Experimental Research into the Repair of High Temperature Damage to Cement Mortar Samples Using Microbial Mineralization Technology

: Experiments such as microbial activation culture, subculture selection, and ﬁre damage repair of cement mortar specimens were conducted to investigate the repairing effect of Sporosarcina pasteurii as a repair agent for ﬁre-damaged cracks in cement mortar specimens. In addition, multi-scale parameters such as compressive strength and chloride ion migration coefﬁcient of cement mortar specimens before and after restoration were compared. The effect of microbial mineralization technology on the repair of ﬁre-damaged cracks in cement mortar specimens was investigated, and the microstructure and mineral composition of the products were analyzed. The results showed that the strong alkaline environment in the cracks of the cement mortar specimens after a high temperature of 500 ◦ C inhibited the activity of bacteria and weakened the mineralization effect; the compressive strength of the repaired cement mortar specimens was 22.8% higher than that of the unrepaired ﬁre-damaged specimens; the compressive strength of the repaired cement mortar reached 78.2% of the strength of the original cement mortar specimen without high temperature; after restoration, the chloride ion penetration resistance of the cement mortar specimens decreased by about 16.9% compared with that before restoration.


Introduction
Fire is currently the most significant danger to buildings [1].When concrete structures are exposed to high-temperature fires suddenly, they experience degraded material properties, increased deformation and cracks, weakened structural strength, and reduced safety and durability.Fire and high temperatures result in severe damage to concrete materials due to a series of physical and chemical changes, such as phase transitions in cement stone, increased cracks, and a porous and loose structure.Furthermore, the cracking and disjointness of the cement stone-aggregate interface contribute to the damage.As a result, concrete damage caused by fire tends to progress gradually from the surface to the interior, with most of the damage defects manifesting as cracks [2,3].
Although many repair materials are available for fixing concrete cracks, most of them consist of organic polymer materials, which are incompatible with cement-based materials.Additionally, some of these materials have harmful and pathogenic characteristics.Biomineralization, which is dependent on Biologically Induced Calcium Carbonate Precipitation Technology (MICP), uses biological metabolic activities to induce calcium carbonate precipitation and repair cracks in concrete.Biomineralization technology has the advantage of being environmentally friendly, and the calcium carbonate generated is highly compatible with cement-based materials [4][5][6].Qian Chunxiang and Wang Ruixing [7][8][9][10][11] conducted research to investigate the formation mechanism of microbially mineralized calcite in self-healing concrete.They discovered that concrete crack self-healing bacteria produced calcite-type CaCO 3 as a mineralization product and identified the necessary factors and chemical changes in the mineralization process.The study on the yield of calcite mineralized by microorganisms revealed that different substrate concentrations, medium concentrations, n(MgSO 4 )/n(CaCl 2 ), and inoculum amounts significantly influence the calcite yield.Their research also revealed that with an increase in age, the number of calcium carbonate crystals slightly larger than 1 mm in concrete cracks increased, while the crack width gradually decreased.After around 40 days, calcium carbonate crystals had effectively filled the cracks on the surface.
Although urease activity has been found in numerous species of microorganisms, only in a few microorganisms, represented by Bacillus cereus, has very high urease activity been found and induced biomineralization reactions.At the beginning of the 21st century, Whiffin [12] was the first to carry out a study on the differences in urease activity among multiple species and found that microbial urease activity was generally higher than that of plants and animals, but urease activity in microorganisms also shows significant interspecies differences exist.Among them, the urease activity of Helicobacter pylori is at a high level among microorganisms [13,14], while the urease activity of Aspergillus spp. is relatively low [15] and even comparable to that of plants.The species with the highest known urease activity is Sporosarcina pasteurii [16], whose overall expression of urease genes reaches even 1 percent of its cell dry weight [17].
Wang Ruixing and Qian Chunxiang [18] utilized microbial-induced calcium carbonate deposition technology for repairing the surface of cement-based materials with defects, where they also studied the surface formation.Moreover, they applied a high concentration of bacteria to the surface of the material with defects and noted the formation of a sediment film about 100 µm thick on the top horizontal layer.As a result, the water absorption rate of the component decreased by approximately 85%, and the repair effect of the topmost layer was superior to that of the side.The researchers also tested the strength of the cracks repaired using microbial technology.Their findings indicate that after 28 days of repairing cracks, the compressive strength increased by 76% compared with that of the unrepaired cracks.The tensile strength of the repaired cracks increased to 84% of the parent body's strength.
In MICP technology, microorganisms decompose carbonate, which transforms into CO 2− 3 and combines with Ca 2+ to form the precipitation of CaCO 3 .According to the reaction mechanism expressed in Formulas (1) and (2), the combination of 1 mol of Ca 2+ in 1 mol of CO 2− 3 generates 1 mol of CaCO 3 .
In this experiment, the microbial group with the best growth activity within 72 h was selected.Using C 4 H 6 CaO 4 •H 2 O as an external Ca 2+ source, mineralization repair tests were carried out to investigate the strength of cement mortar specimens before and after high temperature damage and the influence of microbial activity on their repair performance.The effect of microbial mineralization on fracture repair was reflected by the comparison of chloride ion migration parameters before and after repair.Meanwhile, the microstructure and mineral composition of the repair products were analyzed.To provide a theoretical basis for microbial mineralization technology to repair high temperature damage defects in cementitious sand specimens and to provide a preliminary research basis for further application of MICP technology to concrete repair.

