3.2.1. Typical Stress–Strain Behaviors
In order to define the strength alteration of stabilized local sands over time, specimens were made by mixing dry sand with FA. The FA contents ranged from 5% to 30% at intervals of 5%. The stabilized specimens were tested, and the stress–strain responses of the specimens were recorded after curing at predetermined CTs starting from 7 up to 56 days. Each subsequent CT was set twice as long as the previous CT.
The typical stress–strain curve of unconfined compressive strength (UCS) and direct shear strength (DSS) tests on fly-ash-stabilized fine sands, under different curing times and fly ash contents, are shown in
Figure 2. Both UCS and DSS test results clearly show that, at low fly ash (FA) contents of less than 20%, stresses incrementally increase along with strain elongation up to the peak failure state of the specimen’s stresses at any curing time. By contrast, both UCS and DSS tests indicate that those stresses increase sharply up to that corresponding to higher fly ash content, particularly at longer curing times. The higher the CT and FA content, the higher the peak stress reached. As aforementioned, the peak occurred at a very low strain. At a low FA content and CT, this strain was about 1.5%, particularly for shear strength curves, while at a higher FA content and CT, that strain was at 2%, as for the case of both the UCS and DSS stress–strain relationships in
Figure 2. This fact illustrates that fly-ash-stabilized sand with a higher FA content and longer CT is more resistant to shearing up to a certain strain of 2%. The increase in FA content binds sand grains into larger and stronger particles as a result of the cementitious, pozzolanic reaction, and the self-hardening occurring during curing. This mechanism continues over time, and at the same time, there is an upgrade in the mechanical properties of the specimen. This prospect is more obvious in the specimen with greater CT and FA content, particularly the shearing specimen. In the case of the compressed specimen shown in
Figure 2b, there is an exception: the peak stress of the specimen with the highest FA content and longest CT was at a lower strain of 1%. Those stresses, UCS and DSS, decrease continuously after the 2% strain has been exceeded, except for untreated sand (FA content = 0). The shear stress of untreated sand remains stable, even though the shear strain is extended, showing a failure state of the untreated sands. For stabilized sands, the stress decreases smoothly at lower fly ash contents. However, the stress alleviates sharply at higher fly ash contents, indicating brittle behavior of the bonding prepared by fly ash. The beneficial effect of fly ash through binding sand particles seems to have disappeared. A strain of approximately 2% is able to abolish this advantage.
On the basis of
Table 5, UCS
max increases along with the increases in FA content and CT for both fine and coarse sand. Both natural and clean sands also show the same trend. This improvement with increases in FA content is the result of the bonding of grain into larger particles owing to increased FA. Along with the time of curing, self-hardening takes place on the generated bond, producing a stronger specimen. These prospects were also noted by Bell (1989), Ola (1978), and others [
30,
37,
38,
39,
40]. In terms of the different saturations during sample preparation, UCS
max increases with decreasing S
r, as shown from the available data in
Table 5. Specimens were prepared under three different S
r of 30%, 50%, and 100%. It was found that the highest value of UCS
max occurred for clean fine sand with a larger FA content and CT, and lower S
r. It seems that, at low saturation, there is a significant improvement in the mechanical properties of fly-ash-stabilized sands. This trend agrees with the test results of the investigations detailed in Jalali et al. (1997), Jha et al. (2009), and Sezer et al. (2006) [
30,
31,
32], who made a valuable prediction; that is, that the observed improvements in mechanical properties might be caused by decreased saturation during curing. A similar observation conducted using another binder agent of precipitated calcite under various degrees of saturation was reported by Simatupang and coworkers, and Cheng et al. (2013) [
8,
9,
34,
41]. They found that the mechanical properties of calcite-treated sands increased with a decrease in saturation during curing. The test results attained in this research along with the previous test results indicated that S
r had an important role in improving the mechanical properties of fly-ash-stabilized sands.
In addition, the shear strength of the fly-ash-stabilized sand is higher than that of untreated sand, as presented in
Figure 2d. This amelioration is conferred by the fly ash that is added to the sand specimen. Fly ash binds sand grains, creating a matrix among them, and hardens during curing, enhancing the mechanical properties of that sand. Regarding the stress–strain relationship, the shear strength of fly-ash-stabilized sands displays a similar trend to the compressive strength in terms of both the improvement rate and the fragility of fly ash binding. Specimens cured at a higher FA content and CT, but at lower S
r, show a dramatic increase in their strength until they reach their peak strength at a specific strain. In further shearing, the peaks of the strengths show a drastic fall in the small strain rate, demonstrating their fragility.
