## 3 Results and Discussion
### Hydraulic Conductivity
#### 3.1.2 Hydraulic Conductivity of Olivine-Admixed Soil after Carbonation  
The hydraulic conductivity test results of the olivine-admixed clay at various carbonation times and carbonation pressure are shown in Table 4. As seen from the table, an increase in olivine content, carbonation time, and carbonation pressure reduces the hydraulic conductivity. The lowest hydraulic conductivity of 6.34 \(\times\) 10\({}^{-11}\) m/s was recorded for a sample containing 30% olivine content after a carbonating duration of 23.5 h and under a carbonation pressure of 150 kPa. This hydraulic conductivity reduction may be due to the formation of hydrated magnesium carbonates, including usebuonite (MgCO\({}_{3}\).3H\({}_{2}\)O), hydromagnesite ((Mg)\({}_{5}\)(CO)\({}_{3}\))\({}_{4}\)(OH)\({}_{2}\).4H\({}_{2}\)O), and dypingite ((Mg)\({}_{5}\)(CO)\({}_{3}\))\({}_{4}\)(OH)\({}_{2}\).5H\({}_{2}\)O), as depicted in Equations (4)-(8) [38; 40], which fills the pores and binds the clay particles together, forming packed and compacted clay particles. However, it is worth noting that the formation of the various hydrated magnesium carbonate phases is a function of the carbonation pressure and carbonation duration.  
\[\text{Mg}_{2}\text{SiO}_{4}+4\text{H}_{2}\text{O}\rightarrow\text{2 Mg(OH)}_ {2}+\text{H}_{4}\text{SiO}_{4} \tag{4}\]  
\[\text{Mg(OH)}_{2}+\text{CO}_{2}\rightarrow\text{MgO}_{3}\text{H}_{2}\text{O} \tag{5}\]  
\[\text{Mg(OH)}_{2}+\text{CO}_{2}+2\text{H}_{2}\text{O}\rightarrow\text{MgO}_ {5}\text{-3H}_{2}\text{O}\text{ (}\text{usequehouite}\text{)} \tag{6}\]  
\[5\text{Mg(OH)}_{2}+4\text{CO}_{2}+\text{H}_{2}\text{O}\rightarrow\text{(Mg)}_ {5}\text{(CO}_{3})_{4}\text{(OH)}_{2}\text{-5H}_{2}\text{O (}\text{dypingite}\text{)} \tag{7}\]  
\[5\text{Mg(OH)}_{2}+4\text{CO}_{2}\rightarrow\text{(Mg)}_{5}\text{(CO}_{3})_{4 }\text{(OH)}_{2}\text{-4H}_{2}\text{O (}\text{hydromagnesite}\text{)} \tag{8}\]  
[PERSON] et al. [47] suggested that magnesium oxide (MgO) carbonation products could promote the filling of pores and soil particle cement, resulting in structural pores being refined, void ratio reduced, and hydraulic conductivity subsequently reduced. The hydrated magnesium carbonates also form massive crystals with well-ramified networks and effective binding ability, hence resulting in hydraulic conductivity reduction.  
A comparative analysis of the hydraulic conductivity of the carbonated olivine-admixed clay and the cured olivine-treated clay was investigated. It is seen from Table 4 and Figure 3 that the hydraulic conductivity of both carbonated olivine-admixed clay and the cured olivine-treated clay samples decline with an increase in olivine content. Nonetheless, the carbonated olivine-admixed clay specimens' hydraulic conductivities are lower than that of cured olivine-admixed clay specimens at the various olivine contents. For instance, after 14.8 h of carbonation, the percentage decrease in the hydraulic conductivity of both 20% (9.08 \(\times\) 10\({}^{-11}\) m/s) and 30% (8.07 \(\times\) 10\({}^{-11}\) m/s) olivine-admixed specimens, compared with the corresponding 7 days cured 20% (4.05 \(\times\) 10\({}^{-10}\) m/s) and 30% (2.31 \(\times\) 10\({}^{-10}\) m/s) olivine-stabilized specimens, were approximately 78% and 65%, respectively. The lower hydraulic conductivity values recorded for the carbonated specimens is due to the formation of extra magnesium carbonate phases, including usebuonite, hydromagnesite, and dypingite, which filled the pores and bind the soil particles together. The findings demonstrate that carbonated olivine-admixed soils could, in a few hours, achieve significant hydraulic conductivity reduction, which is suitable for liner application. The study further confirms olivine as a sustainable soil binder through the sequestration of CO\({}_{2}\). It is worth noting that the hydraulic conductivity recorded for both carbonated and cured olivine-admixed clay specimens satisfy the Malaysian regulatory requirement of \(\leq\)10\({}^{-8}\) m/s for liner utilization specified by the regulatory authority [48].