1 Introduction

Champlain Sea clay, also known as Leda clay, is commonly found along the St. Lawrence Lowlands regions in the provinces of Ontario and Quebec in eastern Canada (Penner and Burn 1978). It is one kind marine sediment formed about 8000–12,000 years ago (Karrow 1961). The most well-known feature of Champlain Sea clay is its transformation from a relatively brittle material to liquid when it is disturbed (Crawford 1968). Destruction of the fabric of the clay may be due to induced stress or remolding. Weak particle bonds and high water contents are also the reasons behind its sudden change in state to fluid (Quigley 1983). Due to its peculiar properties, Champlain Sea clay is commonly subject to flow-type landslides (Gillot 1970), which resulted in property damages and loss of lives (Tavenas et al. 1971; Eden et al. 1971). Other engineering challenges include excess settlement over time and considerable shrinkage on dewatering (Penner and Burn 1978; Gillot 1970).

Ground improvement is normally required for construction in soft marine clays. Deep soil mixing (DSM), also known as cement mixing, shallow soil mixing, etc., is a popular ground improvement technique where the binder, normally cement or lime, is mixed in-situ with soil to improve its properties. The main objective of DSM is to improve the strength, stiffness, and permeability of the soil by creating bonding between soil particles. When soil is mixed with cementitious binders, the binder provides the necessary calcium ions to be exchanged with the monovalent cations such as Na+ and K+, which causes a significant reduction of the double layer thickness and hence a reduction in the plasticity along with an increase in shear strength. Cement hydration produces calcium silicate hydrate (CSH), calcium hydroxide, and also calcium aluminum hydrate (CAH) (Bergado et al. 1996). There are numerous research on the applications of DSM for treating difficult soils around the world (Okumura et al. 1972; Okumura and Terashi 1975; Terashi et al. 1980; Bergado et al. 1996; Miura et al. 2001; Åhnberget al. 2003; Lee et al. 2005; Shen et al. 2008; Horpibulsk et al. 2011; Liu et al. 2012; Kitazume et al. 2013; Sasanian and Newson 2013; Paniagua et al. 2019). Recently, there are some studies regarding the microstructural analysis of treated soils using advanced techniques, such as scanning electronic micrograph (SEM), X-ray diffractions (XRD), mercury intrusion porosimetry (MIP), and thermal gravity analysis (TGA). Al-Rawas et al. (2002) applied slag and cement/slag to treat an expansive Na-montmorillonite clay and found the formation of aggregations in the SEM analyses. Chew et al. (2004) and Kamruzzaman et al. (2009) investigated the development of microstructure in cement-treated Singapore marine clay by performing SEM and MIP analyses and concluded an increase in the degree of flocculation at higher cement dosages. Horpibulsuk et al. (2010) investigated the strength development in cement-stabilized silty clay from Thailand using SEM, MIP, and TGA and found an optimum water content at which cement is more effective in treating that clay. Du et al. (2014) investigated the changes in phases of major hydration products and microstructural characteristics of the zinc-contaminated kaolin clay treated with cement and found the major hydration reaction products formed at a low contamination. More recently, Liu et al. (2019) found a large number of ettringite and CSH crystals enwrapped in cement-treated Lianyungang marine clay and the changes in morphology and the amount of CSH with time by the SEM and XRD analyses. Cement is by far the most popular binder used in practice due to its readily availability. The cement dosages were reported to vary from about 100 kg/m3 to 400 kg/m3 for DSM projects around the world (Bruce et al. 2013). In addition to traditional cement or lime, other binders were also used in the studies, in particular the industry wastes. For example, Yoobanpot et al. (2017) compared the effect of cement, cement kiln dust (CKD), and CKD/fly ash on the strength and microstructures of the stabilized Bangkok clay. They found that cement led to a rapid increase in strength in the short-term curing period and tended to have a gradual increase in strength for the long-term curing period. However, the CKD-only mixture had the same trend as cement with a lower compressive strength.

