Microbial-inspired self-healing of concrete cracks by sodium silicate-coated recycled concrete aggregates served as bacterial carrier

Abstract

Microbially induced carbonate precipitation (MICP) is a promising technique for the autonomous healing of concrete cracks. In this study, the effect of pH on MICP was investigated. The results indicate that the MICP process was inhibited when the pH was higher than 11. Both vaterite and calcite were produced when the pH was < 8, whereas only calcite was produced when the pH was > 8. Recycled concrete aggregates (RCA) coated with sodium silicate have been proposed as protective carriers for microbial healing agents. Although the presence of the coated RCA resulted in a loss of the splitting tension strength of the concrete, the loaded healing agents were highly efficient in self-healing cracks. Concrete incorporated with 20% RCA loaded with healing agents exhibited the best self-healing performance. When the initial crack widths were between 0.3 and 0.4 mm, the 7-d mean healing rate was approximately 90%. At 28 d, the crack area filling ratio was 86.4%, while its water tightness recovery ratio was 74.4% and 29.8%, respectively, for rapid and slow absorption. This study suggests that RCA coated with sodium silicate is an effective method for packaging microbial healing agents and has great potential for developing cost-effective self-healing concrete.

Graphical abstract

Keywords

self-healing / microbial-induced carbonate precipitation / water uptake / recycled concrete aggregate / protective carrier

1 Introduction

Concrete is a widely accepted civil engineering material owing to its high compressive strength, ease of formation, and low cost. Unfortunately, concrete is prone to crack owing to its poor volume stability [1,2]. Although microcracks have slightly negative effects on the mechanical performances of concrete, they provide paths for the penetration of corrosive media, such as Cl⁻, S{\text{O}}_{\text{4}}^{2-}, CO₂, and eventually lead to the deterioration of concrete. The timely repair of microcracks is required to extend the service life of concrete structures. Traditional repair methods, including regular detection and manual maintenance, are typically labor-intensive and expensive. To address this issue, autonomous healing of concrete cracks was proposed based on the concept of bionics, in which organisms can repair themselves. Because of the continuous hydration of the unhydrated clinker particles or carbonization of the hydration products, concrete can automatically heal ultra-fine cracks (~ 100 μm), which is also called autogenous self-healing. However, additional healing agents were required to form larger cracks. Many self-healing strategies have been developed such as electrochemical deposition [3,4], microcapsule-enabled methods [5,6], and bacteria-based technique [7–10]. Based on classical nucleation theory, Chen et al. [11] proposed a novel and promising crack repair method by the in situ synthesis of ettringite for underground structures, which greatly improved the healing rate of the traditional electrochemical deposition method in a water environment.

Among the healing agents developed to date, microbially induced carbonate precipitation (MICP) is a promising technique in the past decades. The principle of concrete self-healing via MICP involves the filling of cracks with calcium carbonates produced by bacteria. Dormant bacterial spores, nutritional substrates, and calcium sources were added to the concrete mixture during fabrication. These healing agents are released upon cracking. Dormant spores germinate by the ingress of oxygen and water via cracks and then produce calcium carbonates by metabolism to heal the cracks. Compared with typical polymer-based healing, calcium carbonate precipitated by bacteria is nontoxic and highly similar to concrete components. Therefore, they are environmentally friendly and compatibility [12–16].

Although studies have confirmed the feasibility of using MICP to achieve concrete self-healing, the metabolic pathways of MICP are limited [17–20]. Among the existing pathways, ureolytic hydrolysis is the most efficient, as the reaction rate of enzymatic hydrolysis is approximately 10¹⁴ times faster than the corresponding chemical reaction rate [21]. During the ureolytic MICP process, urease is first secreted by bacteria, which then catalyzes urea to induce calcium carbonate in an alkaline environment. The effectiveness of concrete self-healing is significantly influenced by the rate of MICP, which is determined by the ureolytic activity of bacteria. Prior studies have investigated factors contributing to the MICP process in bacteria, such as temperature, pH, ion species, and ion concentrations. However, most of these studies were performed in culture medium environments. Because crack healing occurs in concrete, further studies on the MICP process conducted in a simulated concrete environment are necessary.

Another major concern with the MICP-based self-healing strategy is the harsh environment required for bacteria. The high shear stress during the concrete mixing stage and the squeeze stress during the hardening stage of concrete can severely threaten the survival of bacteria pre-added to concrete [22–24]. Therefore, in most cases, protective carriers are needed to protect the bacteria in concrete. Porous materials are competitive as protective carriers because of their extensive range of sources and low cost [25–31].

Recently, recycled concrete aggregates (RCA) was proposed as a potential protective carrier candidate [32,33]. Derived from construction and demolition waste (CDW), RCA is also characterized by a high porosity and water absorption ratio owing to the adhered old mortar and large fissures formed by crushing [34–38]. Previous studies have shown that the efficiency of crack healing in concrete is enhanced if bacteria are immobilized in the RCA [39]. However, the addition of RCA can result in the loss of workability, strength, and durability of concrete owing to the high water demand and numerous defects of RCA [40]. In addition, the self-healing agents loaded in RCA would suffer from the risk of leakage into concrete because the pores of RCA always remain open. To overcome these drawbacks, it is imperative to coat porous RCA with a protective layer of organic or inorganic materials. One promising material is sodium silicate, which is typically used in a solution. Sodium silicate reacts with Ca(OH)₂ in the RCA to produce a C-S-H gel that can be deposited on the surface of the RCA [41].

(1){\mathrm{N}\mathrm{a}}_2{\mathrm{S}\mathrm{i}\mathrm{O}}_3+\mathrm{C}\mathrm{a}{\left(\mathrm{O}\mathrm{H}\right)}_2+ {\mathrm{H}}_2\mathrm{O} \to \mathrm{C}-\mathrm{S} -\mathrm{H}+\mathrm{N} \mathrm{a}\mathrm{O}\mathrm{H}

Therefore, a water-repellent coating layer is expected to form and prevent agent leakage. Unfortunately, concrete self-healing using sodium-silicate-coated RCA as a protective carrier for ureolytic bacteria has not yet been explored.

The purpose of this study was twofold. First, the effects of the types of nutrients and pH values on bacterial reproduction, urease activity, and calcium carbonate precipitation were studied in a culture medium with a simulated concrete pore solution, as nutrients and pH are the most important factors contributing to MICP. Second, RCA coated with sodium silicate was prepared as a protective carrier for the bacterial healing agents. The healing effectiveness was then assessed by crack closure, water tightness, and microstructural analyses of the healing products.

