Unconfined Mechanical Properties of Nanoclay Cement Compound Modified Calcareous Sand of the South China Sea

Calcareous sand is characterized by low strength, multiporosity, and high-pressure shrinkage.)is often leads to poor engineering properties. )erefore, it is often necessary to reinforce the calcareous sand in order to meet the actual engineering demand for bearing capacity. To study the modification effect of calcareous sand of nanoclay by cement, mechanical tests and microscopic tests of the cement calcareous sand (CCS) modified by different nanoclay content and cement content were conducted. )rough the unconfined compressive test, the mechanical properties of nanoclay and cement compoundmodified calcareous sand (NCCS) were studied. )e micromechanism of nanoclay and cement composite modified calcareous sand was analyzed by SEM. Representative curves of data groups of stress and strain data were obtained by the fusion algorithm, which effectively circumvented the discrete distribution of determined data. Test results showed that the following: (1) )e compressive property of CCS could be improved by the admixture of nanoclay, and the optimum admixture ratio was 8%. (2) )e admixture of nanoclay could enhance the deformationmodulus of CCS and improve the resistance of CCS against external load deformation capacity. (3) Nanoclay could increase the density of the internal structure of CCS and improve its mechanical properties.


Introduction
In the category of rock soil, calcareous sand belonged to carbonate soil or calcareous soil, which was mainly composed of bone fragments of sea organism skeleton under the impact of sea water, such as cay, seaweed, shell, and so on [1][2][3]. Because calcareous sand did not undergo long distance transportation during the sedimentation [4], there were lots of small pores in sand particles with irregular shapes and sharp edges and corners.
In the 1960s, the unfavorable effect of calcareous sand gradually occurred in practical engineering [5]. e pile sliding of the petroleum platform in Iran did not cause an unfavorable effect on practical engineering. us, the engineering problem caused by calcareous sand did not draw great attention. Until the 1970s, with the surging number of the petroleum crisis, the engineering problem caused by the calcareous sand gradually draw attention [6].
us, researchers at home and abroad began to study the basic engineering property of calcareous sand [7][8][9]. Zhang et al. [10] studied the mechanical properties of calcareous sand through a drained triaxial test. e results showed that the dilation phenomenon was obvious and the particle breakage index was the smallest under the same consolidation pressure. Shen et al. [11] showed that the particle morphology of calcareous sand was irregular and multiangle. Changing the coarse particle content was the most effective way to decrease the compressibility of calcareous sand. Rasouli et al. [12] showed that not only the shear strength but also the plastic deformation of calcareous sand was larger than those of quartz sands. Many researches [13][14][15] showed that calcareous sand still has the characteristics of low strength, easy breakage, and high compressibility in practical engineering. To prepare the calcareous sand for further usage in the construction of superstructures, necessary stabilization is required [11,16]. Cement, as a common gelatinizing agent, can greatly improve the mechanical properties of soft soil. Liu et al. [17,18] studied the effect of the concentration of gelatinizing agent on the deep cemented soil strength and further discussed the thermal conductivity property of cemented rock soil engineering material. Wang et al. [19,20] found that cement performed better than lime when improving the unconfined compressive strength of dredged marine clay. us, part of the researchers began to study the mechanical properties of cement solidified calcareous sand. Sharma and Fahey [21] demonstrated that cement could enhance the shear strength of calcareous sand. Ismail et al. [22] studied the influence of different cementation materials on the mechanical properties of calcareous sand through a triaxial test. e results demonstrated that compared with calcite in powder and pulverized lime, Portland cement could form better cementation of calcareous sand. us, in this work, cement was chosen as the gelatinizing agent of calcareous sand. Nanotechnology and nanomaterial had a broad research prospect in the 21 st century [23,24]. Yao et al. [25,26] explored the influence of nano-MgO on the strength of cement silty clay through a direct shear test. e results demonstrated that the appropriate admixture of nano-MgO could improve the mechanical performance of cement silty clay. Wang et al. [27] found that nano-MgO can modify the shear properties of cement silty clay through a direct shear test, and the optimal mixing ratio was 5%. Nanoclay was one of the common nanomaterials which could be widely applied in the cemented materials. Allalou et al. [28] demonstrated that nanoclay could be used as a catalyst to promote the cement hydration reaction of low clinker cement mortar. Mohamed et al. [29] obtained a similar conclusion. Li et al. [30] found that the substitution of 5%-10% nanoclay for cement could enhance the compressibility of cement iron tailings through an unconfined compression test. Zaid et al. [31] demonstrated that the solidifying effect of soft soil by the admixture of nanoclay was the best through a triaxial test. Irshidat et al. [32] explored the influence of nanoclay on the mechanical properties of modified cement-based materials. e results showed that the bending strength was the best with the 2% admixture of nanoclay at 400°C. e above researches showed that nanoclay could improve the mechanical performance of cemented material.
In sum, although there were lots of studies at home and abroad about the mechanical performance of calcareous sand, the studies about the mechanical performance of the nanomaterial modified calcareous sand were few. In this work, the influence of the nanoclay admixture on the compressive strength of cement solidifying calcareous sand was mainly studied. e resistance against external load deformation capacity of the nanoclay modified cement calcareous sand was studied, which was anticipated to provide a reference for the usage of nanoclay and cement compound modified calcareous sand in the job site.

