# A Comprehensive Review on Clay Soil Stabilization Using Rice Husk Ash and Lime Sludge
## 1 Introduction
### Conventional Stabilizers  
In construction projects, Ordinary Portland Cement (OPC) is primarily utilized as a binding agent in producing concrete, mortar, and grout. It is mixed with aggregates and water to create a paste that hardens and develops strength over time. OPC-based concrete structures are widely utilized for building structural elements [38]. In the soil stabilization process, OPC can be deployed as an additive to improve the engineering properties of the soil. Adding OPC to the soil can enhance mechanical characteristics [39]. The process typically involves thoroughly mixing the OPC with the soil, forming a cementitious matrix that binds the soil particles together. This improves soil cohesion and increases its resistance to erosion [40]. The effectiveness of OPC as a soil stabilizer depends on factors such as the soil type, moisture content, and the desired outcome of the stabilization. It is commonly used for stabilizing cohesive soils such as clay or silt [41]. Calcium silicate hydrate (CSH) is the main product of the hydration of Portland cement [42]. CSH is the primary binder and the main component responsible for the strength and durability of cement-based materials [43]. It forms through a chemical reaction between cementitious materials such as Portland cement and water. CSH is responsible for developing the hardened cement paste's strength. It reacts with water and the soil particles, improving cohesion, increasing strength, reducing permeability, and improving stability. It helps to bind the soil particles together and create a more stable and compacted soil matrix [44]. Calcium aluminum silicate hydrate (CASH) is another hydrated mineral compound formed in cementitious materials. CASH is a specific type of calcium silicate hydrate that contains additional alumina (Al\({}_{2}\)O\({}_{3}\)) in its structure [45]. It forms when cement containing alumina-rich materials, such as fly ash or slag, undergoes hydration. It is the primary product of the chemical reaction between cementitious materials, such as Portland cement and water, during hydration [46]. It is responsible for developing the strength and durability of hardened cement paste. CASH is an amorphous material composed of calcium, aluminum, silicon, and oxygen atoms, with water molecules trapped within its structure [47]. It forms a gel-like substance that fills the spaces between cement particles, providing cohesiveness and strength to the cement matrix. While both CSH and CASH contribute to the strength and durability of cementitious materials, CASH can exhibit different properties compared to traditional CSH due to the presence of alumina [48]. The addition of alumina can affect the structure and mechanical properties of the hydrated phases, influencing the overall performance of the cementitious material [49]. Conventional stabilizers, including cement, are widely used in construction but contribute significantly to carbon emissions, raising serious environmental concerns. As sustainability becomes a priority, finding eco-friendly alternatives is essential to reduce the environmental impact of such materials. Consequently, efforts are being made within the cement industry to reduce its carbon footprint and mitigate carbon dioxide (CO\({}_{2}\)) emissions by:  
1. Using alternative fuels, such as biomass or waste-derived fuels, which can help lower CO\({}_{2}\) emissions. Additionally, using alternative raw materials, such as industrial byproducts, can reduce the need for limestone extraction and decrease associated emissions [50; 51]. 2. Implementing energy-efficient technologies and practices can reduce the overall energy consumption and emissions from cement production. This includes optimizing kiln operations, improving heat recovery systems, and employing more efficient grinding processes [52]. 3. Carbon capture and storage (CCS) technologies aim to capture CO\({}_{2}\) emissions from cement plants, store them underground, or utilize them in other industrial processes.