Investigation of Strength and Microstructural Characteristics of Blended Cement-Admixed Clay with Bottom Ash

Abstract

This research presents an experimental study of the strength and microstructural characteristics of cement-bottom ash-admixed Bangkok clay, paying special attention to the efficiency of adding up the bottom ash (BA) of different finesses as a cementitious material and the role played by BA in enhancing the strength of the mixture. The obtained results were discussed with cemented clay mixed with other industrial ashes (i.e., fly ash and risk husk ash). The pozzolanic reaction and packing effect of BA on strength development were also discussed with tests of mixtures with insoluble material. The experimental study was performed through unconfined compression (UC), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) tests. The obtained results demonstrate that the BA could be advantageously supplemented as cementitious material into the cement-admixed clay mixture to improve the strength characteristic. The finer particle size of BA could be beneficial for achieving a high strength due to the pozzolanic reaction and packing effects. By adding up a BA content of larger than 15% when the base cement content is not less than 20%, the strength of the mixture increased efficiently with the increasing BA content. Compared with fly ash of a similar grain size, the higher efficiency of BA is obtained when a BA content of greater than 15% is considered. Finally, the microstructure and changes in elemental composition/distribution were analyzed by TGA and SEM tests to explain the mechanism to improve the strength of cement–BA-admixed clay.

1. Introduction

For environmental sustainability, studies on the utilization of industrial waste materials as pozzolanic materials in the construction sector have been increasingly performed in the past three decades (e.g., [1,2]). For the coal combustion process of power plants, two main pozzolanic materials of by-products; namely, fly ash (FA) and bottom ash (BA); are released from manufacturing. Those ashes are generally composed of 80% FA, which has been widely utilized as a replacement for ordinary Portland cement in soil stabilization [3] or concrete construction [4]. The remaining 20%, whose majority is the BA, has currently been disposed of to landfills, and the amount tends to increase every year. Several problems such as the cost of disposal, loss of natural sites for disposal and environmental impact surrounding the disposal area have become increasingly acute. Attempts to utilize BA as a secondary raw material for Portland cement production (e.g., [5]) and as an aggregate replacement in concrete or mortar (e.g., [6,7]) have been increasingly conducted during the last two decades. For applications in geotechnical engineering, the BA is primarily recommended as a partial substitution of aggregates for road and pavement works (e.g., [8,9]) and for filling material (e.g., [10]). The use of BA to stabilize the expansive soil has been also studied (e.g., [11,12]). With the direct utilization of BA in the form of products from the power plant (without modification), the main applications are thus for a fine aggregate replacement in concrete and filling material and to improve the soil gradation in earthworks. The cement replacement or pozzolanic function of the BA are rarely considered. Compared to FA, which is widely used as a cement replacement with high economic value, the BA exhibits a low value with the current applications despite the chemical composition of BA being rather similar to that of FA when both materials were obtained from the same source of coal and power plants [13]. This is mainly attributed to the particle size and porosity of the BA, which are much larger than those of FA. With modifications, such as by the grinding or sieving processes, the fine BA can be more economically utilized as a cement replacement in concrete works [14,15]. The sieving process before grinding can eliminate the large particles that have a lower amorphous content and higher insoluble waste content. The grinding process could convert the BA from an inactive pozzolanic material to a more active form [16]. With the improved value of BA to be used as a cementitious material instead of as a filling material, it becomes feasible to prepare a finer BA through milling or grinding. However, no attempt has been made to utilize the fine BA as a cementitious material in cement-stabilized soils, particularly the cement admixed clay.

Among several applications in geotechnical engineering, soil stabilization by chemical additives generally consumes a large amount of cement. For deep stabilization, the utilized cement content can be as large as 450 kg per cubic of in situ soil [17] due to the high-water content in the native clay. Because the strength of the cement-admixed clay does not linearly increase with the cement content [18], the utilization of BA in partially replacing cement or adding up could substantially reduce the use of cement, particularly when the high-strength cemented clay is required. A previous study [19] revealed two different mechanisms involving the strength increase of BA-added composite geomaterials: particle bonding and fabric change. The first one is attributed to the pozzolanic reaction, and the latter one deals with the packing effect. Unlike cement mortar, the matrix of cement-admixed clay comprises a very fine aggregate (clay particles whose size is small compared to the BA particle size) and a large void. Particularly at high water content, the void is very large. It is thus interesting to observe how the BA particles play a role in the packing effect function. The main objective of this study is to investigate the effective utilization of BA as a cementitious material in cement-admixed clay. BA was ground to prepare three different grain size distributions to determine the effect of BA fineness. Sand as an insoluble material was also ground to achieve a similar gradation of BA and mixed in cement-admixed clay to obtain insight into the role of BA in the strength improvement of cement–BA-admixed clay.

