# 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  
5.1.1. Linear Shrinkage  
Linear shrinkage is a key parameter for fired bricks. It describes the contraction or expansion behavior of bricks during heat treatment. It can be determined by measuring the length of the sample either before and after drying or before and after firing. In addition, it can be evaluated over the whole process using a caliper.  
\[\text{Linear Firing Shrinkage}(\%)=\frac{\text{L}_{\text{dried}}-\text{L}_{ \text{fired}}}{\text{L}_{\text{dried}}}, \tag{1}\]  
For the bricks produced using ashes, several factors could affect the linear shrinkage after firing either by contraction or by expansion. In general, the contraction phenomenon is related to the quantity of glassy phase formed during firing. As a result of a glassy phase being filled into the continuous voids and holes in the porous structure of ceramic material, a densification phenomenon takes place, resulting in the contraction of the elaborated product. The flux of materials such as K\({}_{2}\)O and the auxiliary flux materials such as CaO, MgO, and Na\({}_{2}\)O, contained in the lignocellulosic biomass ashes, could play an important role during the glassy phase. Indeed, these oxides play fluxing or melting promoters' roles allowing the cooking temperatures to decrease and to generate a greater quantity of liquid phase which contributes to the formation of the glassy phase. However, the expansion could be attributed to the dilation of pores due to the gases emitted during the firing process (mainly CO\({}_{2}\) and O\({}_{2}\)). In the studies [60; 61; 62], It was observed that the addition of olive bottom ash (OPBA), olive stone ash (OSA), olive oil wastewater ash (OMW), and olive pomace fly ash (CVLL) to fired clay bricks causes an expansion condition because there is a greater effect of gaseous materials in olive waste than flow materials on the brick. In fact, as shown in Table 3, these wastes are rich in significant amounts of gaseous metals and, more specifically, significant amounts of carbonates, which decompose and release CO2 in the pores where the phenomenon of expansion was observed for all the bricks made from the ashes of waste olives [60]. For palm oil fuel ash (POFA), Ref. [70] observed shrinkage of different samples when the POFA content increased. This shrinkage varied from 0.3% for pure clay to 0.6% and 0.9% when 1% and 10% of POFA were used, respectively. Nevertheless, the linear shrinkage increases progressively with increasing percentages of ash addition for all used ashes.  
It is to be highlighted that shrinkage depends strongly on the nature and chemical composition of the biomass ash used and mainly on the content of melting oxides and continuous gaseous components in the ash, so that no real comparison could be made between the different types of ash (Figure 2). However, it is recommended that the curing shrinkage should be between 2.5% and 4% to maintain good mechanical performance [71]. Indeed, excessive shrinkage causes stresses and breaks of the prepared material as is exhibited in Figure 2 despite that all samples were manufactured with different types of lignocellulosic ashes respecting the standard conditions.