Strength-increase mechanism and microstructural characteristics of a biotreated geomaterial

Abstract

Microbially induced calcite precipitation (MICP) is a recently proposed method that is environmentally friendly and has considerable potential applications in artificial biotreated geomaterials. New artificial biotreated geomaterials are produced based on the MICP technology for different parent soils. The purpose of this study is to explore the strength-increase mechanism and microstructural characteristics of the biotreated geomaterial through a series of experiments. The results show that longer mineralization time results in higher-strength biotreated geomaterial. The strength growth rate rapidly increases in the beginning and remains stable afterwards. The calcium ion content significantly increases with the extended mineralization time. When standard sand was used as a parent soil, the calcium ion content increased to a factor of 39 after 7 days. The bacterial cells with attached calcium ions serve as the nucleus of crystallization and fill the pore space. When fine sand was used as a parent soil, the calcium ion content increased to only a factor of 7 after 7 days of mineralization. The nucleus of crystallization could not normally grow because of the limited pore space. The porosity and variation in porosity are clearly affected by the parent soil. Therefore, the strength of the biotreated geomaterial is affected by the parent soil properties, mineralization time, and granular material pore space. This paper provides a basis for theory and experiments for biotreated geomaterials in future engineering practice.

Keywords

biotreated geomaterial / microbially induced calcite precipitation / strength-increase mechanism / microstructural characteristics

Introduction

Microbially induced calcite precipitation (MICP) is a method proposed in recent years that is environmentally friendly and has considerable potential in engineering applications [¹–¹¹]. The innovation is used to bond sand grains together to produce a new artificial biotreated geomaterial with good mechanical properties in engineering practice [¹²–¹⁴].

Several studies have been conducted to evaluate the engineering properties of biotreated geomaterials [¹⁵–²⁶]. Calcite precipitation was induced through an artificially controlled biological and geochemical process, and it was added to the parent soil as a cement component. Through ureolysis, MICP can stabilize loose sand to generate a new artificial biotreated geomaterial and achieve the desired strength and stiffness properties. The 1D finite-element simulation for the biogrouting process in sand columns has been used to analyze the effect of several factors on the distribution of calcium carbonate precipitation along the sand column and precipitation rate after the reaction. The effectiveness of MICP has been analyzed on the cyclic resistance as a function of the cementation solution (CS) content, effective confining pressure and cyclic stress ratio (CSR) through the cyclic triaxial test. The strength behavior of biocemented sand has been assessed with various cementation levels and provides insight into the mechanism of MICP treatment sand [²⁷–⁴¹]. However, there are few reports on the strength-increase mechanism of biotreated geomaterials. The factors that affect the strength formation and development are critical, and an in-depth study must be conducted on the artificial biotreated geomaterial, which has considerable application potential in engineering practices.

In this study, the mechanism responsible for the strength increase of an artificial biotreated geomaterial and its microstructural characteristics has been investigated through a series of experiments. The strength increase mechanism is explored based on test results. The relationships between the strength development of the biotreated geomaterial and the parent soil particle characteristic, parent soil pore properties, and mineralization time are analyzed. The porosity and variation in porosity are analyzed for different parent soil types. The purpose is to provide theoretical and experimental foundations for future engineering practice involving artificial biotreated geomaterials. The authors believe that this study is useful for the evaluations of the strength formation and development of biotreated geomaterials and the expansion of its applications in engineering practice.

Materials and methods

Materials

The material is an artificial biotreated geomaterial produced using the MICP technology. The primary materials, including the microorganism, cementation medium and parent soil, were prepared in a laboratory.