There are only limited research available on the DSM technique in treating Champlain Sea clay. Locat et al. (1996) conducted a laboratory investigation on the quicklime stabilization of this clay. It was found that even at a higher water content above the liquid limit, a significant strength increase was obtained when enough time and lime were provided. More investigations have been recently conducted on the effectiveness of cement in treating this clay. Li et al. (2016) applied a cement dosage of 144 kg/m3 and increased the unconfined compressive strength (UCS) of treated samples by 10 times that of the untreated ones. Afroz et al. (2018) investigated both strength and compressibility of general use Portland cement in treating Champlain Sea clay and found significant strength and compressibility improvement within a short period of 7 days. However, there is no information regarding the optimum dosage of cement in treating Champlain Sea clay. The application of industry waste, like CKD, may also have significant economical and environmental benefits. In addition, it is also important to study the microstructural change due to the cement mixing and investigate the relationship between the microstructural change and strength improvement of the treated clay. This study was conducted to address these issues by investigating the efficiency of different cement-based binders and the associated microstructural changes using both SEM and XRD techniques. The binder used in this study include general use Portland cement and a mix of cement blended with either slag, CKD, or silica fume. The UCS test was used to investigate the strength development. The chemical reaction products were identified using the XRD technique. SEM was used to qualitatively evaluate the microstructural change in the treated clay specimens.

2 Design of Experimental Program

2.1 Champlain Sea Clay Samples Used in this Study

The Champlain Sea clay samples used in this study were obtained from a dam site approximately 60 km west from the city of Ottawa, Ontario, Canada. The soil samples were large size undisturbed Laval samples. Physical property tests were conducted on the clay sample, including the Atterberg limits, grain size, density tests (Liu et al. 2017). The detailed physical properties of Champlain Sea clay used in this study are shown in Table 1. According to the Unified Soil Classification System, the clay used in this study is classified as CH (Clay with high plasticity).

Table 1 Physical properties of soil samples from a depth of 21.6 m
Full size table

XRD was conducted on the same clay sample. The combined bulk and clay XRD results, consists mainly of illite clay with lesser amounts of quartz, chlorite, plagioclase feldspar, kaolinite, hornblende, potassium feldspar, and pyrite, as shown in Table 2.

Table 2 Summary of XRD analysis on the clay sample used in this study (SNC-Lavalin 2017)
Full size table

2.2 Cement-Based Binders Applied in this Study

A total of five cement-based binders, obtained from a local cement manufacturing plant, were applied in this study: General use Portland cement (C), a mix of cement and CKD at a mass ratio of 1:1; a mix of cement and CKD (3:1); a mix of cement and slag (CS) (3:1); and a mix of cement and silica fume (HSF) (8% by weight).

CKD, a significant by-product of the cement manufacturing process, is a particulate mixture of partially calcinated and unreacted raw feed, clinker dust and ash, enriched with alkali sulfates, halides and other volatiles. Its size distribution, chemical, and physical properties depend on several production factors such as raw feed material, type of kiln operation, dust collection system, and fuel type. The use of CKD has been evaluated as an addition to Portland cement and found that CKD has a comparable effect on stabilizing expansive clay with similar results as Portland cement, fly ash, and lime (Zaman et al. 1992; Sayah 1993; Yala et al. 2012).

Blast furnace slag, a by-product during the conversion process of iron to steel, can be crushed or milled to very fine particles and make it a suitable partial replacement for or additive to Portland cement. The chemical composition of slag is quite uniform. It consists primarily of silicates, aluminosilicates, calcium-alumina-silicates, and some oxides such as iron oxide and manganese oxide (Yuksel 2018). Using slag in soil stabilisation is a cost-effective and environmentally friendly applications for this industrial by-product.

Silica fume, a by-product of the silicon smelting process, is known for its production of high-strength concrete. In the concrete industry, it is used for two different reasons: to replace a portion of the cement to reduce the cost; and as an additive to improve concrete properties under both fresh and hardened conditions. In this study, the use of silica fume is to investigate its effect on the strength development of soil–cement mixture.