2 Materials and methods

2.1 Bacterial culture and sporulation procedure

The typical alkali-tolerant ureolytic bacterium Sporosarcina pasteurii (S. pasteurii), was obtained from the China General Microbiological Culture Collection Center (CGMCC 1.3687). The bacterial strain was stored in 15% (v/v) glycerol at –80 °C. Upon activation, the bacteria were cultured in a nutrient medium containing 5 g/L soy protein, 15 g/L casein, and 20 g/L urea. Except for urea, other components were sterilized at 120 °C up to 20 min. As urea is heat sensitive, it was sterilized separately by filtration, mixed with other components, and subjected to autoclaving. The pH of the nutrient was modulated to 9.0 ± 0.2 with 1 mol/L sterile HCl or NaOH. The medium was inoculated with bacteria (2% v/v) and then incubated in a 120 r·min⁻¹ shaker up to 48 h at 30 °C. After incubation, cells were collected by centrifugation at 4000× g. The collected cells were rinsed and suspended with the sterile 0.9% NaCl solution, followed by preserving in a 4 °C fridge for future use.

For sporulation, the bacteria were cultivated in a sporulation medium consisting 5 g/L glucose, 10 g/L yeast extract, 0.5 g/L K₂HPO₄, 0.1 g/L MgSO₄, 10 mg/L MnSO₄·H₂O, 7.5 m g/L CaCl₂, and 10 g/L (NH₄)₂SO₄. The pH of the sporulation medium was modulated to 9.0 ± 0.2 by the method mentioned earlier. The medium was inoculated by 2% v/v cell suspension and then incubated at 28 °C, 100 r/min until > 90% of the cells transformed into spores, which were also collected by the centrifuge at 4000× g. Subsequently, the spores were rinsed and re-suspended with the sterile 0.9% NaCl solution. To obtain spores without live cells, the suspension was heated to 80 °C for 20 min in a water bath. The spore concentration was approximately 10⁹ spores/mL. The obtained spores were stored in a 4 °C fridge until further use. By using Schaeffer−Fulton endospores staining method [42], the bacterial cells and spores were observed under an optical microscope using the Schaeffer−Fulton endospore staining method [42], as shown in Fig.1.

2.2 Effect of cultural media on bacterial reproduction and urease activity

The culture medium provided essential nutrients for bacterial germination, growth, metabolism, and urease secretion. Three types of media were used to compare their effects on bacterial reproduction and urease activity. The composition of the medium is shown in Tab.1. Except for ammonium sulfate or urea that are heat sensitive, other culture media ingredients were autoclaved at 121 °C for 20 min. The ammonium sulfate or urea solution was sterilized by filtration, as described in Subsection 2.1. The pH of the media was modulated to 9.0 ± 0.2 by the method, as described in Subsection 2.1. Subsequently, S. pasteurii was inoculated into each medium and incubated at 30 °C, 120 r/min up to 48 h.

A visible spectrophotometer V-1200 was used to measure the bacterial concentration. During measurement, the culture was diluted to a range 0.2 of 0.8 of absorbance. Bacterial concentration Y (cells/mL) was calculated as follows [43]:

(2)Y=8.59\times {10}^7\times {{OD}_{600}}^{1.3627} ,

where OD₆₀₀ is the absorbance degree at 600 nm. The urease secreted by S. pasteurii decomposed urea into \text{N}{\text{H}}_{\text{4}}^{\text{+}} and \text{C}{\text{O}}_{\text{3}}^{\text{2}-}. This increased the electrical conductivity of the solution. Therefore, the urease activity of the bacteria was determined based on the rate of increase in electrical conductivity. After placing 1 mL bacteria suspension in 9 mL urea solution (1.67 mol/L), the electrical conductivity was tested at 25 °C up to 5 min. According to the experiential formula, the specific urease activity (SUA, mM·min⁻¹·cells⁻¹) is given by [44]

(3)SUA=\frac{111.1\sigma }{Y},

where σ is the electrical conductivity, mS·cm⁻¹·min⁻¹.

2.3 Effect of pH on microbially induced carbonate precipitation

To evaluate the effect of the pH of the crack environment on the MICP process, a culture medium containing a simulated concrete pore solution and nutrients with varied initial pH values was used. First, cement paste with a water-to-cement ratio (w/c) of approximately 10 was placed on a shaker at 120 r/min for 2 h. The paste was then vacuum filtered to obtain a mimic concrete pore solution. Using the mimicking concrete pore solution as the solvent, the medium was prepared by adding 15 g/L casein, 5 g/L soy protein, and 20 g/L urea. The sterilization process for the medium was the same as that described in Subsection 2.1. To modulate the initial pH of the medium from 6 to 12, 1 mol/L sterile NaOH or HCl was used. Spores were inoculated into the medium and incubated at 30 °C, 120 r/min for 8 d. Additionally, a control group (denoted as C) with an initial pH adjusted to 12 but without a spore inoculum was set. The variation in pH of each group was monitored using a pH meter (SevenExcellence S400 Benchtop, Mettler-Toledo). Spore viability was evaluated based on the amount of urea degraded according to Douglas and Bremner [45]. The concentration of calcium ions was determined using the ethylenediaminetetraacetic acid disodium (EDTA-2Na) titration method to reflect the formation of CaCO₃ during the MICP process. The initial calcium ion concentration was 0.0227 mol/L. Each group comprised of three replicates.

2.4 Immobilization of healing agents in recycled concrete aggregates

The details of the RCA used in this study can be found in Ref. [32]. The size of the RCAs varied from 0.075 to 5 mm, and the water absorption ratio of the RCAs was 10%, as measured according to the Chinese Standard GB/T 14685-2022. During the immobilization process, the RCA was immersed in a spore suspension containing 45 g/L casein and 15 g/L soy protein until the RCA were fully saturated. After absorption, the RCA were dried in a 45 °C oven to attain a constant weight. The absorption and drying processes were repeated twice. Subsequently, the RCA was coated twice with sodium silicate to prevent leakage of spores and nutrients. After immobilization, 100 g of RCA containing 0.90 g casein, 0.30 g soy protein, and 4 × 10⁹ spores were obtained. The morphology of RCA before and after the coating treatment was observed using scanning electron microscopy (SEM, Nova NanoSEM 450, FEI). Large open pores were found on the surface of the RCA, and sufficient space was provided to store the healing agents (Fig.2(a) and Fig.2(b)). After treatment, the surface was completely covered with a relatively smooth substance, that is, hardened sodium silicate (Fig.2(c) and Fig.2(d)).