Experimental Materials.
Calcareous sand in this work was from one district of Yongxing Island, Sansha City, Hainan Province, which was loose and noncemented sand particle. e particle size distribution curve is shown in Figure 1. Compared with common terrestrial sand, calcareous sand was pale. ere were coral fragments in calcareous sand. us, there were some red particles in white sand particles. e basic physical mechanical indexes of calcareous sand are shown in Table 1.
In this experiment, common Lanting PO 32.5 Portland cement produced by Zhaoshan Architectural Limited Co. in Shaoxing City was used, whose basic physical mechanical indexes are shown in Table 2.
Nanoclay which was a light yellow powder was produced by Jingxi Montmorillonite Technology Limited Co. in Hubei, whose main technical indexes are shown in Table 3. e microstructure of calcareous sand, cement, and nanoclay is shown in Figure 2. As could be seen from Figure 2(a), there were many pores on the surface of the microstructure of calcareous sand, which indicated that calcareous sand has high compression properties and is easy to be broken. As could be seen from Figure 2(b), the size of cement particles was different and the edges and corners were clear. As could be seen from Figure 2(c), the microstructure of nanoclay was relatively dense and the particle size was small, which indicated that it has a good size effect.

Experiment Preparation
e unconfined compressive test used 01-LH0501 automated, multifunctional unconfined compressive machine, which was produced by Nanjing TaikeAo Scientific Limited Co., as shown in Figure 3. In this experiment, the load rate of the calcareous sand sample was set as 1 mm/min.

Experiment Scheme.
e experiment considered the influence of not only the admixture of nanoclay but also the curing time on cement calcareous sand. e average natural moisture content of calcareous sand used in the test was 30%. Combined with "design specification for cement soil mix proportion" [33] (JGJ 233-2011) and practical engineering application, the mixing ratio of cement was 10%. In the experiment, with controlled cement content and the water  content, the influence of nanoclay admixture and the curing time (7 d, 14 d, and 28 d) on the unconfined compressive strength of calcareous sand were studied. e experimental design scheme is shown in Table 4. According to the Geotechnical test standard of GBT 50123-2019 [34], not until 3-5% strain after the peak in the unconfined test did the test end. In this work, not until 5% axial strain did the test end.

Sample Preparation and Curing.
According to the Geotechnical test standard of GBT 50123-2019 [34], in this experiment, unconfined samples were all cylinders with a diameter of 39.1 mm and a height of 80 mm. e detailed calcareous sand sample preparation procedures were as follows: (1) Calcareous Sand Preparation. Before experiments, the air-dried calcareous sand was oven-dried at the temperature of 105°C. Impurities of little shells and cay fragments in calcareous sand were removed by 2 mm sieve. e residuals were saved in air-tight storage box as shown in Figure 4(a).
(2) Sample Molds Preparation. Sample molds were washed and oven-dried, whose inner wall was then coated with Vaseline evenly. en, it was assembled as shown in Figure 4 ishing of the calcareous sand sample, it was necessary to smooth the sample surface. en the sample was put into the saturated apparatus of the sample mold. Taps of the curing time and the admixture amount were labeled and as shown in Figure 4(e). More importantly, drip stones were put at two ends of the sample. e filtration paper was put between the drip stone and the sample to prevent the calcareous sand particle from adhering to the drip stone. Due to the characteristic of the loose property and low cohesion, the calcareous sample was needed to be cured for 4 d with mold, which was removed subsequently. After mold was removed, the sample was wrapped by preservative film and cured in the curing room. e molded calcareous sample was shown in Figure 4(f ).