The final strength of the treated soil is influenced by many factors, such as soil type, binder type, binder dosage, curing time, mixing method (Kitazume and Terashi 2013). According to a previous study on Champlain Sea clay by Li et al. (2016), a minimum dosage of 50 kg/m3 per mixed volume was required to obtain a meaningful strength increase. Cement dosages ranging from 50 to 200 kg/m3 were selected to see the impact of cement dosage in this study. The design of binder dosage was mainly based on the cement content, as shown in Table 3, where the type and the number of samples for each test (bracketed) are also included in the table.

Table 3 Summary table of binder types and dosages used in this study
Full size table

3 Soil Sample Preparation and Testing Procedures

3.1 Sample Preparation for UCS Samples

Both dry mixing and wet mixing methods are commonly used in practice, though dry mixing can be applied only to soils with natural water contents above 60% (Bruce et al. 2013). The wet mixing method following the procedures recommended by the US Federal Highway Administration Design Manual (Bruce et al. 2013) was used in this study. The detailed procedures can be found in Monsif et al. (2020).

The UCS test was conducted in accordance with ASTM D2166 standard. The load rate of 1.25 mm/min was applied to the sample and the test was terminated until a limit strain of 15% was reached.

3.2 Sample Preparation for SEM Specimens

The specimens for the SEM analyses were prepared from the same mix for the UCS samples. A 12-mm diameter by 10-mm high ring was used to host a SEM specimen. The soil-binder mix was pressed into the specimen ring and carefully trimmed to a flat surface. The specimen was then kept in a sealed plastic bag and cured under the same curing conditions as the UCS samples. After a desired curing time, the SEM specimen was oven-dried under a temperature of 105 °C for 24 h. Last, the specimen was polished with a sandpaper to a flat surface to be coated with a thin layer of gold. The coating is crucial for preventing charging of the specimen to obtain a high quality SEM image.

A JEOL-6380 LV SEM machine was used to obtain the SEMs of treated clay specimens under different binder and curing time conditions. The accelerating voltage ranged between 20 and 30 keV, with a working distance varying between 10 to 12 mm.

3.3 Sample Preparation for XRD Specimens

The XRD specimens were prepared the same way as the SEM specimens except a different specimen size of 35-mm in diameter and 7-mm in height and an air-dry procedure. The XRD analyses were performed using a PANalytical X’Pert Pro diffractometer equipped with a Cu-Kα X-ray tube. An input voltage of 45 kV and a current of 40 mA were used in this study. The specimens were scanned for 2 Theta values ranging from 5° to 90°, with a step length of 0.02°, a scanning rate of 2°/min, and a slit width of 0.3 mm. The XRD results were analyzed using X’Pert HighScore Plus software.

4 Test Results and Analysis

4.1 Change of Unconfined Compressive Strength Due to Cement-Mixing

The stress–strain curves of the UCS test of Champlain Sea clay before and after cement mixing are shown in Fig. 1. The average value of peak UCS, qu, for three undisturbed clay samples was 84 kPa with a standard deviation of 6.0 kPa. The variation in the qu was mainly caused by sample disturbance and test preparation. A significant strength improvement was observed after cement mixing. After mixing with a cement dosage of 50 kg/m3, as shown in Fig. 1a, the average qu of two tests for each condition increased substantially from 84 to 590 kPa in just 7 days. The qu further increased with curing time to 842 kPa after 28 days of curing, approximately 12 times that of untreated soil. In addition to the strength increase, the failure strain, εf, reduced from around 3% to about 1%, which means that the failure mode of clay samples changed from a relatively ductile mode to a brittle one. The qu of treated samples increased substantially with increasing cement dosage, as shown in Fig. 1b for 100 kg/m3 cement dosage and Fig. 1c for 200 kg/m3 dosage cases. For example, under the same curing period of 28 days, the qu of treated samples increased from 842 kPa for 50 kg/m3 samples in Fig. 1a, to 1948 kPa for 100 kg/m3 samples in Fig. 1b, and further improved to 2472 kPa for 200 kg/m3 samples in Fig. 1c. For cement-treated samples, the more prominent strength increase was observed when the cement dosage increased from 50 kg/m3 in Fig. 1a to 100 kg/m3 in Fig. 1b compared to the dosage increase from 100 to 200 kg/m3 in Fig. 1c. This may imply the optimum cement dosage would be around 100 kg/m3 in terms of strength increase and cost effectiveness. The qu also increased with increasing curing time. For the 100 kg/m3 samples in Fig. 1b, the qu increased substantially from 1948 kPa at 28 days of curing to 2578 kPa at 300 days. The strength increase of Champlain Sea clay due to mixing is similar to findings in other soils (Kawasaki 1981; Lorenzo and Bergado 2006).