2.5 Preparation of self-healing concrete specimens

Five groups of concrete specimens containing different amounts of RCA were fabricated by Portland cement P·I 42.5, natural fine aggregate, and water. The w/c ratio was 0.5, and the sand-to-cement ratio (s/c) was 2.5. Group R was the reference group without the addition of healing components. In Groups B-10, B-20, B-30, and B-40, natural fine aggregates were replaced by 10%, 20%, 30%, and 40% by volume, respectively, of RCA immobilized with microbial healing agents. Except for group R, urea and Ca(CH₃COO)₂·H₂O as nutritional substrate and calcium source were directly incorporated to achieve MICP. A summary of the mix proportioning of the concrete is presented in Tab.2. In each group, two types of specimens–discs and cubes–were fabricated for different testing purposes. The discal specimens with dimension of Φ80 mm × H20 mm were used for the splitting tension test and crack visualization, and four discs were prepared in each group. Cubic samples with dimensions of 50 mm × 50 mm × 50 mm were used for the water uptake test, and three cubes were prepared for each group. For cubes, basalt fibers with 3% cement in mass were added to maintain the integrity of the specimens during the tests. The mixing procedure followed the Chinese standard GB/ T17671-1999. After curing for 24 h at 20 °C and ≥ 95%R.H., the samples were demolded, and then put back until testing.

2.6 Effect of self-healing components on mechanical properties of concrete

To assess the effect of the addition of self-healing components, including coated RCA and healing agents of urea and calcium sources, on the mechanical properties of concrete, splitting tension tests conforming to ASTM C 496 were conducted on the disc specimens at 28 d. In the splitting tensile test, the disks were subjected to a compressive load along two axial lines. The loading was applied at 0.8 MPa/s until the specimens failed. The compressive stress produced a uniform transverse tension stress along the vertical line.

2.7 Crack creation and self-healing incubation

At the end of the splitting tension test, the disc was divided into two halves and rejoined using stainless steel tape. A nail was fixed at one end of the crack to reduce its final width to less than 0.8 mm. The crack creation process is described in our previous study [32]. The cubes with fibers were subjected to compression to produce cracks. A compressive load was applied to the cubes at a computer-controlled speed of 0.001 mm/s. When the compressive load reached its maximum value, the loading was immediately stopped.

After cracking, the specimens were incubated with wet–dry cycles for up to four weeks. During each cycle, the specimens were immersed in water for 16 h and subsequently exposed to air for 8 h. The incubation was conducted in an air-conditioning room at 20 °C, 60% R.H.

2.8 Evaluation of the self-healing efficiency

2.8.1 Visualization and quantification of crack healing

Cracks in the specimens were observed and recorded using an optical microscope equipped with a digital camera. During incubation, the specimens were removed every week and exposed to the air until the surface was completely dry for observation. The size of the cracks was determined using ImageJ software. The crack-healing ratio, CH_r, is given by

(4){CH}_{\text{r}}=\frac{W_{\mathrm{i}}-W_{\mathrm{t}}}{W_{\mathrm{i}}},

where W_i (mm) is the initial crack width and W_t (mm) is the width of the crack after incubation for t days. The crack-area filling ratio CAF_r was assessed as follows:

(5){CAF}_{\text{r}}=\frac{A_{\text{i}}-A_{\text{t}}}{A_{\text{I}}} ,

where A_i (mm²) is the initial area of the crack and A_t (mm²) is the area of the crack after incubation for t d.

2.8.2 Water uptake test

Capillary water uptake tests were performed on 28-d cubic specimens to assess the self-healing efficiency in terms of the water tightness of the concrete. Before the test, the specimens were dried in a 40 °C oven until the mass variation was < 1% within 24 h. The specimen was then immersed in deionized water 3 cm above the upper surface. The cubes were removed at regular intervals to record their weights. Before weighing, a wet towel was used to wipe water drops from the surfaces of the specimens. After weighing, the specimens were placed in a water bath. The water uptake coefficient k, g/(cm²·min²) was calculated according to [46]

(6)\frac{Q}{S}= k\sqrt{t},

where Q (g) is the mass of the specimen at time t (min) and S (mm²) is the surface area of the specimen in contact with water. To quantify the recovery of the water tightness of the specimens by self-healing, the capillary water uptake of the cubes before cracking, after cracking, and after healing was tested sequentially.

2.8.3 Self-healing products analysis

After 28 d of healing, the specimens were broken into pieces and the healing products were collected. The deposits were collected from the wall of crack close to the surface, and then ground to powder with particle size < 75 μm. To identify the mineralogy of the precipitates, the X-ray diffractometry (XRD, DX-2700BH, Haoyuan) with Cu Kα radiation was used. The operating conditions were 40 kV and 40 mA. The scanning rate was set at 5°/min.

The morphology of the healing products was determined using scanning electron microscopy (SEM, Nova NanoSEM 450, FEI). Before measurements, the fragments from the wall of crack was dried in a 40 °C oven up to 3 d. Fragments were mounted on stubs, coated with gold, and observed.

3 Results and discussion

3.1 Effect of cultural media on urease activity

Applying ureolytic bacterial S. pasteurii to self-healing concrete requires high urease activity. The optimal selection of the culture medium from the three types of media based on bacterial reproduction and SUA was performed, as shown in Fig.3. The highest cell concentration was obtained using YAM medium, and the cell concentration increased from 2.07 × 10⁸ cells/mL at 24 h to 3.18 × 10⁸ cells/mL at 48 h. A noticeable increase in cell concentration from 5.83 × 10⁷ to 1.48 × 10⁸ cells/mL after 24 h was also observed in the YUM medium. Although a relatively low cell concentration was observed for the CSM medium, the SUA of bacteria in the CSM medium was the highest, increasing from 24 h to 48 h. In comparison, the SUA levels of bacteria in the YAM and YUM media were relatively lower and decreased from 24 h to 48 h.

According to a previous study [47], when the cell concentration is low, each cell decomposes urea and serves as a nucleation site to form CaCO₃. Once precipitation is initiated, CaCO₃ crystals continue to grow, mainly because of microbial activity. In this process, the specific CaCO₃ precipitation rate, which determines the efficiency of healing of concrete cracks, is highly dependent on SUA but not on the total amount of bacteria. The SUA of bacteria in CSM medium was the highest among the three types of media and increased with time, indicating a high potential for urea decomposition and CaCO₃ precipitation capacity for each cell in CSM. Therefore, CSM was selected as the culture medium for the following experiments.

