3.1 Characterization of paddy field soil
The collected soil sample was classified as fined-grained, consisting of 1.40% gravel-sized, 40.40% sand-sized, and 6.80% silt-sized particles, along with 51.40% clay-sized particles (Fig. 1), following ASTM Standard D422 [14]. X-ray diffraction analysis with the courtesy of previous research by Islam et al. [11], carried out on soil passing through a #200 sieve (74 µm), following Terzano et al. [24], results are plotted in Fig. 2, indicating the presence of quartz, orthoclase, illite, and kaolinite.
Table 2
Geotechnical properties of soil used in the investigation.
Properties | Test values |
Specific gravity (-) | 2.58 |
Field moisture content (%) | 43.13 |
Gravel (%) | 00.83 |
Sand (%) | 37.76 |
Silt (%) | 45.04 |
Clay (%) | 16.37 |
Liquid limit, LL (%) | 54.48 |
Plastic limit, PL (%) | 29.68 |
Plasticity index, PI (%) | 24.79 |
Optimum moisture content, OMC (%) | 20.30 |
Maximum dry density, MDD (g/cm3) | 1.615 |
pH (-) | 6.7 |
Liquid limit (LL) and plastic limit (PL) tests were carried out by following the four-point method and hand-rolling method, respectively, according to ASTM Standard D4318 [16]. The values of LL and PL for the soil sample are 54.48% and 29.68%, respectively (Table 2). The specific gravity of the soil was measured as 2.58, following ASTM Standard D854 [15]. In accordance with the Unified Soil Classification System (USCS), the collected soil is classified as inorganic silts of high plasticity (MH) or organic clays of medium to high plasticity (OH).
3.2 Behavioral changes of stabilized soil in the presence of fly ash and cement
3.2.1 Influence on the Atterberg limits of the soil
The Atterberg limits test demonstrates unique effects of cement and fly ash on the soil's properties, as illustrated in Fig. 3. In both liquid limit and plastic limit cases, soil-fly ash mixtures exhibit a higher slope. For nearly all binder combinations, the liquid limit decreases, and the plastic limit increases (Fig. 3) with the rising binder content, whether it be fly ash, cement, or a combination of both. Up to approximately 7% binder content, the slope of the liquid limit curve for the soil-cement mixtures exceeds that of the soil-fly ash mixtures. However, beyond 7% binder content, the situation reverses. Meanwhile, the plasticity limit remains relatively constant for the soil-cement mixtures up to 15% (Fig. 3). Figure 4 illustrates that all data points consistently fall within the regions of MH or OH for all binder content combinations in this study.
3.2.2 Impact on maximum dry density (MDD) and optimum moisture content (OMC) of the soil
The impact of fly ash and cement on the compaction efficacy of soil was illustrated through the standard proctor test as depicted in Fig. 5. The MDD of the soil-binder mixtures reduced with an increase in all combinations of binder content (Fig. 5), and the slope exhibited an opposite trend to that of the liquid limit (Fig. 3). Specifically, the slope of the curve for soil-fly ash mixtures was steeper than that of soil-cement mixtures until around 7% binder content in soil, after which the situation reversed. However, the overall reduction in MDD can be explained by the incremental addition of fly ash and cement in soil, as their specific gravity is relatively low compared to the original clay soil [9, 11, 25, 26]. The MDD of 10% binder content was 1.56 g/cm3, 1.52 g/cm3, and 1.54 g/cm3 for soil-fly ash mixture, soil-cement mixture, and soil-fly ash-cement mixture, respectively, where unstabilized soil’s MDD was 1.62 g/cm3. In both individual and combined addition of binders to the soil, a decrease in MDD was observed, consistent with findings from previous research [6, 9, 27–29].
Conversely, the OMC rises with higher binder content in all combinations (Fig. 5). The increasing rate is higher for soil-cement mixtures than the soil-fly ash mixtures. The OMC for soil-cement mixtures is almost constant throughout the range of 14–18%. The OMC of 10% binder content was 21.4%, 27.5%, and 24.4% for soil-fly ash mixture, soil-cement mixture, and soil-fly ash-cement mixture, respectively, where unstabilized soil’s OMC was 20.3%. The increase in OMC is typically associated with the presence of finer particles and the bonding formed between soil particles and binders, leading to greater water retention and plasticity in the soil-binder mixtures [11, 19, 25, 27, 28]. Additionally, another study observed that the presence of large, hollow spheres in fly ash leads to a rise in the OMC value as the binder content in the soil increases [26].
3.3 Optimum fly ash and cement content for maximum unconfined compressive strength of clay soil
The test results presented in Figs. 6 and 8 illustrate the progression of UCS of soil-binder against curing time for various ratios of fly ash and cement content. The UCS of untreated soil samples are 270.99 kPa and 289.07 kPa after 7 and 28 days of curing, respectively. The plots indicate that the soil's strength increased with curing time when fly ash and cement were added individually, as expected. Interestingly, it was observed that the strength improvement rate increased until reaching a specific percentage of fly ash and cement content. This phenomenon was also evident when both fly ash and cement were used in the soil.