Microbial Sample Preparation and Characterization
Sporosarcina pasteurii DSM 33 from the German Collection of Microorganisms and Cell Cultures was activated in a solid medium containing casein (15.0 g), soy protein (5.0 g), NaCl (5.0 g), agar (15.0 g), deionized water (1000.0mL), and urea (20.0 g) to prepare the microbial sample.A 1 mol/L NaOH solution was used to adjust the pH of the solid and liquid media.The solid medium's pH was 7.3, and the aerobic incubation was carried out at 30 • C for 48 h.A 1 mol/L NaOH solution was prepared to adjust the pH of the solid and liquid media.The pH of the activated solid medium was 7.3, and aerobic incubation was carried out at 30 • C for 48 h.Table 1 shows the medium mixing ratios.The samples were taken from the solid medium and sub-cultured into the liquid medium, as shown in Table 1, with a pH of 7.3.The incubation process was conducted under the settings of a shaker at 30 • C and 170 r/min.The activity of microorganisms was characterized based on the sample conductivity, which was measured using the DZS-708L multi-parameter instrument.The researchers mixed 18 mL of 1 mol/L urea solution with 2 mL of bacterial solution, and the change in the mixed solution's conductivity was determined by measuring for 5 min.The result was expressed in µs/cm/min.The turbidity of the bacterial solution was determined using a UV-1800 ultraviolet spectrophotometer, specifically by measuring its absorbance value at a wavelength of 600 nm.The samples' conductivity and OD 600 values were measured every hour for 72 h.

Cement Mortar Test Piece Preparation and High-Temperature Damage Test Method
A 40 × 40 × 160 mm cement mortar specimen was prepared according to the mix ratio specified in Table 2.The specimen was left to cure for 28 days in an environment with a relative humidity of over 90% and a temperature of 20 • C ± 1 • C. Six specimens were prepared for each of the following groups: the original group, the high-temperature unrepaired group, and the high-temperature repaired group.The temperature inside the test piece was maintained at the set temperature of the resistance furnace for 15 min.Subsequently, the specimen was removed from the furnace and cooled using a method that simulated firefighting conditions.Immediately after removal from the oven, the specimen was cooled in a running basin of water.The test block was then dried indoors and set aside.The high-temperature damage test was conducted using a box-type resistance furnace with a rated voltage of 220 V, an output power of 15 kW, and a maximum operating temperature of 1200 • C. The heating rate was set at approximately 10 • C/min.The furnace dimensions were 600 mm × 400 mm × 400 mm (L × W × H).The target temperature for the test was 500 • C. The heating process is as follows: the cement mortar specimen was heated in the furnace.The internal temperature of the sample was measured using a thermocouple.When the temperature of the specimen was equal to the temperature of the resistance furnace, the furnace door was opened to allow the specimen to cool for 1.5 h.The load and repair test was carried out after natural cooling to room temperature.Compressive strength was evaluated using an electro-hydraulic servo universal testing machine with a loading rate of 2.4 kN/s.Results were accurate to 0.1 MPa.

