Research Article Experimental Study on the Aeolian Sand Solidification via MICP Technique

This study solidi ﬁ es the aeolian sand by microbial-induced carbonate precipitation (MICP) technique. The e ﬀ ects of cementation solution with di ﬀ erent concentrations, particle size, and grouting batches are examined via the bender element, uncon ﬁ ned compressive test, and scanning electron microscope (SEM). The bender element results show that the wave speed of loose aeolian sand is 200m/s; however, after solidi ﬁ cation of the aeolian sand, the speed of P-wave is about 450-600 m/s and S-wave is about 350-500m/s. Additionally, the uncon ﬁ ned compressive strength (UCS) results indicate that when the concentration of cementation solution is 0.75mol/L, the UCS of biosolidi ﬁ ed sand sample is the highest. Then, compared with the aeolian sand with original grade, the particles ranging from 0.1 to 0.4mm have a better cementation e ﬀ ect. Moreover, the UCS of biosolidi ﬁ ed sand samples increases along with the grouting batch. From the SEM images, it can be seen that when the grouting batch reaches to ﬁ ve times, the particles are almost completely covered by CaCO 3 crystals compared with the three batches and four batches.


Introduction
The mechanism MICP is that the urease produced by microorganisms and the ammonia gas and carbon dioxide produced by the hydrolysis of urea can be converted into ammonium ion and bicarbonate ion in the alkaline solution environment. Then, the bicarbonate ion will attract calcium ions in solution thus precipitating calcium carbonate. Moreover, as reported, the microorganism is generally negatively charged thereby absorbing positively charged cations such as calcium ions, making the microorganism become the crystal nucleus in the crystallization process of calcium carbonate.
The researchers realized the importance of microbial mineralization as early as 1970s [1,2]. The researchers studied on the urease activity of bacteria as the research object and study the effects of nickel ions, grouting methods, and different calcium salt concentrations on the strength of microbial grouting solidified sands [3,4]. Researchers found that the concentration of nutrients is negatively correlated with the strength of the sample. When the concentration is low, the strength of the sample is higher and the sample integrity is better [5]. Scholars in the United Kingdom have discovered through research that step-by-step grouting can improve the uniformity of calcium carbonate spatial distribution. The calcium carbonate generated in the early and midterm will help to fix the microorganisms and induce the formation of new calcium carbonate [6]. The research team in Saudi Arabia discussed the influence of culture medium, bacterial concentration, and different buffers on the compressive strength of cement mortar. The strength can reach up to 39.6 MPa [7]. Researchers found that when the particle size distribution contains 75% coarse aggregate and 25% fine aggregate, the maximum uniaxial compressive strength is about 575 kPa. Adding fine aggregate to the coarse aggregate can reduce the size of the coarse aggregate particles. Provide more bridge contact [8,9].
Ivanov and Chu applied MICP technology to plugging and improving soil strength in geotechnical engineering [10]. Harkes et al. optimized the MICP grouting technology by the clogging of the grouting port and the unevenness of the sample solidification that often occurred in the experiment [11]. Dejong et al. used microbial solidification to gel the concrete and monitored the shear wave velocity during the repair process [12]. The researchers in Canada successfully reduced the permeability of oilfield sandstone by cementing sandstone particles [13]. Some of the research teams used spraying, immersion, and infiltration methods to conduct indoor or on-site microbial coating tests on limestone, marble, and sandstone stone samples or actual cultural relics [14][15][16][17][18][19].
Some scholars have also pay attention to how to improve the economy of the MICP [20]. And other scholars began to pay attention to adding some other additives to improve the strength of MICP. For instance, Dhami et al. and Choi et al. added polyvinyl alcohol fiber and found that the UCS and splitting tensile strength of the sample were increased by 138% and 186%, respectively, and the permeability was reduced by 126% [21,22]. Xu et al. found that the uniaxial compressive performance of the MICP-treated sand column has been improved by add-ing magnesium ions [23]. Cheng and Cord-Ruwisch tried the surface infiltration method and compared it with the continuous grouting method [24].
Another scholars have studied the MICP by the numerical simulation. The research team in the Netherlands has established a complete mathematical model of the microbial grouting process [25][26][27]. The Swiss Federal Institute of Technology established a model to consider the coupling of various factors, including biology, chemistry, fluids, and mechanics [28]. The research team used the three-dimensional discrete element method (DEM) to simulate the mechanical behavior of siltstone after MICP reinforcement [29]. Cheng et al. use CT scanning technology to quantify the key microscopic properties of MICP, such as the size of crystals [30].
The model tests are of great help to MICP research [31][32][33]. In 2010, scholars in the Netherlands applied the technology of microbial induction to generate calcium carbonate in the on-site sandy gravel soil reinforcement project [34,35]. Van Paassen et al. used microbiological methods to carry out 1 m 3 and 100 m 3 sand samples for prototype sand foundation reinforcement tests [36]. Kalkan analyzed the reinforcement mechanism and influencing factors of MCP [37].   In summary, some research results have been made in the MICP technique for solidifying various saturated sand and unsaturated sands, including metallogenic mechanism of sand, grouting reinforcement method, and nutrient concentration for the effect of mineralization. However, the aeolian sand is different from the solidifying various saturated sands     Note: the black font in Table 3 represents the basic plan.
3 Geofluids as well as unsaturated sand, because of the much lower moisture content and the much poorer particle size distribution. Meanwhile, there is no effect on solidifying aeolian sand by using the traditional foundation reinforcement methods. In addition, there are few researches on the MICP for aeolian sand solidification. Therefore, it is very necessary to carry out research on aeolian sand solidification via MICP.