2. Materials and Methods

For evaluating the efficiency of BA to replace the cement or add up in the cement-admixed clay material, the cement–BA-admixed clay specimens were prepared from the soft Bangkok clay with various mixing proportions of BA contents (B_c) in the range of 5–35%, cement contents (C) of 10–30% and the remolded clay with the water contents (w_r) corresponding to 130% and 200%. The mixing proportions of cement–BA-admixed clays performed in this study are very close to some tests of cement-admixed clays with FA and RHA materials as reported in the previous studies [20,21]. Therefore, the efficiency of using BA as a cementitious material compared with other pozzolanic materials (i.e., FA and RHA) can be discussed and highlighted. To demonstrate the BA particle size effect and role play of BA on the cement–BA-admixed clay characteristics, the BA and insoluble material (i.e., sand in this study) are prepared with three different particle size distributions. The detailed information regarding the experimental tests are explained as follows.

2.1. Materials

To prepare the cement–BA-admixed clay specimens, three kinds of materials were used including cement, soft clay, and BA. Portland cement (OPC) type I with a specific gravity of 3.14 was employed in this study.

Table 1. Chemical composition of Portland cement, BA, FA (data from [20]), and RHA (data from [21]).

Chemical Composition (%)	Portland Cement	BA	FA	RHA
Silicon dioxide (SiO2)	20.20	35.70	48.00	93.00
Alumina oxide (Al2O3)	5.40	20.10	26.00	0.17
Ferric oxide (Fe2O3)	2.90	12.00	10.00	0.35
SiO2 + Al2O3 + Fe2O3	28.50	68.10	84.00	93.50
Calcium oxide (CaO)	63.80	15.50	5.00	0.91
Sulfur trioxide (SO3)	2.30	1.41	0.70	0.11
Magnesium oxide (MgO)	1.50	2.34	2.00	0.42
Sodium oxide (Na2O)	2.72	1.44	0–2	0.63
Potassium oxide (K2O)	0.30	2.28	0–5	2.82
Other	–	6.10	–	–
Loss of ignition (LOI)	2.00	2.83	3.00	4.70

and Figure 1 present, respectively, the chemical compositions and particle size distribution of OPC. The natural soft clay utilized in this study is the typical soft Bangkok clay sampling from the areas close to King Mongkut’s University of Technology Thonburi (KMUTT), located in the southwestern part of Bangkok, Thailand. Clay samples were collected for the soft clay layer within the depth of 4–5 m beneath the ground surface. The physical properties of the sampled clay were summarized as follows [20,21,22]: specific gravity of 2.68, total unit weight of about 14.1 kN/m³, natural water content of 80%, liquid limit of 103%, plastic limit of 43%, undrained shear strength of about 16–17 kPa (determining from unconfined compression tests), and high plasticity clays (CH) according to the Unified Soil Classification System. The particle size distribution of natural soft clay is presented in Figure 1.

The BA used in this research was obtained from Mae Moh Electric Power Plant, Thailand. The chemical compositions of BA are listed in Table 1. The main chemical composition of BA was SiO₂, Al₂O₃, Fe₂O₃, and CaO, which were, respectively, 35.7%, 20.4%, 12%, and 15.5% by weight. The BA samples were collected from the disposal area and then sun-dried roundly for 1–2 days to decrease the moisture content to approximately 1%. After that, they were ground to be smaller in size using the Los Angeles abrasion machine. The BA samples were prepared into three different grain size distributions: coarse-grain size (BA-C), fine-grain size (BA-F), and very fine-grain size (BA-VF), as plotted in Figure 1. The gradation of BA-F was prepared to obtain a relatively similar grain size distribution of both FA-F [20] and RHA-F [21] adopted in the previous studies for comparison, as illustrated in Figure 1. Note that the ranges of grain diameters for all considered size distributions are included in this figure. Although the ranges of grain diameters between fine-grain and very-fine-grain sizes are relatively close, the very-fine-grain size contains more fine aggregates than the fine-grain size, particularly for grain diameters in the range of 0.6–10 μm. To investigate the role of BA in the strength improvement of the cement-admixed clay, a clean sand was also ground to achieve similar gradations to those of the BA as depicted in the figure.

2.2. Specimen Preparations

Before the mixing process, the natural soft clay was remolded to achieve the desired remolding water content (w_r) by adding the additional water (Δw_w) for increasing the flowability and workability of the mixture [20]. To obtain the desired w_r of 130% and 200%, the Δw_w was added using the following fundamental equation [20,22]:

Δw_w = [W_s/(1 + w₀)] × (w_r − w₀),

(1)

where W_s is the total weight of the clay sample before remolding and w₀ is the initial water content of the clay sample.

