An Investigation into Performance of Cement-Stabilized Kaolinite Clay with Recycled Seashells Exposed to Sulphate

Sulphate attack is one of the key issues in geotechnical engineering. This study aims to investigate the efficacy of the seashell to reduce negative impacts of the magnesium sulphate concentration on the cement-stabilized clay mixtures by performing a series of unconfined compressive strength (UCS) tests. Three percent of cement (3, 5 and 7%) was utilized in this study. In addition, the benchmark and exposed specimens were cured for 7, 14, and 28 days before testing and exposure, respectively. A series of the compaction tests were conducted and the optimum moisture content (OMC) and maximum dry density (MDD) values were achieved. In the next stage, the UCS tests were performed on the specimens containing 10, 20, or 30% seashell contents and the specimens were exposed to sulphate concentration. Scanning electron microscope morphology had indicated that seashells are a suitable replacement for cement. Qualitative X-ray diffraction had shown that the presence of magnesium sulphate reduces the formation of calcium silicate hydrate, which causes durability issues in cement-stabilized soils. The results indicated that seashell is effective to improve the sulphate resistance of cement-stabilized soil.


Introduction
The concentrated magnesium sulphate in soil has destructive impacts on infrastructure, especially in the transport sector. This phenomenon can be seen in various areas in particular around the coastal regions, near the groundwater, landfills, and wastewater facilities. As far as there is no contact between magnesium-sulphate-contaminated soil with air, there is no issue with this soil, however, when these types of soil are subject to construction of new infrastructures, like roads or highways, destructive impacts can be shown over time. For instance, Snedker and Temporal [1] reported that a roadway has been bumped due to exposure to sulphate concentration. Another roadway damage was reported by Rollings et al. [2] that was stabilized by Portland cement (PC) and deteriorated over time.
Sulphate attacks occurred in cement-stabilized soil when the main hydration product of soil stabilization, calcium silicate hydrate (CSH) reacted with the sulphate ions present in the soil when the sulphates such as magnesium sulphate occurred naturally in the environment. This phenomenon is known as the decalcification, and a new product is generated out of this process which is known as magnesium silicate hydrate (MSH), which reduces the strength of the soil [3][4][5]. The decalcification occurred due to the replacement of the calcium element in CSH with the magnesium element which led to the reduction in unconfined compressive strength (UCS) of cement-stabilized soils [5,6]. It is also known that the decalcification process still occurs even at low sulphate concentration that still led to the reduction in UCS of cement-stabilized soils [7]. Besides the formation of MSH, it is also known that ettringite is formed in sulphate attacks for stabilized soils that also cause damage to the cement-stabilized soils [8]. Therefore, the effect of sulphate attacks in soils is a major concern for pavements and roads as sulphate attacks reduce the operation duration of the infrastructure and increase the need for maintenance. Despite that, many research studies on sulphate attacks have been conducted to investigate the impacts of the sulphate attack on concrete structures [9][10][11][12][13]. It is known that lesser considerations have been given towards its impacts from a geotechnical point of view, particularly on the application of seashells as partial replacement of cement on stabilized clays to improve the UCS properties of the cement-stabilized soils.
Improvement of the soil mechanical behavior using by-product material is a current topic within geotechnical engineering [14,15]. This approach is not only cost-effective but also aligns with the United Nation's sustainable development goals (UNSDGs) which help to reduce the carbon footprint from the planet. The use of cement is also known to emit large quantity of carbon dioxide emission, with Australian industry emission levels to be 700 kg of carbon dioxide released with every 1000 kg of cement produced. In addition, the use of cement also has led to the use of large quantities of virgin limestone material for the production of cement, which does not align with UNSDG's aim. For instance, application of the by-products, such as recycled aggregates and seashells, has been widely investigated in the literature to aid the reduction in carbon dioxide and increase the application of renewable materials for civil works [16,17].
Seashell is an abundant material, in particular in the coastal region which provides a good potential of application in the construction sector [18][19][20]. Previous studies showed that application of the seashell can be promising to improve soil mechanical behavior [21][22][23][24][25][26]; however, no studies have been conducted to investigate its effect on the improvement of the sulphate contaminated soils. A review of the literature showed that no previous studies investigated the effect of seashell addition on the improvement of the sulphate contaminated soils. Therefore, this study aims to investigate the effect of seashell addition on the improvement of the soil UCS when exposed to magnesium sulphate.

