Mechanical Properties of Self-Healing Concrete with Dawson Microcapsule

Abstract

Concrete structure integrity is significantly compromised by the primary problem of cracking. Typically, surface cracking (predominantly shrinkage-induced and thermal microcracking) is rectified using costly and time-consuming repair methods involving mortar and other techniques. Research efforts have recently shifted towards developing smart materials to reduce concrete’s propensity for cracking, enhance its structural stability, and prevent damage to its framework. Concrete designs with self-healing capabilities can safeguard against degradation and enhance long-term durability. Despite extensive research, a consensus on the optimal preparation and mechanical properties of self-healing concrete has yet to be reached. Within self-healing concrete that utilizes microcapsules, repair agents are dispersed throughout the matrix to form a bond and seal cracks as damage develops. From the viewpoint of a sustainable society, this approach appears to promote the use of construction materials. This study examined the impact of Dawson/urea–formaldehyde microcapsule-based self-healing concrete using strength tests, where the effectiveness of different microcapsule quantities (0.5–2% microcapsule by weight of cement) was assessed. Following the data and data analysis, it becomes evident that among all samples, the 1% microcapsule sample yields outstanding results for both 7-day and 28-day compressive strength.

1. Introduction

Concrete is prone to cracking, which greatly reduces its stability and has a substantial influence on its manufacturing and application processes [1,2]. Every year, a significant amount of money and resources are spent on repairing cracks in concrete throughout all countries [3,4]. In recent years, researchers have introduced smart materials capable of significant achievement recognition and autonomously repairing cracks in order to tackle concrete’s critical cracking issue. For the first time, the notion of self-healing concrete was expressed by Carolyn Dry [5] in 1994. Self-healing in concrete is categorized into two types: autonomous and autogenous healing [6]. Autonomous healing concrete primarily focuses on the inherent self-healing properties of the concrete, which primarily involve further hydration of cement-based materials through processes like carbonation and pozzolanic reaction [7]. Generally, the inherent healing capabilities of the matrix are not sufficient to counteract additional cracking and deterioration of the concrete. Artificial designs of autogenous healing involve the supplementation of additional components with the specific purpose of facilitating the healing process. The designs encompass non-encapsulated crack self-healing, encapsulated self-healing, and microbial self-healing. Capsule methods are typically either chemically based or biologically based in nature. The self-healing capabilities of the capsule method require a chemical reaction between the core material and the surrounding environment to function effectively [8,9]. In this instance, the process of packaging container damage is a distinct factor in healing, and it is classified independently; it mechanistically replicates the rupture of a blood vessel in the event of an injury, thereby releasing healing compounds. Capsule methods can be constructed using either liquid tubes or microcapsules, depending on the type of container employed. The primary components of the liquid tube method are organic polymer substances. Previous research on microcapsule techniques has employed inorganic materials in restorative applications [10,11]. In the microcapsule process, the healing agent is embedded within the solid matrix substance. By applying concentrated tension, the microcapsules can effectively seal the crack tip and then release the healing agent, which enables the repair of damage to occur. White initially examined the microencapsulation approach using urea–formaldehyde as the shell material and dicyclopentadiene as the sore salve material [12]. Following this process, the substance osmoses into the matrix, undergoes a gradual hardening, and eventually fills the crack. This process efficiently modifies material specifications. Advances in microcapsule technology have led to the development of a broad variety of microcapsules. Sodium silicate microcapsules are frequently used as core materials [13], epoxy resin microcapsules [14,15], isocyanate microcapsules [16], and other repair agents [17]. The wall materials most frequently utilized include urea–formaldehyde resin [16,17] and polyurethane [18]. The cement’s self-healing functionality is influenced by several factors, including the amount and scale of microcapsules present, the level of damage incurred, and the length and circumstances of the healing process itself. Studies have shown that nano metal oxides and nano oxides possess the capacity to substantially boost the compressive strength of both concrete and cement mortars. As a result, they are highly recommended for use as additives in cement and concrete formulations [18,19,20]. Polyoxometalates are metal oxides that exhibit a range of characteristics and structures. These include nano-sized conduction–metal–oxygen clusters and classical solid oxides, classified as polyoxometalates [21]. Many polyoxometalates offer cost advantages and environmentally friendly properties, making them highly beneficial from both economic and environmental viewpoints [22]. A novel approach could be developed, potentially by incorporating them into the production process, due to their ability to enhance the structural properties of concrete. In our previous work, we explored the potential of Preyssler polyoxometalate in advancing nanotechnology research [23], and with our continued focus on expanding the uses of polyoxometalates, we recognized it as a priority to examine their effectiveness in enhancing the compressive strength of concrete. This research aimed to investigate the impact of Dawson heteropolyacid utilization as microcapsules in concrete, a crucial building material. The primary goal of this study was to establish whether microcapsules of Dawson could enhance compressive strength and contribute to a more robust concrete. The article is divided into six main sections, including Introduction, Experiment, Morphology of Microcapsules, Mechanical Properties of Concrete Containing Microcapsules, Strength Repair Performance of Self-Healing Concrete, and Conclusion, and seven sub-sections, including Materials, Preparation of Microcapsules, Preparation of Concrete, Reactive Powder Concrete Samples, Evaluation of Mechanical Properties and Strength Recovery after Damage, Effect of Microcapsule Amount on Compression Test, and Before and After Repair. In Figure 1, a diagrammatic microcapsule-based self-healing method is displayed.

