FAN Yanan, DU Hongxiu, WEI Hong

(College of Civil Engineering, Taiyuan University of Technology, Taiyuan 030024, China)

Abstract: The C60 concrete blocks with surface crack damage under high temperature environment were soaked by adding appropriate amount of soybean urease into the CO(NH2)2-CaCl2 solution, the soybean urease mineralized calcium carbonate were characterized, and the effect of repairing concrete surface crack damage were evaluated by the surface sedimentation of C60 concrete blocks in the study. The experimental results showed that the activity of soybean powder was statistically significant, and its productivity of urease was comparable with that of urease-producing bacteria. After immersion in a soybean solution, a layer of complete and continuous white sediment covered the concrete surface. The cracks on the concrete surface were completely shielded, and the rising temperature on infrared thermal image of the concrete after repair was lower than before. Besides, through analysis by SEM, EDS, and XRD, the products formed after repair were found to be calcite-type CaCO3 with high purity, and the crystals exhibited different morphological features. The above results indicate that soybean urease can regulate and induce the formation of calcium carbonate, and the precipitate is innocuous and harmless, suitable for a new type of concrete crack repair material.

Key words: soybean urease; concrete; calcium carbonate; remediation; damage

1 Introduction

The concrete structure is currently and will continue to be the major type of building structure.However, due to the nature of low tensile strength of concrete materials, cracks become a common quality problem, affecting the performance of components or structures[1]. There are many repair materials available for concrete cracks, but most of them are harmful to the environment, and are not compatible with cement-based materials[2]. With the combination of biotechnology and civil engineering disciplines, the technology of using biology to induce calcium carbonate to repair cracks has been developed[3-8]. This technology has good compatibility with concrete, and possesses the advantages of environmental friendliness and high durability. The research focus is to use urease to decompose urea to produce carbonate ions, which then combine with the calcium source in the surrounding environment to continuously produce and accumulate calcium carbonate crystals with certain adhesive strength, achieving the effect of sealing cracks[3,5,9-12].

Urease crystals were originally extracted from legumes, and in recent years microorganisms have gradually become the main source of urease. Bang[13]used different immobilised carriers to carry urease bacteria to induce calcium carbonate to crystallize,which not only repaired cracking in concrete, but also improved its strength. Wanget al[14-16]introduced urease bacteria into concrete. Combined with the carriers, cracks were eventually repaired. Compared with untreated cracked specimens, the capillary water absorption rate was reduced by 70%. Jonkerset al[17]added expansive clay containing urease bacteria to concrete, significantly improving the repair capacity,and reducing the water permeability of the specimen.After Muyncket al[18]used Bacillus X as a repair agent for concrete, the capillary water absorption rate,the air permeability, the anti-carbonisation, chloride ion permeability, and frost resistance increased. Yu and Qianet al[19-21]studied the self-repairing ability of concrete cracks by using urease bacteria. It was concluded that, concrete cracks were almost completely filled by calcium carbonate crystals, and the capillary water absorption rate was reduced by 90%.

The preliminary theoretical study on the technology of using microorganism to precipitate calcium carbonate has been conducted, and great progress has been made in the aspects of impermeability, durability, and defect repair of concrete.But urease from microorganisms must be through the process of bacterial culture, and it is affected by environmental factors, such as temperature, pH, the instability of urease activity, the high pathogenicity and the high cost of high-yield urease[22,23]. So, it will be a new research direction by directing extraction of urease from plants for mineralization and deposition of calcium carbonate. At present, there are few studies on the direct use of soybean urease mineralized calcium carbonate at home and abroad, even fewer studies on the use of it to plug concrete cracks，and the studies on the repairing damage after high-temperature exposure are rare.

Building fire is the most frequent and most serious disaster of all catastrophes. Concrete fire damage is the result of multiple factor at varying scales from the surface to the interior. It is mainly characterized by a large number of cracks, looseness, spalling, and other defects. In this paper, the concrete fire damage crack was prepared by simulating fire temperature with resistance furnace, which could represent the real crack damage and distribution. The core content was to repair the surface cracks of concrete after fire by soybean urease. After high temperature treatment, the blocks were soaked by adding appropriate amount of soybean urease into the CO(NH2)2- CaCl2solution.CO32-was produced through continuous hydrolysis of urea by urease, and the surface of concrete blocks appeared the calcium carbonate precipitation. Through the crack widths measuring instrument and infrared thermal image technology, the characteristics before and after crack repair are compared, and the repaired product was qualitative analyzed by using SEM, EDS and XRD. The purpose of this paper is to analyze the characteristics of calcium carbonate induced by soybean urease and to provide theoretical basis for its application in concrete surface crack repair.

