Experimental research on mechanically and thermally activation of nano-kaolin to improve the properties of ultra-high-performance fiber-reinforced concrete

1.1 Objective and significance of the present study

This study explores the impact of nano-kaolin clay, mechanically and thermally activated, as a cement substitute in UHPC at percentages of 0.5, 0.10, 0.15, 0.20, and 0.25% by binder weight. A uniform quantity of double-hooked end steel fibers was added to all the mixes. It addresses a gap in research by assessing UHPC’s engineering properties, including strength (compressive and indirect tensile strength, modulus of elasticity [MOE], and modulus of rupture [MOR]), durability (acid resistance, sorptivity, temperature endurance, and water absorption), and porosity. The focus is on how different proportions of activated nano-kaolin affect UHPC’s performance. Results aim to enhance the understanding of sustainable UHPC development, providing insights into structural performance and environmental resilience and informing optimized mix designs for advanced concrete materials in construction.

5.1 Flowability test

The flowability characteristics of UHPC, including different proportions of nano-kaolin, are presented in Figure 3. The significance of Figure 3 lies in its depiction of how mechanically and thermally activated nano-kaolin impacts the concrete’s fresh properties. When up to 0.20% of the cement is replaced with either MAK or TAK, there is a noticeable improvement in the UHPC’s flowability. This increase in flowability can be primarily ascribed to the pozzolanic reaction initiated by kaolin. This reaction produces supplementary C–S–H gel, which diminishes the water requirement needed to achieve the desired consistency, as noted in past studies [42]. Another essential side to consider is the particulate nature of kaolin itself. These kaolin particles serve a dual role as they not only facilitate improved lubrication amongst cement grains but also enhance the overall fluid dynamics of the concrete mix. Consequently, this increases flowability, which remains prominent until the kaolin content reaches 0.20%. However, a sloping point was observed in flowability when the nano-kaolin inclusion surpassed 0.20%, specifically at 0.25%. At this stage, flowability experiences a decline. The likely motivation behind this diminished flowability is the excessive concentration of kaolin particles. Their excessive presence might prompt an accumulation, thereby upsetting the tangential motion of the cement grains, as discussed in past studies [43]. Such diminishing is counterproductive and compromises the concrete mix’s flowability. The flowability of the reference mixture stands at 145 mm. However, a 0.5% cement substitution with either MAK or TAK increases flowability by around 19.3 and 29% for 173 and 187 mm, respectively. However, there is a reversal in the trend when the replacement level increases to 0.25%. Specifically, flowability records a drop of about 6.2% for B2-MAK-25 and 3.4% for B3-TAK-25 when compared with the reference mixture.

Figure 3

The flow of fresh UHPFRC.

When comparing MAK with TAK in terms of their influence on flowability, it becomes evident that TAK outperforms MAK. Specifically, mixtures incorporating TAK consistently recorded the highest flow values compared to any other mixture with kaolin content. The improved performance of TAK in enhancing flowability can be traced to its production process. The thermal activation, through controlled heat treatment, enhances the pozzolanic reactivity of the kaolin. This enhanced reactivity translates into a more noticeable microstructural improvement within the UHPC matrix. Consequently, there is a notable improvement in the lubrication dynamics amongst the concrete particles, as observed in the past study [44]. This enhanced lubrication characteristic inherently gives TAK-incorporated mixtures an edge in flowability over those with MAK. Furthermore, TAK showed an enhanced pozzolanic reactivity and a keen affinity to synergize with other constituents in the concrete. This proactive interaction raises a composite effect, resulting in better flowability and amplified concrete strength. Additionally, the inherent properties of TAK serve as an internal lubricant in the concrete mix, paving the way for a more optimal incorporation of all the ingredients.

Corroborating the current study findings, it is worth noting that the observed results are consistent with prior research, reinforcing the reliability of the observations. As highlighted in the study of Piasta et al. [45], there is an established precedent for the superior performance of thermally activated nano-kaolin in enhancing concrete’s flowability and overall structural integrity.

