Enhancing the Mechanical and Permeability Properties of Bentonite Plastic Concrete Using Pozzolanic Additives

Concrete is the most commonly used building material worldwide and serves as the foundation for infrastructure development because of its accessibility, workability, and mechanical strength. Notwithstanding these benefits, conventional Portland cement concrete has several drawbacks, most notably brittleness, microcracking vulnerability, high permeability, and issues with long-term durability in harsh environments. Recent studies have focused on altering the concrete matrix by adding cementitious materials and nano/micro-scale additives to address these shortcomings. These modifications improve the mechanical and permeability-related properties of concrete [1]. Because of its distinct physicochemical characteristics, bentonite has drawn more attention from researchers in recent years than any other additive. Montmorillonite, the primary component of bentonite, is a naturally occurring clay with a large specific surface area, expansive behavior, and high water absorption. Owing to these properties, bentonite can improve the distribution of pore sizes in the cement matrix and decrease pore connectivity. Consequently, bentonite-containing concrete exhibits notable enhancements in impermeability and crack sealing properties. By encouraging the swelling of microcracks and densifying the microstructure, researchers showed that adding up to 5% bentonite improved the self-healing capacity of cementitious composites [2]. In addition to bentonite, several pozzolanic materials are commonly used to enhance the performance of concrete. Fly ash, a class F pozzolan with a high concentration of amorphous silica and alumina, is a byproduct of burning coal in thermal power plants. Long-term strength gain and decreased permeability are caused by the reaction of FA with calcium hydroxide released during cement hydration to form more calcium silicate hydrate (C–S–H). According to studies, adding fly ash in place of 15–30% of cement improves resistance to sulfate attack, alkali-silica reaction, and chloride ion penetration, in addition to increasing compressive and tensile strength over time [3, 4]. Another high-performance pozzolan with a fine particle size and high reactivity is metakaolin, which is produced by calcining pure kaolinite clay at temperatures between 600 and 800℃. Metakaolin improves the early age and long-term strength of concrete by densifying the interfacial transition zone (ITZ) between the aggregates and cement paste. The researchers verified that adding 10–20% metakaolin by weight of cement lowers total porosity, improves resistance to chemical attack, and improves the microstructure of concrete, which lowers permeability and capillary suction [5, 6]. An ultrafine amorphous silica byproduct produced during the manufacturing of silicon and ferrosilicon alloys is silica fume, which is sometimes referred to as micro silica. Silica fume is a highly reactive pozzolan and filler, with particle sizes almost 100 times smaller than those of cement. By filling tiny voids and quickly reacting with free lime to create more C-S-H gel, the porosity and permeability of concrete are dramatically lowered. The inclusion of 5–10% silica fume increases the resistance to sulfate and chloride ion ingress, decreases water absorption, and improves the compressive and flexural strengths. A new method for improving concrete performance is provided by the combination of bentonite and various SCMs such as fly ash, metakaolin, and silica fume. Although SCMs aid in matrix refinement and pozzolanic reactivity, bentonite adds sealing and swelling properties that reduce permeability and microcracking. Because Portland cement is partially replaced by this dual mechanism, concrete may have better mechanical performance, a longer lifespan, and a smaller environmental impact [7].

To improve the mechanical strength, decrease the permeability, and refine the interfacial transition zones (ITZ), bentonite and pozzolanic additives are predicted to interact to decrease the pore connectivity and promote the formation of C–S–H and C–A–H gels. The anticipated performance improvement examined in this study was based on these mechanisms.

Although much research has been conducted on how fly ash, metakaolin, and silica fume affect concrete performance separately, few thorough studies have compared how they affect bentonite-modified plastic concrete. This study fills this knowledge gap by assessing the impact of these additives at different dosages on the mechanical and permeability properties of plastic concrete with a consistent 10% bentonite content. Finding the best pozzolanic material to overcome the drawbacks of bentonite, specifically its propensity to increase porosity and decrease mechanical strength, is the goal. It is anticipated that the combination of bentonite and specific pozzolanic materials will result in concrete with denser microstructure and greater resilience. The structure of the paper is as follows: the materials and experimental procedure are described in detail in Section 2, the results are shown in Section 3, the discussion is given in Section 4, and the conclusions are summarised in Section 5.

The purpose of the experimental program was to assess how pozzolanic replacement—specifically, fly ash, metakaolin, and silica fume—affects the mechanical strength, permeability, and water absorption of bentonite plastic concrete. As shown in Table 3, ten concrete mixes were prepared, comprising one control mix (CTRL) and nine modified mixes that included pozzolans at different replacement levels (5%, 10%, and 15%).