Stress-Strain Curves.
To study the influence of nanoclay content and curing time on the mechanical performance of cement calcareous sand, the unconfined compressive experiment was conducted in this work. e stress-strain curves of CCS and NCCS samples on 7 d, 14 d, and 28 d are shown in Figures 5-7, respectively, among which nos. 1-5 represented test curves of five parallel samples.
Combined with Figures 5-7, the stress-strain curves of unconfined experiments of CCS and NCCS were both softening curves. To circumvent the deviation from the peak   Advances in Civil Engineering value brought in by the single test, 5 repeated tests were conducted. However, 5 repeated tests were independent, which resulted in some deviation of the data. e certain discrete results could not realize the consistency. To facilitate the further analysis of the influence of nanoclay admixture and curing time on the compressibility of cement calcareous sand, it was very important to integrate 5 stress-strain curves in the test to one curve. Combined with related references [35], the test curves were treated by normalization in this work. e detailed method procedures were as follows:  (1) e stress deviation was confirmed. q i and q j (i, j � 1, 2, 3, 4, 5) represented the corresponding peak stress of five groups of unconfined tests, respectively. r ij represented the absolute value of the difference between the stress of q i and q j , respectively. e calculation was shown in the following equation: (2) e correlation degree of the stress was confirmed. c ij was the correlation degree of stress. e calculation method was shown in equation (2). c ij and the stress deviation r ij should be in a relationship of inverse proportion. e larger the stress deviation, the smaller c ij . If the stress deviation r ij was 0, the correlation degree c ij was 1, illustrating that the correlation degree of the stress with itself was 1: (3) e stress correlation degree matrix was established.
In Matrix C, c ij represented the correlation degree between two stresses of the ith and jth as shown in equation (3). However, it could not completely demonstrate the correlation degree between one stress and all other stresses: (4) e weighing factor was confirmed. w j should comprehensively consider influence factors of all correlation degrees. From amalgamating theory of probability theory, there existed a group of non- 5 , which permitted the establishment of the following equation: (5) e weighing matrix was established. Equation (4) was transformed to matrix as shown in equation (5).
In equation (5), (6) e stress weighing coefficient was confirmed. Since the correlation degree matrix C is nonnegative, there exists an eigenvalue λ, which makes the equation λV � CV hold. en the eigenvector V can be obtained by the eigenvalue λ. e calculation method of weighing coefficient was shown in the following equation: (7) e fusion stress was confirmed. w i was the weighing coefficient of the ith value. e fusion result of 5 stress was shown in the following equation: From Figures 5-7, 5 stress-strain curves with each admixture ratio basically demonstrated a consistent changing principle. According to the above-described method, the weighing coefficient of each curve was obtained by the application of the fusion algorithm for the peak values of five stress-strain curves. e density p was obtained by the comparison of the fusion stress and the stress average. e weighing coefficient calculation results of correlated curves of CCS and NCCS on 7 d, 14 d, and 28 d are shown in Table 5.
To verify the feasibility, the obtained stress-strain curves were compared with the original data, which is shown in Figure 8. Results showed that the stress-strain curve obtained by the fusion algorithm had a relatively good correlation with original unconfined compressive strength data. e weighed stress-strain cures of CCS and NCCS with a different admixture of nanoclay by fusion algorithm are shown in Figure 9. From Figure 9, the stress-strain curves of CCS and NCCS on 7d, 14 d, and 28 d were all softening curves. e peak stress of NCCS with different ages was all larger than those of CCS, illustrating that the admixture of nanoclay could enhance the peak stress of CCS. As shown in Figure 10, the trends of stress-strain curves with different ages and different admixture contents were approximately the same, which could be approximately divided into four stages. Zone I was the precompressive stage, which corresponded to precontact between the sensor and CCS and NCCS samples.
Advances in Civil Engineering e stress variation was very small. In addition, the increasing speed was slow. (2) Zone II was a linear stage where the stress increasing speed of CCS and NCCS accelerated.
e corresponding stress and strain demonstrated linear increasing relationship. Under different ages, the stress increasing speed of NCCS was apparently higher than that of CCS. (3) Zone III was the elastoplastic stage where the stress increasing speed of CCS and NCCS changed from rapid to slow with occurred apparent peak stress. e peak stresses of NCCS with a different admixture of nanoclay were all larger than those of CCS. (4) Zone IV was the attenuation stage where the stress of CCS and NCCS began to attenuate with occurred sample destroy. When the axial strain of CCS reached 3%, the stress curve began to slow down and approached 0. When the axial strain of CCS reached 4%, the stress curve began to slow down and approached 0.

