Innovative approaches to enhancing crack repair: Performance optimization of biopolymer-infused CXT

2.1 Cement

The cement used to prepare CXT was Ordinary Portland Cement 43 grade conforming to ASTM C1019 specifications. This cement exhibited a fineness of 225 m²·kg⁻¹, a specific gravity of 3.15, and a soundness of 8 mm, ensuring suitability for its application as a repair material.

2.2 XG

XG was sourced from H R Chemicals, Mumbai – 400019, Maharashtra, India. XG is defined as a biopolymer with unique rheological properties derived from microbial fermentation. It is classified as a polysaccharide, consisting of repeating units of glucose, mannose, and glucuronic acid, and is produced by the bacterium Xanthomonas campestris. The molecular formula of XG is (C₃₅H₄₉O₂₉) _n . Its chemical structure consists of a main-chain pentasaccharide repeat of linear β-1,4-d-glucose, with each molecule containing approximately 7,000 pentamers. These repeating units are composed of glucose, mannose, and glucuronic acid. The side chains of XG contain alternating glucose monomers with mannose and glucuronic acid, as well as pyruvic acid (Figure 1). This intricate structure gives XG its unique properties, such as high solubility in both hot and cold water and its ability to form highly viscous solutions at low concentrations [19].

Figure 1

Chemical structure of XG [20].

The chemical structure of XG also imparts significant stability to the polymer. It remains stable over a wide range of temperatures and pH levels, and its viscosity is not significantly affected by changes in the temperature or pH. Additionally, XG exhibits excellent compatibility with various salts, enabling it to maintain its rheological properties even in the presence of high ionic strength environments.

This complex structure imparts unique rheological properties to XG, such as high viscosity and pseudoplasticity, which make it highly effective in enhancing the flowability and workability of cementitious materials. These properties are particularly advantageous in concrete applications, where XG helps improve the dispersion of particles, reduce the water content, and prevent washout. Additionally, XG remains stable in the presence of acids, alkalis, salts, and high temperatures, making it a robust additive in various industrial applications, including the petroleum sector where it is used extensively as a powerful viscosifier.

Furthermore, XG’s ability to form strong, cohesive matrices with cement hydrates enhances the mechanical strength of the repair material, while its pseudoplastic behavior ensures that the material remains workable during application but becomes highly viscous once applied, reducing the risk of flow and washout.

2.3 TA powder

TA acts as a retarder in CXT by interfering with the early hydration reactions. Careful dosage is essential to prevent strength reduction and potential interactions with other admixtures. This study incorporates TA powder (0.2–0.6% by weight of cement) to explore its potential synergistic interactions with biopolymers. This dosage range was selected to investigate the potential effects of CXT while minimizing risks to the repair material’s ultimate strength and durability.

2.4 Superplasticizer

In this study, sulfonated melamine formaldehyde (SMF) served as a superplasticizer, effectively dispersing cement particles to enhance the workability and flow characteristics of sealant mixes. By reducing the water content while improving the fluidity, it contributes to the enhanced strength and durability of CXT.

2.5 Water

A constant w/c ratio of 0.5 was initially maintained for all mixes to provide a consistent starting point for evaluating the effects of biopolymer and TA.

3.1 Cement hydration (simplified)

In simplified cement hydration, C₃S and C₂S react with water to form C₃S₂H₃ and calcium hydroxide (Ca(OH)₂).

The equations representing these reactions are the following:

These reactions are crucial for the early stage of cement hydration, where C₃S and C₂S undergo dissolution and subsequent precipitation of hydration products. C₃S₂H₃ contributes to concrete strength and structure, while Ca(OH)₂ provides pore solution alkalinity, aiding in corrosion protection and hydration of supplementary cementitious materials.

3.2 Dissolution of C₃S and C₂S

In biopolymer-modified CXT, the dissolution of C₃S and C₂S involves their reaction with water, resulting in the release of Ca²⁺, SiO 4 4 − , and hydroxide ions (OH⁻) into solution. The equations representing these dissolution reactions are the following:

These reactions are essential in the initial stages of cement hydration, where water penetrates the cementitious matrix, interacts with C₃S and C₂S particles, and triggers their dissolution. The released ions contribute to the formation of pore solution alkalinity and provide the necessary constituents for subsequent hydration reactions, ultimately leading to the development of hydrated cementitious phases and the setting and hardening of the repair material.

3.3 Formation of Ca(OH)₂

In the formation of Ca(OH)₂, Ca²⁺ in aqueous solution react with OH⁻ to produce solid Ca(OH)₂. The chemical equation representing this reaction is:

This reaction plays a crucial role in cement hydration, as Ca(OH)₂ is a key hydration product. It contributes to the strength and durability of CXT.