The graphs presented in
Figure 2d are redrawn in
Figure 3 in the form of shear strength parameters of cohesion “c” and friction angle “θ”. They were measured based on the graph connecting the peak shear stress at each normal stress applied at 0.12, 0.24, and 0.36 kg/cm
2 according to the Mohr–Coulomb theory, as depicted in
Figure 3. The graphs depicted in this figure clearly show that the shear strength parameters increase with FA content. In the case of the specimens cured at FA contents of either 5% or 10%, as included in
Figure 3, their cohesions almost coincide. This is probably because of the fabric effect, as the FA contents at sample preparation were dissimilar [
42].
Moreover, on the basis of the data available in
Table 6, shear strength parameters increase with the increase in CT and the decrease in S
r. The increase in those parameters is the result of the increase of the amount of FA binding sand particles at the surface into larger grains as a consequence of the addition of FA into the specimen and the effectiveness of decreasing S
r. A larger grain produces a higher friction angle. The pozzolanic reaction, self-cementitious parameters, and loss of saturation during curing are other main factors resulting in the improvement of those parameters. Self-hardening makes the specimen stronger, which is directly related to the cohesion improvement. Other researchers have included these aspects in their considerations [
30,
31,
32]. Simatupang and coworkers (colleagues) conducted observations on other binders of precipitated calcite and obtained a similar trend [
8,
9,
41]. The results of their observations showed that the cohesion of the precipitated calcite increased by increasing calcite content and decreasing saturation during curing. Furthermore, they found that, at a CT of around 6 h, the precipitated calcite reached its optimum strength. This was determined by measuring the stiffness of the treated specimen at a small strain level during curing.
The shear strength parameters of fly-ash-stabilized sands are also influenced by the type of the sands used and their grain sizes. As shown in
Table 6, they are distinguished based on their types as natural or clean sands, and their grain sizes as fine or coarse sands. It was found that the shear strength parameters of the clean sands were higher than those of the natural sands for both fine and coarse sands. The same trend was outlined by the grain size of sand, in which the friction angle “θ” for coarse sand was larger than that for fine sand. However, this is not the case for coarse sand in terms of cohesion; the cohesions of coarse sand are below those of fine sand. These trends apply to all cases of tested sand, and the results are tabulated in
Table 6.
If stabilized sands are sheared further, their strength fails and comes close to the residual shear strength of untreated sands, as clearly shown in
Figure 2c,d. The main factor of this drawback is bond deterioration. At strain levels of more than 2%, the beneficial effect of fly ash bonds disappears. In this state, the fly ash bond is destroyed along with shearing, as aforementioned. A similar trend is noted in the test results previously attained by researchers, including Delfosse-Ribay et al. (2004), Lin et al. (2016), and Simatupang et al. (2019) [
3,
41,
43]. They prepared a deformation characteristic test on a treated specimen and found that the shear modulus of treated specimens decreased sharply along with strain increases and approached that of untreated specimens. The same conclusion is also shown here; that is, that the key point is the decline in the function of the binder. More detailed information regarding both the UCS and DSS test results for different test conditions is listed in
Table 5 and
Table 6, consecutively.
3.2.2. Effect of the Fly Ash Percentage
The effects of the fly ash (FA) content—which varied from 5% to 30% at intervals of 5%—on the UCS maximum and shear strength parameters of stabilized sand under the investigated parameters are tabulated in
Table 5 and
Table 6 respectively and shown typically in
Figure 4. Some of the data plotted in the figures presented in this research were scattered, but their trends were represented by either solid lines or dashed dot lines, as shown in each figure. Both
Table 5 as well as
Table 6 and
Figure 4 illustrate that the UCS
max tends to increase as the fly ash percentage increases in all cases of tested sands, as expected. The improvement in the UCS
max is a consequence of the bonding owing to FA. As the FA content increases, the sand particles become larger and stronger over time owing to self-hardening during curing. Previous research on fly-ash-stabilized soils prepared by Harichane et al., (2011), Ola (1978), Prabakar et al. (2004), Sezer et al. (2006), and Sridharan et al. (1997) showed the same behavior [
18,
28,
30,
37,
39]. Other research on other binders of precipitated calcite showed a similar trend. The further increase in the content of the precipitated calcite generates a stronger material; it becomes rock-like with a higher UCS of typically more than several MPa [
7,
44,
45].