Fig. 1
figure 1

The stress–strain curves of unconfined compressive strength tests on Champlain Sea clay without and with cement treatment a 50 kg/m3; b 100 kg/m3; c 200 kg/m3

Full size image

The stress–strain curves of clay samples treated with other cement-based binders are similar to those of cement-treated samples, but with different magnitudes of qu, as shown in Fig. 2a for CKD 67 kg/m3 dosage cases, Fig. 2b for CKD 100 kg/m3 dosage cases, Fig. 2c for HSF 50 kg/m3 dosage cases, and Fig. 2d for CS 67 kg/m3 dosage cases. The samples treated with other cement-based binders became more rigid as curing time increased. For example, the εf reduced from 1 to 2% for curing times up to 28 days to around 0.5% at the curing time of 300 days. The effect of the curing time on the qu values is shown in Fig. 3 for different binder types, where qu is the average value of two samples tested for each curing and binder condition. Generally, the qu value increased with increasing curing time. The qu value is mainly controlled by the cement dosage at the short curing time. For example, a very similar qu was found among different binder samples at the short curing time up to 28 days under the same cement dosage (50 kg/m3 except 46 kg/m3 for HSF samples in Fig. 2c) even with different additive conditions. With the increasing curing time, the increase of strength over time became more prominent in samples treated with the cement/slag mix (Fig. 2d) or the cement/CKD mix (CKD100 in Fig. 2b). The qu values from these two binder types were much higher than the C50 samples, which means CKD and slag can help pozzolanic reactions. These results show the feasibility and efficiency of using CKD or slag to replace a portion of cement in treating Champlain Sea clay. More details about the UCS test results can be found in Table 4.

Fig. 2
figure 2

Stress–strain curves of unconfined compressive strength tests on Champlain Sea clay treated with other cement-based binders

Full size image
Fig. 3
figure 3

Effect of curing time on the qu of treated Champlain Sea clay

Full size image
Table 4 Summary of unconfined compressive strength tests conducted on natural and treated Champlain Sea clay samples
Full size table

The plot of qu against the ratio of total water content to binder content, w/c, is shown in Fig. 4, where qu reasonably follows a function of w/c at a certain curing time. The w/c parameter was first proposed by Miura et al. (2001) to represent the strength behavior of cement-stabilized clay. This qu change with w/c for treated Champlain Sea clay is similar to the trends found in other soils (Terashi 1977; Lorenzo and Bergado 2004).

Fig. 4
figure 4

Unconfined compressive strength qu versus the total water to binder ratio

Full size image

The secant moduli, E50, was determined at a stress level at the half of the qu, as illustrated in Figs. 1 and 2. The relationship between E50 and qu of each sample is shown in Fig. 5. The figure demonstrated that the E50 of treated Champlain Sea clay almost linearly increased with the qu regardless of different binder types and curing period. The E50 can be taken as 70 qu–180 qu, with an average ratio of 123 for cement-treated Champlain Sea clay. This ratio of E50/qu for treated Champlain Sea clay is within the lower range reported on mixing treated soils, as shown in Table 5.