3.4 TA dissociation

TA, represented as H₂T, undergoes dissociation in aqueous solution in two stages. Initially, it dissociates into a hydrogen tartrate ion (HT⁻) and a hydrogen ion (H⁺), as shown in the equation:

Subsequently, the HT⁻ further dissociates to yield a tartrate ion (T²⁻) and another H⁺:

These dissociation reactions are essential in controlling the pH of solutions containing TA and play a crucial role in various chemical and industrial processes, including food production and pharmaceuticals.

3.5 Calcium–tartrate complex formation

In the formation of a calcium–tartrate complex (CaT), Ca²⁺ in an aqueous solution react with T²⁻ to produce a soluble CaT. The chemical equation representing this equilibrium reaction is as follows:

This reaction occurs in solution and involves the binding of Ca²⁺ with T²⁻ to form the complex.

4.1 Optimization process of CXT

To optimize the formulation of the biopolymer-modified CXT, it is crucial to meticulously adjust the proportions of XG and TA. The concentration of XG was fine-tuned to achieve an optimal balance between workability and strength, as higher concentrations enhanced the viscosity but may impede the flowability. Similarly, the TA dosage was calibrated within a range of 0.2–0.6% by weight of cement to effectively control the setting time without compromising the overall strength. Maintaining a consistent water–cement ratio of approximately 0.5 is essential, and incorporating a superplasticizer, such as SMF, significantly improves the flowability while reducing the water content.

Comprehensive workability tests, including slump cone and Marsh cone tests, were conducted to assess flow characteristics, while Vicat needle tests were performed to determine the initial and final setting times. Mechanical properties were evaluated through compressive and flexural strength tests at 7, 14, and 28 days to ensure long-term durability. Based on these test results, the proportions were iteratively adjusted to minimize the shrinkage and enhance the performance. The optimized formulation was validated through real-world crack repair applications and monitored for long-term performance to ensure its sustainability and robustness.

4.2 Slump cone test

The slump cone test, a widely recognized method for evaluating the workability of fresh concrete, was employed to investigate the effects of varying the XG to cement (XG:C) ratios and TA concentrations on the flow behavior of CXT. The slump values for the different trial mixes of CXT are shown in Figure 2. This test involved filling a metal cone, with a bottom diameter of 20 cm, a top diameter of 10 cm, and a height of 30 cm, with a fresh sealant in three layers. Each layer was tamped 25 times, slightly deviating from the standard practice to ensure thorough compaction. After filling, the cone was carefully lifted vertically, allowing the sealant to slump. The vertical displacement of the sealant, known as the slump, was measured to assess its workability. This process was systematically repeated for each trial mix ratio to accurately determine their respective workability characteristics. By comparing the slump values, the influence of different XG:C ratios and TA concentrations on the flow behavior and workability of the CXT mixtures was comprehensively analyzed.

Figure 2

Graph representing the slump value for each trail.

The results demonstrated a significant positive correlation between the XG:C ratio and the slump value. The 1:2 XG:C ratio consistently exhibited the highest slump values, indicating superior flowability compared to other ratios tested. This enhanced flow can be attributed to the thickening and viscosity-modifying properties of XG. Additionally, the inclusion of moderate amounts of TA (e.g., TA0.4–0.4% weight of cement) exhibited a synergistic effect, further increasing slump values. This is likely due to TA’s retarding action, which delays cement hydration and extends the sealant’s workable time.

While the 1:2 XG:C ratio with moderate TA addition demonstrated significant potential for crack-sealing applications, a comprehensive assessment necessitated further investigations which was performed next to slump tests. Supplementary tests to evaluate the setting time and mechanical strength are essential to ensure that the enhanced workability does not compromise the long-term repair performance. The slump value for each trail is tabulated in Table 1.

4.3 Marsh cone test

The Marsh cone is a standardized tool used to measure the flowability of repair material and similar cementitious mixes. It involves filling a metal cone with CXT and measuring the time it takes for a specific volume to flow out as shown in Figure 3.

Figure 3

Images representing the Marsh cone test.

Faster Marsh cone times indicated greater flowability. This simple test provides valuable insights into CXT’s workability, making it relevant for applications like crack sealing where easy flow is essential. The Marsh cone values for different trail mixes of this biopolymer-modified CXT are shown in Figure 4, and the value for each trail is tabulated in Table 2.

Figure 4

Marsh cone test values for each trail.

The Marsh cone test results reveal a complex interplay between the XG concentration, TA inclusion, and CXT’s flowability. XG emerges as the dominant factor driving flow enhancement, with increasing XG:C ratios consistently yielding lower Marsh cone values (indicating faster flow times). This can be attributed to XG’s ability to thicken the sealant, modifying its viscosity and promoting better flow behavior.

