# Stabilisation of Soft Clay, Quick Clay and Peat by Industrial By-Products and Biochars
## 1 Introduction  
Soil stabilisation is used extensively throughout the world to improve soil characteristics. One application of soil stabilisation involves the improvement of engineering properties of thick soft soil deposits, typically referred to as deep soil stabilisation or deep mixing, to enable the development and construction of buildings, roads, railways and other land use. Deep soil stabilisation is performed by mixing a binder into the soil, whereupon chemical reactions strengthen the soil matrix. Traditionally, cement (CEM) and quicklime (QL) have been the preferred binders. However, their production is well known to emit considerable amounts of carbon dioxide (CO\({}_{2}\)). Globally, cement production alone contributes to approximately 8% of global anthropogenic CO\({}_{2}\) emissions [1, 2]. To the best of the authors' knowledge, there is no estimation of the total CO\({}_{2}\) emissions generated from the deep soil stabilisation of soft soil deposits. However, it is safe to assume that it is considerable. There are major research efforts being made across the world to decrease CO\({}_{2}\) emissions in the soil stabilisation industry as well as in the cement and concrete industries. One way of achieving this is to substitute CEM and QL with alternative binder types and, since society is creating large amounts of unused industrial by-products (IBPs), these could be a potential substitute. IBPs often come in the form of ashes from the combustion of the original waste product. In the soil stabilisation industry, research and testing on a laboratory scale have been conducted using a wide variety of IBPs, e.g., cement kiln dust [3; 4; 5], lime kiln dust [6; 7], fly ash [8; 9; 10], bottom ash [11], gypsum [12; 13], ground-granulated blast-furnace slag (GGBS) [14; 15; 16; 17; 18] and ladle slag [12]. In addition, other binders or additives have been studied, such as biopolymers [19; 20], geopolymers [21], fibres [22], reactive magnesia [5], colloidal silica [23] and natural or synthetic zeolites [24; 25]. These studies have illustrated the potential to reduce emissions when using IBPs. However, despite all the research efforts, it is difficult to establish a common framework for their stabilisation effect. This might be attributed to the fact that IBPs can differ considerably in their composition and to the high variability of soils in terms of, e.g., grain size distribution, mineralogical composition, water content, organic content, etc. It is, therefore, vital that studies on the stabilisation effect of different IBPs in different countries and regions continue to be performed. Recently, there has also been increased interest and research on the use of biochar as an alternative binder. Biochar is manufactured by the pyrolysis of biomass, i.e., combustion in an inert atmosphere (i.e., the absence of oxygen), resulting in a non-biodegradable carbonaceous material with typical carbon contents exceeding 75%. It is characterised by a high porosity and large surface area and, hence, a high water-absorption capacity [26; 27], and has several potential benefits, including soil fertility improvement and contaminant immobilisation [28]. Recent studies have also shown that biochar in mixtures with cement can have a positive stabilisation effect in soils [27; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39], meaning that it can be used as an alternative binder along with its carbon sequestration potential. Since 1.0 kg of biochar contains a minimum of approximately 0.75 kg of carbon (C), it can sequester over -2.7 kg of CO\({}_{2}\)eq as no decomposition or erosion takes place in low permeable thick clay deposits. This amount of sequestered carbon is substantial, considering that corresponding CO\({}_{2}\)eq emissions for CEM and QL, as examples, are approximately 0.65 kg and 1.0 kg CO\({}_{2}\)eq, respectively, per kg binder [6]. To make a CEM-biochar-based binder climate neutral, i.e., resulting in net zero CO\({}_{2}\)eq emissions, one thus needs only to replace approximately 20-30% of the CEM with biochar.