Fig. 5
figure 5

Modulus of elasticity versus unconfined compressive strength of different cement-based binder treated Champlain Sea clay

Full size image
Table 5 The value of \({E}_{50}\)/\({q}_{u}\) for various soil-binder samples
Full size table

A non-destructive test was performed using a Pundit Lab ultrasonic NDT device on the cement-treated samples. Frequency of transmission pulses was kept at 50 kHz and travel time at the microsecond level was read directly on the screen of the device. The ultrasonic pulse velocity was calculated by dividing the sample length by the travel time for the ultrasonic pulses. Previous studies showed that the relationship between the ultrasonic velocity and qu can be estimated by an exponential relationship proposed by Ravindrarajah (1997). The correlation between the ultrasonic pulse velocity, v, and qu is shown in Fig. 6. The close relationship is similar to the finding for cement-treated sand by Choobbasti and Kutanaei (2017). This verifies the possibility of applying ultrasonic NDT to assess the strength of cement-treated Champlain Sea clay. More research is required in the future to confirm this finding.

Fig. 6
figure 6

The ultrasonic pulse velocity and qu of cement-treated Champlain Sea clay

Full size image

4.2 Microstructural Change of Champlain Sea Clay Due to Cement Mixing

The elemental composition analysis of the natural clay was conducted using energy dispersive spectroscopy (EDS) technique. The average composition of a specific area (Spectrum 6 in Fig. 7a) is shown in Fig. 7b. This area is composed mainly of sodium (Na) and chlorine (Cl) with about 37.6% by weight. The high amount of sodium and chloride can be attributed to the marine sedimentation environment of Champlain Sea clay.

Fig. 7
figure 7

Spectra of EDS analysis (a) and average elemental composition of natural clay (b)

Full size image

The microstructure of a natural clay specimen was observed on a view parallel to its bedding plane using the SEMs under two magnifications of 5000 (Fig. 8a) and 10,000 (Fig. 8b, c). The observed structure is an aggregated structure with the aggregate diameter of approximately 1–2 μm. The aggregates are composed of small platelets separated by a porous network. Some silt particles are scattered among the aggregates. Figure 8b highlights a silt particle with a diameter of around 6 μm, to which some platelets are stuck by the side and the edge. The texture of the natural clay consisted of many sheet-like particles (Fig. 8c). The flaky and plate-like particles could be identified as illite with some scattered kaolinite minerals. The sample exhibits a matrix of overlap between particles where slightly curled flakes arises (Fig. 8c).

Fig. 8
figure 8

SEM micrographs of natural clay and clay treated with 50 kg/m3 cement

Full size image

The impact of cement mixing on the microstructure of the clay is shown in Fig. 8d and f. Microstructure of the 7-day cured specimen (Fig. 8d) did not show a significant difference from the natural one mainly due to short curing time. However, with increasing curing time, the hydration products clearly change the microstructure of the cemented specimens. The soil clusters and the pores were covered and filled by the hydration products at 14 day curing (Fig. 8e). Over time, the hydration products in the pores became more prominent and the soil–cement clusters became larger because of the growth of cementitious products over time (Fig. 8f). No significant change can be easily observed between the 14-day and the 28-day cured specimens, which is in agreement with the similar qu values, as shown in Table 4. This could be due to the insufficiency of cement for further hydration after 14 days of curing.

The effects of cement dosage on the pore size distribution of the cemented specimens are shown in Figs. 8 and 9. An early formation of hydration products was observed in the 7-day cured specimens under higher cement dosages (Fig. 9a and d). Substantial growths of reaction products can be observed as the curing time increased to 14 days (Fig. 9b and e), where cementitious bridges in the form of needle-like crystals, also called ettringite (marked by rectangles) are formed between clay aggregates and clay particles. In the 28-day cured specimens (Fig. 9c and f), substantial pores were closed and more hydration products were hardened on the surface of the clay particles. These hydration products bind together the clay particles or clusters of clay particles and created a stronger soil–cement matrix of soil. With increasing cement dosage, the cementitious products fill small pores and formed large soil–cement clusters at 7-day curing. With increasing curing time, the volume of small pores decreased and more cementitious products formed larger aggregates and interconnected the particles together.