Microorganism

The microorganism in this study is the bacterium Sporosarcina pasteurii (American Type Culture Collection 11859) [³⁰]. The bacterial cell concentration was determined by measuring the absorbance (optical density) of the suspension using a spectrophotometer at a 600 nm wavelength (OD₆₀₀). An optical density of 600 nm (OD₆₀₀) is suggested to be 0.5–0.8 [¹⁸,²⁸,³²]. In this study, the optical density of 600 nm (OD₆₀₀) was used as 0.6, and the bacteria cell concentration was determined to be 4.3 × 10⁷ cells/mL [⁴²]. During the preparation, the temperature and relative humidity were 20°C and 40%, respectively [¹⁹,³⁰,⁴²,⁴³].

Cementation medium

The cementation medium included urea (30 g/L), CaCl₂∙H₂O (73.5 g/L), NH₄Cl (10 g/L), NaHCO₃ (2.1 g/L), and nutrient broth (3.0 g/L). This medium provides nutrition for bacterial growth and propagation and provides the calcium source for the MICP process. The cementation medium concentration was set at 0.5 mol Ca, and the urea-Ca²⁺ molar ratio was set at 1:1 [²⁸]. The cementation medium pH was adjusted to 6.0 to prevent the formation of amorphous CaCO₃ before the MICP process.

Parent soil

The parent soils are standard sand and fine sand, which have different particle sizes and pore properties. The particle shape and gradation are important factors that can affect the mechanical behaviors and strength development of sand [²³,²⁴,⁴²]. The purpose in this study is to compare the effects of these characteristics on the strength-increase mechanism of the biotreated geomaterial. The particle size distribution curves are shown in Fig. 1. The diagram of loose sand particles and their photomicrograph are shown in Figs. 2(a) and 2(b).

The unit weight of the standard sand (99.7% quartz) was 15.5 kN/m³, and the specific gravity was 2.62 (SL237-1999). The standard sand was uniformly graded with a median particle size of 0.9 mm. The coefficient of uniformity (C_u) of the standard sand was 1.96, and its coefficient of curvature (C_c) was 0.91. The sand is classified as poorly graded sand according to the Unified Soil Classification System (USCS).

The proportion of fine sand particles, which are smaller than 0.25 mm, was 93%. The median particle size was 0.17 mm. The sand was uniformly graded with a coefficient of uniformity (C_u) of 1.8 and a coefficient of curvature (C_c) of 1.09. The unit weight of the fine sand was 15.85 kN/m³, and the specific gravity was 2.64 (SL237-1999). The fine sand is classified as poorly graded sand (SP) according to the USCS.

Before being used, the standard sand and fine sand were soaked with 0.1 mol NaOH solution for 12 h and subsequently washed and soaked with 0.1 mol HCl solution for another 12 h. The sands were cleaned with deionized water until its pH was 7. After drying, the parent soil was ready for use.

Experimental method

The specimens were prepared in full contact flexible molds (FCFM), which were made of geotextile, as described in Ref. [¹⁹]. The geotextile was a polypropylene, staple-fiber, needle-punched nonwoven material. This geotextile has many pores, which enable the rapid chemical penetration of the cementation media into the sand pores to promote the MICP process. Triplicate specimens were prepared, and the specimens to test the unconfined compressive strength (UCS) were 50 mm in diameter and 50 mm tall (specimen volume: 98.17 cm³). The mold consisted of an annular part, a bottom and a cover, as shown in Fig. 2(c). CaCO₃ was well-distributed in the samples, and the samples could be easily removed from the mold [¹⁹,³⁰].

All specimens were prepared in a continually stirred batch reactor, as shown in Fig. 2(d). The reactor consisted of a plastic box, a shelf, a magnetic mixer and an air pump. The box contained the soil specimens and cementation medium, and the specimens in the FCFM were placed on the shelf and fully immersed in the cementation medium. Small holes of 5 mm in diameter were punched in the shelf to allow water to flow through the shelf. For example, 6 specimens on the shelf were immersed in 24 L of cementation media. The magnetic mixer was used to maintain a uniform solution. The air pump provided oxygen for bacterial growth.