2. Experimental Procedure

The authors introduce all the materials, tools, facilities, and methods that were used in this section.

2.1. Materials

The raw materials necessary for the synthesis of microcapsules comprise resorcinol (C₆H₆O₂), formaldehyde (CH₂O), ammonium chloride (NH₄Cl), urea (CO(NH₂)₂), hexane (CH₃(CH₂)₄CH₃), span 60 (C₂₄H₄₆O₆), sulfonic acid (R-C₆H₆-OSO₃H, R=C₁₀-C₁₃), sodium tungstate dehydrate (Na₂WO₄.2H₂O), phosphoric acid (H₃PO₄), and potassium chloride (KCL).

2.2. Microcapsules Preparation

Key components of microcapsules are their central core and outer wall. Optimizing the ratio of core to wall within a suitable range is essential for the efficient creation of self-healing microcapsules. The established ratio guarantees that damaged microcapsules successfully repair fractured concrete without compromising the integrity of those that remain unbroken. The core and wall need to have a certain level of chemical stability and be resistant to reactions with surrounding materials [24,25,26]. During the microcapsule preparation process, it is essential that the wall material undergoes complete polymerization and is securely bonded to the core material. The wall must possess sufficient strength to maintain the core material’s integrity throughout concrete preparation without exerting excessive force that could compromise the microcapsule’s integrity when the concrete cracks. The Wells–Dawson species, with the formula [P₂W₁₈O₆₂]⁶⁻, was prepared as per the method outlined in reference [27]. An aqueous solution of the α/β K₆P₂W₁₈O₆₂·10H₂O salt was first prepared, then treated with ether and a concentrated 37% HCl solution. The structure is approximately 1 nanometer in diameter and is composed of WO₆ units. It comprises a cluster which shares the WO₆ units at the edges and corners, encompassing two central tetrahedral PO₄ units [28].