Fig. 9
figure 9

SEM micrographs of clay specimens treated with cement dosages of 100 and 200 kg/m3

Full size image

It is worthy noting that the poor quality of SEM micrographs were observed for treated clay specimens compared to natural clay specimens. The 24-h oven-drying method might leave a high amount of water inside the treated clay specimens, which could cause this image quality issue. This issue became less prominent in the specimens by increasing curing time and cement dosage, which allows more consumption of water for hydration reactions and reduces the charging effect for a better image quality. In addition, the liquid nitrogen freezing method used by other researchers shall be investigated to improve the quality of SEMs in the future studies.

The SEM micrographs of specimens treated with two CKD dosages are shown in Fig. 10. For CKD67 specimens, Fig. 10a–c show quite similar surface morphology without many soil-binder aggregations formed under different curing times. The main reason can be contributed to insufficient cement to generate enough cementitious reactions. This was also associated with similar qu values obtained in the UCS tests. With the CKD dosage being increased, a significant amount of large soil-binder aggregates was observed in the SEMs of CKD100 specimens. At the curing time of 7 days, the clay particles started to get coated with the hydration products. As the curing time increased, a large amount of aggregate clumps in the size of approximately 5 µm started to get formed (indicated by the yellow ellipse in Fig. 10e), in particular, in the 28 day-cured specimen (Fig. 10f).

Fig. 10
figure 10

SEM micrographs of clay specimens treated with CKD dosages of 67 and 100 kg/m3

Full size image

The SEM micrographs of CS67 and HSF50 specimens are shown in Fig. 11. Significant changes could be observed with increasing curing time in the microstructures of CS67 specimens (Fig. 11a–c). At the 7-day curing, the clay particles were coated by the early stage formation of the hydration products and resulted in a reduction in voids in the CS67 specimen (Fig. 11a), while not much found in the HSF50 specimen (Fig. 11d). As the curing time increased to 14 days, the clay-binder aggregates were observed in the SEMs of both the CS67 and HSF50 specimens (Fig. 11b and e). At the curing time of 28 days, these aggregates grew further to about 7 µm in length in the CS67 specimen (Fig. 11c) and about 6 µm in length in the HSF50 specimen (Fig. 11f), which are about three times that of natural clay. Besides the aggregations, the aggregate interlock was observed in the SEMs, which could be another mechanism for the shear strength improvement. These microstructure changes also explain the mechanism behind the qu increase obtained in the UCS tests.

Fig. 11
figure 11

SEM micrographs of clay specimens treated with CS67 and HSF50 binders

Full size image

In summary, the SEM micrographs of the treated specimens show both hydration products and cementitious products significantly increased over time, though the intensities of the changes varied depending on the types of binder and dosage. The hydration products and cementitious products not only fill the pore space but also enhance the inter-aggregate bonding strength, which result in an increase in the shear strength. Based on the microstructural analysis, a cement dosage of 50 kg/m3 is required to show the microstructural change and cementitious product growth over time. With additives, more hydration products and cementitious products were observed in the treated clay specimens. The significant microstructure changes observed in the CKD100 (Fig. 10d–f) and CS67 (Fig. 11a–c) specimens also correlated well with the substantial strength increase over time in these samples observed in the UCS tests. The microstructure analysis confirmed the feasibility and efficiency of using CKD or slag to replace a portion of cement in treating Champlain Sea clay.