The UCS of the soil exhibited a rising trend with the rise in fly ash content up to a certain percentage, specifically 5% (Fig. 6). For 7- and 28-day curing periods, this binder (5% fly ash) provided UCS of 349.15 kPa and 430.96 kPa, respectively, representing a notable increment of 28.84% and 49.08% compared to untreated soil. However, beyond the 5% fly ash content, a considerable depletion in strength was observed. These results delineate two distinct zones (Fig. 7): an active zone (0–5% fly ash) and a deterioration zone (above 5% fly ash) [11]. Other researchers have observed this phenomenon across varying percentages of fly ash in several kinds of soil [26, 30–32]. The impact of the deterioration zone is significant, resulting in a reduction in the soil's strength below its normal level. Notably, at 3 and 7 days of curing, 20% (SF20) and 25% (SF25) fly ash content provided less strength than the original soil's strength alone. Similar observations of a deteriorating phenomenon in different percentages of fly ash content in clay soil have been reported by other researchers [9, 29]. The decline in strength can be attributed to two main factors. Firstly, the limited strengthening capacity of fly ash is a critical factor contributing to its inability to enhance the strength of this type of soil. Secondly, insufficient coverage of contact points between the soil and fly ash further exacerbates the soil-fly ash particles bonding [11, 30]. While adding fly ash to a maximum of 5% may adequately cover all available contact points in the soil, exceeding this percentage results in a decline in strength.
The strength results from the experiment demonstrate that the use of cement as a binder material in soil leads to a better strength development compared to fly ash, as depicted in Fig. 8 - a result consistent with Sumesh et al. [9]. However, the findings reveal that the rate of development for soil-cement mixtures decreases beyond a certain binder content for this specific soil type. With an 8% cement content, the soil-cement mixture generates a UCS of 775.45 kPa over 28-day curing periods, representing a significant increment of 168.26% compared to the unstabilized soil strength. The increment rate remains substantial up to 12% cement content, yielding a UCS of 883.42 kPa. However, beyond 8% cement content, the rate slows down, reaching 933.28 kPa for 18% cement content. Consequently, these results delineate two distinct zones (refer to Fig. 9): an active zone (0–8% cement) and an inert zone (above 8% cement). While a comparable active zone was identified for soil-cement mixtures as seen with soil-fly ash mixtures, a new inert zone emerges, differentiating it from soil-fly ash mixtures, where no significant strength development occurs. This observation aligns with a similar zone pattern reported by Horpibulsuk et al. [12], where an active zone was identified up to 10% cement content.
In the context of combining both fly ash and cement binder (Fig. 10), the strength increment of soil was smooth for the entire curing period up to 10% binder content (SF5C5: 5% fly ash and 5% cement). At 10% binder content (SF5C5), the produced UCS of soil-binder was 689.60 kPa, which was 90.21% higher than the unstabilized soil’s strength for the 28-day curing period. Beyond 10% binder (SF5C5), slightly irregular strength development occurred throughout the entire curing period. The maximum increase of 34.42% increment rate was found for 6% binder content (SF3C3) in soil, generating 520.47 kPa UCS. The increment rate remained favorable up to 12% binder content (SF6C6). Starting from 12% binder content (SF6C6), the strength increment rate decreased rapidly and continued until 16% binder content (SF8C8). Beyond 16% binder content (SF8C8), the strength started to diminish. Overall, the stabilized soil strength for the 28 days produces three zones for different binder contents (Fig. 11): an active zone (0–12% binder content: fly ash and cement), an inert zone (12–16% binder content: fly ash and cement), and a deterioration zone (above 16% binder content: fly ash and cement). A group of researchers have found similar results, indicating that strength develops up to 25% fly ash content, irrespective of the amount of cement mixed [33].
This study demonstrates that all soil samples treated with fly ash and cement exhibit enhanced UCS compared to untreated soil. This strength improvement is attributed not only to the pozzolanic [29] and hydration reactions [7] but also to the presence of iron and aluminum oxides in the soil [9]. Interestingly, approximately 50% of the strength increase occurs within the first 7 days of curing. Several factors influence the rate and level of strength development, including the type of clay minerals present, the type of fly ash, the proportion of fly ash and cement, ambient temperature, and curing environment [34]. Figures 6, 8, and 10 illustrate that the UCS of the soil increases over curing time. This is primarily due to the pozzolanic reaction of fly ash and the hydration reaction of cement. While the hydration reaction is faster, the pozzolanic reaction requires more time to develop a bond [35]. The free lime in fly ash reacts with alumina and silica in the existence of water to form calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH) gels, which act as binders connecting soil particles [9]. Similarly, the hydration and pozzolanic reactions of cement also contribute to the binding of soil particles, resulting in a higher UCS compared to fly ash alone [36].