Kaolinite Clay
The kaolinite clay was sourced from Sibelco Australia Kaolinite clay, with the clay selected due to its suitability to be stabilized with cement and seashells. The clay was a mixture of 93% kaolinite clay and approximately 7% of quartz. Based on ASTM D 2487, the clay is classified as a highly plastic clay group (CH). The index tests represent that the soil samples the plastic limit of 31%, liquid limit of 58% and bears the plasticity index of 27%. The particle size distribution (PSD) of soil can be seen in Figure 1. Kaolinite clay has a specific gravity of 2.8 and a pH of 7. The kaolinite clay is a plate-like crystal shape with the length of 0.2 µm to 2 µm and the thickness of 0.05 µm to 0.2 µm. The chemical composition of the kaolinite clay is 70-95% kaolinite, 5-15% of crystalline quartz and <15 % of traces of other minerals.

Portland Cement (PC)
The Australian general purpose (GP) cement was used as the cementitious material in this study where it is equivalent to Type II Portland cement (PC) in the U.S. The cement is selected as the binder for stabilization due to its availability and low cost. The PC was sourced locally from Cockburn Cement. The specific gravity was between 2.9 and 3.2. The pH was 12. The bulk density ranged from 1400 kg/m 3 to 1800 kg/m 3 . The chemical composition from X-ray Fluorescence is shown below in Table 1.

Seashell
Crushed seashells (SS) were sourced from L'Haridon mining, a local supplier in Perth. All the seashells were sourced in a crushed state. The seashells had a specific gravity of 2.35 and water absorption of 0.8%. In accordance to the manufacturer, calcium oxide (CaO) consisted of one of the main ingredients of the used seashell in this study. The PSD of the seashells used in this study is shown in Figure 1.  Table 1 shows the testing program followed in this study to prepare the mixture and conducting the tests. The applied Portland cement (PC) contents were 3, 5 and 7%, while the seashell (SS) dosages of 5, 10 and 15% were utilized to prepare the specimens. The proportions added were based on the percentage by weight of the solids. The samples were cured for 7, 14 and 28 days when they thoroughly mixed. A series of compaction tests were conducted to obtain the optimum moisture content (OMC) and maximum dry density (MDD) values to prepare the samples in the unconfined compressive strength (UCS) testing section. Each test has been conducted 5 times with ± 2% tolerance.

Methodology and Specimens Preparation
To prepare the specimens, initially, each material was thoroughly mixed in a dry condition and then was mixed with water according to the acquired OMC value. Then, the specimens were tamped in 3 layers until MDD values were achieved for each specimen. After that, the specimens were wrapped and stored in moisture and a temperature-controlled room at 25 • C and 50% humidity to fulfil the curing period. Two batches of each variable were produced with one batch designated as the benchmark and the other exposed to sulphate attack. This allows the comparison of the resistance of seashells substituted in cement for the stabilization of the kaolinite soil. The specimens that are exposed to sulphate are designated as E and benchmark specimens as B, with the proposed mixes summarized in Table 2. Table 2. Proposed mixtures used in this study.

Standard Proctor Test
Australian Standard 1289.5.1.1 [27] was followed as a guideline to perform the compaction tests in this study. The optimum moisture content (OMC) and maximum dry density (MDD) were derived from the tests. The results of the tests can be seen in Table 3. For this test, the soil sample was mixed thoroughly with its admixtures (i.e., Portland cement and seashells). The sample was compacted in 3 layers using rammer and 25 uniform blows from the height of 300 mm with the compaction energy of 596 kJ/m 3 .