Experiment and Characterization of Microbial Mineralization Restoration of High-Temperature Damaged Cement Mortar Specimens
The repair test was conducted by immersing the high-temperature damaged specimen in a mixture of bacteria solution, urea, and C 4 H 6 CaO 4 •H 2 O for 24 h.It was then dried at room temperature, and its multi-scale performance before and after restoration was compared.The multi-scale performance of the specimen before and after restoration was compared.A compression test was used for macroscopic characterization.The chloride-ion transfer coefficient test was conducted utilizing the vacuum negative pressure adsorption method.The RCM-6H multifunctional concrete durability comprehensive tester was used for the test.The instrument had six channels and could measure six specimens simultaneously.It was equipped with temperature and current monitoring sensors and can display temperature and current in real time.The temperature display had an accuracy of 0.1 • C, while the current display had an accuracy of 0.01 mA.A SHZ-III vacuum pump was utilized, with a maximum vacuum degree of 10 kPa.The defoaming barrel was a custommade cast iron vacuum cylinder with a transparent barrel cover that allowed observing the inside of the barrel at all times.The specimen was prepared according to the mixing ratio given in Table 2.The mold was a 100 × 150 mm cylinder during molding.After it cured to the designated age, the test piece was processed, and both forming surfaces at the ends were carefully cut off.As a test specimen, a cylinder was cut from the center of the specimen with a height of 50 ± 2 mm.The generated product underwent microscopic characterization through SEM and XRD experiments.

Microbial Growth Curve and Selection
Sporosarcina pasteurii DSM 33 is a commonly used microorganism for biomineralization, and its activity and concentration should be monitored at various time intervals during its growth process.The study selected the optimal time period during the growth curve to maintain an effective biomineralization process.The microbial growth was monitored over a 72-h period in this experiment.Sample conductivity and OD600 values were measured separately for L-21~10 at 1 h intervals for the first 12 h, 2 h intervals for the 12th-24th, 4 h intervals for the 24th-48th, and 12 h intervals for the last 24 h.Each data point is the mean value of the sample data in 10 tubes, as shown in Figure 1. Figure 1 indicates that during the 72-h culture of microorganisms, there is a linear correlation between OD600 and conductivity, and both values progressively increase from the initial stage to 48 h.At the early stage of bacterial growth, the conductivity and OD600 values of the bacterial solution grow slowly, and the microbial activity is low.At around 4 h bacterial growth enters the logarithmic phase and activity increases significantly; at 7 h, it enters the stable growth phase and microbial activity continues to rise; at 48 h, incu- Figure 1 indicates that during the 72-h culture of microorganisms, there is a linear correlation between OD 600 and conductivity, and both values progressively increase from the initial stage to 48 h.At the early stage of bacterial growth, the conductivity and OD600 values of the bacterial solution grow slowly, and the microbial activity is low.At around 4 h bacterial growth enters the logarithmic phase and activity increases significantly; at 7 h, it enters the stable growth phase and microbial activity continues to rise; at 48 h, incubation activity reaches its maximum; it then enters the bacterial growth decline phase and activity starts to decrease.The microbial activity reached its maximum at 48 h before diminishing gradually.The reason for the above-mentioned phenomenon has been related to the amount of microbial inoculum and the content of nutrients in the culture solution.At the point when microbial conductivity reached its peak, the highest decomposition rate of CO 2− 3 resulted from urea.At this stage, an increased quantity of Ca 2+ was introduced, and more precipitates were produced.Due to microorganism loss resulting from growth conditions during remediation, it is advisable to use microorganisms cultured for 48 h during the remediation test.