3.2 Effect of pH on microbially induced carbonate precipitation

The variations in urea decomposition, Ca²⁺ consumption, and pH value as a function of the initial pH in the culture medium containing the simulated concrete pore solution and nutrients are shown in Fig.4. No urea decomposition was observed in control group C. In the presence of spores, approximately 90% of urea degraded in one day when the initial pH was 7 or 8, whereas the percentage of urea decomposition in one day decreased to 74.7% and 76.7%, respectively, if the initial pH was set as 6 and 9. When the initial pH was 10, the urea degradation was apparently retarded that only 9% of urea was decomposed after one day, but over 90% of urea was consumed in two days. A pH value greater than 11 led to the inhibition of urea degradation (Fig.4(a)). These results are in line with prior studies showing that urease activity dropped considerably when the pH was over 10 [47,48]. The variation in Ca²⁺ consumption agrees with that of urea degradation, indicating the transformation of free Ca²⁺ into CaCO₃ during the MICP process (Fig.4(b)). However, there was a slight depletion of Ca²⁺ (up to 10% of total Ca²⁺ in 5 d) in the medium with an initial pH of 11 and 12, even though urea degradation was not observed. This could be due to carbonation occurring in the alkaline medium, as the depletion of Ca²⁺ was also found in the control group without any microorganisms. A similar phenomenon was observed in other studies [49].

The pH evolution showed an interesting tendency; the final pH remained at 9 after one day regardless of the initial pH from 6 to 10 (Fig.4(c)). As confirmed by urea degradation, the spores have good pH adaptability and can germinate, reproduce, and decompose urea. The overall biochemical reactions were as follows [50].

(7)\mathrm{C}\mathrm{O}{\left(\mathrm{N}{\mathrm{H}}_2\right)}_2+{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{B}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{s}\mathrm{e}}{\to }\mathrm{N} {\mathrm{H}}_2\mathrm{C}\mathrm{O}\mathrm{O} \mathrm{H}+{\mathrm{N}\mathrm{H}}_3

(8){\mathrm{N}\mathrm{H}}_2\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H} +{\mathrm{H}}_2\mathrm{O}\to {\mathrm{N}\mathrm{H}}_3+ {\mathrm{H}}_2{\mathrm{C}\mathrm{O}}_3

(9){\mathrm{N}\mathrm{H}}_3+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{N}\mathrm{H}}_4^{+}+{\mathrm{O}\mathrm{H}}^{-}

In solution, urea is first decomposed into carbamic acid and ammonia by bacterial urease (Eq. (7)). The unstable carbamic acid was immediately hydrolyzed to ammonia and carbonic acid (Eq. (8)). The ammonia dissolved in water and formed ammonium and hydroxyl ions (Eq. (9)). The buffering effect of the {\mathrm{N}\mathrm{H}}_4^{+}/NH₃ system was responsible for the stable pH of the solution at approxi-mately 9 [51,52]. In this environment, carbonic acid is deprotonated to form bicarbonate and carbonate ions.

(10){\mathrm{H}}_2{\mathrm{C}\mathrm{O}}_3\leftrightarrow {\mathrm{H}}^{+}+{\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}

(11){\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}\leftrightarrow {\mathrm{C}\mathrm{O}}_3^{2-}+{\mathrm{H}}^{+}

When the ionic activity product (ICP) of calcium and carbonate ions exceeded the solubility constant of CaCO₃, precipitation occurred due to oversaturation. Bacterial cells serve as nucleation sites.

(12){\mathrm{C}\mathrm{e}\mathrm{l}\mathrm{l}-\mathrm{C}\mathrm{a}}^{2+}+{\mathrm{C}\mathrm{O}}_3^{2-}\to \mathrm{C} \mathrm{e}\mathrm{l}\mathrm{l}-\mathrm{C} \mathrm{a}{\mathrm{C}\mathrm{O}}_3\downarrow

To further verify the biochemical process described above, the pH value was manually adjusted to 9 using HCl after 5 days for culture media with an initial pH of 11 and 12 (Fig.4). The urea was completely consumed after one and two days, respectively, for the initial pH 11 and 12. Spores are alkali-tolerant and can survive in highly alkaline environments. Although the activity of spores was temporarily restrained by high alkalinity (pH > 11), the spores were not damaged because urea could still be decomposed if the pH was adjusted to a moderate value (pH = 9). However, spore germination and urea decomposition were delayed after pH adjustment. To address this issue, protective carriers should be used to avoid direct contact of spores with a highly alkaline environment. Our previous study showed that after 7 d of alkaline treatment, spores loaded in porous carriers maintained their urease activity [32].

The products from MICP were collected for XRD analysis. When the pH was > 8, the precipitate was calcite. When the pH was decreased from 6 to 8, vaterite was observed in the precipitates (Fig.5(a)). The ratio of the CaCO₃ polymorphs in the precipitates was then calculated using the Rietveld method based on the XRD data. The effect of the initial pH on the ratio of CaCO₃ polymorphs is illustrated in Fig.5(b). The molar contents of vaterite in products at initial pH 6, 7, and 8 were 13.8%, 18.6%, and 8.6%, respectively, which were significantly lower than calcite.

When the pH was in the range between 6–8, the precipitates contained a small amount of vaterite. At lower pH values (6–8), S. pasteurii has relatively higher urease activity to promote the production of dissolved inorganic carbon (DIC) by urea decomposition [44]. Additionally, less dissolved organic carbon (DOC) was released from the EPS of bacteria in lower pH environments. Therefore, only a small amount of calcium ions was complexed by ionized acidic polar groups in the DOC, resulting in increased calcium ion activity in the solution [53]. Higher DIC and calcium ion activity result in a high degree of supersaturation of CaCO₃, which favors the formation of vaterite [54]. Although vaterite is a thermodynamically unstable mineral that tends to transform to calcite under most natural conditions [55,56], it can remain stable if organic compounds are adsorbed on the surface of the crystals and inhibit the phase transition [57,58]. When the pH exceeded 8, the decrease in urease activity and the increase in ionized acidic polar groups in the DOC favored the formation of calcite [44,53]. An earlier study [59] revealed that the mineralogical phase of microbial products varied from a mixture of vaterite and calcite to calcite with increasing pH.

3.3 Effect of self-healing components on concrete

The 28-d splitting tensile strengths of the discs are shown in Fig.6. The incorporation of self-healing components negatively impacted the tensile strength of concrete. The strength value decreased by 19.6% for specimens containing both coated RCA and healing agents (urea and calcium sources) compared with the reference specimens. When the amount of coated RCA increased up to 40%, the splitting tensile strength decreased monotonically, and the decreasing ratio was 29% compared to the reference.