4.3 XRD Analysis of Phase Composition Change Due to Cement Mixing

The XRD patterns indicated that the dominant minerals for natural clay are illite (2K2O. 3MgO. Al2O3. 24 SiO2. 12H2O), kaolinite (Al2O3. 2SiO2. 2H2O), and quartz (SiO2), as shown in Fig. 12a. After cement mixing, more phases were observed in the cement-treated specimen, as shown in Fig. 12b, where the specimen was mixed with 50 kg/m3 cement and cured for 28 days. The major reaction products were identified as calcium silicate hydrate (CSH, Ca1.5SiO3.5.H2O) and tecto-dialumodisilicate tetrahydrate (CASH, CaAl2 Si2O8 (H2O)4). Calcium hydroxide (Ca(OH)2), a major product of cement hydration, could not be detected in the treated-clay specimen cured at 28 days because of its rapid dissolution as Ca2+ and (OH)− into pore solutions after cement hydration, which leads to subsequent dissolution of some silicate and aluminate from clay minerals and pozzolanic reaction. The absence of Ca(OH)2 from the XRD patterns and the drop of the kaolinite and illite peaks could explain the occurrence of the pozzolanic reaction.

Fig. 12
figure 12

XRD patterns for a natural clay and b cement-treated clay

Full size image

Figure 13 shows the elemental compositions of natural clay and the cement-treated specimens (50 kg/m3 cement and 28-day curing). It can be noted that the amount of calcium (Ca) element increased from 3.5% in natural clay to 16.9% in the cemented clay specimen. However, there was a little change in the amount of silicon and aluminium elements. The increase in the amount of calcium element in the treated specimen is due to the cementitious products, which in turn results in a strength increase observed from the UCS tests.

Fig. 13
figure 13

Elemental compositions of a natural clay and b cement-treated clay

Full size image

5 Summary and Conclusions

An experimental investigation was conducted to understand the strength development and the microstructure changes associated with Champlain Sea clay when treated with cement-based binders. A total of five different cement-based binders were applied in this study: general use Portland cement (C), a mix of cement and cement kiln dust (CKD) (1:1 by mass); a mix of cement with CKD (3:1); a mix of cement with slag (CS) (3:1); and a mix of cement and silica fume (HSF) (8%). The unconfined compressive strength tests were conducted to measure the shear strength development under different binder type, binder dosage, and curing time conditions. The scanning electronic microscopy (SEM) and X-ray diffraction (XRD) analyses were conducted to investigate the microstructural changes in the treated clay specimens. Based on the test results, the following conclusions can be drawn:

  • In general, cement is efficient to treat Champlain Sea clay resulting in a significant strength increase. The optimum cement dosage was found to be around 100 kg/m3 in terms of strength improvement and cost-effectiveness.

  • Among the five different binders, cement is the most efficient in improving the shear strength of treated clay after a short curing time. The clay samples treated with CKD100 (50 kg/m3 cement plus 50 kg/m3 CKD) and CS67 (50 kg/m3 cement plus 17 kg/m3 slag) showed a substantial shear strength increase over time due to pozzolanic reaction. These results proved the feasibility and efficiency of using CKD or slag to replace a portion of cement in treating Champlain Sea clay.

  • Based on the SEM observations, natural Champlain Sea clay exhibits an open structure with flocculated particles with sizes of around 1 µm. The links were observed between the microstructural changes due to artificial cementation. The SEM images of the cement-treated clay specimens indicated the presence of a reticulated matrix within the material. As the cement dosage increased, the microstructure of cement-treated clay gradually transformed from a dispersed structure into a flocculated state. The cementitious products were affixed to the particles, coated the particle surfaces, and formed large clay-binder aggregates with sizes up to 7 µm. The hydration products and cementitious products filled the voids between the aggregates and enhanced the bonding between the aggregates, which resulted in the shear strength improvement in the treated clay.

  • Based on the XRD analyses, cementitious products such as calcium silicate hydrate (CSH) (Ca1.5 SiO3.5. H2O) and calcium tecto-dialumodisilicate tetrahydrate (CASH) (CaAl2 Si2O8 (H2O)4) were formed in the clay specimen treated by 50 kg/m3 general use Portland cement and cured for 28 days, which explains the strength improvement of Champlain Sea clay due to cement mixing.