The Dawson microcapsules were synthesized via the use of water-in-oil suspension polymerization, where the shell material was composed of urea–formaldehyde. The microcapsules’ shell materials and the Dawson healing agent comprised the monomer phase. To create an aqueous solution, combine 5 g of urea, 0.5 g of ammonium chloride, 10 g of Dawson compound, 13 g of formaldehyde, and 0.5 g of resorcinol in 50 g of water. The mixture should then be mixed at 750 rpm for a duration of 1–3 h. For the oil phase preparation, 0.1 g of sulfonic acid, 180 g of hexane, and 0.5 g of Span60 were blended over a 24 h period at a temperature of 40 °C and a rotational speed of 800–1500 rpm. Following this, sonicate the obtained solution for a period of 10 min at a 50% amplitude in a pulsed mode. The reaction rate is directly affected by the sulfonic acid catalyst. Hexane was selected for the oil phase due to its affordability and low weight. The ability of the substance to change quickly into a gas is helpful for collecting the microcapsule. The placement of the vial under a vapor hood facilitates the simple removal of hexane via evaporation. To achieve a successful water-in-oil emulsion, it is essential to keep the water-to-oil ratio at approximately 1:3. The oil phase must be carefully heated to a temperature between 40 and 50 °C before being well-mixed with a high-shear mixer. High temperatures must be avoided as they can disrupt the chemical reaction rate and cause the shell wall to form too quickly. Once the preferred temperature is reached, the aqueous phase should be added gradually, one drop at a time, to initiate the polymerization process. The time taken for heating typically falls within a 1–3 h timeframe before the urea–formaldehyde in situ condensation process is complete, resulting in the conversion of aqueous phase liquid droplets into microcapsules with a solid polymer coating. After the reaction is finished, hexane is isolated and the mixture of microcapsules is filtered. After that, the oven was used for drying the filtered microcapsules for 48 h at 50 °C. The synthesis procedure of microcapsules is illustrated in Figure 2. In Figure 3, synthesized microcapsules are displayed. Dawson microcapsules used in concrete are typically spherical particles with smooth or slightly rough surfaces, ranging in size from 10 to 200 μm, and exhibit a relatively low bulk density due to their hollow core structure. Physically, they are stable under normal concrete mixing and curing conditions and can encapsulate a significant fraction of healing agents, allowing controlled release when cracks form. Chemically, the microcapsule shell is usually such as that of urea–formaldehyde, providing resistance to alkali environments and moisture in concrete, while preventing premature reaction or degradation of the encapsulated agent. Their shell can interact with the concrete matrix, ensuring compatibility and sustained release of the healing agent over time, which enhances the self-healing capacity of the cementitious material.

2.3. Concrete Preparation

Reactive Powder Concrete Specimens

Concrete known as Reactive Powder Concrete (RPC) was first introduced in 1990 in France. This type of concrete is renowned for its exceptional strength and malleability. The compressive strength of RPC exceeds 200 MPa. Utilizing very fine grains with low water–cement ratios is a defining feature of this particular type of concrete [29]. The primary components of RPC include Portland cement, very fine silica powder with a particle size of less than 600 micro meters, micro-silica, water, superplasticizer. The article used Portland cement type II with a density of 3.15 gr/cm², which was sourced from local markets in the area where the experiments took place. The data presented in

Table 1. Concrete mix design.

Materials	Amount in kg/m3
Cement (kg)	840
Water (kg)	200
Silica sand (kg)	1104
Admixture1 (Superplasticizer polycarboxylate-based product)	42
Admixture2 (Microcapsule concentration 1 (%))	0.5, 1, 1.5 and 2%
Micro-silica (kg)	202

¹ the quantity is expressed as a weight percentage of cement.

illustrate the blend composition that used RPC with a water-to-cement ratio of 0.23. All the superplasticizers (polycarboxylate-based admixture) were combined with the necessary amount of water; then, the dry materials were mixed for 5 min. At 1000 rpm for 3 min, the mixer combined the materials and 65% water and concrete lubricant were then added. The mixing process continued at 1000 rpm for a further 8 min, with the remaining water and superplasticizer being added. The polymerized microcapsules were then added to the mixture at the end. After completing the slump test, cube specimens of 50 × 50 × 50 mm³ in size were prepared for the measurement of the 7- and 28-day compressive strength, with three specimens for each percentage. The specimens were then wrapped in wet sackcloth and left for 24 h (Figure 4). The specimens were fully submerged in a water–lime solution for the curing process, as depicted in Figure 5. The curing medium was a 0.5% Ca(OH)₂ solution by mass, providing a stable alkaline environment for hydration and self-healing reactions.

Five different types of concrete samples were prepared as follows:

Mode I: The mixture consists of concrete, with 0.5% of microcapsules, denoted as D0.5%.

