# Stabilisation of Soft Clay, Quick Clay and Peat by Industrial By-Products and Biochars
## 4 Discussion  
### Stabilisation Effect for Different Binders in Clays  
To further analyse the comparative stabilisation effects of biochars and IBPs, the relative strengths for both the Onsey and the Tiller-Flotten clays when the biochars and IBPs were added to CEM are plotted in Figure 7. It was observed that most of the biochars had a negative effect on strength development (see Figure 7a), apart from BC1 and BC4 in the Onsey clay, where the strength increase was \(\sim\)20-40%. Notably, BC1 and BC4 also had the least negative effect in the Tiller-Flotten clay, where the strength only decreased \(\sim\)15%. The largest negative effect was observed for BC2, where the strength in the Tiller-Flotten clay decreased by a considerable \(\sim\)65%. These results indicate that the use of biochar in the stabilisation of clays results in relatively few benefits, which can be expected since it does not contain any calcium-based cementitious properties. However, previous tests have revealed positive stabilisation effects on the Tiller-Flotten clay [29], although with a different type of biochar. Notably, the stabilisation effect of biochar in soils has been observed to be highly dependent on the type of biochar and its properties such as SFA, water content and grain size distribution, and the resulting stabilisation effect thus varies greatly [37, 84]. Research on mortars, concrete and asphalt has also shown that biochar can be beneficial as a substitution or additive, although negative effects also have been reported [85, 86, 87, 88, 89]. In those cases where the biochar has shown positive effects, this has mostly been attributed to the water retention properties of biochar. This reduces water evaporation during curing, reduces microcracking and provides a continuous source of water for the cement hydration occurring over time [85, 86]. In addition, the micropores of the biochar can provide space for the cementitious compounds resulting from pozzolanic reactions [90, 91]. Whether a positive or negative effect is obtained depends on the biochar content in relation to the cement content, the water saturation of biochar prior to mixing and the grain size distribution. Generally, finer-grained and pre-soaked biochar is preferred in cement mortar mixtures [38]. Typically, positive effects have been reported for biochar contents up to \(\sim\)2-3%, beyond which, a negative effect is often observed, especially if the content is over \(\sim\)10% [88, 92]. How these findings translate to soil stabilisation has thus far not been properly investigated. As clays typically have a large amount of water readily available for cement hydration, the potential benefits from water retention properties might be lower compared to those from mortars and cement. On the other hand, biochar could enable densification in clays with high porosity and, thus, both the initial porosity and amount of biochar per unit volume of clay would affect the stabilisation effect. Previous tests on soils have shown  
Figure 7: Average strength of mixtures of biochars and IBPs relative to mixtures with 100% CEM as a binder for the Onsey and Tiller-Flotten clays, (**a**) biochars, and (**b**) IBPs. that an optimal amount of biochar can be found [29; 38], which should be studied further with the soils used herein. Another important parameter that highly affects the stabilisation effect is the binder uniformity, i.e., the distribution of binder in the soil that is obtained after mixing. The higher binder uniformity of calcium-based binders such as CEM and QL was observed to be beneficial [93]. How these results apply to biochar is not known and, although it could be assumed to have the same effect, this effect must be studied further. Based on simple linear regression analyses, there appeared to be no correlations between the stabilisation effect and the mineralogical or chemical oxide compositions of the biochars as shown by the XRD and XRF analyses.