5.2 Compressive strength and MOE

The test results about compressive strength and MOE at the respective intervals of 28 and 90 days are outlined in Figure 4, revealing insightful patterns and correlations. A noticeable compressive strength and MOE enhancement are apparent in samples containing 0.20% mechanically activated nano-kaolin (B2-MAK-20) and 0.20% thermally activated nano-kaolin (B3-TAK-20). This enhancement can be unraveled by diving into kaolin’s intrinsic properties and reactive capabilities as a pozzolanic material with high quantities of amorphous silica and alumina. When integrated into the concrete mix, kaolin reacts with calcium hydroxide, a byproduct of OPC hydration. The resultant calcium silicate hydrate gel, which forms, played a pivotal role as a primary contributor to the strength of cementitious materials. As such, the improved interlocking microstructure concludes in improved compressive strength. Peeling back the layers on the differential performance between mechanically activated nano-kaolin and thermally activated nano-kaolin, especially at an identical 0.20% replacement level, brings to the fore the distinct roles of reactivity and particle characteristics specific to each type. Mechanically activated nano-kaolin was subjected to a thorough grinding, increasing its surface area and instigating defects within the particles, thus rendering it highly amenable to the pozzolanic reaction. Conversely, thermally activated nano-kaolin was exposed to a controlled heat treatment, which increased its reactivity by transforming a more significant proportion of the amorphous phase into the crystalline phase. As a result, thermally activated nano-kaolin catalyzes a more proficient pozzolanic reaction, leading to demonstrable gains in strength, as corroborated by Song et al. [46]. At 28 days, the compressive strength of the reference mixture stands at 122.7 MPa. Introducing 0.20% mechanically activated kaolin (B2-MAK-20) has an approximate 7% increment in strength, culminating in a measured strength of 130.4 MPa. Exploring these tangible improvements in strength further illuminates the potential of kaolin as a valuable addition in modifying and enhancing the mechanical properties of UHPC, opening ways for refined material design and potential applications in more demanding structural scenarios. These enhancements and previous findings underpin the increasing interest and research in kaolin’s role in optimizing concrete mixes for specialized applications, particularly where enhanced strength and durability are paramount.

Figure 4

Compressive strength and MOE of UHPC at 28 and 90 days.

Incorporating 20% thermally activated kaolin (B3-TAK-20) results in a noticeable improvement in compressive strength, evidencing an increase of approximately 9.5% over the baseline, bearing a strength value of 134.5 MPa. Also, at the 90-day mark, the reference mixture’s strength rises to 125.1 MPa. The introduction of 0.20% mechanically activated nano-kaolin (B2-MAK-20) into this mix sees the strength surge by an impressive 14.3%, settling at 142.5 MPa. However, the incorporation of 0.20% thermally activated nano-kaolin (B3-TAK-20) showcases a peak of performance, registering a remarkable increase of around 21.6%, achieving a standout strength of 152.4 MPa. This observable inclination toward improved MOE in UHPC specimens, incorporating 0.20% mechanically activated nano-kaolin (B2-MAK-20) and 0.20% thermally activated nano-kaolin (B3-TAK-20), aligns intricately with the foundational principles of material science. The augmentation in MOE can be attributed to the enhanced pozzolanic reaction and particle packing density facilitated by the nano-scale modification of kaolin through mechanical and thermal activation. These modifications significantly improve the interfacial transition zone between the cement matrix and the aggregates, leading to a denser, more homogenous microstructure. Consequently, this densification contributes to the increased mechanical properties, including the MOE, by optimizing the distribution of stresses and minimizing microcracks under applied loads. Further, the nano-scale size of the activated kaolin particles aids in filling the voids within the concrete matrix more effectively than conventional materials, thereby enhancing the overall durability and strength characteristics of the UHPC. The underlying mechanisms supporting this phenomenon are well documented within nanotechnology’s application in advanced composite materials, illustrating a significant correlation between nanoparticle modification processes and the resultant mechanical properties of composite materials.

Mechanically and thermally activated nano-kaolin are intrinsically pozzolanic, with amorphous (or shapeless) alumina and silica. These components are introduced to the concrete mix to instigate a pozzolanic reaction, synergizing with the calcium hydroxide (CH) birthed during OPC hydration, as outlined in reference [47]. This catalytic process arranges the formation of supplementary calcium silicate hydrate gel, a foundation phase with a pivotal role in increasing the strength and rigidity of the cementitious matrix, as corroborated by past studies [48]. With kaolin’s incorporation, there’s an increase in the C–S–H gel volume, producing a more intricate interlocking microstructure and, consequently, enhanced MOE values. At 28 days, the result reveals the reference mixture’s MOE was 22.3 GPa. In 20%, mechanically activated nano-kaolin (B2-MAK-20) results in nearly 20%, achieving 26.8 GPa. Meanwhile, introducing 20% thermally activated nano-kaolin (B3-TAK-20) pushes the envelope even further, marking an ascent of around 22.3% to 27.3 GPa. At 90 days, the reference mix’s elasticity modulus advanced to 24.4 GPa. The blend with 0.20% mechanically activated nano-kaolin (B2-MAK-20) reflects a growth of approximately 19.3%, reaching 29.2 GPa. Yet, it is the mixture with 20% thermally activated nano-kaolin (B3-TAK-20) that truly showcased a significant increase of around 25.5%, settling at 30.7 GPa.