Physical Properties of Aeolian Sand
(1) Natural Moisture Content Calculation. According to GB/ T50123-1999 geotechnical test standards of China, the natural moisture content test needs to be carried out twice and its arithmetic average is taken. The formula of natural moisture content for aeolian sand is as follows:

Geofluids
m0 is the quality of wet soil (g), and md is the quality of dry soil (g). The testing results of natural moisture content of aeolian sand are shown in Table 1. The natural moisture content of aeolian sand is about 0.80% by calculation.
(2) Particle Size Distribution Analysis. The particle size distribution is presented in Figure 1. As can be seen in Table 2, the particle size distribution of the aeolian sand is mainly between 0.1 mm and 0.4 mm.
According to Figure 1, the particle size distribution of the aeolian sand is mainly between 0.1 mm and 0.4 mm. Additionally, the effective particle is d 10 = 0:11, the median is d 50 = 0:17, and the limit particle size is d 60 = 0:19 and d 30 = 0:14. The uneven coefficient of aeolian sand is C u = d 60 /d 10 = 1:73; and the curvature coefficient is C C = d 30 2 /ðd 60 × d 10 Þ = 0:74. According to GB/T50123-1999 geotechnical test standards of China, the pH value of suspension water of aeolian sand is 8.77.

Bacteria.
The microbe used in the current study is Sporosarcina pasteurii, which has a strong survivability as well as a high urease production ability. The cell surface is negatively charged, the spores are oval or spherical, and the cell rod is 2-3 μm long and 1-2 μm in diameter. The process of bacterial culture is presented in Figure 2 [23]. The average urease activity is about 15.01 mmol/min, and the average OD600 = 2:62.

Fixation and Cementation Solution.
The cementation solution herein is composed of CaCl 2 and urea, which is analytically pure and was provided by Shanghai Titan Reagent Co., Ltd. In this study, the detailed experimental scheme is shown in Table 3.

Sample Preparation.
For each sample, 115 g dry sand particles were packed into a column. The grouting and solidification process of aeolian sand is carried out in a medical syringe. Other instruments mainly include balance, geononwoven fabric, labeled sand, funnel, beaker, purified water, tray, retainer, and peristaltic pump. The geononwoven fabric can make the CaCO 3 not block the grouting path and flow easily. The peristaltic pump can send the CaCl 2 to the sand, and the rotation speed is about 5 r/min.
The effects of different concentrations of CaCl 2 , particle size distribution, and different grouting batches, respectively, are being studied.

Particle Size Distribution.
The particle all-size distribution and particle size from 0.1 mm to 0.4 mm are selected under the premise of the basic scheme. As can be seen from Figure 3, the range from 0.1 mm to 0.4 mm of particle size is more particles in test. The particle size range that is too large (>0.4 mm) and too small (<0.1 mm) is removed. 7 Geofluids diameter. The syringe is divided into two parts with a tool, from which the aeolian sand column is taken out and labeled.