Our previous study revealed that the strength of concrete was improved by the addition of 3% urea and 4.5% calcium source in mass of cement [60]. In this study, the same amounts of urea and calcium were incorporated into the concrete. The urea and calcium sources did not adversely affect the splitting tensile strength. Therefore, the decrease in the strength of the specimens with self-healing components can only be ascribed to the incorporation of weak RCA. Nevertheless, previous studies confirmed that the addition of RCA had a negligible impact on the strength of concrete, even when the replacement ratio of RCA was up to 30%. Based on the above analysis, the reduction in the splitting tension strength in this study can be attributed to the weak bonding between the treated RCA and the paste matrix. As shown in Fig.2, the treatment of RCA with sodium silicate resulted in a smooth RCA surface, which in turn impaired the bond between RCA and the paste [61]. The splitting tension is sensitive to the weak bonding of the aggregate–paste interface in concrete.

3.4 Investigation of the self-healing effectiveness

3.4.1 Visualization and quantification of crack healing

The evolution of the crack width during crack healing was recorded weekly using an optical microscope (Fig.7). Specimens without microbial healing components (Group R) exhibited very limited crack healing. In the typical microbial group (group B-20), the cracks almost completely healed for up to 3 weeks.

The variations in the crack widths of each group of specimens are summarized in Fig.8. The blue dots represent the initial crack widths, whereas the white and orange dots represent the crack widths after two and four weeks, respectively. The movement of the dots toward the x-axis indicates crack healing. After 2 weeks of wet–dry cycle incubation, limited amount of crack healing was observed for group R specimens, as a few dots moved from their original positions toward the X-axis (Fig.8(a)). A slight decrease was observed over subsequent weeks. The largest width of the cracks that could be healed in group R was approximately 0.253 mm. The crack healing of the specimens without any healing components was attributed to autogenous healing of the concrete matrix. Autogenous healing mechanisms primarily include the continuous hydration of unhydrated clinker and the carbonation of hydration products [62]. Nevertheless, the upper limit of crack widths that can be healed by autogenous self-healing is usually low (less than 0.3 mm in most cases) [62].

Notably, the specimens with microbial healing components achieved more pronounced crack healing after 2 weeks of wet–dry cycles incubation as compared with the one without microbial healing components (Fig.8(b)–Fig.8(e)). Sustained healing was also clearly observed for up to 4 weeks. After 28 d, the largest crack widths were 0.431, 0.580, 0.582, and 0.632 mm for groups B-10, B-20, B-30, and B-40, respectively. These results verified that the sodium silicate coating formed on the RCA surface could break upon concrete cracking. The bacterial spores and nutrients loaded into the RCA were then released. In the presence of water and air, the spores germinated into vegetative cells and triggered MICP to heal the cracks.

Fig.9 summarizes the mean crack-healing ratio as a function of both the initial crack width and incubation time. As the initial crack width increased, the healing ratio decreased. The microbial groups achieved much better healing effectiveness than the reference group, which is consistent with the results mentioned earlier. Specifically, the increase in the mean healing ratio with time for group R did not exhibit a pronounced tendency until the initial crack widths less than 0.3 mm. The mean healing ratio of group R achieved 57% if the initial widths of crack were from 0.2 to 0.3 mm, and it dropped to only 11% if the initial widths of crack were > 0.6 mm. The microbial groups had a significantly higher mean healing ratio than group R, particularly for initial crack widths of < 0.5 mm. For instance, the mean healing ratio was approximately 90% from 7 to 28 d for the group B-20, with initial widths of crack between 0.3–0.4 mm. In addition, a continuous increase in the mean healing ratio was observed for microbial groups with initial crack widths of < 0.6 mm. When the initial crack were > 0.6 mm, the mean healing ratio increased, except for group B-40, whose healing agent was sufficient to fill larger cracks. For group B-40, the mean healing ratio of cracks with initial widths larger than 0.6 mm was over 50% at 28 d. Nevertheless, a higher content of self-healing components is harmful to concrete.

The crack area filling ratio after healing for four weeks is presented in Fig.10. Notably, the specimens with microbial healing components showed significantly higher crack area filling ratio compared with reference specimens. At 28 d, a maximum ratio of 86.4% was achieved in group B-20, whereas only 33.7% of crack area was filled for reference. Nevertheless, increased RCA replacement did not result in an increased crack-filling ratio, even though more microbial healing agents were used. This could be because more voids and defects, which formed along with the addition of coated RCA, also required filling by MICP. Similar findings have been previously reported [63,64].

3.4.2 Water uptake

The water-uptake curves of the intact specimens are shown in Fig.11(a). The specimens without healing components exhibited the highest water-uptake capacity. The incorporation of healing components significantly reduced the water uptake, and group B-10 showed the lowest water uptake capacity. However, water uptake gradually increased with increasing amounts of healing components. Water uptake is a liquid transport process caused by capillary suction and is highly dependent on the connectivity and volume of pores or cracks in a system [65–67]. The reduction in the water uptake in the presence of healing components indicates the improved compactness of the concrete matrix. This could be attributed to the improved fluidity of the concrete mixture, such as urea, which is a part of the healing components that has a water-reducing effect. In addition, the surface of the RCA after the treatment was smooth, which also had the effect of water reduction. By contrast, the treated RCA was weakly bonded to the matrix, leading to an increase in interfacial cracks and an enhanced capacity for water uptake.

Once cracked, each specimen exhibited five to seven cracks on its surface. The water uptake capacity of all specimens significantly increased owing to the presence of large cracks (Fig.11(b)). After healing for 28 d, all the specimens showed a reduction in water uptake (Fig.11(c)).

A closer inspection of Fig.11(a)–Fig.11(c) reveals that all the water uptake curves deviate from the linear relationship with t^1/2, which is inconsistent with the theoretical Eq. (5). Previous studies have discussed this anomalous absorption behavior of cement-based materials and ascribed it to the expansion of hardened cement paste upon water uptake; however, their experimental period was up to 400 h [68]. In this study, the anomalous water uptake over a much shorter period can be attributed to water transportation in pores or cracks of various sizes. According to the slope of the curves, the process can be divided into rapid absorption, transition, and slow absorption stages (Fig.11(d)). Rapid absorption and slow absorption are related to the penetration of water into large voids or cracks in the short-term and small capillary pores in the long-term, respectively.