After finishing the preparation of remolded clay for a few hours, a cement–BA slurry with the desired base cement and BA contents was added to the remolded clay and mixed using a portable mechanical mixer. A water-to-cement ratio (w/c) of 1:1 was employed in this study for creating the cement–BA slurry used for the mixing process. Therefore, the final water content (w_f) of the cement–BA-admixed clay mixture just after the mixing process is equal to the summation of water from remolded clay and additional water from the cement–BA slurry. Hence, w_f can be estimated as follows:

w_f = w_r + [(w/c) × C],

(2)

where C is the desired cement content, which is determined as the percentage ratio of the cement weight to the dry weight of clay for each mixing batch.

After completing the mixing process, the homogenous cement–BA-admixed clay mixture was poured systematically into the 35 mm-diameter and 90 mm-long polyvinylchloride (PVC) molds. Each specimen was placed by four lifts and compacted periodically by tapping against a table to avoid air bubbles within the cement–BA-admixed clay paste [22]. Note that, before pouring, the internal surface of the PVC molds was coated by lubricant (Vaseline) oil for making a smooth and slippery contact between the specimen and mold. Later, the PVC molds with cement–BA-admixed clay specimens were wrapped by the plastic films and were then cured in the humidity room (maintaining a humidity of 97% and ambient temperature of 25 °C) for a period of 5 days. Afterward, the cement–BA-admixed clay specimens were carefully removed from the molds and wrapped in plastic films again to prevent moisture loss for a further curing period within a humidity room until reaching a target curing time. Three specimens were prepared for each mixing proportion. In addition to the curing times of 7 and 28 days, which are commonly considered for deep stabilization applications, the properties at a curing time of 60 days are also observed. It should be mentioned that, before the unconfined compression test, the top and bottom surfaces of the specimens were meticulously trimmed to attain a sample height of 70 mm. By using this approach, it can confidently avoid the bleeding effect of cement paste and also provide the smooth and perpendicular surface of both ends of the specimen, which is useful for testing [22].

2.3. Testing Program and Methods

To assess the efficiency of using BA as the cementitious material, the cement–BA-admixed clays for different w_r of 130% and 200% with four different values of B_c and three C (i.e., 10%, 20%, and 30%) were performed, as summarized in

Table 2. Summary of the program for UC tests.

Name of Set	Cement Content (%)	BA Content (%)	Water Content (%)	Curing Time (Days)
1	10	5, 15, 25, 35	130, 200	7, 28, 60
2	20	5, 15, 25, 35	130, 200	7, 28, 60
3	30	5, 15, 25, 35	130, 200	7, 28, 60

. In this study, B_c is defined as the percentage ratio of BA weight to the dry weight of clay used for each mixture. For assessing the strength characteristic, the unconfined compression (UC) tests were conducted following ASTM D2166 [23], in which the constant shearing rate of 1.14% per minute was adopted [21,22]. The unconfined compressive strength (q_u) is determined as the maximum compressive stress. For each mixing proportion, an average value of q_u was reported based on three specimens. To investigate the consumption of calcium hydroxide, Ca(OH)₂ by the pozzolanic reaction introduced by BA, the thermogravimetric analysis (TGA) was carried out to observe the remaining Ca(OH)₂ content through mass loss. In the heating process, a temperature in the range of 20 to 980 °C under a heating rate of 10 °C/min is used [24]. Moreover, scanning electron microscopy (SEM) tests were also conducted to analyze the microstructure and the changes in elemental composition/distribution induced by hydration and pozzolanic reactions of cement-admixed clays with and without BA. The small block-shaped samples (approximate dimension of 5 mm × 5 mm × 2 mm) were picked from the failure plane of the tested specimens after finishing the UC tests. Before analyzing with SEM, the SEM samples were dried by the oven and also coated with platinum over 30 s at a current of 50 mA. SEM micrographs with 2500 times magnification were obtained by Do SEM JSM-5410 LV equipment.

