# Mechanical Properties of Cement-Treated Soil Mixed with Cellulose Nanofibre
## 3 Effect of CNF on Strength Increase
### Unconfined Compressive Strength
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**(a)**} & \multicolumn{3}{c}{**Soil (Kawasaki Clay)**} & \multicolumn{3}{c}{**Anounts of Component**} & \multirow{2}{*}{**Total**} & \multirow{2}{*}{**CNF \(-\)No Addition-**} & \multirow{2}{*}{**Total**} & \multirow{2}{*}{**CNF \(-\)No Addition-**} & \multirow{2}{*}{**Content**} \\ \cline{2-3} \cline{8-10}  & \multicolumn{1}{c}{**Total**} & & & & & & & & & & \\  & \multicolumn{1}{c}{**Particle**} & \multicolumn{1}{c}{**Water**} & \multicolumn{1}{c}{**(**Early Strength)**} & \multicolumn{1}{c}{**Total**} & \multicolumn{1}{c}{**Solid**} & \multirow{2}{*}{**Water**} & \multirow{2}{*}{**Ratio (\%)**} & \multirow{2}{*}{**Content**} \\ \hline g & 2000 & 1237 & 763 & 325 & 124 & 0 & 0 & 0 & 2449 & & \\ g/cm\({}^{3}\) & - & 2.671 & 1.023 & 1.00 & 3.14 & - & 1.5 & 1.0 & 1.556 & 0.0 & 78.6 \\ cm\({}^{3}\) & 1209 & 463 & 746 & 325 & 39 & 0 & 0 & 0 & 1574 The strength at the age of 7 days was 1.4-1.5 times higher in the soil treated with CNF than in the soil without CNF treatment, indicating the effect of CNF. The strength of the soil at 28 days of age was 1.1-1.3 times higher than that of the soil without CNF treatment, whereas the ratio of these strengths was smaller than that of the soil at the age of 7 days. At the age of 112 days, the trend was more pronounced, and the strength of the treated soil with TEMPO-CNF was lower than that of the treated soil without CNF. The strength of the soil treated with ACC-CNF was similar to that of the treated soil without CNF. This tendency can be clearly observed in Figure 6b, where the strength ratios of the treated soil without CNF are shown. It was found that the CNF mixture increased the strength at the initial age of the treated soil but reduced the strength development in the long term. This trend is more evident for the TEMPO-CNF. Increasing the initial strength and reducing the long-term strength is advantageous in cement treatment. As mentioned in Section 1, one of the challenges of cement treatment is the low short-term strength, and if the long-term strength is too high, the treated soil can hinder the rehabilitation of geotechnical structures. CNFs have the potential to solve these problems. It is not clear how the strength development was suppressed with age, although it was possible that in TEMPO-CNF, the carboxylic groups of CNF reacted with the calcium ions of cement and aggregated, suppressing the cement hydration reaction and reducing the increasing rate of long-term strength. On the other hand, the failure strains (axial strains at the peak value) of the soil treated with TEMPO-CNF were not very large, whereas those of the soil treated with ACC-CNF were large. The addition of ACC-CNF increased the failure strain by a factor of 2, indicating that some types of CNFs could mitigate the brittleness, which is a major problem in cement-treated soil. Figure 5: Relation between stress and strain in unconfined compression tests (Method 1). Figure 6: Time change of unconfined compressive strength (Method 1). The soil treated with CNF was photographed using an optical microscope. Figure 7 shows micrographs of the sheared surfaces (peeled and sliding surfaces) produced by the unconfined compression tests. In the photographs of the soil treated with CNF, no fibrous material could be observed in the TEMPO-CNF specimen, whereas it could be observed in the ACC-CNF specimen. The fibrous material was considered to be a CNF. TEMPO-CNF has a high degree of friability, which cannot be captured by optical microscopy, but a bunch of CNFs was seen in the void. Fibrous material could be observed at any given position in the ACC-CNF specimen, suggesting that the CNF was uniformly agitated. It was assumed that the uniformly mixed CNFs contributed to the strength increase. In addition, a number of small voids appeared in the soil treated with CNF, especially in the case of the TEMPO-CNF mixture. This was attributed to the reduced fluidity of the CNF mixture, which allowed air to enter the soil.