# A Comprehensive Review on Clay Soil Stabilization Using Rice Husk Ash and Lime Sludge
## 2 Materials
### 2.2. Lime Sludge (LS)  
Lime sludge is a byproduct produced in various industrial processes, mainly in the paper industry and water treatment plants. In the pulp and paper industry, a significant source of lime sludge, approximately 100-200 kg of LS, can be generated per ton of pulp produced. Global pulp production is around 185 million tons annually, which could imply around 18.5 to 37 million tons of lime sludge per year from this sector alone [91]. Lime sludge can vary in composition depending on the specific industrial process and the impurities in the treated lime. However, it primarily contains calcium carbonate (CaCO\({}_{3}\)) [92]. LS can have beneficial applications as a waste material, as adopted and illustrated in Figure 1, particularly in soil stabilization and construction materials [93].  
Lime sludge can be deployed as a stabilizing agent to amend the engineering properties of soil or as a raw material in the production of cementitious materials [94]. However, proper waste management of lime sludge is necessary to prevent environmental pollution. Lime sludge can be deployed to change the engineering properties of fine-grained soils and the fine-grained fractions of granular soils [95]. It can reduce the plasticity of fine-grained  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline
**Source** & **CaO** & **SiO\({}_{2}\)** & **Na\({}_{2}\)O** & **MgO** & **Al\({}_{2}\)O\({}_{3}\)** & **Fe\({}_{2}\)O\({}_{3}\)** & **K\({}_{2}\)O** & **TiO\({}_{2}\)** & **LOI** \\ \hline
[84] & 0.578 & 96.235 & 0.054 & 0.269 & 0.281 & 1.366 & 0.454 & - & - \\
[85] & 0.82 & 80.82 & 0.96 & - & 0.25 & 0.38 & 1.25 & - & 12.70 \\
[86] & 0.37 & 87.49 & 0.01 & 0.01 & 0.05 & 0.04 & 0.8 & 0.01 & 11.21 \\
[87] & 0.62 & 89.69 & 1.02 & 0.57 & 1.01 & 0.23 & 0.81 & 4.06 & - \\
[88] & 0.82 & 80.02 & 0.01 & 0.74 & 1.81 & 0.79 & 1.01 & 0.43 & 10.62 \\ Mean & 0.6416 & 86.8508 & 0.4108 & 0.39725 & 0.6804 & 0.5612 & 0.8648 & 2.245 & 11.51 \\ SD & 0.1675 & 5.96 & 0.474 & 0.279 & 0.861 & 0.387 & 0.265 & 2.11 & 1.07 \\ \hline \end{tabular}
\end{table}
Table 1: Chemical composition of RHA obtained by XRF (in percent).  
Figure 1: Utilization possibilities of LS [93].  
soils, such as clay, resulting in reduced potential for shrinkage and swelling and improved soil workability, strength, drainage, and permeability, making the soil more stable [96]. The lime sludge treatment effectively stabilizes plastic clays and other fine-grained soils with high water storage capacity. Soils with higher plasticity or water content generally require a higher lime sludge dosage for effective stabilization [97]. However, as a general guideline, a lime sludge content of 3-10 percent by dry weight is sufficient to achieve adequate stabilization [98]. The results of the chemical analysis of LS using the X-ray fluorescence (XRF) test are given in Table 2.  
The results obtained from different studies demonstrate that calcium oxide (CaO) is a major component, reflecting the high calcium content derived from the lime used in treatment processes. The concentration can vary widely but often ranges from 40% to 95% depending on how much lime was used and how completely it reacted. Silica (SiO\({}_{2}\)) is present in varying amounts, from 0% to 5%. Iron oxide (Fe\({}_{2}\)O\({}_{3}\)) is present in smaller quantities, less than 1%, and can come from various sources, including the treatment process or impurities in the lime. Magnesium oxide (MgO) is found in smaller amounts, less than 1%. It can come from the mineral composition of the lime or other additives used in the treatment process. Sodium oxide (Na\({}_{2}\)O) is in smaller amounts, less than 3%. The exact composition of lime sludge can vary based on factors such as the source of the water being treated, the type of lime used, and the specifics of the treatment process. XRF provides a detailed elemental analysis that helps understand and manage lime sludge for various applications or disposal methods [103; 104].