The samples were prepared from a mixture of parent soil and bacteria. The dry weights of the standard sand and fine sand were 155 and 158.6 g, respectively. The two types of parent soils were uniformly mixed with 40 mL of bacteria solution. The mixture of parent soil and bacteria was divided into triplicates and evenly placed into the mold. Then, air was pluviated into the FCFM molds to create a medium dense condition (D_r: 44%–55%).

In the reaction, only air was continuously pumped into the reactor; no bacteria, growth medium, or cementation medium was added to or pumped out of the reactor. After 7 days, the shelf and MICP-treated soil specimens were removed from the reactor. The MICP-treated specimens were removed from the FCFM mold by cutting the mold and subsequently stored in the reaction room. Details of the sample preparation are described in Refs. [¹⁹,³⁰]. Figures 2(e) and 2(f) show the MICP-treated specimen after 7 days of air-dried treatment at room temperature. As shown in Figs. 2(e) and 2(f), the specimens were well-molded via the MICP process. The specimens displayed good strength, good uniformity, and a lack of stratification and fragile layers.

The specimen porosity was measured using an AutoPore IV 9510 testing system and a Mercury porosimetry analyzer (Version 5.0). Because of the unwettability of mercury on the solid surface, pressure was created to overcome the resistance of surface tension of mercury. Mercury was injected into the pores of the materials. According to the theory of capillary pressure, a smaller capillary pore size requires a larger pressure. In fact, the pressure required to inject mercury into the pores of the specimens is inversely proportional to the pore size. The mercury pressure was 0–60000 psi, and the pore size was 50 Å -1000 µm. In the mercury injection test, the porosity, pore characteristics and main pore distribution of the specimens can be measured based on different pressures used to inject mercury. Figures 3(a) and 3(b) shows the relationship of the mercury injection volume and pore size in the specimens. A smaller pore size requires a larger pressure and makes it more difficult to inject additional mercury into the specimens. Figures 3(c) and 3(d) show the relationship of the log differential injection (dV/dlogD) and pore size and the internal pore volume distribution in the specimen. Macropores occupied most of the distribution, whereas mesopores and micropores occupied a smaller percentage.

Results and discussions

Strength increase and development of the biotreated geomaterial

To determine the strength development of the biotreated geomaterial with the mineralization time, UCS tests were conducted on the MICP-treated standard sand and MICP-treated fine sand under strain-controlled conditions at a uniform loading rate of 1.0%/min in accordance with Test Method ASTM D2166. The tests were halted when the axial strain reached 20%. The UCS results at the mineralization times of 1, 2, 3, 4, 5, 6, 7, and 8 days are summarized in Table 1. The UCS variation between consecutive days (units of MPa/d) is analyzed in Fig. 4.

Based on Table 1, the MICP process developed with increasing mineralization time. The UCS of the MICP-treated specimens increased with the increase in mineralization time. Among the MICP-treated standard sand specimens, the UCS increased from 0.15 to 2.38 MPa with mineralization time of 1 of 8 days. Figure 4 shows the UCS variation between consecutive days for the two different parent soils. For MICP-treated standard sand, the specimen strength relatively slowly increased in the first 3th days and reached its maximum strength on the 5th day; then, the increase slowed until there was no distinct increase till the 8th day. Thus, the desired strength of the MICP-treated standard sand specimen was achieved after 7 days. However, the MICP-treated fine sand specimens had a different strength-increase curve. The strength of these specimens significantly increased in the first two days, and the rate of increase subsequently slowed. After the 5th day, the strength increase was small, and the MICP process stopped. This difference is due to the particle characteristics and pore properties of the parent soil. Because fine sand has small particle size and low porosity, the nutrients cannot be transported among the pores and supplied to the bacteria when the calcium carbonate crystals, which are created by the MICP process, fill the limited pore space. The insufficient MICP mineralization directly affects the strength formation and development of the MICP-treated fine sand.