2 Experimental

2.1 Soybean urease

2.1.1 Preparation of soybean urease solution

The soybeans were purchased from the market.The soybean urease solution was prepared according to Table 1. After standing, the supernatant liquid was removed to test soybean urease activity.

2.1.2 Measurement of soybean urease activity

Table 1 Soybean solution solubility ratio/(g·L-1)

The urea hydrolysis chemical reaction is shown in Formula 1, wherein the ability of urease to promote hydrolysis of urea directly determines its ability to form calcium carbonate:

In the experiment, it was difficult to detect urea or NH4+and CO32-directly. According to the literature[24],an electrical conductivity method was used to determine urease activity, and the mechanism is such that during hydrolysis of urea, the originally non-conductive urea is hydrolysed into conductive ammonium ions and carbonate ions, so that the conductivity of the solution is increased. The change in solution conductivity was measured by a conductivity meter, and the rate of change is proportional to the rate at which the enzyme decomposes urea, thereby reflecting enzymatic activity.The amount of urea hydrolysis is proportional to the change in solution conductivity: urea hydrolysis amount (mmol/L) = conductivity change (mS / cm) ×11.11 (R2= 0.998 8), reflecting the ability of soybean to hydrolyse urea. The measurement method is as follows:2 mL of the aforementioned soybean clear liquid was mixed with 18 mL of 1.1 mol/L urea solution,and the conductivity of the solution was measured by a conductivity meter over a 5-minute period, and the average conductivity change value (mS/cm·min)measured within 5 minutes was multiplied by the dilution factor (10 times), which gave the soybean urease activity (in unit mmol/L·min).

Fig.1 shows the change in soybean urease activity extracted from soybean powder solution of different mass concentrations. Fig.2 demonstrates the relationship between the activity of soybean urease per unit mass (soybean urease activity/soya powder mass concentration) and the concentration of soybean powder solution. The bacterial activity in MICP tests is generally within 3.3 to 20 mM/min. It can be seen from Fig.1 that the soybean urease activity can be compared with the bacterial activity, and the activity increases linearly with the increase in concentration of the soybean powder solution. However, it can be seen from Fig.2 that with the increase of soybean powder concentration, the soybean urease activity per unit shows a downward trend. This is in accordance with the fact that monomer urease activity of ureaseproducing bacteria is not consistent with bacterial concentration[24-26]. According to the fact that, in studies of the microbial deposition of calcium carbonate technology, many scholars[24,25,27]found that the low urea hydrolysis rate was more stable than the high rate to form stable calcite-type calcium carbonate crystals. In addition, considering the cost, a ratio of 1:100 soybean solution was carried out in subsequent experiments.

Fig.1 The relationship between soybean urease activity and soybean powder mass concentration

Fig.2 The relationship between soybean urease activity per unit mass and soybean powder mass concentration

2.2 Preparation of cracked concrete specimens

2.2.1 Materials and mix ratio of concrete specimen

Table 2 Mix ratio of C60 HPC/(kg/m3)

Concrete specimens were prepared with ordinary Portland cement, P.O. 42.5, with 51.18% C3S, 22.16%C2S, 10.06% C3A, 13.15% C4AF, 0.19%f-Cao, China.Slag powder, level S95, 28 days activity index 110%,China. Fly ash, grade II, 0.67% f-Cao. Naphthalene water reducer, water-reducing ratio 15-20%. Coarse aggregate, limestone, 5-20 mm continuous grading. Fine aggregate, local natural sand, fineness modulus 2.95,medium sand Ⅱ area, grading qualified. The concrete mixes were prepared, as summarized in Table 2.

2.2.2 Preparation of concrete specimens

The C60HPC standard cube specimen was prepared according to Chinese National Standard GB/T 50081-2002 Standard for Test Method of Mechanical Properties of Ordinary Concrete. After the specimen was demoulded, it was placed in a curing tank containing Ca(OH)2saturated solution at (20±2) ℃ for 28 days.

2.2.3 Concrete strength specimen crack treatment

An SRJX type box resistance furnace was adopted, with a rated maximum temperature of 1 200℃, a rated power of 15 kW, and heating rate of 10 ℃/

min. The simulated fire temperature was set to 600 ℃.When the temperature of the pre-buried thermocouple in the centre of the specimen was consistent with the set temperature, the temperature was kept constant for 15 minutes, so that the temperature inside and outside the concrete was consistent indicating that the concrete would be evenly heated. Specimens kept at room temperature (20 ℃) were used as the comparison group. After high-temperature exposure, the specimen was removed from the high-temperature environment and placed on dry ground (indoors) and allowed to cool[28].