3.1 Compressive strength​

Compressive strength tests were conducted using 100 mm × 100 mm × 100 mm cubic concrete specimens in compliance with BS 12390-3 [10]. To evaluate the strength development over time, tests were conducted at a curing age of 28 d. A uniaxial compressive load was applied at a regulated rate of 0.5 MPa/s until failure using a calibrated universal testing machine (UTM). To guarantee dependability and reduce experimental error, three specimens were tested for each mix and curing age, and the average compressive strength was reported.

3.2 Splitting tensile strength

In compliance with ASTM C496 [11], a splitting tensile test was used to ascertain the indirect tensile strength of the concrete. Standard cylindrical specimens measuring 150 mm in diameter and 300 mm in height were used for this test. A compressive line load was applied along the length of the specimen, and each cylinder was positioned horizontally between the platens of a universal testing machine (UTM). The specimen failed in tension along its vertical diameter owing to this configuration, which created tensile stresses perpendicular to the direction of the applied load. Three specimens were tested for each mix and age at 28 days after curing, and the representative value was the average splitting tensile strength. This technique offers important information about the tensile strength and resistance to cracking of bentonite plastic concrete mixtures with different pozzolanic additives.

3.3 Water permeability

The test was conducted in accordance with DIN 1048 Part 5 [12], using cylindrical specimens measuring 100 mm in diameter and 200 mm in height. After 28 d of water curing, the specimens were subjected to a constant water pressure of 5 bar (0.5 MPa) for 72 h. A chisel and hammer were used to split the specimens along their longitudinal axis after they had been exposed to a water pressure of 0.5 MPa for 72 h (150 × 300 mm). Subsequently, the penetration depth was measured. A calibrated scale was used to record depth. At the end of the testing period, the specimens were split longitudinally, and the maximum depth of water penetration was measured using a graduated scale. Three specimens were tested for each mix and age, and the average penetration depth was recorded as the water-permeability index. This method is particularly suitable for assessing the efficacy of bentonite and pozzolanic additives in lowering capillary porosity and improving the impermeability of the concrete matrix.

3.4 Absorption reduction

Concrete cubes measuring 100 × 100 × 100 mm were used to measure water absorption in compliance with ASTM C642 [13]. After 24 h of oven drying at 100–110℃ until a constant weight (W₁) was reached, each specimen was cooled to room temperature in a desiccator. After 48 h of complete submersion in water, the saturated surface dry weights (W₂) of the specimens were measured. The following formula was used to determine the percentage of water absorption:

Water Absorption (%) = ((W₂ – W₁) / W₁) × 100

This process provides supplementary information to the permeability test results by enabling a reliable estimation of the open porosity and moisture ingress capacity of concrete mixes.

3.5 Field-emission scanning electron microscopy

Field-emission scanning electron microscopy (FESEM) was used to investigate the microstructural properties of the hardened concrete composites. Small fractured fragments from the interior of the 28-day cured specimens were gathered to avoid surface contamination. These fragments were first oven-dried at 50℃ to remove residual moisture and subsequently covered with a thin layer of gold using a sputter coater to ensure adequate conductivity and clarity during scanning.

This study shows that adding pozzolanic materials to bentonite-modified plastic concrete, specifically silica fume, metakaolin, and Class F fly ash, can significantly improve the mechanical and durability properties of the material. Because of the expansive nature and high water retention capacity of bentonite, the control mix with 10% bentonite and no pozzolanic replacement showed the lowest compressive and tensile strengths as well as the highest permeability and water absorption values. 10% silica fume (SF-10) produced the best overall results among the tested additives, with the lowest values for water permeability (2.1 × 10⁻⁷ m/s) and water absorption (4.28%), a 36% increase in compressive strength, and a noticeable improvement in tensile strength. Significant improvements in strength and pore refinement were also achieved with 10% replacement metakaolin (MK-10), which decreased absorption to 5.32%. The same dosage of fly ash (FA-10) produced slight but steady improvements, which were particularly advantageous for long-term durability. These results were corroborated by the FESEM images, which showed a moderately compacted matrix in MK-10, a dense and homogeneous microstructure in SF-10, and spherical fly ash particles inside a partially densified matrix in FA-10. In contrast, the control mix displayed weak ITZ zones and a porous, poorly bonded structure with noticeable CH crystals. Overall, the findings demonstrate that pozzolanic materials can mitigate the negative effects of bentonite, particularly its increased porosity and decreased strength. Because of its high reactivity and ultrafine particles, silica fume was the most effective, followed by metakaolin. Despite its slower initial reactivity, fly ash improves the microstructural integrity over time. The combination of bentonite and SCMs offers a sustainable method for creating concrete with enhanced mechanical performance, impermeability, and potential for use in structures requiring durability under environmental stress.

The created mixes show promise for use in cut-off walls, tunnels, and other subterranean or hydraulic structures where water resistance and durability are crucial because of their enhanced mechanical and impermeability qualities.