A Strengthening Mechanism and Inuencing Factors of Coal Fly Ash using Microbial-Induced Carbonate Precipitation

Coal y ash (CFA) that is discharged from coal-red power plants has characteristics of ne particles, a small specic gravity and a large pore ratio. The dust suppression and strengthening of CFA is usually carried out by spraying water in the ash storage yard, but this practical approach is unsatisfactory. The strength of the CFA lling material affects the construction safety and operational safety of the ash dams. To carry out effective CFA material utilization and dust-pollution control, this paper applied the microbial-induced carbonate precipitation (MICP) method to enhance the strength of CFA under two curing conditions of natural evaporation (NE) and in a moisture cylinder (MC). The microbial-reaction mechanism, strengthening characteristics and inuencing factors were studied. The results show that: 1) the calcium carbonate that is produced by microorganisms in the CFA is calcite, and the calcium-carbonate content increases from 7% to 18.9% and 15.3% under the MC and NE curing conditions, respectively; 2) MICP can improve the CFA strength, and the unconned compressive strength can be increased by 6.55 times to 97.63 kPa under the MC curing conditions; 3) the solidication strength of CFA varies signicantly with the concentration of CaCl 2 .2H 2 O and CH 4 N 2 O nutrients. The peak stress increases initially and then decreases with an increase in nutrient salt concentration. The optimum nutrient concentration was 0.5 mol/L and 1.0 mol/L for the MC and NE curing conditions, respectively; 4) calcium carbonate that was induced by microbial reaction can reduce the water loss in the ash specimen, and allow the ash surface to form a better water-retention effect, with good prospects for dust suppression. group with an average stress of only 26.14 kPa. The average peak stresses for the 0.10, 0.25, 0.50, 0.75, 1.00, 1.25 and 1.50 mol/L nutrient concentration test groups were 26.14, 53.45, 80.70, 68.08, 64.74, 50.53 and 45.92 kPa, respectively. The results showed that the peak stress of CFA specimens increased initially and then decreased with the increase of nutrient concentration from 0.10 mol/L. According to the maximum compressive strength, an optimum nutrient concentration existed for the MC curing condition. MICP-treated


Introduction
China is the largest global producer of coal, with nearly 12% of total proven reserves of 114×10 9 tons. China is also one of few countries with coal as the main energy source. Primary coal demand is projected to increase from 1308×10 6  CFA that is discharged from coal-red power plants has characteristics of ne particles, a small speci c gravity, a large void ratio, and an easy evaporation of water in dry environments. The practical effect of surface solidi cation and dust suppression by extensive water irrigation is used commonly, but is ineffective, expensive, and di cult to meet the long-term effective dust-proof needs of ash storage yards.
The dust-raising phenomenon of ash storage yards under natural conditions is common (Fig. 2) and serious dust pollution occurs frequently.
Dams in ash storage yards are formed by initial and later sub-dams that become lled with CFA. With an increase in service life and the continuous expansion of low-calorie coal utilization, the stacking quantity of CFA increases signi cantly, and sub-dam heights can be increased continuously as required. Therefore, the strength of CFA, as the main lling material, affects the dam slope stability, and has an important impact on construction and operational safety.
Compared with the increasing construction of CFA storage yards, studies on CFA strengthening, which has an important impact on dust suppression and sub-dam slope stability, are lagging behind. For better material utilization and dust pollution control of CFA, effective strength-enhancement methods must be studied, which have an important practical signi cance to increase stringent environmental protection requirements and rapid developments of construction and reinforce ash dam projects.  Table 1.

Bacteria solution
Sporosarcina pasteurii, which is a chemoheterotrophic bacteria, was from the Institute of Microbiology CAS. The cells were rod-shaped, 2-3 µm in length and 0.5-1.5 µm in diameter, gram-positive, and sporeround. Optimized #136 medium was made up by 20.0 g yeast powder, 10.0 g ammonium sulfate and 1.0 mL nickel chloride per liter of water. The pH of the medium was adjusted to 9.0, and sterilized in a highpressure steam sterilizing pot at 120 ℃ for 20 min. After inoculation, the bacteria solution was cultured in a shaking incubator at 30 ℃ and 170 r/min for 24 h. The measured data show that the initial OD 600 of the Sporosarcina pasteurii solution was 3.70, and the urease activity was 1.86 mS/cm/min at 26.9 ℃.