By using Eq. (6), the initial sorptivity coefficient and the secondary sorptivity coefficient, which refer to the rapid and slow water uptake, respectively, were then calculated, and the results are shown in Fig.12(a) and 12(b). To further analyze the effect of cracking and self-healing on water uptake, an indicator named the water tightness recovery ratio r_C, % was calculated according to

(13)r_{\mathrm{C}}= \frac{k_{\mathrm{a}\mathrm{c}}-k_{\mathrm{a}\mathrm{h}}}{k_{\mathrm{a}\mathrm{c}}-k_{\mathrm{b}\mathrm{c}}}\times 100 \mathrm{\%},

where k_bc, k_ac, and k_ah denote the initial and secondary sorptivity coefficients before and after cracking and healing, respectively. A higher r_C value indicates better healing effectiveness in terms of resistance to water ingress. The r_C values are shown in Fig.12(c). For rapid water uptake over a short period, a significantly higher r_C was observed for the microbial groups than for the reference group. The highest r_C value was obtained in group B-20 (74.4%), which was approximately 1.6 times as much as that of the reference (46%). The calculated r_C for the initial sorptivity coefficient was highly consistent with the crack-area-filling ratio, which further verified that large cracks were healed effectively by MICP, impeding the rapid water uptake process.

As for the slow water uptake in the long-term, the r_C value of group R was close to zero, indicating an unchanged capillary pore structure after the healing of specimens with microbial healing components. The r_C of groups B-10 and B-20 were 18.0% and 22.9%, respectively. Microbial precipitation during the crack healing process is responsible for improving r_C of slow absorption, as the microbial precipitates could block the capillary pores to some degree. However, an increased amount of RCA resulted in a negligible r_C value during long-term water uptake, as an excessive amount of weak bonding between RCA and the matrix was formed.

3.4.3 Analysis on self-healing precipitates

The healing products collected from the walls of the cracks were analyzed using XRD. As shown in Fig.13, the main mineralogical phases of the precipitates in both the reference and microbial groups were quartz and calcite. Except for the quartz that comes from sand, the presence of calcite on the crack wall of the reference specimens was attributed to the carbonation of Ca(OH)₂, one of the major hydration products in concrete. Although the dissolution of CO₂ usually limits the carbonation process from the air and the leaching of Ca(OH)₂ from the matrix [62], the absence of Ca(OH)₂ indicates the full carbonation of the hydration products on the crack wall. For group B-20, a large amount of microbially induced calcite was produced, as higher peaks corresponding to calcite were observed. No vaterite was found on the cracked wall because the pH of the concrete environment was often higher than 12 [9].

The morphologies of the cracked walls after incubation are shown in Fig.14. Reticular networks were identified on the crack walls of Group R, indicating the presence of C-S-H gel as the main constituent of the hydration products. In addition, small rhombohedral particles were observed, indicating the presence of calcite. In contrast, several irregular rhombohedral crystals were observed on the crack walls in Group B-20. The SEM images are consistent with the XRD results. In addition, the microbial healing products were closely packed on the crack wall and blocked voids inside the crack, which provided evidence to explain the phenomenon that the water-tightness recovery ratio was reduced for long-term water uptake [32,69].

4 Conclusions

1) The MICP process was inhibited when the pH was above 11 because the urease activity was restrained in a highly alkaline environment. Both vaterite and calcite was produced by MICP when the pH was lower than 8, whereas only calcite was produced when the pH was higher than 8.

2) The self-healing components consist of RCA loaded with bacterial spores, nutrients, and precipitation precursors, including urea and calcium sources. The RCA was coated with sodium silicate to prevent the leakage of healing agents. The addition of self-healing components resulted in a reduction of the splitting tension strength of the concrete specimens from 19.6% to 29% when the replacement ratio of RCA to sand increased from 10% to 40%.

3) The microbial self-healing specimens incorporating 20% RCA achieved a mean healing ratio of approximately 90% as early as 7 d if the initial crack widths were between 0.3 and 0.4 mm, whereas its crack area filling ratio reached 86.4% at 28 d, which was the highest among all the groups. Accordingly, the highest water tightness recovery ratio was also obtained for the microbial self-healing specimens with 20% RCA, regardless of the short- or long-term water uptake.

4) This study suggests that RCA coated with sodium silicate is an effective method for packaging microbial healing agents and has the potential to develop cost-effective self-healing concrete.

References

[1] Zhang Y, Zhang S Q, Jiang X, Chen Q, Jiang Z W, Ju J W, Bauchy M. Insights into the thermal effect on the fracture toughness of calcium silicate hydrate grains: A reactive molecular dynamics study. Cement and Concrete Composites, 2022, 134: 104824

[2] Zhang Y, Ju J W, Zhu H H, Yan Z G. A novel multi-scale model for predicting the thermal damage of hybrid fiber-reinforced concrete. International Journal of Damage Mechanics, 2020, 29(1): 19–44

[3] Zhu H H, Chen Q, Yan Z G, Ju J W, Zhou S. Micromechanical models for saturated concrete repaired by the electrochemical deposition method. Materials and Structures, 2014, 47(6): 1067–1082

[4] Yan Z G, Chen Q, Zhu H H, Ju J W, Zhou S, Jiang Z W. A multi-phase micromechanical model for unsaturated concrete repaired using the electrochemical deposition method. International Journal of Solids and Structures, 2013, 50(24): 3875–3885

[5] Zhu H H, Zhou S, Yan Z G, Ju J W, Chen Q. A two-dimensional micromechanical damage-healing model on microcrack-induced damage for microcapsule-enabled self-healing cementitious composites under tensile loading. International Journal of Damage Mechanics, 2015, 24(1): 95–115

[6] Zhou S, Zhu H H, Ju J W, Yan Z G, Chen Q. Modeling microcapsule-enabled self-healing cementitious composite materials using discrete element method. International Journal of Damage Mechanics, 2017, 26(2): 340–357

[7] Xu J, Yao W. Multiscale mechanical quantification of self-healing concrete incorporating non-ureolytic bacteria-based healing agent. Cement and Concrete Research, 2014, 64(1): 1–10

[8] Xu J, Tang Y, Wang X, Wang Z, Yao W. Application of ureolysis-based microbial CaCO3 precipitation in self-healing of concrete and inhibition of reinforcement corrosion. Construction & Building Materials, 2020, 265: 120364

[9] Xu J, Wang X, Wang B. Biochemical process of ureolysis-based microbial CaCO₃ precipitation and its application in self-healing concrete. Applied Microbiology and Biotechnology, 2018, 102(7): 3121–3132

[10] Zhuang X Y, Zhou S. The prediction of self-healing capacity of bacteria-based concrete using machine learning approaches. Computers, Materials & Continua, 2019, 59(1): 57–77

[11] Chen Q, Xie L J, Huang A L, Li B, Sun Y X, Jiang Z W, Li W T, Zhu H H. Healing of concrete cracks by in-situ synthesis of ettringite induced by electric field. Construction & Building Materials, 2022, 352: 128685

