3.3.1. UCS Development
The UCS results of untreated peat and 8 designated treated mixes are presented in
Figure 3,
Figure 4 and
Figure 5. The untreated Teluk Intan peat showed a low strength of about 42.94 kPa due to the high amount of organic matter, which imparted compressible and collapsible characteristics in the parent peat, thus yielding a low compressive strength—as also stated in previous studies [
32,
33,
34].
Figure 1.
(a) Standard compaction curves of untreated and treated peat with 10%, 15%, and 20% SF; (b) effects of increasing SF dosage on the dry density and Wopt of treated peat.
Figure 1.
(a) Standard compaction curves of untreated and treated peat with 10%, 15%, and 20% SF; (b) effects of increasing SF dosage on the dry density and Wopt of treated peat.
Figure 2.
(a) Standard compaction curves of untreated and treated peat with varying percentages of SF and OPC; (b) effects of varying SF–OPC dosages on the dry density and Wopt of treated peat.
Figure 2.
(a) Standard compaction curves of untreated and treated peat with varying percentages of SF and OPC; (b) effects of varying SF–OPC dosages on the dry density and Wopt of treated peat.
Figure 3.
The unconfined compression strength of 10%, 15%, and 20% SF-stabilized peat under varying curing periods: (a) 7 days, (b) 14 days, and (c) 28 days.
Figure 3.
The unconfined compression strength of 10%, 15%, and 20% SF-stabilized peat under varying curing periods: (a) 7 days, (b) 14 days, and (c) 28 days.
Figure 4.
The unconfined compression strength of SF- and OPC-stabilized peat under varying curing periods: (a) 7 days, (b) 14 days, and (c) 28 days.
Figure 4.
The unconfined compression strength of SF- and OPC-stabilized peat under varying curing periods: (a) 7 days, (b) 14 days, and (c) 28 days.
Figure 5.
The unconfined compression strength of untreated peat, 15% OPC-stabilized peat, and 15% SF- and 15% OPC-stabilized peat under varying curing periods: (a) 7 days, (b) 14 days, and (c) 28 days.
Figure 5.
The unconfined compression strength of untreated peat, 15% OPC-stabilized peat, and 15% SF- and 15% OPC-stabilized peat under varying curing periods: (a) 7 days, (b) 14 days, and (c) 28 days.
Various factors, including the type of peat, water content, mineral content, organic, content, fiber content, and pH, affect the strength gain of cement-stabilized peat. The UCS results of the Teluk Intan peat stabilized with varying SF contents (10, 15, and 20%) and cured for 7, 14, and 28 days are presented in
Figure 3a–c. it can be observed that the compressive strength significantly increased as the amount of SF and curing period increased. The highest strength was recorded by mixing 20% SF in all curing periods. The highest values of UCS after 7, 14, and 28 days of curing were 296.6, 534.7, and 1063.94 kPa, respectively, which also indicated the development of strength as the curing duration was prolonged. The strength enhancement of stabilized peat using silica fume was due to the formation of calcium silicate (C–S) and calcium silicate hydrate (C–S–H) gels [
35]. Similarly, the effectiveness of OPC in peat soil has been previously reported [
8,
36,
37]. However, the strength enhancements of SF- and OPC-stabilized peat with curing were different due to the differences in hydration rate. Comparatively, SF led to higher strength enhancements than OPC due to peat’s acidic nature, which hindered the stabilization caused by OPC.
Additionally, the detrimental environmental effect of OPC has hindered its applications. For the sake of comparison, OPC was replaced by different amounts of SF for peat stabilization, as shown
Figure 4 and
Figure 5. To assess the combined effect of SF and OPC, three different mix combinations were made: 5% SF and 15% OPC, 10% SF and 10% OPC, and 15% SF and 5% OPC. The cumulative quantity of binders was kept at 20% because the highest strength of Teluk Intan peat was achieved with 20% SF content. Moreover, Kalantari et al. [
8] suggested a 20% cumulative OPC and SF binder for peat stabilization.