Influence of the Nanoclay Admixture on the Compressive
Strength. To facilitate the analysis of the compressibility modification effect of nanoclay and cement modified calcareous sand, the maximum of axial stress was chosen as the unconfined compressive strength of NCCS in this work. e relationship curve between the nanoclay admixture ratio and the NCCS compressive strength is shown in Figure 11.
From Figure 11, the admixture of nanoclay could enhance the compressive strength of cement calcareous sand. With the curing time of 7 d and with the increase of nanocaly content, the compressive strength of NCCS increased accordingly. Compared with the compressive strength of CCS (858.97 kPa), the compressive strength of NCCS-4, NCCS-6, NCCS-8, and NCCS-10 increased 10.8%, 17.7%, 43.9%, and 46.4%, respectively. When the nanoclay content was between 0 and 8%, the increasing speed of the compressive strength of NCCS increased gradually. When the nanoclay content exceeded 8%, the increasing speed of NCCS compressive strength slowed down, illustrating that the appropriate admixture of nanoclay could enhance the CCS compressive strength. However, when the nanoclay content exceeded a certain limit, the good modified effect was not guaranteed. For example, in this experiment, the increasing amplitude of CCS compressive strength of NCCS-10 compared with that of CCS was only a little higher than that of NCCS-8. When the curing time was 14 d and 28 d, with the increase of nanoclay content, the compressive strength of NCCS demonstrated a trend of initially increasing and then decreasing and a maximum occurred with the nanoclay content of 8%. On 14 d, the compressive strength of NCCS-4, NCCS-6, NCCS-8, and NCCS-10 increased 14.1%, 31.3%, 64.4%, and 60.5%, respectively, compared with that of CCS. On 28 d, the compressive strength of NCCS-4, NCCS-6, NCCS-8, and NCCS-10 increased 26.7%, 66.3%, 77.6%, and 69.7%, respectively, compared with CCS.
From the above conclusions, in samples on the short ages of 7 d, the compressive strength of NCCS increased with the increase of nanoclay content. In samples on the middle/long age of 14/28 d, the compressive strength of NCCS demonstrated a trend of initial increasing and then decreasing with the increase of nanoclay content. e reason might be as follow: nanoclay in this experiment was nanoderivative of montmorillonite with hydrophilicity and weak expansibility. With low nanoclay content, water in the interior of the sample was sufficient. On the one hand, nanoclay adsorbed water swelled and filled in the internal pore of NCCS. On the other hand, the nucleation effect of nanoclay could promote the cement hydration reaction of NCCS [35]. With too high nanoclay content, hydroscopicity of nanoclay could consume the internal water of NCCS and thus prohibit the cement hydration reaction in the interior of NCCS, illustrating that there existed an optimum admixture content when modifying the CCS compressive strength by nanoclay. In this experiment, comprehensively considering factors of economy and curing time, the optimum admixture content for the modification of the CCS compressive strength by nanoclay was 8%.