Comparison of Compressive Strength and Chloride Ion Migration Coefficient before and after Restoration
The compressive strength of the original group, the 500 • C damage, and the repaired specimens were compared with provide a more intuitive macro-repair effect, as shown in Figure 2. The results from Figure 2 showed that, on average, the compressive strength of cement mortar sand specimens after 500 °C high-temperature damage was 36.3% lower than that of the undamaged specimens.However, the compressive strength of the repaired specimens was 22.8% higher than that of the damaged specimens and recovered to 78.2% of the undamaged specimens.This was due to the CaCO3 crystals generated by microbial mineralization, which filled the cracks and pores caused by high-temperature damage.The microbial mineralization product calcium carbonate has a certain strength and adhesion, and the particle size is smaller than the defect size of the cementitious sand specimen, which can fill the defect effectively.This increased the cohesive force of the specimen, ultimately improving its compressive strength.Figure 3 compares the chloride migration parameters of the specimens before and after repair.The results from Figure 2 showed that, on average, the compressive strength of cement mortar sand specimens after 500 • C high-temperature damage was 36.3% lower than that of the undamaged specimens.However, the compressive strength of the repaired specimens was 22.8% higher than that of the damaged specimens and recovered to 78.2% of the undamaged specimens.This was due to the CaCO 3 crystals generated by microbial mineralization, which filled the cracks and pores caused by high-temperature damage.The microbial mineralization product calcium carbonate has a certain strength and adhesion, and the particle size is smaller than the defect size of the cementitious sand specimen, which can fill the defect effectively.This increased the cohesive force of the specimen, ultimately improving its compressive strength.Figure 3 compares the chloride migration parameters of the specimens before and after repair.mineralization, which filled the cracks and pores caused by high-temperature damage.The microbial mineralization product calcium carbonate has a certain strength and adhesion, and the particle size is smaller than the defect size of the cementitious sand specimen, which can fill the defect effectively.This increased the cohesive force of the specimen, ultimately improving its compressive strength.Figure 3 compares the chloride migration parameters of the specimens before and after repair.Figure 3 shows that there was a 16.9% reduction in the chloride ion migration parameters of the repaired specimens, compared with the unrepaired ones.This happened because the mineralization products filled up high-temperature damaged micro-cracks and micro-pores, reducing the diffusion path of chloride ions and thereby enhancing the antipenetration ability of the repaired specimen to chloride ions.The chloride ion migration parameters of the repaired specimens were significantly different from the undamaged original specimens.The cementitious sand specimens were damaged at 500 • C and to a greater extent, resulting in a lower degree of improvement in resistance to chlorine penetration after restoration, making it difficult to achieve better results by relying solely on microbial mineralisation techniques for restoration.

SEM Photos and XRD Analysis of High-Temperature Damage Products of Cement Mortar Specimens Repaired by Biomineralization
The SEM photograph of the restoration product is shown in Figure 4. No. 1 is a 1-10 µm spherical calcium carbonate crystal with some agglomeration.Meanwhile, No. 2 is a C 4 H 6 CaO 4 •H 2 O long strip crystal that does not react, whereas No. 3 is an agglomerated flake SiO 2 , and both participate only in partial filling.During the repair process, the morphology and yield of the microbially induced precipitated calcium carbonate were affected by the high alkaline environment inside the defective, high-temperature damaged cementitious sand specimens.The morphology was spherulite, the yield was reduced, and the impurities inside the defective cementitious sand specimens formed polymers with the calcium carbonate to fill the defect.Since the high-temperature damaged cracks of the cement mortar specimen belong to an alkaline environment, the growth of microorganisms involved in the remediation process was restricted due to the use of a solution.This significantly affects the production of CaCO 3 required for remediation, as shown in the mineral composition analysis of the restoration product (Figure 5).Therefore, as shown in Figure 5, the mineral composition of the restoration product was analyzed.
According to Figure 5, the analytical grade C 4 H 6 CaO 4 •H 2 O used in the test showed a small amount of Ca(OH) 2 contamination, while the repair products included calcium carbonate of both calcite-type and vaterite-type, in addition to calcium silicate hydrate gel.The mineralized product calcium carbonate is calcite as the main crystalline phase, with a small amount of spherulite crystalline phase present, with a strong diffraction peak at 2θ = 29.The experiments showed that the high-temperature damage environment did not affect the types of substances produced through microbial mineralization.However, it did affect the microscopic morphology of CaCO 3 in addition to impacting the repair speed and effectiveness of microbial mineralization.flake SiO2, and both participate only in partial filling.During the repair process, the morphology and yield of the microbially induced precipitated calcium carbonate were affected by the high alkaline environment inside the defective, high-temperature damaged cementitious sand specimens.The morphology was spherulite, the yield was reduced, and the impurities inside the defective cementitious sand specimens formed polymers with the calcium carbonate to fill the defect.Since the high-temperature damaged cracks of the cement mortar specimen belong to an alkaline environment, the growth of microorganisms involved in the remediation process was restricted due to the use of a solution.This significantly affects the production of CaCO3 required for remediation, as shown in the mineral composition analysis of the restoration product (Figure 5).Therefore, as shown in Figure 5, the mineral composition of the restoration product was analyzed.According to Figure 5, the analytical grade C4H6CaO4•H 2O used in the test showed a small amount of Ca(OH)2 contamination, while the repair products included calcium carbonate of both calcite-type and vaterite-type, in addition to calcium silicate hydrate gel.The mineralized product calcium carbonate is calcite as the main crystalline phase, with a small amount of spherulite crystalline phase present, with a strong diffraction peak at 2θ