3. Results and Discussions

3.1. Unconfined Compressive Strength

Figure 2 presents some stress–strain curves of tested specimens obtained from the UC tests for various mixing proportions. It is noted that the BA with a fine grain size distribution, BA-F, was considered for investigating the strength characteristics of specimens in this subsection. As seen in Figure 2a, for both cement-admixed clays with and without BA involving w_r of 130% at t_c of 28 days, the maximum stress or strength increases with the increase in both C and B_c. This is mainly attributed to the cement hydration reaction, resulting in the dissociation of calcium ions, which further creates calcium silicate hydrate (CSH) gel and Ca(OH)₂. The CSH gel acts as a cementitious substance that provides adhesive properties to clay particles for forming a bond together with the clay particles or clusters of clay particles, while Ca(OH)₂ is a byproduct of the cement hydration procedure [20,24,25]. Subsequently, due to the SiO₂ components in BA absorbing the Ca(OH)₂ given by the cement hydration reaction, the pozzolanic reaction is then developed, which also generates the augmentative CSH gels to enhance more bonding development between clay particles [21,26]. This phenomenon is confirmed by comparing the results of specimens with the mixing proportions of C = 20% by varying the B_c from 0 to 35%, as depicted in Figure 2a. Interestingly, cement-admixed clay with C = 20% and B_c = 15% can provide a relatively similar strength to the mixture with C = 30% without BA. This indicates that the additional BA can significantly improve the strength of the mixture. At the same strength, cement–BA-admixed clays exhibit a strain at a peak strength that is less than cement-admixed clays. In other words, by adding more BA, the mixture becomes much more brittle, in which the stresses are dropped abruptly with the developed strain after peak stress.

In addition, not only the C and B_c, but the t_c and w_r are also important for the strength characteristic of cement–BA-admixed clays, as illustrated in Figure 2b. In this figure, the tested results from the mixtures with C of 20% and B_c of 15% by varying the t_c and w_r are compared. At the same mixing proportion, it is inferred that the strength develops with the t_c, especially for the small water content of remolded clay (i.e., w_r = 130% for this study). For w_r = 130%, the strength at t_c = 28 days can be larger than almost 2 times that at t_c = 7 days. A similar tendency can be obtained when making a comparison between cases at t_c of 28 and 60 days. Again, this phenomenon is primarily caused by the continuous process with the times of cement hydration and pozzolanic reactions [21,24,26]. Although water is a necessary portion of the hydration process, excessive water can be inferior to the strength development due to the increase in distance between inter-particles or inter-clusters of particle spacing as reported by previous studies [25,27,28]. Figure 2b also shows that the strength of the mixture at higher w_r was significantly less than that at lower w_r (beyond 2 times smaller) with the same cementitious content (i.e., C and B_c), particularly for larger t_c.

The obtained q_u at t_c = 28 days concerning w_r of 130% and 200%, C of 10–30% and B_c = 5–35% are plotted in Figure 3. It is seen that the use of only cement yields a lower q_u than the cement–BA-admixed clay because the CSH products were solely obtained from the cement hydration reaction without a pozzolanic reaction induced by BA. Referring, respectively, to Figure 3a,b for both w_r of 130% and 200%, it can be observed that the increasing rate of q_u at an identical B_c is significant when adding C from 10% to 20%, particularly for the results with small water and large BA contents (i.e., w_r = 130% and B_c = 25–35%, see Figure 3a). After that, q_u is developed slightly. For investigating the effect of adding BA, it is seen that adding BA of larger than 15% can be improved effectively by the q_u. This tendency result is more prominent when the large BA content is considered together with small water content (i.e., C = 20–30% at w_r = 130%, see Figure 3a). From the overall obtained results, in the case of base cement content beyond 10%, the efficiency to gain the large q_u by adding the BA is successfully achieved. Therefore, the obtained result of this study implies the possibility of utilizing BA for replacing or adding up cement in deep cement mixing applications. Note that this tendency result is also similar to the obtained q_u results at t_c of 7 and 60 days, and hence the q_u results at t_c of 28 days are solely presented herein.

3.2. Efficiency of BA as Cementitious Material

The results of cement–FA and –RHA-admixed clays (FA content, F_c and RHA content, R_c in the ranges of 5–35%) as reported in past studies [20,21] were presented herein to compare the strength characteristics considering various kinds of ashes into cement-admixed clay mixtures with the obtained results of the current study. Figure 1 shows the grain size distributions of FA-F and RHA-F as used in the previous studies, and it was found that the grain size distributions of these two ashes were not much different from the considered BA-F of this study, especially for the grain sizes of larger than 4 μm, which are approximately 50% of the total portion.

3.2.1. Discussions in Terms of Unconfined Compressive Strength Increased

The comparisons of q_u at t_c of 28 days in cases of w_r = 130% and 200% for various pozzolanic material contents are, respectively, illustrated in Figure 4a,b for both the base cement contents of 10% and 20%. By comparing all ashes at identical w_r, it is found that the q_u of cement-admixed clays blending with RHA are larger than those with FA and BA for the same pozzolanic material contents. This tendency is very clear for the mixtures with large cement contents (i.e., C = 20%, see Figure 4b). This is probably because the SiO₂ component in the RHA is higher than that in the other ashes, as presented in Table 1. However, for the cases of C = 10% with ash contents of larger than 25% (Figure 4a), it is interesting to mention that the cement–BA-admixed clays yield a slightly larger or equal q_u to the cement–FA-admixed clays for both w_r of 130% and 200%, even though the SiO₂ of FA (48%) is higher than that of BA (35.7%). Since the cost of BA is currently much cheaper than FA, this suggests using BA with a content larger than 25% for blending with the cement-admixed clay instead of FA for achieving a superior strength in the case of low cement content (i.e., 10% for this study).