Mode II: Consists of 1% (D1%) concrete.

Mode III: This mode includes the concrete, into which 1.5% by weight of microcapsules (as represented by D1.5%) has been incorporated.

Mode IV: This mode includes concrete that contains 2% microcapsules, specifically D2%.

Mode V: This mode utilizes ordinary cement without the addition of extra materials to manufacture the blank samples (BC).

2.4. Assessment of Mechanical Properties and Post-Damage Strength Recovery

The experimental setups utilized for compression tests are depicted in Figure 6. The compressive strength of the concrete was tested at both 7 days and 28 days.

3. Morphology and Composition of Microcapsules

Characterization parameters for microcapsules include permeability constant, wall strength, particle size, and wall thickness, with the latter being referenced in [30]. This study concentrates on self-healing microcapsules in concrete, particularly examining their large-scale test results and investigating only their key properties. FTIR analyses were performed on the samples using a Fourier transform infrared spectrometer within the range of 400–4000 cm⁻¹. Figure 7 displays the FTIR spectra of Dawson and its microcapsules. The tensile bond in the urea–formaldehyde polymer between hydrogen and nitrogen (H-N) and hydrogen and oxygen (H-O) atoms can be identified through a vibrational band located at approximately 3350 cm⁻¹. The detection of characteristic peaks at 959.67 and 1091.89 in both Dawson and the microcapsule suggests that Dawson is present within the microcapsule, as inferred from the FTIR analysis of Dawson and the microcapsules containing it. The results indicated that the microcapsules had an average D50 of approximately 100 μm and D90 of 150 μm, confirming a relatively narrow distribution.

To examine the microcapsules and concrete structure, Scanning Electron Microscope (FE-SEM) was utilized. Figure 8a shows that the majority of the microcapsules have a spherical shape, although there are also particles with irregular shapes present between them. The Energy-dispersive X-ray spectroscopy (EDS) analysis shows that the Dawson microcapsule contains elements such as C, W, O, and N. This indicates the presence of both Dawson (core material) and urea–formaldehyde (shell material). As shown in Figure 8b, the high peak of Tungsten (W), which is the primary component of Dawson, suggests that the core of the microcapsule is well-formed and contains an active substance. Hence, the analysis revealed the presence of urea–formaldehyde (shell material) and calcium nitrate (core material). As depicted in Figure 8b, the EDS analysis chart shows a very high peak intensity of Tungsten (W). This indicates that the core of the microcapsule contains Dawson and is well-formed. To ensure the production of spherical microcapsules (Figure 8c), the Transmission Electron Microscope (TEM) was utilized. According to this test, the production of spherical microcapsules was investigated, which showed that the microcapsule was spherical.

4. Mechanical Properties of Microcapsule-Based Self-Healing Concrete

Effects of Microcapsule Content on Compressive Test

Compressive strength tests were conducted on all specimens. For the 7- and 28-day compressive strength, three cube specimens were created for each percent of hybrid, and the average outcome of each set of three specimens was chosen as the primary result. All test results are displayed in Figure 9.

According to Figure 9, by comparing the compressive strength of the control sample with different percentages, the following results are obtained as Figure 10. It is noticed that these amounts were gained from the formula (${\displaystyle \frac{fmax-fcontrol}{fcontrol}}\times 100\%$), where f control is the mean compressive strength of BC at the same curing age.

The compressive strength of concrete has a direct relationship with its density. Concrete that is denser will typically exhibit greater compressive strength. Consequently, a higher density value signifies that the sample in question exhibits greater resistance. The data in Figure 11 indicate that samples with 1.5% microcapsules have greater strength compared to the other samples.

Previous studies have also investigated the impact of Preyssler hybrid, Preyssler–Calcium Nitrate, and Calcium Nitrate microcapsules on self-healing concrete [23]. Comparing the results of Figure 12 and Figure 13 with those of Dawson microcapsules will help determine which microcapsules exhibit the highest compressive strength. The discussion section now includes an explanation highlighting that Dawson-type heteropolyacids react more actively with cement hydration products (Ca(OH)₂ and C–S–H gel) compared to other heteropolyacid types, thereby promoting additional secondary binding phases that enhance 28-day strength.