2.3 Mixed solution

8 L calcium chloride solution was prepared in a plastic container. After fully dissolving, the calcium solution was kept until its temperature reached room temperature. Urea was added to the calcium solution.The final concentration of the mix solution of urea and calcium chloride is 0.5 mol/L. Then the soybean powder (at a ratio of 1:100) was added, stirred, and then stood for 5 minutes, into which the completely cooled concrete specimen was placed, and left for 3 days. Meanwhile, as a correlation group, two cooled specimens were slowly put into water instead.

2.4 Experimental method

2.4.1 Repair effect characterization and evaluation

a) The evaluation of crack repair width

A crack mirror was used to measure the crack width on the concrete surface after high-temperature exposure. The measurement position was marked, and the data recorded. When the repair was due, the crack repair phenomenon was described, and the crack width at the marked position was measured again. To quantify the ability of soybean urease to repair cracks, the crack repair rate is characterized by crack repair rate[5], as given by:

where,δrepresents crack repair rate, %;k0represents the initial crack width, mm;ktis the crack width at timet, mm.

b) Infrared thermal imaging technology

Infrared thermal imaging technology is a nondestructive test method that uses infrared detectors and optical imaging objectives to receive the infrared radiation energy distribution of the concrete being tested[29]. Here, infrared thermal images of the same specimen before and after crack repair were captured by TH9100WV infrared thermal imager. The radiance was set to 0.92 during the experiment. The external heat source was an infrared light bulb, and the distance between the infrared thermal imager and the concrete specimen was 1 m, and the heating time was 3 min.After the images were acquired, the data were analysed and processed to obtain the average temperature rise in the specimen.

2.4.2 Sediment composition and morphology

The white sediment formed on the surface of the specimen was scraped off with a sharp blade, and dried naturally. The composition thereof was analysed by X-ray diffractometer (Ultimav, Japan), and the sediment composition and morphology were analysed by a Phenom-XL scanning electron microscope (The Netherlands).

3 Results and discussion

3.1 Crystal analysis

Fig.3 Morphology of white sediment crystals

The specimen was immersed in the mixture of soybean powder, urea, and calcium chloride. After 3 days, a layer of white sediment was found on the surface. A relatively complete sample of white sediment was taken from the surface of the concrete specimen for crystal analysis.

Fig.4 EDS spectrum

According to the SEM results (Fig.3), there was a significant stratification phenomenon in the side section of the white precipitate sample, which was mainly divided into three layers, and there were large differences in crystalline morphology and characteristics between layers. Here, the white precipitate that is in contact with the concrete surface is defined as the bottom layer, on top of which are the middle and surface layers. Fig.3(a) shows the crystalline morphology at the bottom layer. It can be seen from the figure that the crystals deposited by the mineralisation of soybean are mainly spherical and double hemispherical. The surface of the spheres is smooth and solid, and their size is uniform, with diameters between 20 to 30 μm. The ends of the spheres are symmetrically distributed with columnar and fibrous materials, which are closely packed and stacked on each other, while being tightly bound to the spheres. Figs.3(b) and 3(c) show the morphology of the crystals in the middle layer. It can be seen from the figure that the crystalline morphology of the middle layer is spherical, elliptical, agglomerated flower clusters and new pineapple shape. The difference from the bottom layer is that the surface is rough, with diameters between 3 to 10 μm. Fig.3(d) shows the crystal morphology at the surface layer. It can be seen from the figure that the morphology of the sediment is spherical as a whole, the surface of the spherical particles is rougher and the diameter is between 1 and 5 μm. Granular crystals are seen protruding from each sphere, and the quasi-spherical particles appear as numerous smooth hexahedra or polyhedral which are interspersed or stacked.

According to the results of SEM, energy spectrum analysis was respectively applied to the crystals of different morphological features in the white sediment. EDS element analysis was conducted on 13 morphological types, including large-diameter spheres with smooth surfaces, small spheres with smooth surfaces, small spheres with rough surfaces, particles with protruding surface elements, columnar particles,fibrils,etc(Fig.4). The results show that, regardless of the type of crystalline morphology, three elements C, O, and Ca (with a small amount of Cl and N) are present.

Fig.5 XRD spectrum

For further qualitative analysis, X-ray diffraction analysis was applied to the bottom, middle, and upper samples, showing that the spectra of precipitates were almost identical. As shown(Fig.5), the spacing and strength of the diffraction peaks’ crystal faces observed from these spectra are consistent with the CaCO3calcite standard spectrum. They are all calcite structures and belong to the hexagonal system. In the spectrogram, characteristic diffraction peaks appear near 2θ=23.2°, 29.5°, 36.1°, 39.5°, 43.3°, 47.5°, 48.6°,and 57.5°, respectively, corresponding to the diffractive surfaces (012), (104), (110), (113), (024), (018), (116),and (122): miscellaneous peaks are rare, indicating that the sediment is CaCO3of higher purity[25-27,30], which proves that, as a concrete crack repair material, the product of soybean powder is non-toxic and harmless,and the compatibility with the concrete substrate is good.