Chemical composition analysis on biotreated geomaterial

An XRF 2400 super X-ray fluorescence spectrophotometer from the Tianrui Company (China) was used to determine the chemical compositions of the specimens. C_u K_α radiation (0.154056 nm) was used with a scanning range 2θ of 10° to 65°, a scanning voltage of 40 kV and an electric current of 40 mA.

Specimens of MICP-treated standard sand and MICP-treated fine sand were selected for XRF testing before and after 7 days of mineralization. The results are shown in Fig. 5. Based on Fig. 5, the calcium concentration significantly increased after the MICP mineralization in both parent soils. After 7 days of mineralization, the calcium concentration increased by factors of 39 and 7 in the MICP-treated standard sand and MICP-treated fine sand, respectively. This difference is due to the low porosity of the fine sand and the insufficient MICP mineralization process, which produced a relatively small number of calcium carbonate crystals. Therefore, there is calcareous bonding in the loose sand, and microbially induced calcium carbonate is generated and filled the pores, which causes an intrinsic strength increase and artificial biotreated geomaterial development.

To determine the amounts of calcium carbonate induced by the MICP as a function of mineralization time, the specimens of MICP-treated standard sand and MICP-treated fine sand were dried for 7 days in air and subsequently heated in an oven at 50°C for 48 h. The specimens were weighed after being dried (m₁) and subsequently soaked and washed with 0.1 mol diluted HCI. The specimens were dried a second time in the oven and weighed (m₂) until no bubbles were observed. The weight loss (m₁–m₂) was calculated, which represents the amount of calcium carbonate induced in the MICP process. The chemical reaction equation is as follows:

\mathrm{C}\mathrm{a}{\mathrm{C}\mathrm{O}}_3+2\mathrm{H}\mathrm{C}\mathrm{l}=\mathrm{C}\mathrm{a}{\mathrm{C}\mathrm{l}}_{\mathrm{2}}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{C}\mathrm{O}}_2

The generation rate of calcium carbonate (w) is defined as the ratio of the amount of calcium carbonate after mineralization and the amount of parent soil before mineralization in a certain mineralization period (Table 2). The staged formation rate of calcium carbonate (w_n–w_n−1) is defined as the change in generation rate of calcium carbonate between two consecutive mineralization times. The effect of the type of parent soil on the staged formation rate of calcium carbonate is shown in Fig. 6. In the MICP-treated standard sand, the amount of calcium carbonate induced in the MICP process increased with increasing mineralization time. The staged formation rate of calcium carbonate was maximal on the 5th day and subsequently gradually decreased. In the MICP-treated fine sand specimens, the staged formation rate of calcium carbonate rapidly increased at the beginning and decreased after 3 days of mineralization because of the lower porosity and limited pore space. To solve the difficulties for MICP-treated fine sand, new experimental devices and methods have been examined and improved in recent research results [⁴²].

Microscopic pore testing analysis on biotreated geomaterial

The porosity of the MICP-treated standard sand and MICP-treated fine sand specimens was tested at the mineralization time of 7 days using an AutoPore IV 9500. The porosity (n) at consecutive mineralization times (n_n−1–n_n) are listed in Table 3. The effect of the parent soils on the porosity variation is shown in Fig. 7. For the MICP-treated standard sand, the amount of calcium carbonate induced in the MICP process increased with the mineralization time. As calcium carbonate continually filled the pore space, the porosity gradually decreased, and the porosity variation increased to a maximum on the 5th day. In the MICP-treated fine sand, the porosity decreased with the adequate MICP process. A significant change in porosity appeared at the beginning of the mineralization reaction, and the variations in porosity were not obvious after 3 days of mineralization because of the fine particles, uniform particle size distribution, and smaller porosity of the parent soil.