An additional aspect contributing to the observed enhancement in performance metrics is the inherent particle characteristics of the nano-kaolin. The process of mechanical activation yields nano-kaolin with ultra-fine granularity. In contrast, thermal activation results in a crystalline structure of the kaolin, a transformation induced by the thermal treatment. These distinct physical states enable the nanoparticles to penetrate, fill, and strengthen the interstitial spaces within the concrete matrix effectively, a phenomenon corroborated by previous research findings [49]. The superior packing density achieved through these nanoparticle dispersions leads to a microstructure marked by reduced porosity, minimized air entrapment, and fewer structural weaknesses. This refined micro-architecture enhances the distribution and transfer of loads across the concrete matrix, thereby increasing its stiffness. Such modifications are directly observable in the elevated MOE values, indicating a significant improvement in the material’s rigidity and structural integrity. This intricate relationship between the intrinsic properties of nano-kaolin particles and the mechanical performance of the concrete underscores the importance of material selection and processing techniques in achieving desired outcomes in advanced construction materials.

5.3 Indirect tensile strength (ITS)

Figure 5 depicts the test results for ITS at 28 and 90 days. Analyzing the data reveals a substantial enhancement in ITS in the specimens containing 20% mechanically activated kaolin (B2-MAK-20) and 0.20% thermally activated nano-kaolin (B3-TAK-20). The observed increase in ITS up to 0.20% replacement level for both mechanically activated nano-kaolin (B2-MAK-20) and thermally activated kaolin (B3-TAK-20) can be attributed to the positive effects of the pozzolanic reaction and microstructural modifications. The increase in ITS is associated with the densification of the concrete microstructure and the reduction in porosity achieved with kaolin addition. At lower replacement levels (up to 0.20%), the fine particles of mechanically activated kaolin and the thermally activated kaolin’s crystalline structure fill the voids and spaces within the matrix, reducing the overall porosity [50]. This results in improved load transfer mechanisms and enhanced tensile strength. At 28 days, the ITS of the reference mixture is 9.87 MPa. Replacing cement with 0.20% mechanically activated nano-kaolin (B2-MAM-20) increases the ITS by approximately 38% to reach 13.2 MPa, while 0.20% thermally activated nano-kaolin (B3-TAK-20) results in an increase of about 39.1% to reach 13.7 MPa. At 90 days, the ITS of the reference mixture increases to 11.7 MPa. With 20% mechanically activated nano-kaolin (B2-MAK-20), the ITS increases by approximately 35.9% to reach 15.45 MPa, while 20% thermally activated nano-kaolin (B3-TAK-20) exhibits an increase of about 38.5% to reach 16.2 MPa.

Figure 5

ITS of UHPC at 28 and 90 days.

However, the reduction in ITS observed at a 0.25% replacement level for mechanically activated nano-kaolin (B2-MAK-25) and thermally activated kaolin (B3-TAK-25) can be attributed to a few factors. At higher replacement levels, excessive kaolin might lead to an overcrowded microstructure with an increased amount of unreacted kaolin particles [51]. This could cause particle agglomeration and hinder the pozzolanic reaction, leading to a less effective strengthening mechanism [31]. The ITS decreases at a 0.25% replacement level compared to the 20% level. At 28 days, the ITS for 0.25% mechanically activated nano-kaolin (B2-MAK-25) is reduced by approximately 10.7% compared to B2-MAM-20, and for 0.25% thermally activated nano-kaolin (B3-TAK-25), the reduction is about 11.8% compared to B3-TAM-20. At 90 days, the decrease in ITS values for B2-MAM-25 and B3-TAM-25 compared to their 0.20% counterparts is approximately 2.8 and 4.6%, respectively. The excessive presence of kaolin might also lead to additional pores and voids due to incomplete filling, creating weak zones within the concrete matrix. These vulnerable zones can act as stress concentrators, reducing the overall ITS values.