Figure 4a–c shows the 7, 14, and 28 day curing-strengths of SF (5, 10, and 15%) and OPC (5, 10, and 15%), respectively.
Figure 5. indicates the strength obtained by using 15% SF, as advised by Kalantari et al. [
8], and a trail mix of adopting 15% OPC and 15% SF. It can be observed that the achieved strength was higher than the parent peat due to its fine pozzolanic nature. However, the gained strength was still lower than the strength obtained when using 20% SF. Comparatively, the strength gained by using SF was higher compared to that gained by using OPC; this may have been due to the extremely fine size of silica fume, which is also approximately 120–200% more pozzolanic than OPC [
35]. Moreover, the presence of humic acid in peat soil reduces the efficiency of OPC by retarding strength enhancement. For this reason, Axelsson et al. [
38] suggested utilizing a surplus amount of OPC to neutralize the humic acid. Thus, using SF (a waste material and effective binder) to enhance the strength of peat is a more cost-efficient and sustainable solution than using OPC. Combining SF and OPC for strength development is associated with pozzolanic reactions and the development of calcium silicate (CS), calcium aluminate hydrate (C–A–H), calcium silicate hydrate (C–S–H) bonds, and ettringite (AFt) formation [
7,
8,
39].
3.3.2. Strength Development by Various Mix Designs
ASTM D 4609 (Standard Guide for Evaluating Effectiveness of Admixtures for Soil Stabilization) specifies a UCS value of 345 kPa for a stabilized soil to be considered an effective binder, as shown in
Figure 6 [
29]. As such, the strength development index (SDI) obtained with Equation (1) is used, to assess the influence of various mix designs on peat strength [
40].
where Max. UCS
(stabilized) and Max. UCS
(parent) indicate the ultimate unconfined compressive strength of treated and untreated peat, respectively.
Figure 7 shows the SDI results of stabilized and parent peat cured for 7, 14, and 28 days. Based on the untreated peat UCS value (42.94 kPa) and the targeted UCS value (345 kPa), an SDI of 7.03 was calculated, as indicated in
Figure 7. It was observed that the parent peat and almost all stabilized peat cured for 7 days fell below the targeted unconfined compression strength. After extending the curing period to 14 days, several mixes met the minimum strength criteria: peat and 20% SF; peat, 10% SF, and 10% OPC; peat, 5% OPC, and 15% SF; and peat, 15% OPC, and 15% SF. All the stabilized peat mixes met the minimum UCS criteria advised by ASTM D 4609 after 28 days of curing, as shown in
Figure 6 and
Figure 7. A slower development of UCS when using OPC compared to when using SF was observed here and previously in [
41]. Considering the targeted SDI value of 7.03, both OPC treated peat cured for 28 days and SF treated peat cured for 14 days encouraged subgrade improvement.
3.3.3. Binder’s Effect on the Failure Modes
The presence of fibers and partially decomposed vegetation induces structural anisotropy in peat, thus affecting the compression modes of failure [
42]. The failure modes of the parent Teluk Intan peat and stabilized peat are presented in
Figure 8. It can be observed that the failure pattern changed with the stabilization of peat. Moreover, the failure behavior changed with binder dosages due to changes in density [
43].