3.2.2. Discussions in Terms of Equivalent Cement Content

To evaluate the efficiency of cementitious material for deep cement mixing applications, the quantity of BA content to be added is transformed into a supplementary amount of cement content apart from the cement base through the equivalent cement content (C_eq) index. Based on the concrete strength analysis concept, C_eq can be estimated by back-calculation of the obtained q_u of cement-admixed clay with pozzolanic material using the modified equation proposed by Papadakis and Tsimas [29], as expressed in the following equation:

q_u = K × [((C + kP)/W) − b],

(3)

where k is an efficiency factor of any pozzolanic materials needed for replacing or adding up depending on the chemical composition, grain size distribution, curing time; C, P, and W are, respectively, the adding cement, pozzolanic, and water contents. K and b are, respectively, the factors depending principally on the cement type (kPa) and curing time. By replacing the term kP with C_eq, the parameter C_eq can be estimated as

C_eq = [((q_u/K) + b) × W] – C.

(4)

Before obtaining the C_eq of cement-BA-admixed clays, the parameters K and b must be determined with the help of mixtures without pozzolanic material results. By considering Equation (3) without the term of kP, the relationship between the q_u and C/W parameters of the mixtures without BA can be plotted as illustrated in Figure 5. It is worth noting that the parameter W is the total water content deducted by a constant. According to the analysis results of past studies [20,21], this constant is defined to be equal to 80, which closely corresponds to the natural water content used in this study. In this study, the K parameters are defined as the slopes of the fitting lines as presented in Figure 5, which correspond to 1180.4, 2035.6, and 2831.9 kPa for t_c of 7, 28, and 60 days, respectively. For the b parameters, they can be estimated as the ratio of slopes over the y-intercept values of the fitting lines. Thus, the b values are, respectively, equal to 0.0653, 0.0583, and 0.0477 for the t_c of 7, 28, and 60 days. Consequently, by substituting the q_u obtained from cement–BA-admixed clay mixtures with the mixing proportions and both parameters K and b as determined above in Equation (4) for each t_c, the parameter C_eq can be thus obtained.

Figure 6 presents the relationship between the actual BA content, B_c in each mixture and the calculated C_eq for different w_r of 130% and 200% in association with various t_c of 7, 28, and 60 days. The equivalent line with a slope 1:1 is also included in the figure. As seen in this figure, it is demonstrated that the efficiency of adding up the BA into the cement-admixed clays can be revealed. The C_eq of cement–BA-admixed clays increase with increasing C and t_c, but decreases with an increase in w_r. For w_r = 200% (Figure 6b), adding BA enhances the strength of the cement–BA-admixed clay mixtures with the inferior equivalent efficiency in which most data points are located under the equivalent line. However, for a t_c of 28 days and high cement content (i.e., C = 30%), adding BA with a content of 35% can improve the strength of the mixture involving w_r of 130% better than Portland cement as the point is over the equivalent line (Figure 6a). Furthermore, in a long-term situation (i.e., 60 days for this study) for the mixtures with low water content (i.e., w_r = 130%), it is indicated that the high efficiency of adding up BA together with large cement content (i.e., C = 20–30%) is obtained as all points of these data sets are beyond the 1:1 line, as depicted in Figure 6a. This reveals the possibility of using BA to replace or add up the Portland cement in the cement-admixed clay applications for cost-effectiveness.

In this part, the comparison in the efficiency of adding up the various pozzolanic materials including BA, FA, and RHA into cement-admixed clay is presented in Figure 7. The discussions are made for the cement-admixed clays blending with BA-F (obtained results of this study), FA-F [20], and RHA-F [21] at w_r = 130% with different curing times of 7 and 28 days. At an early age (i.e., t_c = 7 days, see Figure 7a), the C_eq of cement-admixed clays blending with all ashes are located under an equivalent line, resulting in low equivalent efficiency. This is probably because the pozzolanic reaction has not fully taken place due to an insufficient amount of product from hydration processes at an early age. Except for the case where B_c = 20%, adding the RHA content up to 35% can improve the strength equally to the use of Portland cement, i.e., C_eq is slightly above the 1:1 line. It may be caused by the amount of Ca(OH)₂ produced by the cement hydration reaction, which seems high enough to generate more pozzolanic reactions in conjunction with the large amount of SiO₂ from RHA.