5.4 MOR

The MOR offers significant insights into the flexural strength of concrete materials, essentially determining their resistance to breakage under a bending load. Figure 6(a) explains the MOR for all samples over 28 and 90 days. When analyzing the trends from the MOR data, an apparent pattern emerges. Incorporation of both MAK and TAK by up to 0.20% enhances the MOR. However, any further increase in their percentages seemingly inverts this trend, causing a decrement in MOR. This phenomenon underscores kaolin’s pivotal role in augmenting the cementitious material’s structural matrix. As a pozzolanic agent, kaolin collaborates with the calcium hydroxide developing during cement hydration, precipitating a reaction that conjures additional calcium silicate hydrate gel. This gel intensifies the adhesion and interlocking within the UHPC, a factor confirmed by another research [52]. A closer examination of the data at the 28 days reveals the MOR of the reference mixture of 6.4 MPa. Introducing a 0.20% proportion of mechanically activated kaolin (B2-MAK-20) improves the MOR to 46.9%, with values reaching 9.4 MPa.

Figure 6

(a) MOR of UHPC at 28 and 90 days. (b) Statistical relationship of MOR between compressive and ITS at 28 and 90 days.

In contrast, the same percentage of thermally activated kaolin (B3-TAK-20) takes the MOR on an even higher increase, with an almost doubling increase of around 93.8%, reaching a value of 12.4 MPa. At a 90-day interval, the MOR of the reference mixture reached 7.1 MPa. Yet, adding 0.20% mechanically activated kaolin (B2-MAK-20) propels the MOR further by a commendable 58.5%, settling at 11.3 MPa. Meanwhile, the mix modified with 0.20% thermally activated nano-kaolin (B3-TAK-20) increased by about 101.4%, with an MOR of 14.3 MPa. In addition to the pozzolanic reaction, kaolin’s PSD and morphology play a crucial role in influencing MOR. Fine particles can potentially fill the voids between cement particles, creating a denser matrix less prone to cracking under bending loads. Moreover, the thermal treatment in TAK could lead to a more homogeneous PSD, which might be another reason for its superior performance compared to MAK in enhancing the MOR of the concrete.

However, at the 0.25% replacement level for mechanically activated nano-kaolin (B2-MAK-25) and thermally activated nano-kaolin (B3-TAK-25), there was a reduction in MOR compared to the 0.20% level. This reduction could be attributed to several factors. Excessive nano-kaolin leads to a congested microstructure at this higher replacement level, causing particle accumulation and potentially hindering the pozzolanic reaction. This may create weak zones or less effective bonding within the concrete matrix, decreasing the MOR values. The MOR reduces at a 0.25% replacement level compared to the 0.20%. At 28 days, the MOR for 25% mechanically activated nano-kaolin (B2-MAK-25) is reduced by approximately 6.4% compared to B2-MAK-20, and for 25% thermally activated nano-kaolin (B3-TAK-25), the reduction is about 12.1% compared to B3-TAK-20. At 90 days, the decrease in MOR values for B2-MAK-25 and B3-TAK-25 compared to their 0.20% counterparts is approximately 15.4 and 10.5%, respectively. Additionally, the microstructural homogeneity achieved with both types of kaolin is crucial in enhancing the distribution of the pozzolanic reaction products throughout the concrete matrix. This uniformity further contributes to better load distribution and stiffness [53]. Furthermore, the synergistic relationship between the increase in MOR and the corresponding rise in MOE is unmissable. When the MOR, a measure of the concrete’s resilience to bending stresses, increases, it indicates its enhanced capacity to bear loads. This capacity is shown in the improved MOE values.

The present study also employed linear regression analysis to predict the MOR (MPa) values at 28 and 90 days, utilizing the compressive and ITS values at the corresponding time points. The analysis revealed a strong linear relationship (Figure 6(b)), as evidenced by high regression coefficient values (R-squared) of 94.6 and 91.5% for 28 and 90 days, respectively. These high R-squared values indicate that approximately 94.6 and 91.5% of the variability in the MOR values can be explained by the linear relationship with the compressive strength and ITS values, respectively. Such a high level of explained variance suggests the results’ robustness and accuracy, bolstering the reliability of the estimated MOR values at both 28 and 90 days.