TA demonstrates a synergistic, yet more subtle effect. Within a selected XG:C trail ratio, increasing TA concentrations tend to result in slightly improved flowability. This aligns with TA’s known ability to retard cement hydration, extending the workable time of the repair material CXT and contributing to its flow characteristics. However, there is a crucial caveat: excessive XG concentrations, as demonstrated by the 2:1 ratio, appear to negate these flowability benefits. This highlights the need for a balanced formulation, as overly high XG levels can increase the overall CXT viscosity to the point where the flow is hindered, even with TA present. For crack sealing applications, the 1:2 XG:C ratio, particularly with moderate TA levels, holds significant promise due to their demonstrated superior flow.

4.4 Setting time of CXT

The Vicat needle test (ASTM C191) is a standardized method for evaluating the setting time of cementitious materials. In this study, the Vicat needle test was performed with a load of 300 g to determine the initial setting time of the optimum mix ratio XG 1:2 by varying the TA powder concentration between 0.2 and 0.6% by weight of the cement biopolymer formulation. This test measures the penetration resistance of a standardized needle into the fresh sealant, providing insights into both initial and final setting times.

The initial setting time of the biopolymer-modified CXT was observed to be within the range of 21–23 min. Specifically, the initial setting time for the mix with 0.2% TA was 21 min, for 0.4% TA it was 22 min, and for 0.6% TA it was 23 min. This range indicates that the material remains workable for a sufficient period, allowing for adequate application and adjustments during the repair process.

The final setting time of the repair material CXT was observed between 24 and 27 h. The mix with 0.2% TA had a final setting time of 24 h, the mix with 0.4% TA had a final setting time of 26 h, and the mix with 0.6% TA had a final setting time of 27 h. This extended setting period ensures that the material undergoes a gradual hardening process, reducing the risk of cracking and ensuring a more durable repair. The setting time values for different trail mixes of this biopolymer-modified CXT are shown in Figure 5, and the results are summarized in Table 3.

Figure 5

Setting time values for different trial mixes.

These findings indicate that the optimal mix (XG 1:2 with TA0.2/0.4/0.6) provides a balanced performance in terms of workability and setting times. The initial setting times ensure that the material can be easily applied and adjusted, while the final setting times allow for a thorough and gradual hardening process. This balance is essential for the durability of the repair, making the biopolymer-modified CXT with the optimal formulation of XG 1:2 and TA0.2–0.6 a promising solution for effective and durable crack repair in concrete structures.

4.5 Casting of CXT cubes

CXT cubes (5 cm × 5 cm × 5 cm) were prepared using a mix ratio (outlined in Table 4) of XG:C (1:2), as this ratio was identified as optimal in previous experiments. To ensure accuracy, precise quantities of cement, biopolymer (XG), TA, and superplasticizer were calculated to yield the required CXT volume for each cube. The dry components (cement, XG, and TA) were meticulously mixed in a clean container for approximately 2 min to ensure complete homogenization. This mixing was performed using a low-speed mechanical mixer to facilitate consistency.

A stepwise addition of water and superplasticizer followed during the mixing process. Initially, roughly 70% of water was added, along with continuous mixing. The superplasticizer was then incorporated, and mixing was continued. The remaining water was gradually added, with adjustments made as necessary to achieve a homogenous mixture with the desired workability. This mixing process typically lasted for 3–5 min, ensuring complete hydration of the cement, a uniform distribution of the biopolymer, and activation of the superplasticizer. This homogenous mixture was placed into cube molds as shown in Figure 6. After molding, the cubes were wrapped with a polythene sheet to prevent moisture loss and ensure proper curing, which is essential for achieving the desired mechanical properties.

Figure 6

Preparation of CXT cubes.

4.6 Compressive strength

The compressive strength of CXT was assessed using standard cubes 5 cm × 5 cm × 5 cm, in accordance with the established procedure outlined in IS 4031 Part 7, and the results are tabulated in Table 5.

The compressive strength analysis as shown in Figure 7 revealed a significant increase in strength for all CXT mixes over the 28-day curing period. The most rapid strength development occurred between 7 and 14 days, followed by a slower rate of gain from 14 to 28 days. This strength evolution pattern aligns with typical repair material behavior. Notably, mixes incorporating higher TA concentrations (XG 1:2 TA0.6) exhibited slightly greater increases in strength throughout the curing period.

Figure 7

Compressive strength values for 7, 14, and 28 days.