Preparation of UCS Tests
To run the UCS tests the guidelines in ASTM D2166 [28] were followed. The samples were compacted in 3 layers to achieve the desired OMC and MDD as obtained in compaction tests. Following the relevant curing time, the benchmark samples were tested. Similarly, the exposed samples were tested, and the results were compared with the benchmark to investigate if seashell can improve the resistance against sulphate attack. All the UCS testing was run with a 0.1 mm/min rate to assure the accuracy of the results. Additionally, a concentration of 42.2 g/L of magnesium sulphate was added to the samples for sulphate testing, the sulphate exposure was conducted following ASTM C1012 [29].
The used mold had an approximate diameter of 50mm and a height of 100mm. A total of 74 UCS tests (without considering repeated and verification testing) were conducted on the specimens. As indicated, for each specimen, two samples were prepared one for benchmark testing and the other one for the exposed testing. Each benchmark specimen was tested immediately after fulfilling the curing period. However, the exposed specimen batches were submerged in the sulphate basin (with a 42.2 gr/L concentration as per standard testing) for 5 h and then subjected to the UCS testing. At least 5 replicates were used for each variable.

X-ray Diffraction
An X-ray diffractometer with a cooper tube had been used for the qualitative analysis of a benchmark and exposed specimens. The cement-stabilized soil was grounded in a micronizer to finely grounded powder of 5 µm and below for good quality particle statistics. The crystalline phases were determined with EVA 11.0 and cross referenced to the ICDD (International Centre of Diffraction Data) PDF4+ crystalline phases database.

Scanning Electron Microscope
A scanning electron microscope is used to determine the morphology of the cement-stabilized soil. For sample preparation, specimens were selected and attached onto a stud with carbon coating applied on the specimen to reduce electron charge buildup in the material. High vacuum mode with secondary electron mode is utilized for the data collection of the morphology of the specimen. Beam voltage is set to 15 kV, which is sufficient to generate secondary electron images.

Compaction Test
The results of compaction tests are tabulated in Table 3. As can be seen in Table 3, the trend of results shows that the addition of Portland cement had led to reduced MDD, and increased OMC values. This behavior can be attributed to the increased number of voids that were formed due to the presence of seashell particles and generation of a porous medium for the mixture to absorb more moisture [29][30][31][32][33][34][35]. However, contrary behavior was recorded for the MDD values. The maximum dry density values showed a plunge by addition of seashell content at about 2-3%. This behavior can be explained due to a lower specific gravity of seashells in comparison with the clay.