5.5 Acid attack test

The acid attack test on all UHPC samples at 90 days is presented in Figure 7(a) and (b). For UHPC samples with up to 0.20% kaolin replacement level, both mechanical and thermal activation processes demonstrated substantial improvements in the acid resistance. Specifically, the mechanically activated nano-kaolin (B2-MAK-20) increased residual compressive strength to 129.6 MPa, surpassing the reference mixture by 17.6%. On the other hand, the thermally activated nano-kaolin (B3-TAK-20) achieved an even more impressive performance, with its compressive strength elevating by 24.4% to reach 137.6 MPa. The mass loss, a crucial indicator of material degradation under acid attack, was mitigated by including kaolin at the 0.20% replacement level. Figure 7(b) shows that mechanically activated nano-kaolin (B2-MAK-20) curtailed the mass loss by 32.3%, resulting in an 8.4% mass loss. More notably, the thermally activated nano-kaolin (B3-TAK-20) exhibited a significant reduction in mass loss of around 46.4%, recording a final loss of just 6.7%. However, an observation has occurred when the kaolin content is increased to a 25% replacement level. The performance benefits were reversed, pointing toward an optimal threshold for kaolin content in UHPC. Both types of kaolin (B2-MAK-25 and B3-TAK-25) observed a rise in mass loss and a decline in residual compressive strength. Specifically, the mechanically activated nano-kaolin (B2-MAK-25) recorded a mass loss of 9.3%, marking a 25.0% increase from its 20% counterpart. Meanwhile, the thermally activated nano-kaolin (B3-TAK-25) witnessed a 13.4% increase in mass loss, resulting in a final value of 7.6%. Regarding compressive strength, the B2-MAK-25 recorded a reduction of 2.3% compared to B2-MAK-20, whereas B3-TAK-25 observed a 3.7% reduction compared to B3-TAK-20. These findings underline the importance of optimizing the kaolin content in UHPC mixes to harness the maximum benefits of acid resistance. The 0.20% replacement level appears optimal, with diminishing returns observed beyond this threshold.

Figure 7

Acid attack test of UHPC: (a) residual compressive strength (MPa) and (b) mass loss (%).

The enhanced acid resilience of the UHPC can be drawn from incorporating kaolin into its matrix. As a recognized pozzolanic agent, kaolin instigates the formation of additional calcium silicate hydrate gel during the cement’s hydration phase. This added C–S–H gel acts as a protection, develops the densification of the microstructure, and mitigates its inherent porosity. Consequently, this strong and compacted matrix emerges as a tough barrier to acid penetration [27]. Furthermore, the individual kaolin granules serve a dual function. Not only do they complement the hydration reaction, but they also settle into the interstitial spaces of the UHPC, behaving similarly to a protective layer. Their presence hinders the open infiltration of acid, shielding the concrete from the effects of corrosive entities. This protective layer appears to fade when the kaolin substitution surpasses the 0.20% mark, reaching a 0.25% replacement level, especially noticeable in mechanically activated kaolin (B2-MAK-25) and its thermally activated counterpart (B3-TAK-25). This downturn is risked to stem from an over-saturation of kaolin. At such elevated concentrations of 0.25%, the kaolin particles may clump together instead of integrating, leading to a congested and irregular microstructural scene. Such accumulations lead to weak regions undermining the concrete’s acid-resisting ability [28]. Moreover, this increased (0.25%) substitution rate can skew the UHPC’s intrinsic water-to-cementitious material ratio, degrading the durability, particularly when exposed to acidic attack.

5.6 Sorptivity test

The results from the sorptivity test of all UHPC samples are illustrated in Figure 8, showcasing the evolution of the sorptivity coefficient about the kaolin content. A noticeable decrease in this coefficient is apparent as the proportion of both mechanically and thermally activated kaolin increases. The underlying reason for this trend can be rooted in kaolin’s instrumental role in refining the microstructure and pore topology of the concrete, thereby strengthening its resistance against water ingress. Notably, as the kaolin proportion increases from a modest 0.5% to a more substantial 0.25%, the sorptivity coefficient undergoes a consistent decline across intervals of 28, 56, and 90 days. This trajectory underscores the inference that UHPC formulations manifest an augmented imperviousness to water permeation when increased with richer kaolin concentrations. This noticeable reduction in the sorptivity coefficient is linked to kaolin’s pozzolanic reactivity. When calcium hydroxide is a byproduct of cement hydration, kaolin undergoes a pozzolanic reaction. Given that both the mechanically and thermally activated variants of kaolin are pozzolanic, their consolidation into the UHPC leads to the creation of supplemental calcium silicate hydrate gel [54]. This gel acts as an infill, bridging voids and sealing off pores intrinsic to the concrete structure, thereby truncating the interconnected porosity network. The result is a narrowing of the pathways that allow water to infiltrate the concrete matrix, resulting in the noted reduction in the sorptivity coefficient.