With time growth (from 7 to 28 days), the products of cement hydration and pozzolanic reactions are increased continuously. This leads to the increase in C_eq of cement–FA and –RHA-admixed clays having a large base cement content (i.e., C of 20%), as shown in Figure 7b. Unfortunately, the C_eq of cement–BA-admixed clays at both C of 10% and 20% are still under the equivalent line. This may be due to a small SiO₂ amount of BA for inducing the pozzolanic reaction as compared with other ashes (Table 1). However, at the adding C of 10% for the long term, it is interesting to highlight that the BA with content larger than 15% can allow providing better efficiency than the adding FA, as observed in Figure 7b. Moreover, by adding up the pozzolanic content of 35%, the C_eq of mixture blending with BA at C of 20% is significantly larger than that with RHA at C of 10%, even though the SiO₂ amount of RHA (93%) is much greater than that of BA (35.7%). In summary, the overall results revealed that the efficiency of Portland cement replacement of RHA is better than FA and BA for all considered base cement contents and curing times of this study. Interestingly, in the case of small base cement content (10%) for both curing times of 7 and 28 days, adding up BA of larger than 15% can exhibit a superior or similar efficiency in terms of replacing Portland cement as compared with the use of FA.

For long-term strength development, it is revealed that the strength of cement–BA-admixed clays at t_c of 60 days is approximately 1.5 times that at t_c of 28 days. A similar finding is revealed by Karpisz and Jaworski [30]. This demonstrates the advantage of using this mixture for applications that may require long-term properties, such as retaining walls for deep excavation (e.g., [31]). However, further study over a long time (not less than 3 months) is recommended since the long-term degradation of strength and stiffness caused by aggressive factors in the ground is possibly encountered for both cement (particularly in organic clay [32]) and non-standard cement [33] admixed soils with BA.

3.3. Fineness and Packing Effects

This part presents the influence of the particle size of BA considering fineness and packing effects through the strength characteristic of mixtures with w_r of 130% and C of 15% at curing times of 7 and 28 days. The fineness of BA was divided into three cases as illustrated in Figure 1, including BA-VF, BA-F, and BA-C.

The obtained q_u results of mixtures with B_c of 10%, 20% and 30% for t_c = 7 and 28 days are, respectively, shown in Figure 8a,b. It is found from Figure 8a that the q_u values of mixtures blending with BA-VF at an early age (7 days) are the largest and followed by the results with BA-F and BA-C, respectively. When the curing time increased to 28 days (Figure 8b), the q_u of mixtures blending with BA-VF are also greatest. This may be because the BA-VF particles have a larger surface area to provide the silica and alumina compounds, which effectively introduce the pozzolanic reaction. Therefore, the obtained result indicates that the particle size of BA is a significant feature of the strength characteristics of cement–BA-admixed clays. Hence, the use of a small particle size of BA can significantly accelerate the pozzolanic reaction to achieve a large strength characteristic of cement-BA-admixed clay.

In contrast, the obtained results presented that the q_u of mixtures blending with BA-C insignificantly increases with the B_c (particularly for early age, see Figure 8a) and is slightly smaller or larger than the obtained q_u of the mixture with pure Portland cement in some mixtures (B_c = 10%). It is known that the pozzolanic reaction exhibits in the long term. The results from the early age imply that the packing effect strongly depends on the gradation of the BA, which consequently affects the gradation of the mixture fabric.

To investigate the packing effect, an assessment of the strength characteristic of cement-admixed clay blending with insoluble material (without the pozzolanic reaction) is performed. This can be performed by investigating the cement–sand-admixed clays at w_r of 130% and C of 15% (same mixing proportions as those investigating the fineness effect of BA) with different grain size distributions of sand including S-F and S-C, as plotted in Figure 1. It should be noted that particle size distributions of BA and sand for each pair are slightly different (i.e., BA-F vs. S-F and BA-C vs. S-C). The obtained q_u values of cement–sand-admixed clays are also included in Figure 8.

When the curing time increased to 28 days (see Figure 8b), it is found that the q_u of most cement-admixed clay blending with insoluble material are less than the q_u of pure cement-admixed clays. This is attributed to the fact that, by adding insoluble material, the ratio of the cement to the overall soil phase decreases. However, the q_u values of cement–BA-admixed clays for all BA contents are higher than those of cement-admixed clay blending with an insoluble material at the same grain size distribution of BA and sand. This confirms the pozzolanic reaction of the BA. By comparing the results of mixtures between the BA and insoluble material for similar gradations, the differences in the strength of the mixture of BA-F vs. S-F are much larger than those of BA-C vs. S-C. This indicates that the pozzolanic reaction of the BA can be enriched by making it finer.

Interestingly, although the q_u values of cement-admixed clay blending with insoluble material are smaller than those of pure cement-admixed clays in some contents, the small particle size of insoluble material manages to gain a higher strength than the one with the larger particle size. The obtained result of this study can confirm the packing effect of the material, which can be advantageous if the BA is sufficiently fine. This is consistent with the results of concrete works that presented the decrease in the porosity of pastes contributed by the small particle size of the material due to the packing effect [34].