Effect of Crushed Seashell on Sulphate Resistance
The effect of the addition of different seashell contents at 7, 14, and 28 days of curing periods was investigated in this section with the results being presented in Figure 2 , Figures 3-7. For ease of comparison and clarity in the presentation of the results, the UCS results for benchmark and sulphate exposed specimens were presented for each curing time. Therefore, Figures 2 and 3  From Figures 2 and 3, it clearly can be seen that the addition of both seashell content and Portland cement (PC) had to have led to the increase in the peak UCS values in both benchmark and exposed specimens. It can be suggested that the addition of SS increased the peak UCS values too. However, the rate of increase was observed to be higher in benchmark specimens where there is no exposure to the magnesium sulphate concentration. In fact, the addition of Portland cement (PC) causes the formation of hydration products, such as calcium silicate hydrate (CSH) and calcium aluminum hydrate (CAH). The lengthened curing duration is also found to solidify this process and it leads to an increase in the peak UCS values even more where UCS values were found to be higher at 28 days than 7 days. However, when the specimens were exposed to the sulphate concentration, the UCS value was found to be lower than benchmark specimens as the formation of the hydration products suppressed. This led to the UCS value being reduced. Based on the results collected, it can be concluded that the presence of the seashell content in the mixtures has densified the structure amongst the soil particles and the PC. Therefore, this has increased the peak UCS value as the addition of the more reactive CaO that originated from the SS had accelerated the formation of CSH. As both PC and SS are more reactive to a hydraulic setting in the presence of water, the rapid strength gain of the UCS obtained would be predominantly provided by both materials mentioned. The secondary long-term reaction or pozzolanic behavior of the binder formation is believed to have lesser impact on the material in comparison to the short-term hydraulic reactions.
Sustainability 2020, 12, x FOR PEER REVIEW 6 of 14 secondary long-term reaction or pozzolanic behavior of the binder formation is believed to have lesser impact on the material in comparison to the short-term hydraulic reactions.  Similar results were recorded between the benchmark and exposed specimens with 14-day curing period. It is seen from Figures 4 and 5 that the addition of PC and seashell content increased the peak UCS values in both benchmark and exposed specimens where a similar trend can be observed with 7-day curing period. As indicated, this could be attributed to the increased formation of more hydration products by increasing the PC contents along with the effect of heightened CSH formation due to a longer curing period. The production of the hydration products due to the curing period is known as the pozzolanic reactions. Similar to the 7-day curing period case, the sulphateexposed specimens showed a lesser increase to the benchmark specimens due to generated weaker binders formed amongst soil, seashell and cement from the presence of sulphate. This was attributed to the decalcification phenomenon from the calcium cations in the CSH binder being replaced with  Similar results were recorded between the benchmark and exposed specimens with 14-day curing period. It is seen from Figures 4 and 5 that the addition of PC and seashell content increased the peak UCS values in both benchmark and exposed specimens where a similar trend can be observed with 7-day curing period. As indicated, this could be attributed to the increased formation of more hydration products by increasing the PC contents along with the effect of heightened CSH formation due to a longer curing period. The production of the hydration products due to the curing period is known as the pozzolanic reactions. Similar to the 7-day curing period case, the sulphate-exposed specimens showed a lesser increase to the benchmark specimens due to generated weaker binders formed amongst soil, seashell and cement from the presence of sulphate. This was attributed to the decalcification phenomenon from the calcium cations in the CSH binder being replaced with magnesium cations. The cation replacement with magnesium had led to a formation of a different type of binder, magnesium silicate hydrate (MSH). Despite the MSH still functioning as a binder between the PC, SS and clay, MSH is a comparably weaker binder compared to CSH which leads to lower UCS of the specimens tested.
Sustainability 2020, 12, x FOR PEER REVIEW 7 of 14 type of binder, magnesium silicate hydrate (MSH). Despite the MSH still functioning as a binder between the PC, SS and clay, MSH is a comparably weaker binder compared to CSH which leads to lower UCS of the specimens tested.  UCS results for exposed specimens after 14-day curing time.
A similar result was recorded for the benchmark and exposed specimens after 28-day curing period with the data plotted in Figures 6 and 7. It is seen from the results that the peak UCS values have the highest values due to the lengthy 28-day curing period that led to a higher amount of binder evolved. Similar to the 7-day and 14-day curing periods, it seems that the addition of seashell contents had been observed to enhance the UCS properties of the stabilized clay. It is also important to note that 28 days of curing also led to the highest UCS recorded in this study. In addition, the addition of seashells in cement-stabilized soils was found to have a similar level of UCS of C-only specimens,  A similar result was recorded for the benchmark and exposed specimens after 28-day curing period with the data plotted in Figures 6 and 7. It is seen from the results that the peak UCS values have the highest values due to the lengthy 28-day curing period that led to a higher amount of binder evolved. Similar to the 7-day and 14-day curing periods, it seems that the addition of seashell contents had been observed to enhance the UCS properties of the stabilized clay. It is also important to note that 28 days of curing also led to the highest UCS recorded in this study. In addition, the addition of seashells in cement-stabilized soils was found to have a similar level of UCS of C-only specimens, such as 5% PC to 3% PC with 9% SS and 7% PC to 5% PC to 10% SS have comparable UCS recordings of 500 kPA and 615 kPa. This can be explained with the additional binder formed amongst soil particles and PC [36][37][38][39][40]. Similarly, the cation replacement mechanism of the magnesium with calcium or the decalcification reaction could be the main reason for reducing the peak UCS values in the case of the sulphate-exposed specimens.  UCS results for exposed specimens after 28-day curing time.

Microstructural and Crystallography Analysis
To fully understand the interactions amongst the soil, Portland cement and seashell particles at the micro-level, a series of microstructural analysis has been conducted to provide further insight. To do so, a series of work was conducted on the scanning electron microscope (SEM) and powdered X-ray diffraction (XRD) analysis has been applied on selected specimens.