Conclusions
(1) After monitoring the growth of microorganisms for 72 h, a linear correlation between OD600 and conductivity was detected.There was a linear increase within 48 h after the inoculation of the solid medium into the liquid medium, after which there was a decline.At 48 h, the OD600 was 2.585, and the conductivity was 110 μs/cm/min.The bacterial solution obtained at 48 h was used in the mineralization test.(2) The application of microbial mineralization for repairing 500 °C damaged cement mortar specimens resulted in an improvement in both the compressive strength and chloride ion penetration resistance.The repairing rate of the specimen damaged at 500 °C was found to be 78.2%.This percentage was calculated by comparing the compressive strength of the repaired specimen to that of the undamaged specimen.(3) The analysis of the repair products indicated that CaCO3 was mainly responsible for the restoration of high-temperature, damaged cement mortar specimens.The CaCO3 exhibited moderate strength, and a portion of the pores was filled with calcium silicate hydrate.The high-temperature alkaline environment somehow affected the

Conclusions
(1) After monitoring the growth of microorganisms for 72 h, a linear correlation between OD 600 and conductivity was detected.There was a linear increase within 48 h after the inoculation of the solid medium into the liquid medium, after which there was a decline.At 48 h, the OD 600 was 2.585, and the conductivity was 110 µs/cm/min.The bacterial solution obtained at 48 h was used in the mineralization test.(2) The application of microbial mineralization for repairing 500 • C damaged cement mortar specimens resulted in an improvement in both the compressive strength and chloride ion penetration resistance.The repairing rate of the specimen damaged at 500 • C was found to be 78.2%.This percentage was calculated by comparing the compressive strength of the repaired specimen to that of the undamaged specimen.
(3) The analysis of the repair products indicated that CaCO 3 was mainly responsible for the restoration of high-temperature, damaged cement mortar specimens.The CaCO 3 exhibited moderate strength, and a portion of the pores was filled with calcium silicate hydrate.The high-temperature alkaline environment somehow affected the cracks, resulting in a reduced mineralization effect by microorganisms.Several factors influenced the crystal morphology of the repaired CaCO 3 product, for example, temperature, pH, order of introduction of calcium sources, etc. (4) Through the study of microbial repair of high temperature damage to cementitious sand specimens, further microbial repair methods and effects after high temperature damage to concrete specimens can be explored, and after a reasonable construction method, this will potentially make the MICP technique one of the repair methods after fire damage to concrete buildings.

Figure 3 .
Figure 3.Comparison of chloride ion migration parameters.Figure 3. Comparison of chloride ion migration parameters.

Figure 3 .
Figure 3.Comparison of chloride ion migration parameters.Figure 3. Comparison of chloride ion migration parameters.

Figure 4 .
Figure 4. Microbial mineralization repair cement glue test parts high-temperature damage product.

Figure 4 .
Figure 4. Microbial mineralization repair cement glue test parts high-temperature damage product.

Figure 5 .
Figure 5. Mineral analysis of Microbial mineralization repair cement glue test parts high-temperature damage product.

Figure 5 .
Figure 5. Mineral analysis of Microbial mineralization repair cement glue test parts high-temperature damage product.

Table 2 .
Cement sand specimen mix ratio.