The shear strength parameters of the fly-ash-stabilized sands in this research were stated as a function of the cohesion and friction angle, as expressed by the Mohr–Coulomb strength theory. They increase in fly ash content, as revealed in
Figure 4b,d,f. Additional information taken from that figure indicates that the shear strength parameters of the fly-ash-stabilized sands are always higher than those of the untreated sands, showing the bond effect conferred by fly ash. Sand particles are bonded into larger and stronger grains and behave as dense sands. A similar trend was reported by Harichane et al. (2011), Ola, (1978), Prabakar et al. (2004), Sezer et al. (2006), and Canakci et al. (2015) [
18,
30,
37,
39,
46].
The friction angles of untreated sands were around 27.413° and 30.07° for fine and coarse sands, respectively, and were independent of S
r. This condition is in line with the test results obtained by Cheng et al. (2013) [
34]. Their test was conducted under S
r values of 30%, 65%, and 100%; however, they did not influence the friction angle of untreated silica sands, both fine and coarse. The friction angles of fly-ash-stabilized sand performed in this research, for both fine and coarse sand, were around 27.78–35.51° and 30.14–42.12°, respectively. Those friction angles were within the values reported by Cheng et al. (2013) [
34]. Their test results were around 23–40° and 25–42° for fine and coarse silica sand, respectively. Similar performances were reported in other research studies conducted by Bell (1989), Canakci et al. (2015), and Ola (1978) [
37,
38,
46].
The parameter of cohesion “c” showed values of 0–1.80 kg/cm
2 and 0–2.89 kg/cm
2 for both fine and coarse sands, respectively. These values are also in the range of values reported by Cheng et al. (2013) [
34] using silica sands.
3.2.3. The Effect of Curing Time
The effects of curing time on the strength of the fly-ash-stabilized sands at various predetermined indicators are shown in
Figure 5. The figure depicts the importance of curing time in determining strength improvement. The higher the curing time, the larger the UCS of the resulting specimen. A larger FA content of more than 20% produces a significant increase in UCS
max, especially at a later age. A significant loss of moisture at a later age might have occurred, consequently resulting in a notable improvement in UCS
max. This trend is in agreement with the test results shown by other investigators [
30,
31,
32]. By contrast, for an FA content less than 15%, insignificant increases in UCS
max occur along with curing time. This is probably because of the bonding effect. At a low FA content, weak bonding between sand particles results in low strength. The following example, which occurred during the experiment in this study, further supports this trend; coarse sand specimens prepared with low FA contents showed weak bonding and were not strong enough to hold their own weight, as evidenced by their failure after encasing from their molds (shown by the empty values in
Table 5). A similar fact was also demonstrated by Ola (1978), who showed that the increase in the mechanical properties of soil was largely dependent on the amount of the binder agent binding soil particles into larger and stronger aggregates. In such a condition, the soil behaves as a coarse aggregate that was strongly bound [
37].
In addition, there is a considerable improvement in the shear strength parameters of both “c” and “θ” with the progression of curing time. On the basis of
Figure 5, the cohesion increases around four-fold from a curing time of 7 to 56 days, especially for clean coarse sand (CCS) at an FA content of 20% and S
r of 100%. This is predicted to be because of the self-hardening of the mixture between the fly ash and sand particles. Previous research conducted by Gay and Schad (2000), Harichane et al. (2011), and Sezer et al. (2006) reported a similar trend [
30,
39,
40].