Due to the high fiber and organic content, the parent peat underwent bulging with an unrecognized failure plane upon axial compressive loading, as shown in
Figure 8. However, it was observed that the utilization of hydraulic binders increased stiffness and thus led to a change in the failure mode. This type of failure mode is associated with ductile behavior [
44]. The bulging failure mode subsided, and a compressive V-shaped failure pattern was observed in the 10% SF and 15% OPC treated UCS samples, which indicated a reduction in the ductility with the use of a hydraulic binder. On the other hand, a clear shear failure pattern was noticed in the 15% SF treated peat due to its increased stiffness causing brittleness. In the same way, steep sheerness and high brittleness with bursting failure, causing the destruction of the entire UCS specimen, were observed in the 20% SF treated peat and the 15% SF and 15% OPC treated peat, as shown in
Figure 8. This means that the incorporation of higher hydraulic binders caused a predominant failure plane that induced brittleness. This brittle behavior has been previously reported to be caused by increasing amounts of hydraulic binder [
44,
45]. Additionally, non-recognizable compressive collapsible failure was noticed in the 5% OPC and 15% SF, 10% OPC and 10% SF, and 15% OPC and 5% SF treated peat, as shown in
Figure 8, indicating its structural heterogeneity. Thus, additives such as fibers, shredded tires, and geofoam materials are recommended in tandem with hydraulic binders in peat to limit brittle and collapsible failure.
Figure 8.
Failure modes of the stabilized and parent peat.
Figure 8.
Failure modes of the stabilized and parent peat.
3.3.4. CBR and Comparison
The CBR test is an essential test conducted to estimate the shearing resistance (strength) of a subgrade or subbase of pavement. It is used to examine the durability, efficiencies, and thus overall performance of a pavement foundation material. The subgrade can either be a natural existing ground (original subgrade) or an embankment constructed from borrowed material (new subgrade). In both cases, the minimum permissible CBR value for subgrade is 10% [
46], and the relative ratings of the shearing resistance based on CBR values are given in
Table 6 [
47].
The soaked and unsoaked CBR values of treated and untreated Teluk Intan peat cured for 7, 14, and 28 days are presented in
Figure 9,
Figure 10 and
Figure 11, respectively. The parent Teluk Intan peat showed extremely low soaked and unsoaked CBR values of 1.1% and 3.1%, respectively. In the literature, the unsoaked CBR value of undisturbed peat has been shown to be as low as 0.785% [
48] and ranging from 2.43 to 5.66% [
49,
50,
51]. The
Table 6 ratings suggest that Teluk Intan peat very poor in strength for subgrade materials and need to be treated to enhance their shearing resistance.
The treatment of Teluk Intan peat with hydraulic binders (SF and OPC) showed substantial improvements of the CBR value after curing for 7, 14, and 28 days. As noticed in
Figure 9a, all the designated mixes obtained a higher unsoaked CBR value than 10 after 7 days of curing, which indicated their suitability as subgrade materials. The highest CBR value of the specimens cured for 7 days specimens was obtained by treating peat with 15% SF and 15% OPC, as shown in
Figure 9b. Similarly to UCS, CBR was increased after prolonging the curing period due to pozzolanic reactions, as shown in
Figure 10 and
Figure 11. Moreover, the CBR of the stabilized peat increased with the increasing SF and OPC contents. Even the soaked samples of peat; 20% SF and peat; and 15% OPC, and 15% SF achieved CBR values of more than 10, indicating their feasibility as subgrade materials. The observed strength enhancement due to SF may be associated with the formation of calcium silicate hydrate (C–S–H) gel and calcium silicate (CS) [
7,
52], while the combined effect of SF and OPC may be associated with the formation of calcium silicate (CS) and hydrated components as a result of hydration, i.e., calcium aluminate hydrate (C–A–H), calcium silicate hydrate (C–S–H) gel, and AFt formation. [
1]. Moreover, the development of greater interfacial confinement bonding, roughness, contact area, and friction mobilization upon the loading of the stabilized peat yielded higher CBR values [
53]. Comparatively, the strength gains caused by the combined SF and OPC was higher than the other treatments. About 1054%, 1145%, and 1163% CBR was recorded by 15% SF and 15% OPC treated peat after curing for 7, 14, and 28 days of curing, respectively. However, OPC is expensive and causes environmentally hazardous effects, so its adoption is discouraged. Therefore, SF, which is an industrial by-product waste that is cementitious, is a more sustainable and cost-effective binder than OPC.