Scanning Electron Microscopic Characterization
The scanning electron microscopic (SEM) analysis has been conducted on specimen mixed with 7% Portland cement (PC) with sulphate attack from the magnesium sulphate with a 28-day curing period. Figure 8 shows the SEM image that was captured with a backscatter electron (BSE) detector from a 250 µm scanning zoom, as shown in Figure 8. Several locations were selected randomly for energy dispersive spectrometry (EDS) spectrum analysis, however, Spectrum 2 and 3 have been selected as they show a better representation of the sample. The representation of the mentioned spectrums has been plotted in Figures 9 and 10.
As can be seen from Figure 8, the morphology of the sample from SEM is represented with the evidence that the addition of SS is able to form binder with the kaolinite and PC. The EDS spectrum is presented in Figures 9 and 10 with aluminum, calcium and silicon displaying high counts. This is expected as these elements exist predominantly in PC, SS and kaolinite clays. The morphology of the SEM micrograph shows that a fairly dense matrix was formed, suggesting that a binder had been formed between SS, PC and kaolinite. However, a low peak of magnesium element can be seen in the EDS spectrum that suggests that the MSH binder also had evolved during the sulphate exposure. The presence of MSH can be attributed to the cracks and grooves formed from the decalcification process from sulphate attacks, as reported by the previous studies [41][42][43][44][45][46][47][48]. As magnesium does not originate in large quantities from the source material of the stabilized soil, the occurrence of the magnesium peaks can be attributed to the magnesium replacement of CSH.

X-ray Diffraction (XRD) Analysis
The XRD analysis conducted on both the benchmark and exposed stabilized soil specimens that contained 7% Portland cement (PC), and 10% seashell (SS). The curing period of the mentioned specimens was 28 days. Hence, Figures 11 and 12 show the XRD spectrum for the mentioned benchmark and exposed specimens, respectively. Based on Figure 11, there is an amorphous hump observed from 19 • to 24 • 2θ which suggests that CSH is the main component of the benchmark specimen. CSH is largely amorphous without an orderly crystalline line structure where crystalline material tends to appear as distinct peaks in XRD spectrum. However, with sulphate attack on the exposed specimen, two distinctive changes can be observed where distinct peaks of magnesium-based crystals were formed from the presence of magnesium sulphate in the stabilized soil. Secondly, the amorphous hump can be seen to span with a shorter range of 2θ than of the specimens that were not exposed to sulphate ingress. This might suggest that the ingress of magnesium element into the binder of stabilized soils had caused less CSH to be formed which has led to a smaller amorphous hump being observed in the XRD pattern. This is supported by lower UCS recorded with exposed specimens in this study.

Conclusions
Sulphate attack has damaging impacts on roads infrastructures such as roads, highways, and railways. This study investigated the effect of the replacement of cement with seashells to improve the sulphate resistance of soils via a series of experimental unconfined compressive strength (UCS) tests on benchmark and exposed specimens. To do so, a varying amount of seashell (SS) replacement of Portland cement (PC) was mixed with kaolinite clay at different levels of cement to kaolinite. The specimens were cured at varying curing periods before the UCS testing. The compaction testing was conducted to achieve the maximum dry density (MDD) and optimum moisture content (OMC) to use in the sample preparation testing. A total number of 74 UCS tests (excluding failed tests and verification tests) were conducted and the following results can be drawn from the analysis:

1.
The study has shown that the addition of seashell (SS) is effective to improve peak UCS value of the specimens in both benchmark and exposed specimens. However, the rate of increase in exposed specimens was lower than the benchmark specimens.

2.
Increasing the curing time along with increasing the seashell contents enhanced the peak UCS values. The highest UCS values were recorded at 28 days of curing period for both benchmark and exposed specimens. 3.
SEM analysis had shown that the seashells are able to bind with the kaolinite and cement power which supports the replacement of cement by seashell replacement. 4.
XRD analysis had shown that the presence of magnesium sulphate is able to reduce the formation of CSH with a smaller amorphous hump in sulphate-attacked specimens in comparison to benchmark specimens. This provided evidence that specimens under sulphate attacks had reduced formation of CSH, which had led to the deterioration of CSH due to formation of the magnesium silicate hydrate (MSH) after exposure of the specimens to the magnesium sulphate. Funding: This research received no external funding.