Influence of the Curing Time on the Compressive Strength.
It can also be seen from Figure 11, with the increase of the curing time, the compressive strength of CCS and NCCS increased accordingly. For CCS, the compressive strength on 14 d  (8), the deformation modulus [36,37] (E 50 ) was the ratio between stress and strain with 50% axial stress q u of soil under unconfined compressive condition. E 50 was an important parameter evaluating the compressibility, which could represent the resistance capacity against elastoplasticity deformation of soil [38]:

NCCS Deformation Modulus Analysis. As shown in equation
wherein q u was the peak stress of the calcareous sand sample in the unconfined test. 50 was the axial strain with 50% of the peak stress of calcareous sand sample. According to equation (8), the deformation modulus of NCCS was calculated and is shown in Table 6. From Table 5 In other words, with the same deformation requirement, NCCS could bear more external load. Khalid et al. [39] showed that there existed certain linearity between the deformation modulus and itself peak stress. e relationship between the NCCS compressive strength and the deformation modulus is shown in Figure 12. From this figure, the NCCS proportion coefficient between the deformation modulus and peak stress was between 65.87 and 99.46, namely, E 50 � {65.87 ∼ 99.46} q u . By linear

Failure Pattern of Samples
To deeply study the information of nanoclay admixture on CCS destroyed form, in this work, the NCCS sample destroyed form was analyzed. e destroying mode of CCS and NCCS samples are shown in Figure 13. In the test process, there existed little difference between destroying modes of specimens with different curing ages. erefore, the destroying modes of CCS and NCCS on 28 d were analyzed. Here only representative destroying pictures of CCS and NCCS samples were chosen. From Figure 13, on the whole, the destroying modes of CCS and NCCS were inclined crack failure. From Figure 13(a), the destroying mode of CCS was a large tilted fracture that crossed from the upper to the lower base side with few branches of fracture.
ere was no apparent dropping of a calcareous sand particle in the compressive process of CCS. From Figures 13(b)-13(d), the sample fracture of NCCS was different from that of CCS. e angle of the fracture was apparently smaller than that of CCS. In addition, there were many small fractures around the main fracture. With the increase of nanoclay content, the numbers of small cracks on NCCS gradually increased. is showed that nanoclay could improve the compressive strength of CCS but increase the brittleness of CCS.

Mechanism of Modified Calcareous Sand
In order to further analyze the mechanism of nanoclay and cement modified calcareous sand, microscopic tests were carried out on CCS and NCCS on 28 d. e results are shown in Figure 14.
It could be seen from Figure 14(a) that the pores on the surface of calcareous sand were filled with hydration products of cement, which improved the adhesion between calcareous sand particles. It indicated that cement could better cement calcareous sand particles. It can be seen from Figure 14(b) that there were many fine nanoclay   particles and flat block materials in the microstructure of NCCS. Compared with CCS, the pores in the microstructure of NCCS were significantly reduced, and a relatively complete flat block unit appears. e reasons could be attributed to (1) the size effect of nanoclay. e size of nanoclay particles was small, which could fill the pores of calcareous sand well. (2) e nucleation effect of nanoclay. Nanoclay could be used as the core to adsorb calcium ions in the sample and promote hydration reaction. Hamed et al. [40] obtained a similar conclusion. In conclusion, nanoclay could improve the compactness of the internal structure of CCS, so as to improve the compressive strength of CCS.

Results and Analysis
6.1. Results. From the unconfined compressive test and microscopic test, the unconfined mechanical characteristics and microscopic mechanism of CCS and NCCS were analyzed. Conclusions were obtained as follows: (1) With different times, the stress-strain curves of CCS and NCCS were both softening curves. (2) e nanoclay admixture could effectively improve the mechanical performance of CCS. e optimum admixture ratio of 8% was chosen for the nanoclay modified CCS. e mechanical performance with 7 d, 14 d, and 28 d increased 43.9%, 64.4%, and 77.6%, respectively.
(3) e nanoclay admixture could increase the CCS deformed modulus and improve the resistance capacity against plastic deformation of CCS. By the linear simulation for the whole NCCS ratio with different nanoclay admixture, it can provide a reference for the design of practical engineering. (4) e destroying modes of CCS and NCCS were all tilted fracture destroying. With the increase of nanoclay admixture, the numbers of the small cracks of NCCS increased accordingly. (5) Due to the size effect and nucleation effect of nanoclay, the compactness of CCS internal structure and the mechanical properties of CCS could be improved.

Analysis
(1) e study was focused on the effect of nanoclay on the unconfined compressive strength of cement modified calcareous sand. e influence of confining  pressure and cement content on the mechanical properties of calcareous sand needs further study. (2) is study could provide a theoretical basis for practical engineering to use nanoclay and cement composite reinforcement of calcareous sand. However, how to apply it in practical engineering needs further study.

Data Availability
All the data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare no conflicts of interest.