3.4. Thermal Gravity Analysis

The Ca(OH)₂ content in cement–BA-admixed clay is the product of the hydration reaction. Hence, consumption of Ca(OH)₂ adsorbed by the SiO₂ of BA to produce the pozzolanic reaction can be observed through the reduction in Ca(OH)₂ content. To this end, TGA tests were performed to determine the crystalline Ca(OH)₂ content and other hydration (loss of water). Ca(OH)₂ content was calculated based on the weight loss between 450 and 580 °C [35,36], and expressed as a percentage by weight of the ignited sample. During temperatures between 450–580 °C, Ca(OH)₂ will be decomposed into calcium oxide (CaO) and water. Due to the heat, the water is lost, leading to a decrease in the overall weight. The amount of Ca(OH)₂ can be approximated from this loss of water, which is equal to 4.11 times the amount of lost water [35,37]. Then, the change of the cementitious products can be expressed by the change of Ca(OH)₂ since they are the hydration products. In this study, the TGA tests were conducted for the mixtures with and without BA-C and BA-F at w_r of 130% and C of 15%. Additionally, these above-stated mixtures blending with insoluble material were also analyzed by TGA.

Figure 9 presents the relationship between the percentage of mass (m) and the temperature provided by the TGA testing, examples for the mixtures at w_r of 130% and C of 15% with different grain size distributions of BA (one test for BA-C and other two for BA-F) after curing periods of 7 and 28 days. It can be observed that the BA content and particle size are significant parameters on the m and the detailed results and discussions will be provided later.

Table 3. The percentage of weight loss and Ca(OH)₂ of pure cement-admixed clay and cement-admixed clays blending with BA and insoluble material (sand) for various BA contents and sizes at different t_c = 7 and 28 days.

Mixing Proportion *	Weight Loss (%)		Ca(OH)2 (%)
	7 Days	28 Days	7 Days	28 Days
130-15-00	1.872	1.928	7.694	7.922
130-15-30/BA-C	1.638	1.704	6.731	7.002
130-15-10/BA-F	1.724	1.782	7.087	7.325
130-15-30/BA-F	1.561	1.587	6.415	6.521
130-15-30/S-C	1.833	1.908	7.534	7.842
130-15-10/S-F	1.849	1.894	7.563	7.784
130-15-30/S-F	1.822	1.916	7.563	7.875

* w_r-C-B_c/grain size distribution.

summarizes the Ca(OH)₂ content based on the m quantity of the mixtures at different contents and sizes of BA, sizes of insoluble material and curing times. Referring to the results in this table, the amount of Ca(OH)₂ in the cement–BA-admixed clays is lower than that of pure cement-admixed clay and decreases with increasing BA content. This is attributed to the pozzolanic reaction process introduced by BA. A similar result was also obtained from the past study [38], which concluded that the Ca(OH)₂ of the cement paste with FA is always lower than the Ca(OH)₂ of the cement paste without FA. The content of Ca(OH)₂ decreases at longer ages (more than 60 days), which is similar to the cases of cement pastes containing fly ash [38]. In addition, it is also found that the fineness of BA has a significant effect on the reduction rate of Ca(OH)₂. The Ca(OH)₂ of cement-admixed clay at B_c of 30% blending with BA-F dropped more rapidly than that mixture blending with BA-C. This is because the mixture containing finer BA can allow inducing a higher pozzolanic reaction rate than the mixture containing coarse BA.

In addition, by inspecting the amount of Ca(OH)₂ in mixtures blending with insoluble material (sand) as shown in Table 3, it is found that the amounts of Ca(OH)₂ in mixtures blending with insoluble material are larger than those of mixtures blending with BA, and also insignificantly different as compared with pure cement-admixed clays. This confirms the pozzolanic reaction of the BA. The obtained TGA result of cement-admixed clays blending with insoluble materials is consistent with the strength results discussed above.

3.5. Microstructure of Cement-Admixed Clays with and without Pozzolanic Materials

In this part, the changes in microstructures of cement-admixed clays with and without pozzolanic material are observed by SEM. The mixtures as analyzed by TGA as presented above are considered for SEM analysis to investigate the changes of microstructures concerning the variations of BA content and size. A SEM analysis of pure cement-admixed clays was also conducted for comparison.