[12] Boquet E, Boronat A, Ramos-Cormenzana A. Production of Calcite (Calcium Carbonate) Crystals by Soil Bacteria is a General Phenomenon. Nature, 1973, 246(5434): 527–529

[13] Hammes F, Verstraete W. Key roles of pH and calcium metabolism in microbial carbonate precipitation. Reviews in Environmental Science and Biotechnology, 2002, 1(1): 3–7

[14] Castro-Alonso M J, Montanez-Hernandez L E, Sanchez-Munoz M A, Franco M R M, Narayanasamy R, Balagurusamy N. Microbially induced calcium carbonate precipitation (MICP) and its potential in bioconcrete: Microbiological and molecular concepts. Frontiers in Materials, 2019, 6: 126

[15] Lothenbach B, Le Saout G, Gallucci E, Scrivener K. Influence of limestone on the hydration of Portland cements. Cement and Concrete Research, 2008, 38(6): 848–860

[16] Basheer L, Kropp J, Cleland D J. Assessment of the durability of concrete from its permeation properties: A review. Construction & Building Materials, 2001, 15(2−3): 93–103

[17] Jiang L, Jia G, Wang Y, Li Z. Optimization of Sporulation and Germination Conditions of Functional Bacteria for Concrete Crack-Healing and Evaluation of their Repair Capacity. ACS Applied Materials & Interfaces, 2020, 12(9): 10938–10948

[18] Ersan Y C, Hernandez-Sanabria E, Boon N, de Belie N. Enhanced crack closure performance of microbial mortar through nitrate reduction. Cement and Concrete Composites, 2016, 70: 159–170

[19] Zheng T, Su Y, Zhang X, Zhou H, Qian C. Effect and mechanism of encapsulation-based spores on self-healing concrete at different curing ages. ACS Applied Materials & Interfaces, 2020, 12(47): 52415–52432

[20] Xu J, Wang X. Self-healing of concrete cracks by use of bacteria-containing low alkali cementitious material. Construction & Building Materials, 2018, 167: 1–14

[21] Phillips A J, Gerlach R, Lauchnor E, Mitchell A C, Cunningham A B, Spangler L. Engineered applications of ureolytic biomineralization: A review. Biofouling, 2013, 29(6): 715–733

[22] Wang J, Mignon A, Trenson G, Van Vlierberghe S, Boon N, De Belie N. A chitosan based pH-responsive hydrogel for encapsulation of bacteria for self-sealing concrete. Cement and Concrete Composites, 2018, 93: 309–322

[23] Jonkers H M, Thijssen A, Muyzer G, Copuroglu O, Schlangen E. Application of bacteria as self-healing agent for the development of sustainable concrete. Ecological Engineering, 2010, 36(2): 230–235

[24] Tang Y H, Xu J. Application of microbial precipitation in self-healing concrete: A review on the protection strategies for bacteria. Construction & Building Materials, 2021, 306: 124950

[25] Han S, Jang I, Choi E K, Park W, Yi C, Chung N. Bacterial self-healing performance of coated expanded clay in concrete. Journal of Environmental Engineering, 2020, 146(7): 04020072

[26] Yuan H, Zhang Q, Hu X, Wu M, Zhao Y, Feng Y, Shen D. Application of zeolite as a bacterial carrier in the self-healing of cement mortar cracks. Construction & Building Materials, 2022, 331: 127324

[27] Tan L, Reeksting B, Ferrandiz-Mas V, Heath A, Gebhard S, Paine K. Effect of carbonation on bacteria-based self-healing of cementitious composites. Construction & Building Materials, 2020, 257: 119501

[28] Alazhari M, Sharma T, Heath A, Cooper R, Paine K. Application of expanded perlite encapsulated bacteria and growth media for self-healing concrete. Construction & Building Materials, 2018, 160: 610–619

[29] Jiang L, Jia G, Jiang C, Li Z. Sugar-coated expanded perlite as a bacterial carrier for crack-healing concrete applications. Construction & Building Materials, 2020, 232: 117222

[30] Zhang J, Zhao C, Zhou A, Yang C, Zhao L, Li Z. Aragonite formation induced by open cultures of microbial consortia to heal cracks in concrete: Insights into healing mechanisms and crystal polymorphs. Construction & Building Materials, 2019, 224: 815–822

[31] Su Y, Li F, He Z, Qian C. Artificial aggregates could be a potential way to realize microbial self-healing concrete: An example based on modified ceramsite. Journal of Building Engineering, 2021, 35: 102082

[32] Wang X, Xu J, Wang Z, Yao W. Use of recycled concrete aggregates as carriers for self-healing of concrete cracks by bacteria with high urease activity. Construction & Building Materials, 2022, 337: 127581

[33] Zhang Y, Zhang S, Zhao W, Jiang X, Chen Y, Hou J, Wang Y, Yan Z, Zhu H. Influence of multi-scale fiber on residual compressive properties of a novel rubberized concrete subjected to elevated temperatures. Journal of Building Engineering, 2023, 65: 105750

[34] Poon C S, Shui Z H, Lam L. Effect of microstructure of ITZ on compressive strength of concrete prepared with recycled aggregates. Construction & Building Materials, 2004, 18(6): 461–468

[35] Huang F, Zhou S. A Review of lightweight self-healing concrete. Materials (Basel), 2022, 15(21): 7572

[36] Jiang X, Xiao R, Bai Y, Huang B S, Ma Y T. Influence of waste glass powder as a supplementary cementitious material (SCM) on physical and mechanical properties of cement paste under high temperatures. Journal of Cleaner Production, 2022, 340: 130778

[37] Jiang X, Zhang Y, Zhang Y, Ma J, Xiao R, Guo F, Bai Y, Huang B. Influence of size effect on the properties of slag and waste glass-based geopolymer paste. Journal of Cleaner Production, 2023, 383: 135428

[38] Jiang X, Xiao R, Zhang M M, Hu W, Bai Y, Huang B S. A laboratory investigation of steel to fly ash-based geopolymer paste bonding behavior after exposure to elevated temperatures. Construction & Building Materials, 2020, 254: 119267

[39] Khushnood R A, Qureshi Z A, Shaheen N, Ali S. Bio-mineralized self-healing recycled aggregate concrete for sustainable infrastructure. Science of the Total Environment, 2020, 703: 135007

[40] Mefteh H, Kebaïli O, Oucief H, Berredjem L, Arabi N. Influence of moisture conditioning of recycled aggregates on the properties of fresh and hardened concrete. Journal of Cleaner Production, 2013, 54: 282–288