Bender Element Test.
The test uses the bender element BES in conjunction with the data acquisition system. Two size of probes are selected to measure the wave speeds for loose aeolian sand and solidified aeolian sand column, respectively, and the loose aeolian sand is to be the contrast one. The bender element shown in Figure 3.
The electromagnetic wave propagates through the piezoelectric ceramic plates at the ends of two probes of the bender element, one is the transmitting wave device, and the other one is the receiving wave device, completing a test cycle. The degree of different solidification aeolian sand is evaluated through measuring the speed of P-wave and S-wave. The process of measuring is shown in Figure 4.
As can be seen from Figure 2, neither P-wave nor S-wave continuously increases with the increase of nutrient concentration, but there is a peak value. When the concentration is 0.75 mol/L, the solidification of aeolian sand is better than other concentrations by using the MICP. The value for speed   Geofluids of P-wave is bigger than that of the S-wave from the figure.
As can be seen in Figure 5, the speed of P-wave is 1.3 times more than the speed of S-wave. As can be seen in Figures 6 and 7, the wave speed of the particle all-size distribution is smaller than that of the particle size from 0.1 mm to 0.4 mm. The comparative test shows that the particle size is controlled in a relatively uniform range, which can make the pores between particles larger. CaCO 3 is much easier to cement, and the degree of cementation is much better.
There are three different kinds of nutrient concentrations to research, including 0.5 mol/L, 0.75 mol/L, and 1 mol/L. It can be seen from Figure 8 that under the same nutrient concentration, the largest wave speed of the grouting batches is the five batches, and the maximum is the 470 m/s, 550 m/s, and 500 m/s, respectively. It shows that grouting batches are beneficial to improve the solidification aeolian sand. Among the three different nutrient concentrations, the 0.75 mol/L has a relatively large S-wave speed, which is roughly distributed in the range of 450-550 m/s and the average value is higher than the other two concentrations. It also shows that when nutrient concentration is 0.75 mol/L, the aeolian sand solidification is much better.
The values of the P-wave and the S-wave which are tested in the bender element are all reasonable according to the standard for code for seismic design of buildings [38]. And the value of P-wave is one and a half times as large as S-wave.

UCS.
The dried biocemented aeolian sand samples are cut into two parts (A and B); then, their surfaces are ground flat with a grinding machine to eliminate the influence of deviant stress in the compression process. The testing machine is MTS810, and the loading rate is 1 mm/min, controlled by the displacement. It records the process from loading to specimen failure, and the peak stress is the unconfined compressive strength of the specimen. Representative stress-strain curves are presented in Figure 9.
The UCS results are drawn in Figure 10. It can be seen that the UCS of biocemented aeolian sand samples increases when the concentration of cementation solution increases from 0.5 mol/L to 0.75 mol/L. However, when the concentration becomes larger than 0.75 mol/L, then UCS decreases. Additionally, the average value of UCS for the aeolian sand column increases along with the number of injections.
The trend of the UCS are reasonable compared with reference [31], moreover the figure is much more accurate.

SEM.
The SEM images concerning crystal polymorph and microstructures of biocemented sand are presented. Compared with the loose sand particles presented, the aeolian sand particles after MICP treatment are covered by CaCO 3 crystals. It is found that the morphology of precipitated CaCO 3 crystals is mostly cubic. There are some round or slender rod-shaped holes showing on the CaCO 3 surfaces which are identified as the bacterial traces. The diameter of round holes is about 1micron, while the rod-shaped holes are 3-4 microns in length.
It is the precipitated CaCO 3 crystals acting as the cementitious materials which bond the loose sand particles together thus improving the mechanical properties of sand. With the increase of the grouting times, the living environment of microorganisms in the reaction system will be changed, 9 Geofluids which is conducive to the attachment of microorganisms on the surface and joints of sand particles to form crystalline nuclei. It is also conducive to produce the CaCO 3 for increasing the sand strength.
Compared with four batches grouting, the amount of CaCO 3 crystals precipitated by five-batch injection with the same cementation solution concentration is much higher. As for the different concentrations, 0.75 mol/L can induce more CaCO 3 crystals than other concentrations, which corresponds to the figure where 0.75 mol/L can induce the highest UCS (26.09 MPa).
Compared with reference [23], the result of the SEM is similar to it. It is also confirmed that the result of the SEM is reasonable.

Conclusions
Based on the experimental results, the following conclusions can be drawn:

Data Availability
The data and materials are available.

Ethical Approval
All the authors declare that there is ethical approval.

Consent
Everybody consents to participate and to publish.

Disclosure
A preprint has previously been published [39].

Conflicts of Interest
The authors declare that they have no conflicts of interest.