Figure 8

The sorptivity coefficient of UHPC at 28, 56, and 90 days.

Examining the data at the 28 days, the reference mixture had a sorptivity coefficient of 0.196 mm/min^0.5. A 0.5% cement substitution with mechanically activated kaolin (B2-MAK-05) lowers this value by about 2.1% to 0.1912 mm/min^0.5. Its thermally activated counterpart, B3-TAK-05, effects a more pronounced reduction of approximately 5.1%, leading to a coefficient of 0.1907 mm/min^0.5. At 56 days, the reference blend’s coefficient contracts 0.185 mm/min^0.5. B2-MAK-05 further reduces this by roughly 2.7% to 0.18 mm/min^0.5, while B3-TAK-05 observes a decrement of about 3.3%, settling at 0.1801 mm/min^0.5. At 90 days, the reference mix diminishes further to 0.1655 mm/min^0.5. Introducing 5% mechanically activated kaolin (B2-MAK-05) yields a significant drop of around 15.8% to 0.156 mm/min^0.5. In parallel, including 5% thermally activated kaolin (B3-TAK-05) lowers the coefficient by an estimated 14.4% to 0.1581 mm/min^0.5. This downward trend in the sorptivity coefficient continues until the kaolin increases to the 20% threshold for both mechanically and thermally activated forms. However, increasing this threshold and venturing to a 0.25% proportion witnesses a slight increase in the sorptivity coefficient from the 0.20% benchmark. This difference could stem from an overconcentration of kaolin, which, rather than enhancing, could disrupt the microstructural arrangement and cause an increase in the porosity and interconnected void network. Such behavior could compromise the UHPC’s hydrophobic characteristics, rendering it more susceptible to water penetration [55].

5.7 Elevated temperature test

The effects of elevated temperature on all UHPC samples after a curing period of 90 days are illustrated in Figure 9(a) and (b). The changes in the residual compressive strength and mass loss, especially when subjected to a temperature gradient spanning 250–1,000°C, support the critical role of kaolin in moderating the concrete’s microstructural behavior and its inherent thermal characteristics. Notably, when the cement component in UHPC samples was substituted by up to 0.20% with both mechanically and thermally activated kaolin, a visible decline in both mass loss and the residual compressive strength was observed with increasing temperature. This trajectory emphasizes that incorporating kaolin enhances thermo-mechanical resilience in the concrete matrix [56]. The presence of kaolin resulted in the development of additional C–S–H gel over pozzolanic reactions, contributing to a more compact and denser microstructure [57]. This microstructural refinement improved thermal resistance and reduced mass loss at elevated temperatures. The sample with 0.20% thermally activated kaolin (B3-TAK-20) demonstrated the most optimal values regarding residual compressive strength and mass loss compared to the reference mixture and the samples with mechanically activated kaolin. Substituting 0.20% of cement with mechanically activated kaolin (B2-MAK-20) improved residual compressive strength compared to the reference sample at all elevated temperatures. At 250°C, the residual compressive strength is 133.5 MPa, gradually decreasing (Figure 9(a)) to 112.4, 80.4, and 39.7 MPa at 500, 750, and 1,000°C. Similarly, replacing 20% of cement with thermally activated kaolin (B3-TAK-20) resulted in an even more significant enhancement in residual compressive strength compared to the reference mixture at all elevated temperatures. At 250°C, the residual compressive strength is 143.7 MPa, decreasing to 119.7, 85.6, and 40.4 MPa at 500, 750, and 1,000°C, respectively.

Figure 9

Elevated temperature test of UHPC: (a) residual compressive strength (MPa) and (b) mass loss (%).