Compressive strength analysis revealed a rapid strength development profile for the CXT formulation. Approximately 50% of its final compressive strength was achieved within the first 7 days. This accelerated early strength gain highlights the formulation’s potential for applications requiring swift material setting. The majority of strength development occurred within the initial 14 days, with the repair material attaining over 77% of its final strength. A slight increase in strength was observed between 14 and 28 days, reaching its final strength; however, the rate of gain decreased progressively throughout this period.

The accelerated early strength gain of the biopolymer-modified CXT is a critical attribute for applications requiring rapid material setting and early load-bearing capacity. The primary mechanism underlying this accelerated early strength gain involves the modification of the hydration process by the biopolymer additives. XG and TA work synergistically to enhance the dispersion of cement particles, improve the hydration efficiency, and control the setting time. XG’s high viscosity and pseudoplastic properties ensure a uniform distribution of cement particles, promoting more effective hydration. TA acts as a retarder, allowing sufficient time for thorough mixing and application before the material begins to set.

As a result, the initial hydration reactions, particularly those involving C₃S, proceed more efficiently, leading to the rapid formation of C–S–H, which is the primary strength-giving compound in cementitious materials. This accelerated formation of C–S–H contributed to the early strength development of the repair material. Overall, these results suggest that the incorporation of biopolymer and TA into cement mixtures can positively influence compressive strength development over time. The varying percentages of strength achieved at different curing periods offer valuable insights for optimizing mixture designs tailored to crack sealing project requirements and performance expectations.

4.7 Flexural strength of CXT

The determination of the flexural strength of CXT followed the protocol outlined in the Indian standard IS:516. This involved studying the flexural strength using beam specimens measuring 40 mm × 40 mm × 160 mm, with an effective span of 120 mm, subjected to three-point loading. The beams were supported by two roller supports, each with a diameter of 10 mm, spaced 120 mm apart. The specimens were positioned on the roller supports on their bottom faces, with the third point loading applied symmetrically on the opposite and top faces using a third roller.

The two support rollers were set up in a simply supportive manner, facilitating unrestricted rotation of the beam around their line of contact to evenly distribute the line load across the width of the beam as a point load. The flexural tensile strength of the cement grout was determined as the average of three beam tests conducted at intervals of 3, 7, and 28 days. The variations in flexural tensile strength of the cement grout at different ages are depicted in Figure 8, and the results are tabulated in Table 6.

Figure 8

Flexural strength values for 7, 14, and 28 days.

Flexural strength analysis demonstrated a rapid strength development profile for the CXT formulation. Within the first 7 days, the material achieved approximately 51% of its ultimate flexural strength. This accelerated early strength development signifies the potential applicability of the CXT formulation for scenarios demanding swift material setting. The majority of strength development transpired within the initial 14 days, with the repair material reaching over 87% of its final strength. Between 14 and 28 days, a slight increase in strength was observed, culminating in the achievement of material’s final flexural strength. However, the rate of strength gain exhibited a progressive decline throughout this period. Overall, these findings indicate that biopolymer-modified CXT can have a beneficial impact on the development of flexural strength over time.

6.1 Workability and flowability

1. Slump cone test: The slump cone test results (Table 1) demonstrated that the optimal XG ratio was 1:2, with the addition of 0.6% TA, yielding the highest slump values (132 mm). This indicates superior flowability, attributable to the thickening and viscosity-modifying properties of XG. Similar enhancements in flow properties due to biopolymer modifications have been reported, showing significant improvements in the rheological properties of cementitious materials [21].

2. Marsh cone test: Marsh cone test results (Table 2) corroborated with the slump test findings, with the 1:2 XG ratio and 0.6% TA showing the lowest flow times, indicating improved flowability. This aligns with findings that biopolymers enhance the flow characteristics of cementitious mixtures by reducing the water content and improving the particle dispersion [22].

6.2 Setting time

The Vicat needle test results indicated that the optimal formulation (XG 1:2 TA0.6) maintained an initial setting time within 21–24 min and a final setting time of 24–28 h. This delayed setting time provided sufficient workability without compromising early strength gain, consistent with studies showing that TA effectively extends the setting time of cementitious materials [23].

6.3 Mechanical properties

1. Compressive strength: The compressive strength results (Table 5) showed a significant increase over the 28-day curing period, with the highest strength recorded for the XG 1:2 TA0.6 mix (24.8 N·mm⁻²). The rapid early strength gain (77% of final strength within 14 days) observed in this study was supported by research indicating that the inclusion of biopolymers enhanced early-age strength by improving the hydration kinetics and matrix formation [24].

2. Flexural strength: Flexural strength tests (Table 6) indicated that the XG 1:2 TA0.6 mix achieved a maximum strength of 9.4 N·mm⁻² at 28 days. This result corroborates with findings that biopolymers improved the flexural performance by forming stronger cohesive matrices within the cement matrix [25].