However, the strength improvement is delayed at the beginning of the curing time. More time is required for the pozzolanic reaction in forming the cementitious compounds, CSH and CAH, in the mixture of soil particles and fly ash. At a curing time of less than 28 days, the strength increases slowly. It then sharply increases and slows down thereafter. Similar trends were shown in the results of research conducted by Amadi (2014), Amadi and Osu (2018), Consoli et al. (2001), Jha et al. (2009), Jalali et al. (1997), Miller and Azad (2000), Oriola and Moses (2011), Osinubi (2000), Peethamparan and Olek, (2008), Salahudeen et al. (2014), Sezer et al. (2006), and Sreekrishnavilasam et al. (2007) [
21,
22,
23,
24,
25,
26,
27,
30,
31,
32,
47,
48]. In more detail, Jalali et al. (1997) confirmed that the delayed improvement in the strength of stabilized soil was strongly dependent on the curing temperature [
32].
In this research, experiments were conducted under constant room temperature of around 25 °C, either during sample preparation, curing, or testing. It was ensured that there was no acceleration or deceleration on saturation reduction owing to temperature fluctuations during the experimental stages. The delay in the strength improvement at the initial stage is the natural behavior of FA as a binder. Sufficient time is needed for completion of the pozzolanic reaction in establishing cementitious and pozzolanic gel [
21,
22,
23,
24,
25,
26,
27,
30,
31,
32,
38,
47,
48].
In terms of strength improvement, there should be a situation where there is no further change in strength even though the curing time increases. This condition is stated as the maximum strength that can be reached by the specimen. It seems that, in this research, the curing time needed for the specimen to attain the maximum strength was longer than a month, as shown in
Figure 5. The majority of the graphs presented in that figure show that the strength increases significantly from a curing time of 14 to 28 days, but insignificantly thereafter, as observed for up to 56 days. Further studies are needed to determine the optimum curing time to achieve the maximum strength of fly-ash-stabilized sands.
3.2.4. The Effect of Degrees of Saturation
Figure 6 illustrates that the degree of saturation during sample preparation influences the strength of fly-ash-stabilized sand. The specimens were prepared under a degree of saturation of 30%, 50%, or 100%. The fly ash percentages and curing times were varied. Those percentages started from 5% to 30% with 5% intervals, and the curing times were 7, 14, 28, and 56 days.
On the basis of
Figure 6, the mechanical properties of fly-ash-stabilized sand increase with the decrease in the degree of saturation during sample preparation for all cases of the reviewed parameters. This increase is predicted to be because of the effectiveness of fly ash binding that congregates at inter-particle contact and is directly related to the improvement in strength. With a higher degree of saturation during sample preparation, a longer time is needed for evaporation to reduce the saturation. This trend is in line with the test results obtained by others prepared on calcite-treated sands under different S
r [
8,
9,
34,
41]. In terms of the strength improvement shown in
Figure 6, the highest values occur for clean sands with a higher FA content and CT, but with a lower S
r.
With an FA content and CT of 30% and 56 days, respectively, the UCS
max increased by approximately 1.5–2 times when decreasing the S
r from 100% to 30%. On the other hand, the FA content could be reduced by around one-third to one-half to anticipate the expected UCS
max by lowering S
r, as with the previous rate. For example, in the case of natural fine sand (NFS) (as presented in
Table 5), at 14 days of curing, fly-ash-stabilized sand cured at FA contents of either 30% or 20% provided almost the same UCS
max value under S
r of either 100% or 30%, respectively. This mechanism can be explained as follows. At a higher S
r of, for example, 100%, the entire surface of fly-ash-stabilized sand will be moistened with water. In such conditions, keeping in mind the basic nature of the FA in that it will react in the presence of water, the entire surface of the sand will be covered with cementitious material. As a consequence, a lot of the cementitious material formed beyond the contact surface will not be beneficial in increasing the strength of the fly-ash-stabilized sand. The limited cementitious material formed binds sand at inter-particles, which contributes to a small increase in strength. This is also noted by the researchers for the calcite precipitation approach. The spatial distribution of calcite in the calcite-treated soil observed using microscopic images showed that the precipitated calcite was distributed uniformly not only at the contact surface, but also over the whole surface [
7,
43,
49,
50,
51].
