# A Comprehensive Review on Clay Soil Stabilization Using Rice Husk Ash and Lime Sludge
## 2 Materials
### Rice Husk Asl (RHA)  
Rice husk ash is a byproduct obtained from the combustion or incineration of rice husks [73]. Rice husks are the protective coverings of rice grains and are generated during the rice milling process. It constitutes around 20-23% of the total weight of the rice grain [74]. Rice husks are agricultural trash that accounts for nearly one-fifth of the world's yearly gross rice production of 545 million metric tons [75]. Due to the high calorific power of rice husk (approximately 16,720 kJ/kg), it is increasingly used as a fuel source for heat generation in rice drying [76]. Rice husk ash is mainly produced by burning rice husks at around 450-700 \({}^{\circ}\)C for 1-3 h under controlled conditions [77]. During high-temperature combustion, the organic components of rice husk, including cellulose, hemicellulose, and lignin, are more effectively burned, leaving behind a higher percentage of silica-rich ash [78]. In contrast, during lower-temperature combustion, the combustion may be less efficient, and a more significant fraction of the organic matter may remain in the RHA, leading to higher carbon content [79].  
Based on the requirements, the collected RHA may undergo additional processing steps, including grinding or sieving, to achieve the desired particle size distribution and remove impurities. The resulting RHA is a fine, light powder with high silica content (60 to 90%) [80]. RHA primarily consists of amorphous silica (SiO\({}_{2}\)) and is chemically reactive. It contains a high percentage of siliceous elements, indicating its potential for exhibiting pozzolanic characteristics [81]. RHA does not possess inherent cementitious qualities; therefore, it is not used alone for soil stabilization and is commonly utilized as supplementary material in soil stabilization processes [82]. The addition of RHA can significantly influence macro aggregate production, while it has been observed that the addition of RHA decreases the fraction of micro aggregates smaller than 0.25 mm in size. The aggregation of clay particles with RHA can create larger pores, known as macropores, which help to drain the soil better, while it might not significantly impact the smallest-sized pores, known as cryptopores [83]. To sum up, rice husk ash, as agricultural waste, shows remarkable potential for stabilizing clay soils.  
To determine the elemental composition of rice husk ash, the results of the various chemical analyses of RHA performed by X-ray fluorescence (XRF) tests are given in Table 1. Furthermore, the mean and standard deviation (SD) were calculated to determine the average chemical composition and measure the variability of the data for RHA.  
Extracted results indicate that Silica (SiO\({}_{2}\)) is the major component of RHA, ranging from 80% to 96%. Silica in RHA is highly reactive and can contribute to pozzolanic reactions in cementitious materials. Potassium oxide (K\({}_{2}\)O) and sodium oxide (Na\({}_{2}\)O) are found in small amounts, less than 2%. Calcium oxide (CaO) content can vary, but it has been found in small quantities, less than 1%, contributing to the overall pozzolanic activity. Iron oxide (Fe\({}_{2}\)O\({}_{3}\)) content can vary depending on the rice husk source and burning conditions, below 2%. These percentages can vary based on the source of the rice husk, the burning temperature, and the combustion conditions. XRF analysis quantitatively measures these elements, helping to characterize RHA for its potential use in various applications, especially in the construction industry [89; 90].