Nutrient salts
The nutrients used in the experiment were CaCl 2 ·2H 2 O (molecular weight 147.01) and CH 4 N 2 O (molecular weight 60.06), produced by Chemical Reagent Co., Ltd. of China Pharmaceutical Group.
To compare the strength in uencing effects of CFA by the MICP, nutrient concentrations were selected as 0.10, 0.25, 0.50, 0.75, 1.00, 1.25 and 1.50 mol/L, with two parallel specimens for each group. The diameter and height of the columnar specimens were 3.91 cm and 8.00 cm, respectively, and the masses of the nutrient salts that were added to the different groups of specimens are shown in Table 2.
To compare the strengthening enhancement effect, four control CFA specimens were prepared with water addition of the same mass as the bacterial solution under the two different curing conditions.

Specimen preparation and curing
Specimen preparation was based mainly on the ASTM D2166-00 and GB/T 50123 − 2019 standards. The mass mixing ratio of dried CFA and bacterial solution in each specimen of the different test groups was 2:1, and the initial water content was 50%.
Before specimen preparation, to ensure easy demolding, an appropriate amount of Vaseline was smeared on the inside of the specimen mold. According to the designed nutrient salt concentration, during specimen preparation, the CFA was mixed with a certain amount of solid nutrients and stirred evenly. The bacterial liquid was poured into a container that was lled with CFA and nutrient salts, and stirred evenly for 10 min. Mixed specimens were added to a mold with ve layers, which were compacted layer by layer with a hammer. After the nal layer had been compacted, the two ends of the specimens were leveled.
Two kinds of curing conditions were used in the MC and NE, according to the requirements of GB/T 50123 − 2019, and 7-day curing remolded specimen-related tests were carried out. The measured curing temperature in the MC was 24 ℃ and the relative humidity was 75%. The specimens under natural NE curing conditions were placed on a well-ventilated windowsill of the laboratory. Meteorological data showed that the highest temperature in Beijing from May 27 to June 2, 2018 was 35 ℃ and the lowest was 16 ℃. Uncon ned compression tests were carried out after 7 days of curing.

Appearance change
The appearances of the CFA specimens that had been cured under the 7-day MC conditions are shown in Fig. 3. The surface condition differed signi cantly with the nutrient concentration in the MICP test. More mildew was present on the surface of the 0.5 mol/L (Fig. 4) and 0.75 mol/L specimens without any obvious distribution of mildew at a lower or higher nutrient concentration (Fig. 5). The difference in appearance indicates that a different nutrient concentration has a signi cant in uence on the growth of microorganisms in CFA that was cured at room temperature and humidity.
The appearance of the CFA specimen that was cured under the NE condition after 7 days is shown in Fig.  6 and the surface specimens were drier than those cured in the MC. Similar to Fig. 3, the surface conditions of the CFA specimen were also signi cantly different. Samples at 1.00 mol/L nutrient concentration (Fig. 7) had more surface mildew, and there was no obvious mildew on the specimen surface ( Fig. 8) at lower or higher nutrient concentrations, which indicates that different nutrient concentrations have different effects on the growth of microorganisms in CFA under the NE condition.
Compared with the MICP-treated specimens, the control specimens with only water added had no mildew on the CFA surface under the two curing conditions.