Figure 10 illustrates the microstructure of cement-admixed clays at w_r = 130% and C = 15% without pozzolanic material for t_c of 7, 28, 60, and 90 days. It can be seen in Figure 10a that, at t_c of 7 days, some signs of ettringites, which are the cement hydration products (i.e., CSH and hydrated calcium aluminum silicate, CASH), are detected. However, more clay particles and voids are found between clay-cement particles. With a t_c larger than 28 days (Figure 10b), the degrees of hydration and pozzolanic reactions continuously increase, resulting in more products of ettringites by growing from the cement grain connect clay clusters interspersed by large pores and filling voids between the particles by growing hydrates. In the long term, more CSH products are formed and surround the particles of clay, and hence, clay-cement clusters become larger, as shown in Figure 10c and Figure 10d for 60 and 90 days, respectively, because the cation exchange process, which results in Ca²⁺ ions replacing K⁺ cations on the illite particle surface, leading to a decrease in repulsion between successive diffused double layers and more edge-to-face contracts between the successive illite sheets [39,40]. The increase of ettringite of clay-cement particles or clusters can be attributed to an increase in the amount of CSH and a decrease in the number of void spaces, causing increased bonding between the clusters. This can be mainly attributed to the increasing strength gain of the material [41], which is in agreement with the results of development in unconfined compressive strength with time as indicated above.

Figure 11, Figure 12 and Figure 13 show the microstructure of cement–BA-admixed clays at w_r = 130% and C = 15% with different BA contents (10 and 30%) and sizes (BA-F and BA-C). At an early age (7 days) for all considered cement–BA-admixed clays (Figure 11a, Figure 12a and Figure 13a), there are small amount of products introduced by the hydration reaction (CSH and CASH), similar to results of cement-admixed clays without BA as described above. However, for cement–BA-admixed clays, it indicated the particles of cement and BA filled in more voids and also surrounded the clay particles. Consequently, this causes the voids of cement–BA-admixed clays to decrease. Thus, it is one reason to explain why the increased strength of cement–BA-admixed clays is minimal at the initial time (see Figure 3a,b).

At t_c = 28 days, there are more products of the cement hydration and pozzolanic reaction for cement–BA-admixed clays with various contents and sizes because of the continuous process of these two reactions with time, as illustrated in Figure 11b, Figure 12b and Figure 13b. Moreover, it is also found that the surfaces of BA particles are coated with the layers of products of cement hydration and pozzolanic reactions, leading to the deposition of CSH and CASH around the clay clusters. This causes the increase in cluster size together with the reduction of pore size. Hence, it can be concluded that the pore size of cement–BA-admixed clays is smaller than those of cement-admixed clays without BA. As a pozzolanic reaction occurs, cementitious products gradually fill the intracluster voids and strengthen the contacts between soil particles. This is why the strength of cement–BA-admixed clays in the long term is significantly larger than cement-admixed clays without BA as presented in Figure 3a,b.

For the same curing time, by comparing adding BA-F and BA-C at a content of 30% (Figure 11 vs. Figure 13), it is found that the amount of ettringite in the case of a mixture with BA-F is more than that with BA-C. The obtained result is mainly attributed to the smaller particle sizes of BA-F. The BA-F has a larger surface area to provide the silica and alumina compounds for pozzolanic reaction, resulting in a larger strength as confirmed by the results presented in Figure 8. In the case of the same BA size (Figure 12 vs. Figure 13), it can be observed that the amount of ettringite of the cement–BA-admixed clays increases with an increase in the added percentage of BA. These results are consistent with the TGA results as discussed above (Table 3), where the decrease in the amount Ca(OH)₂ was consumed by the pozzolanic reaction.

4. Conclusions

The efficiency of adding disposed waste material, ground bottom ash (BA) into the cement-admixed clay used for deep cement mixing applications was evaluated in this study through a series of UC, TGA, and SEM tests. The fineness and packing effects of BA on strength development were also studied. The obtained results demonstrate that the BA could be advantageously supplemented as a cementitious material into the cement-admixed clay mixture to improve the strength characteristic. According to this assessment, it is confirmed that adding up a BA content of larger than 15% when the base cement content is not less than 20% exhibits higher efficiency. As compared with fly ash of the same grain size at small water content, the potential of using BA is achieved when the content to be added is higher than 15%. The efficiency of BA as a cementitious material is also related to the fineness, which enhances the pozzolanic reaction and packing effects. The finer or smaller particle sizes of BA could successfully be used for achieving a great strength of cement–BA-admixed clay. This result was confirmed through the investigation of the cement-admixed clay blending with insoluble material (i.e., sand) having a similar grain size. Moreover, the obtained TGA and SEM results also prove the strength development of cement–BA-admixed clay, in which the pore spaces within the mixtures can be reduced to enhance the bonding strength through the combined process of cement hydration and pozzolanic reactions.

For future studies, investigations regarding the possibility of the leaching of dangerous compounds (e.g., heavy metals) and possible long-term degradation should be performed.