## 4 Discussion
### Classification of Fine-Grained Soil Containing Ditatoms  
Before classifying the DKM samples, their particle size compositions are obtained from the PSD curves (Figure 12). The diatoms comprise mainly silt, while the kaolin contains more clay than silt. The PSD curves of the DKM samples move downward with increasing diatom content. Higher diatom content in the DKM corresponds to more silt and less clay (Figure 13), and DKM with a diatom content of more than 60% has a dominant composition of silt and is known as silt-dominated soil. Figure 14 shows that the \(d_{50}\) value of the DKM increases with increasing diatom content: \(d_{50}\) increases by 131% when the diatom proportion changes from 60% to 80%. Similar phenomena have been observed in previous studies [6; 32], except for that by [PERSON] and [PERSON] [19], who used clayey silt as a substitute for kaolin to prepare the studied DS. Following the USCS (Figure 7), samples 0D-100K, 20D-80K, and the others were classified as low-plasticity clay (CL), low-plasticity silt (ML), and high-plasticity silt (MH), respectively. According to the Casagrande plasticity chart, the rightward shift of the DKM data points with increasing diatom content indicates the change of soil behavior from clayey to silty, and the DKM samples with \(w_{\text{L}}>50\%\) are of high plasticity. Similar tendencies for types of DKM in other studies can be seen in Figure 7. Considering the classification results, the transition of dominant particle size from clay to silt with increasing diatom content for  
Figure 12: Particle size distribution of DS with different diatom content. Figure 13: Variations of silt and clay particle contents of DKM samples with diatom content. the DKM samples is confirmed by the PSD curves in Figures 12 and 13. However, the results for high-plasticity soil are inappropriate according to the investigation of the microstructure and consistency limits of the DKM samples. For example, sample 100D-0K is non-plastic and should not be classified as soil with high plasticity (Figure 2). Additionally, sample 20D-80K contains dominant clay-sized particles but is misclassified as silt by the USCS. Therefore, the USCS may well result in improper classification of DS because of its unique consistency limits due to the diatom microstructure. [PERSON] and [PERSON] [27] proposed a classification system for fine-grained soil based on pore fluid sensitivity. This classification focuses on the different types of interparticle interaction (i.e., van der Waals and double-layer effects) in fine-grained soil. It uses electrical sensitivity (\(S_{\text{E}}\)) to measure the permittivity and conductivity of the soil. \(S_{\text{E}}\) is calculated using the fall-cone liquid limits, which are measured with different pore fluids of deionized water, 2-M NaCl solution, and kerosene (i.e., \(w_{\text{L,DW}}\), \(w_{\text{L,prime}}\), and \(w_{\text{L,ker}}\), respectively). Figure 15 shows the classification results according to this system for different types of DKM and the DS studied by [PERSON] and [PERSON] [7], along with those for silica silt, kaolin, and other minerals (i.e., mica, bentonite, and illite). With increasing diatom proportion, the permittivity decreases, and the conductivity increases slightly, which results in decreasing \(S_{\text{E}}\) (Figure 15a). As shown in Figure 15b, a DKM with a diatom content of less than 40% is classified as \"intermediate plasticity, intermediate \(S_{\text{E}}\)\", while one with a diatom content of more than 80% is classified as \"high plasticity, low \(S_{\text{E}}\)\". Sample 60D-40K is classified as \"high plasticity, intermediate \(S_{\text{E}}\)\". However, the pore fluid sensitivity classification recognizes pure diatoms as high plasticity soil, although this sample is confirmed to be non-plastic (Figure 2). [PERSON] et al. [12] showed that the silt-sized diatom particles in soil are, in effect, like clay particles, leading to a high value of \(A_{\text{d}}\).