Reaction mechanism
During the solidi cation of CFA by using Sporosarcina pasteurii,bacterial metabolism produces urease with cell dry weight up to 1%, which can decompose urea into carbonic acid (CO 3 2− ) and ammonia (NH 4 + ) (formula 1). Because of the negative charge on the cell wall, Ca 2+ in the CFA sample is adsorbed around the cell. With increasing amount of adsorption, calcium carbonate crystal with gelation is formed around the cell as the crystal nucleus (formula 2), which gradually increases and accumulates in the process of sample curing. Subsequently, a microbial-induced calcium carbonate block with certain strength is formed in CFA.
CO(NH 2 ) 2 +2H 2 O→CO 3 2− +2NH 4 + (1) In particular, it can be seen from Table 1 that the particle diameter of CFA is generally small. The particles with diameter between 0.075 − 0.005 mm or smaller than 0.005 mm accounts for 81.2% and 10.2% of the total mass, respectively. The particle size is in the same order of magnitude with the monomer size of Sporosarcina pasteurii (length 2-3 µm, diameter 0.5-1.5 µm). The particle size of microbial induced calcium carbonate is close to that of CFA particles. Hence, it is more effective for connection and lling of y ash particles and enhance the curing strength of CFA.

Minerals and SEM analysis
Mineral crystals exhibit speci c X-ray diffraction patterns. The intensity of the characteristic peaks in the patterns is correlated positively with the mineral content in the specimens, and the mineral content can be calculated from the intensity of the characteristic diffraction peaks.
To compare the mineral compositions of UT-and MICP-treated CFA specimens of MC and NE, according to SY/T 5163 − 2010 (2010), the X-ray diffraction spectra of non-clay minerals were obtained on a Rigaku TTR III instrument. A comparison of the UT, MC and NE specimens is shown in Fig. 9, and the ordinary non-clay minerals analysis results are shown in Table 3.
After MICP treatment, for rich nutrients of calcium ion and nitrogen sources, the metabolism of Sporosarcina pasteurii can precipitate calcium carbonate crystals of calcite in the CFA. Figure 9 compares the UT specimens, and shows that the calcite content increased for the MC and NE curing conditions. As shown in Table 3 the content of calcite in the UT CFA was 7.0%. After MICP treatment, the calcite content reached 18.9% and 15.3%, respectively, which is an increase of 170% and 119%, respectively.
SEM/EDS images were used to explore the presence and pattern of CaCO 3 precipitation. In Fig. 10 (a), it can be seen that, a great number of pores are formed in the CFA samples, and the calcium carbonate crystals induced by microorganisms are distributed among the CFA particles, with varying patterns at different positions, generally in the form of bundles ( Fig. 10 (b)) and short columns (Fig. 10 (c)). Since the particle size of the microbial induced calcium carbonate is close to that of the CFA particles, the calcium carbonate crystals can connect and ll the surrounding y ash particles more effectively, and improve the curing strength. The EDS analysis con rms that the mineral observed is calcium carbonate (Fig. 10 (d)). Table 4 lists the water-content change statistics of CFA specimens in the different test groups. After 7 days of curing, under the MC curing condition, the water content of the CFA specimens with only water added decreased from 50-33.75% and 33.72%, which is a decrease of 32.50% and 32.56%, respectively. In contrast, the highest water content decrease of the MICP-treated specimens from 50-36.06%, occurred in the test group with a 0.25 mol/L nutrient concentration, which is a decrease of 27.88%, with the lowest reduction in the 0.1 mol/L group, from 50-47.97%, which is a decrease of 4.06%.

Water-content change
Under the NE curing condition, the water content decrease in each group of specimens was similar to that in the MC, but the decrease was larger. The water content of the specimens with only water added decreased from 50-4.90% and 4.36%, respectively, which is a decrease of 90.20% and 91.28%. The highest water content decrease of the MICP-treated specimens was from 50-24.63% in the 1.25 mol/L test group, which represents a decrease of 50.74%. The lowest reduction occurred in the 0.25 mol/L group, from 50-36.70%, which is a decrease of 26.60%. The decrease in water content indicates that the use of MICP in CFA could strengthen the interparticle connection, and reduce the internal water loss.
MICP-treated CFA achieved a better water retention,could worked well in the dust suppression of CFA storage yards.