The improved effectiveness observed at the 0.20% kaolin inclusion level emphasizes the benefits of the heat treatment inherent to thermally activated kaolin. This thermal process instigates a pivotal alteration in the kaolin’s intrinsic architecture, enhancing its compatibility and reactivity with the Ca(OH)₂, a by-product generated during OPC hydration [58]. The result is tangible in the UHPC sample B3-TAK-20, characterized by a more refined microstructure and augmented thermal toughness. These features synergistically work to lower mass loss rates and improve residual compressive strength, rendering it practical to its counterpart, B2-MAK-20, and the baseline reference. However, this phenomenon hit a threshold at a kaolin inclusion of 0.25%. At this stage, both mechanically activated (B2-MAK-25) and thermally activated kaolin (B3-TAK-25) variants experience a regression in performance, particularly regarding residual compressive strength, across all evaluated high temperatures. At 250°C, B2-MAK-25 has a residual compressive strength of 129.7 MPa, reducing to 108.7, 79.2, and 37.2 MPa at 500, 750, and 1,000°C, respectively. For B3-TAK-25, the residual compressive strength at 250°C is 137.4 MPa, decreasing to 119.7 MPa, 85.6, and 40.4 MPa at 500, 750, and 1,000°C, respectively. This behavior might suggest that while introducing kaolin benefits UHPC, an excessive concentration might be counterproductive. The optimal spot appears to be around 0.20% inclusion, beyond which the benefits of kaolin diminish, and its limitations become more pronounced.

The examination of mass loss of UHPC at elevated temperatures reveals insightful findings about its behavior when subjected to heat. When analyzed at different temperature benchmarks, varying outcomes develop based on the composition of the mixture. At 250°C, the reference mixture exhibited a mass loss of 8.6%. However, the mass loss decreased substantially when 0.20% of the OPC was replaced with mechanically activated kaolin (B2-MAK-20). The reduction was 28.7%, resulting in a mass loss of 6.2%. In contrast, 20% thermally activated kaolin (B3-TAK-20) led to an even more significant reduction. The mass loss was reduced by about 33.1% to reach 5.7%. These changes are presented in Figure 9(b). When the temperature was raised to 500°C, the reference mixture demonstrated a mass loss of 29.5%. Incorporating 0.20% mechanically activated nano-kaolin (B2-MAK-20) into the mixture reduced the mass loss by approximately 26.1%, resulting in a final mass loss of 21.6%. On the other hand, using 20% thermally activated kaolin (B3-TAK-20) further reduced the mass loss by 33.4%, with the loss recorded at 19.6%. At an even higher temperature of 750°C, the reference mixture showed a mass loss of 44.2%. When 20% of cement was substituted with mechanically activated kaolin (B2-MAK-20), the mass loss decreased by about 21.9%, leading to a new figure of 34.5%. However, when 0.20% thermally activated kaolin (B3-TAK-20) was used, the reduction in mass loss was approximately 29.4%, with the mass loss registering at 31.2%. Lastly, at the extreme temperature of 1,000°C, the reference mixture had a mass loss of 76.4%. By integrating 0.20% mechanically activated kaolin (B2-MAK-20) into the mixture, the mass loss dropped by roughly 13.1%, settling at 65.8%. Meanwhile, with 0.20% thermally activated kaolin (B3-TAK-20), the reduction in mass loss was about 18.3%, concluding at 62.4%.

However, at a 25% replacement level with both mechanically activated kaolin (B2-MAK-25) and thermally activated kaolin (B3-TAK-25), there was an increase in mass loss and a decrease in residual compressive strength at elevated temperatures compared to the 0.20% level. This trend suggested excessive kaolin content leads to a less favorable microstructure under high-temperature conditions [59]. The overcrowding of kaolin particles may create weak zones and hinder the development of an effective C–S–H gel network, resulting in reduced thermal resistance and increased mass loss [48,56,60]. These findings suggest that the addition of mechanically or thermally activated kaolin can significantly enhance the high-temperature resistance of UHPC. Among the two, thermally activated kaolin offers a more substantial reduction in mass loss across the temperature range evaluated.

