay matrix, which is explained by an increase in RHA's granulometry as a result of which the clay matrix's porosity increases, increasing water absorption. According to [65], the water absorption produced by the addition of 30% blank wood ash (WA) was less interesting (21.9%) than the water absorption produced by the addition of 30% rice husk ash (RHA), which was 32.9%. This discrepancy results from WA ash having a higher proportion of melting oxides than other types of ash. Oxides encourage the formation of the liquid phase, which tends to approach the finer pores of the clay matrix and reduce open porosity. However, as the percentage of WA rises, this mechanism loses its effectiveness. As the percentage of WA ash increases, more open porosity, more water absorption, and less density are produced. In the same context, Ref. [67] observed a lower water absorption for bricks incorporating rice husk (RHA) compared to bricks with sugar bagasse ash (SBA). They cited SBA's porous structure as a potential contributing factor to this behavior.