Uncon ned compression strength
Uncon ned compression tests were performed on all 7-day cured CFA specimens in accordance with ASTM D2166-00. Tests were performed on a CMT 4304 universal testing machine (MTS Systems Corporation). The axial loading rate was 1.0 mm/min. Loading continued until the load values decreased with an increasing strain, or until a 15% strain was reached during the test.
The different colors represent different nutrient concentrations in the stress-strain curves that were obtained from the tests. The three parallel CFA specimens in the same test groups were expressed by solid line a and dotted line b and c, and the corresponding concentrations of 0.10, 0.25, 0.50, 0.75, 1.00, 1.25 and 1.50 mol/L were marked. The control specimens with no numerical value are also shown in which no bacterial solution, but only water, was added. Figure 11 shows the test result for curing in MC. The good cementing ability of calcium carbonate produced by MICP allowed loose CFA particles to be cemented or lled during crystallization, which can improve the compressive strength simultaneously.
The comparison shows obvious peak points in the stress-strain curves of the MICP-treated specimens. A higher peak strength yields a higher corresponding strain, and the overall tendency is similar. During the initial loading stage after the test commences, the stress increases rapidly with an increase in strain and the stress-strain curve increases approximately linearly. After reaching a maximum, the specimen reaches a limit of bearing capacity with a further increase of strain, and the stress decreases rapidly with certain brittle failure characteristics (Fig. 12). For the experiment where only water is added to the control specimens, in contrast, when the axial stress reaches its peak value, the stress decreases slowly with the increase in strain, and the brittle failure characteristics are not obvious.
The uncon ned peak compressive strength of the MICP-treated specimens with different nutrient concentrations is different under the MC curing condition. Table 5 shows that the maximum peak stress of 97.63 kPa occurs in the test group with a 0.50 mol/L nutrient concentration and the average peak stress of this test group was 80.70 kPa. The minimum peak stress was 23.84 kPa in the 0.10 mol/L test group with an average stress of only 26.14 kPa. The average peak stresses for the 0.10, 0.25, 0.50, 0.75, 1.00, 1.25 and 1.50 mol/L nutrient concentration test groups were 26.14, 53.45, 80.70, 68.08, 64.74, 50.53 and 45.92 kPa, respectively. The results showed that the peak stress of CFA specimens increased initially and then decreased with the increase of nutrient concentration from 0.10 mol/L. According to the maximum compressive strength, an optimum nutrient concentration existed for the MC curing condition. At this nutrient concentration, the MICP reacted more fully and the peak strength was higher.
Compared with the MICP-treated specimens, the uncon ned compressive strength of the control specimens with only water added was lowest under the same curing conditions. The maximum was only 18. 20  Under the NE curing condition, the stress-strain curves from the uncon ned compressive tests are shown in Fig. 13. Although the stress-strain curves have obvious peak points, the overall tendency changed partially. Compared with Fig. 11, during the initial loading stage, with the increase in strain, 1.00a and 1.25a show a rapid growth of stress and an approximately linear growth. Before reaching the peak value of 1.00b and 1.50a, the growth of stress is initially fast and then becomes slow; after failure, the stress decreases rapidly with an increase in strain. However, with the increase in strain, specimens 1.00b and 1.50a appeared and another lower peak appeared after the peak value. Unlike Fig. 11, after reaching their peak value, the control specimen stress decreased rapidly with an increase in strain, and specimen b shows a lower peak value after the maximum.
Compared with the specimens cured in MC, the peak of the axial stress appears near 2% of the relatively low strain, and all specimens showed more obvious brittle failure characteristics regardless of whether bacterial solution was added or not (Fig. 14).
Under the NE curing condition, the uncon ned compressive strength with different nutrient concentrations was signi cantly different. 3) The solidi cation strength of the CFA varies signi cantly with variations in CaCl 2 .2H 2 O and CH 4 N 2 O nutrient concentrations. The peak stress increases initially and then decreases with an increase in nutrient salt concentration. The optimum nutrient concentrations were 0.50 mol/L and 1.00 mol/L under the MC and NE curing conditions, respectively. 4) Calcium carbonate induced by microbial reaction can reduce the loss of water in the ash specimen, and allow the ash surface to achieve a better water-retention effect, with good application prospects in dust suppression and seepage control of ash dams.