According to Figure 12, we can say that the between four types of microcapsules, by 0.5 and 1% hybrid Preyssler–Calcium Nitrate, 1.5% Calcium Nitrate, and 2% Preyssler microcapsules have a high value for 7-day compressive strength, respectively. According to Figure 12, the compressive strength after 28 days is high for formulations that include 0.5, 1, and 1.5% Calcium Nitrate and 2% Preyssler–Calcium Nitrate hybrid microcapsules. It was found that when mineral salts such as Preyssler, Dawson, and Calcium Nitrate are combined, the compressive strength may be at its best.

5. Strength Healing Performance of Self-Healing Concrete

The samples with the highest compressive strength were then subjected to a detailed analysis using SEM to investigate the concrete microstructure and potential mechanisms for repairing cracks. The samples comprised various microcapsules. Figure 14 illustrates the outcome of FE-SEM imaging of the concrete specimen embedded with microcapsules following fracture.

Figure 14 illustrates that larger, darker points indicate the presence of natural air voids in the cement matrix, while shallower, spherical points signify the existence of microcapsules.

Before and After Healing

Following the failure, three distinct cracks were selected for observation of the repair process after a 10-day period, as illustrated in Figure 15. The results from the EDS analysis, as depicted in Figure 16, indicate the presence of the repair agent within the crack area. The presence of phosphorus and water elements in the Dawson microcapsules indicates their successful remediation of the fractured area within the cement matrix. The presence of these elements, which are not typically found in cement, provides additional evidence for the effectiveness of the microcapsules. Figure 17 illustrates the effective repair of cracks in concrete that has been treated with microcapsules.

6. Conclusions

The primary objective of this article is to assess the effectiveness of self-healing concrete that incorporates Dawson microcapsules. Using the in situ method, microcapsules are manufactured with a mineral salt core, specifically Dawson, surrounded by a shell made from urea–formaldehyde. An investigation assessed the efficacy of healing concrete with microcapsules, through the evaluation of a strength test and a healing microstructure assessment. Analysis of the data reveals that across all samples, the sample containing 1% microcapsules yields outstanding results for both 7-day and 28-day compressive strength. The findings indicate that the sample containing 1% microcapsules exhibited a 18.45% decrease in compressive strength after 7 days and a 29% increase after 28 days compared to the control sample. The findings showed that the duration of the healing process had a significant impact on the self-healing ability of the microcapsules. Samples of concrete that were repaired for 28 days showed significantly better performance than those that were repaired in 7 days. Microstructural analysis supports capsule-triggered crack filling and local deposition of mineral species consistent with Dawson release, but further chemical phase analysis (XRD) is recommended to confirm phase identity. It is essential to take note that the effects of substituting magnetic water for urban drinking water in this study on the mechanical properties of concrete, specifically the compressive strength, should be considered by those with interest. Further investigation into the potential of other polyoxometalates as microcapsules is a worthwhile direction for expanding this research. Research into concrete is being driven forward on a global basis by the pursuit of increasing the compressive strength of the material through the use of mineral salt microcapsules, including Dawson, Preyssler, and Calcium Nitrate. In other words, The conclusions now include an explicit industry-oriented statement: Dawson microcapsules (at 1% by cement weight) show promise to enhance late-stage compressive strength and assist crack sealing—offering a potentially low-maintenance approach for reducing surface cracking in precast or repair-prone concrete elements. The text recommends pilot-scale trials before adoption.

In conclusion, we can say that self-healing concrete simulation methods have advanced significantly, enabling more accurate prediction of the material’s ability to repair cracks and restore structural integrity. These methods, from finite element analysis to advanced multi-scale modeling, help capture the complex interactions involved in crack healing, including chemical reactions, microcapsule activation, and crack closure. Overall, self-healing concrete simulations provide a powerful tool for understanding and optimizing the performance of sustainable and durable concrete structures.