[41] Shi C, Li Y, Zhang J, Li W, Chong L, Xie Z. Performance enhancement of recycled concrete aggregate—A review. Journal of Cleaner Production, 2016, 112: 466–472

[42] OktariASupriatin VKamalMSyafrullahH. The bacterial endospore stain on schaeffer fulton using variation of methylene blue solution. In: 3rd International Seminar on Mathematics, Science, and Computer Science Education (MSCEIS). Bandung: IOP Publishing Ltd, 2016

[43] Ramachandran S K, Ramachandran V, Bang S S. Remediation of concrete using microorganisms. ACI Materials Journal, 2001, 98(1): 3–9

[44] Whiffin V S. Microbial CaCO3 precipitation for the production of biocement. Dissertation for the Doctoral Degree. Murdoch: Murdoch University, 2004

[45] Douglas L A, Bremner J M. Extraction and colorimetric determination of urea in soils. Soil Science Society of America Journal, 1970, 34(6): 859–862

[46] de Muynck W, Debrouwer D, de Belie N, Verstraete W. Bacterial carbonate precipitation improves the durability of cementitious materials. Cement and Concrete Research, 2008, 38(7): 1005–1014

[47] Stocks-Fischer S, Galinat J K, Bang S S. Microbiological precipitation of CaCO₃. Soil Biology & Biochemistry, 1999, 31(11): 1563–1571

[48] Ersan Y C, Verbruggen H, de Graeve I, Verstraete W, de Belie N, Boon N. Nitrate reducing CaCO₃ precipitating bacteria survive in mortar and inhibit steel corrosion. Cement and Concrete Research, 2016, 83: 19–30

[49] Xu J, Du Y, Jiang Z, She A. Effects of calcium source on biochemical properties of microbial CaCO₃ precipitation. Frontiers in Microbiology, 2015, 6: 1366

[50] de Muynck W, de Belie N, Verstraete W. Microbial carbonate precipitation in construction materials: A review. Ecological Engineering, 2010, 36(2): 118–136

[51] Mitchell A C, Phillips A, Schultz L, Parks S, Spangler L, Cunningham A B, Gerlach R. Microbial CaCO₃ mineral formation and stability in an experimentally simulated high pressure saline aquifer with supercritical CO₂. International Journal of Greenhouse Gas Control, 2013, 15: 86–96

[52] Lauchnor E G, Topp D M, Parker A E, Gerlach R. Whole cell kinetics of ureolysis by Sporosarcina pasteurii. Journal of Applied Microbiology, 2015, 118(6): 1321–1332

[53] Tourney J, Ngwenya B T. Bacterial extracellular polymeric substances (EPS) mediate CaCO₃ morphology and polymorphism. Chemical Geology, 2009, 262(3−4): 138–146

[54] Meldrum F C, Cölfen H. Controlling mineral morphologies and structures in biological and synthetic systems. Chemical Reviews, 2008, 108(11): 4332–4432

[55] Spanos N, Koutsoukos P G. The transformation of vaterite to calcite: effect of the conditions of the solutions in contact with the mineral phase. Journal of Crystal Growth, 1998, 191(4): 783–790

[56] Nehrke G, Van Cappellen P. Framboidal vaterite aggregates and their transformation into calcite: A morphological study. Journal of Crystal Growth, 2006, 287(2): 528–530

[57] Rodriguez-Navarro C, Rodriguez-Gallego M, Ben Chekroun K, Gonzalez-Muñoz Maria T. Conservation of ornamental stone by myxococcus xanthus—Induced carbonate biomineralization. Applied and Environmental Microbiology, 2003, 69(4): 2182–2193

[58] Kawaguchi H, Hirai H, Sakai K, Sera S, Nakajima T, Ebisawa Y, Koyama K. Crystallization of inorganic compounds in polymer solutions. Part I: Control of shape and form of calcium carbonate. Colloid & Polymer Science, 1992, 270(12): 1176–1181

[59] Lv C, Tang C S, Zhu C, Li W Q, Chen T Y, Zhao L, Pan X H. Environmental dependence of microbially induced calcium carbonate crystal precipitations: experimental evidence and insights. Journal of Geotechnical and Geoenvironmental Engineering, 2022, 148(7): 04022050

[60] Xu J, Tang Y, Wang X. A correlation study on optimum conditions of microbial precipitation and prerequisites for self-healing concrete. Process Biochemistry (Barking, London, England), 2020, 94: 266–272

[61] Ozbakkaloglu T, Gholampour A, Xie T. Mechanical and durability properties of recycled aggregate concrete: Effect of recycled aggregate properties and content. Journal of Materials in Civil Engineering, 2018, 30(2): 04017275

[62] de Belie N, Gruyaert E, Al-Tabbaa A, Antonaci P, Baera C, Bajare D, Darquennes A, Davies R, Ferrara L, Jefferson T, Litina C, Miljevic B, Otlewska A, Ranogajec J, Roig-Flores M, Paine K, Lukowski P, Serna P, Tulliani J M, Vucetic S, Wang J, Jonkers H M. A Review of self-healing concrete for damage management of structures. Advanced Materials Interfaces, 2018, 5(17): 1800074

[63] Mirshahmohammad M, Rahmani H, Maleki-Kakelar M, Bahari A. Effect of sustained service loads on the self-healing and corrosion of bacterial concretes. Construction & Building Materials, 2022, 322: 126423

[64] Li X H, Liu R T, Li S C, Zhang C Y, Yan J, Liu Y K, Sun X B, Su P S. Properties and mechanism of self-healing cement paste containing microcapsule under different curing conditions. Construction & Building Materials, 2022, 357: 129410

[65] Hanžič L, Kosec L, Anžel I. Capillary absorption in concrete and the Lucas–Washburn equation. Cement and Concrete Composites, 2010, 32(1): 84–91

[66] Hall C. Water sorptivity of mortars and concretes: A review. Magazine of Concrete Research, 1989, 41(147): 51–61

[67] Benavente D, Lock P, Ángeles García Del Cura M, Ordóñez S. Predicting the capillary imbibition of porous rocks from microstructure. Transport in porous media, 2002, 49(1): 59–76

[68] Alderete N M, Villagrán Zaccardi Y A, De Belie N. Physical evidence of swelling as the cause of anomalous capillary water uptake by cementitious materials. Cement and Concrete Research, 2019, 120: 256–266

[69] Zheng T, Su Y, Qian C, Zhou H. Low alkali sulpho-aluminate cement encapsulated microbial spores for self-healing cement-based materials. Biochemical Engineering Journal, 2020, 163: 107756

RIGHTS & PERMISSIONS

Higher Education Press