Nonlinear relationships are noted among the unconfined compression strength, generation rate of calcium carbonate (w), and porosity (n) because of the inhomogeneity introduced during the preparation of the artificial specimens. However, linear relationships are noted between the UCS and the generation rate of calcium carbonate and between the UCS and the porosity (Fig. 8). When calcium carbonate is induced with sufficient MICP time and continually fills the pore space, the porosity gradually decreases, and the UCS improves with increasing density.

The pores in the specimen are divided into micropores (<2 nm), mesopores (2–50 nm) and macropores (>50 nm). In Fig. 3, the pore size distribution is 3–10⁶ nm with the development of the mineralization process induced by the microorganisms. For the MICP-treated standard sand, the pore size diameters are mostly in the ranges of 10 to 100 nm and 10 to 100 mm. For the MICP-treated fine sand, the pore size diameters are only 10 to 100 mm. The pore size distribution of the MICP-treated specimen reflects the different filling effect from calcium carbonate crystals induced by microorganisms. The biotreated geomaterials based on MICP mineralization have good strength properties and good permeability because of the distribution of pores of different diameters in the MICP-treated specimen.

SEM analysis on biotreated geomaterial

In the laboratory at room temperature (20°C), the specimens of MICP-treated standard sand and MICP-treated fine sand (OD₆₀₀ = 0.6) were pasted on a conducting resin, sprayed with metal and scanned in the vacuum chamber of a QUANTA FEG 650.

The morphological images were obtained after 7 days of mineralization for standard sand and fine sand, as shown in Fig. 9. Based on the SEM images, the crystallization is a continuous process accompanied by the metabolism of the microorganisms. The calcium carbonate crystals exhibited plate, sheet, triangular or spherical shapes following the MICP process. When the parent soil is standard sand, the calcium carbonate crystals evenly covered the surface of the sand particles and formed an interlock connection among the sand particles. There was a significant increase in strength for standard sand after the mineralization process. When the parent soil is fine sand, calcium carbonate crystals were induced and filled the pores of fine sand. The strength of the MICP-treated fine sand increased because of the improved compactness of the parent soil compared to that before mineralization.

In particular, there was less microbial cell disruption on the mineralized surface on the MICP-treated fine sand specimens, vaterite and amorphous calcium carbonate crystals were generated due to insufficient mineralization reaction, and crystal forms of triangular pyramids developed after 7 days of mineralization. Meanwhile, the pore spaces were blocked by the formed crystals, and the nutrients for bacterial growth could not be promptly provided. Thus, the MICP mineralization process slowed and stopped. A clogged and uneven mineralization layer formed after 7 days of mineralization. Effective bonding did not form in the pores, and the strength increase almost stopped.

Conclusions

Based on the MICP mineralization experiments on two types of parent soils, several useful conclusions are drawn.

1)The strength of the biotreated geomaterial increases with the mineralization time. The strength rapidly increases at the beginning and subsequently gradually stabilizes. The strength development varies depending on the type of parent soil. The strength is mainly generated from the calcium carbonate crystals generated by the MICP mineralization. Calcium carbonate bonds form among loose sand particles by filling and joining in the pores. An artificial biotreated geomaterial with higher strength is obtained using the MICP technology.

2)The change in strength is closely related to the porosity and particle characteristic of the parent soil. When the porosity is large, the nuclei of calcium carbonate crystals can properly grow in the pore spaces and create effective bonding among the loose sand particles. Lower porosity results in the extrusion among nuclei, affects the mineralization adequacy, and consequently affects the increase in strength of the biotreated geomaterial. Therefore, the porosity and pore characteristics of the parent soil and the compatibility with microbial-induced crystals are important factors that control the strength increase in biotreated geomaterials.

3)There are nonlinear relationships among the variations in unconfined compression strength, generation rate of calcium carbonate, and porosity because of the inhomogeneity induced by the artificial preparation of the specimens. However, linear relationships are noted between the UCS and the generation rate of calcium carbonate and between the UCS and the porosity of the specimens.

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