The possible cause of stratification of white precipitates is that, at the beginning of the sedimentation process, the soybean urease activity is relatively high, and continuously hydrolyses urea,producing CO32-; the CO32-is less concentrated than Ca2+in the solution, and thus the CO32-is attracted by Ca2+from different directions. Meanwhile, the rates of growth of crystals at different crystal surfaces are practically consistent, and thus the crystal morphology is that of a smooth sphere, and the mass of the big particles is relatively large, and they are prone to being deposited at the bottom layer. With increasing reaction time, the activity of urease gradually decreases, and the rate of urea hydrolysis decreases. In addition, since Ca2+deposited early in the process, lowering the solubility,hexahedral crystals are more likely to be formed at this time. Therefore, the crystal shapes exhibit a transition from smooth spheres to angular hexahedra.

3.2 Characteristic analysis of the crack repair effect

After exposure at 600 ℃, because of the physical and chemical reactions, there were irregular cracks on the surface of C60 concrete specimen. Through measurement, the maximum width of the cracks on the casting surface was about 0.5 mm, and the width of most cracks was 0.1 mm. There was a continuous film-like solid layer at the bottom, but the adhesive strength between the particles was the greatest among the three layers. The white substance on the surface of the specimen was scraped off with a scalpel, and it was found that the crack surface was almost completely filled, and 80% of the white substance was firmly adhered to the specimen surface (Fig.6).

The infrared thermal images of cracks in the same C60 concrete specimen irradiated for 3 minutes before and after repair are shown in Fig.7, after initial infrared irradiation for 30 s, the concrete specimen without surface damage repair exhibited a colour difference,and there were red spots in the middle. After 3 minutes,there were a few white spots, and the red stacked area became bigger, and the average temperature increased by 3.1 ℃. After the repair with soybean urease, the infrared image of the concrete appeared green after 30 s of initial infrared irradiation. The red spots appeared after 3 minutes, but they were discontinuous, mainly yellow, and the average temperature increased by 1.4℃.

Fig.6 The surface of concrete after immersion in soybean urease solution: (a) Before repairing; (b, c, d) After repairing

Fig.7 The infrared thermal images of concrete cracks before and after repairing: (a) Before repairing; (b) After repairing

Comparing the infrared thermal images of concrete before and after repair, it can be found that, at the same temperature, the average infrared temperature increase of the concrete specimen after repair is lower than that without repair, decreasing by about 55%.From the figure, it can be seen that the colour of the concrete infrared image before crack repair is redder than that after repair. The reason for this is that after high-temperature exposure, free water and bound water evaporate and C-S-H gel dehydrates and decomposes,increasing the number and size of pores in the concrete and cracks on the surface[31-33]. Thermal damage and heat accumulation occur on the surface of the concrete.During the same period of irradiation, positions with defects are more vulnerable to heat accumulation:however, after repair, the cracks on the concrete surface are almost completely filled with the white substance.As a result, the temperature increase of the concrete after repair is lower than that before repair. The experiment results show that to use the soybean urease to repair cracked concrete is reliable, and the repair process is simple.

4 Conclusions

According to the experimental study on the damage repair of high-temperature induced concrete cracks by soybean urease mineralisation, the following conclusions can be drawn:

The soybean urease activity increased linearly with the increase of the concentration of soybean powder, but the urease activity per unit mass decreased with the increase of the quality of soybean powder, and its characteristic activities were consistent with those of bacteria producing the urease.

The analysis of SEM, EDS, and XRD can be used to confirm that the white sediments produced by the process of soybean urease repair of damage to the concrete surface were all calcite-type CaCO3materials of high purity. The morphology of the CaCO3crystals showed different characteristics within the sedimentation layers: they were gradually transformed from smooth spheres in the bottom layer to hexahedra with sharp edges in the upper layer, and a new pineapple shape appeared.

After immersion in the solution of soybean urease, urea, and calcium salt, the concrete specimen with cracks on the surface after high-temperature exposure was covered by the complete and continuous white sediment layer. The averagerising temperature on infrared thermal image temperature rise on the concrete surface was reduced by 50% compared to that before repair. It can be observed that the crack repair effect thereof was significant, and the repair process was simple and non-toxic.