At low S
r, however, pore water is concentrated on the contact surface, forming a meniscus where the formed cementitious material will accumulate, which will result in an increase in the strength of the fly-ash-stabilized sand. A similar investigation on different binder agents of calcite-treated sands has been conducted by Simatupang and colleagues with S
r of 30% and 97% [
8,
9,
41], and Cheng et al. (2013) with S
r of 20%, 40%, 80%, and 100% [
34]. They provided scanning electron microscopy (SEM) images for specimens in order to show the more detailed effect of S
r on treated sands. The SEM results show that, for specimens prepared under higher S
r, the precipitated calcite was distributed uniformly on the sand surface. Only a small amount of the precipitated calcite binds sand particles, which results in a small increase in strength. At low S
r, however, the calcite was precipitated locally at the contact surface between grains, which was directly related to the strength improvement. This result, along with the findings of this study, demonstrates that the effectiveness of fly-ash-stabilized sand depends on the formation of cementitious material generated by the FA content present in the specimen. The agglomerate of cementitious material at the interparticle contact surface is more valuable than the total sum of that on the entire surface of the sand.
However, there may be a minimum limit of water content in the specimen that is capable of dissociating the lime (CaO) that exists in the fly ash and establishing the cementitious and pozzolanic gels. The latter will be the next topic for research into the effectiveness of fly ash usage as a cementitious material.
3.2.6. The Effect of Content of Fines
The unconfined compressive strengths and shear strength parameters of fly-ash-stabilized sand with fine grains are presented in
Figure 4,
Figure 5 and
Figure 6, which depict the use of sand with content of fines (natural sand) and without content of fines (clean sand). The content of fines, as referred to here, is the difference between the sand masses before and after being washed. The amount of the content of fines was around 2.88% and 1.33% of the total mass of the dry fine and coarse sand, respectively. The mechanical properties of the clean sand are higher than that of the sand with content of fines (natural sand) for all cases of fly ash content, curing time, and degree of saturation. Despite these increases in fly ash, the value of sand containing content of fines is always smaller than that without content of fines. This indicates that content of fines has a negative effect (inhibition) on the chemical reaction between fly ash and sand particles. It seems that, with a larger fly ash content, the mechanical properties of clean sands are much higher than those of sands with content of fines. The same behavior was mentioned by Axelsson et al. (2002), Canakci et al. (2015), Janz and Johansson (2002), Tremblay et al. (2002), and Hampton and Edil (1998). Their research concluded that content of fines could chemically influence the cementing reaction of the stabilized soil [
46,
52,
53,
54,
55].
The effects of content of fines on the mechanical properties of fly-ash-stabilized sand under any kind of saturation are shown in
Figure 4b and
Figure 5a,b for both various FA contents and various CTs, respectively. Both figures illustrate the drawback of content of fines on the mechanical properties. The UCS
max and the cohesion almost coincide as S
r changes. It seems that the beneficial effect of lowering S
r, which will improve the mechanical properties of fly-ash-stabilized sand, is diminished by the presence of content of fines. At different grain sizes, on the other hand, depicted in panels (c) and (d) of those figures, a similar trend—near coinciding of both UCS
max and cohesion—is also shown. The content of fines degraded the serviceable quality of lower grain sizes and produced a lower UCS
max and cohesion. As mentioned above, a lower grain size generates higher mechanical properties of fly-ash-stabilized sands, but this benefit is reduced with the presence of content of fines in the specimen. Regarding the other parameters of CT and FA content depicted in
Figure 4 and
Figure 5, particularly panels (e) and (f), narrow differences are presented, especially at low values of either CT or FA content. The presence of content of fines is predicted to extend the induction time. As a consequence, the cementitious process as well as pozzolanic reaction have not yet been properly completed, resulting in a low strength. A similar problem occurred when the FA content was low. Binding and hardening processes might be delayed for producing a larger and stronger particle. As content of fines is present in the fly-ash-stabilized sands, the mechanical properties of UCS and cohesion “c” come close to each other, and the serviceable quality of both CT and FA content improved slowly. It seems that this weakness is clearer for specimens cured at low CTs and FA contents. A similar trend is also illustrated in
Figure 6.
On the other hand, the effect of the presence of content of fines in the liquefaction susceptibility of soil was investigated. It was found that liquefaction resistance increased with the increase in content of fines [
56,
57,
58]. This finding, along with the test results obtained in this study, leads to the final remark that the decrease in the mechanical properties of sand with content of fines (natural sand) is not because of the lower strength of the content of fines itself, but is rather a result of both cementitious and pozzolanic reactions being inhibited by the presence of content of fines during curing.