5.8 Water absorption test

The water absorption test, conducted at intervals of 28, 56, and 90 days, offers intriguing insights into the behavior of UHPC samples embedded with varying proportions of mechanically and thermally activated kaolin. The graphical representation in Figure 10 emphasizes the influence of kaolin on the concrete’s microstructural evolution and hydration dynamics. At 28 days, an unexpected difference was recorded across all UHPC specimens, delineating a discernible uptick in water absorption rates when juxtaposed against the reference mix. This phenomenon seemingly contradicts the anticipated performance metrics, where enhanced material processing techniques and the incorporation of nanomaterials aim to reduce porosity and water permeability. The elevation in water absorption suggests an intricate interplay between the microstructural characteristics of the UHPC formulations and their hydrodynamic behavior. Factors such as the distribution and size of pores, the efficiency of particle packing, and the presence of micro-cracks or unreacted pockets within the matrix could significantly influence water ingress. Moreover, the specific ratios of nano-kaolin can alter the hydration kinetics and the development of the cementitious matrix, potentially leading to a more porous structure. Additionally, the increase in water absorption could indicate a higher degree of interconnected porosity within the UHPC matrix, which, while detrimental to the material’s resistance to water penetration, does not necessarily undermine its mechanical properties. This nuanced understanding underscores the complexity of optimizing UHPC formulations for mechanical performance and durability against environmental ingress, highlighting the need for a balanced approach in material design and engineering. This phenomenon can be traced back to the initial pozzolanic reactions between the kaolin and the calcium hydroxide formed during the cement’s hydration phase [61]. Kaolin’s introduction fosters the beginning of supplementary reaction by-products, notably the calcium silicate hydrate gel. This gel plays a pivotal role in refining the concrete’s microstructure. Nonetheless, at this developing stage of 28 days, the concrete’s microstructural maturation is incomplete, leaving open pores and voids. This partly developed microstructure results in an enhanced tendency for water absorption compared to the reference mix.

Figure 10

Water absorption of UHPC at 28, 56, and 90 days.

As the curing extends to 56 and 90 days, a pronounced decline in water absorption is evident. This diminishing trend is supported by the sustained pozzolanic activity between the kaolin and calcium hydroxide [62]. With the C–S–H gel’s mass and the microstructure’s compaction, open porosities are progressively sealed off, regulating the paths for water ingress. Thus, by 90 days, the UHPC specimens’ water absorption tendency has been significantly reduced compared to their 28-day state. At 28 days, the water absorption for the reference mixture was 10.7%. When 5% OPC was replaced with 5% MAK and TAK, water absorption increased by approximately 5.6%, reaching 11.3%. The 0.5% thermally activated kaolin (B3-TAK-05) increased by about 12.6% to reach 12.03%. At 56 days, the water absorption for the reference mixture was 9.9%. With 0.5% mechanically activated nano-kaolin (B2-MAK-05), the water absorption decreased by approximately 8.2% to reach 9.07%, while for 0.5% thermally activated nano-kaolin (B3-TAK-05), this reduction was about 8.4% to reach 8.82%. At 90 days, the water absorption for the reference mixture was 8.68%. Substituting 0.5% of OPC with mechanically activated nano-kaolin (B2-MAK-05) decreased water absorption by approximately 1.5%, reaching 7.63%. The decrease of 5% in thermally activated nano-kaolin (B3-TAK-05) was about 1.2–7.63%.

At the 28-day interval, when 0.25% of cement was replaced by mechanically activated nano-kaolin (B2-MAK-25) and thermally activated nano-kaolin (B3-TAK-25), there was a noticeable increase in water absorption by approximately 17.8 and 19.3%, respectively. This initial increase can be attributed to the preliminary pozzolanic reactions of the kaolin variants. However, as the curing process advanced to 56 days, a prominent reversal in trend was observed. Water absorption for B2-MAK-25 decreased significantly by roughly 20.6% relative to the control mix. Similarly, for B3-TAK-25, the reduction was even more pronounced at approximately 22.9%. Continuing on this path, by the 90-day curing, B2-MAK-25 and B3-TAK-25 observed a decrease in water absorption by around 11.1 and 12.3%, respectively, when compared against the reference mixture. These results showed that the thermally activated kaolin consistently outperformed its mechanically activated counterpart regarding water absorption during the 56- and 90-day intervals. This superiority of thermally activated nano-kaolin can be linked to its structural modifications, a consequence of the heat treatment process. This thermal activation emphasized its pozzolanic responsiveness, thereby allowing it to play a pivotal role in refining the microstructure of the concrete [63]. As a direct fallout, the concrete exhibited diminished porosity, resulting in lower water absorption values during the curing process’s latter stages (56 and 90 days). However, at 28 days, thermally activated kaolin showed slightly higher water absorption than mechanically activated nano-kaolin. This could be due to the initial delay in the pozzolanic activity of thermally activated nano-kaolin during the early curing stages. As the kaolin particles require sufficient time to react with CH and form the C–S–H gel, a slight increase in water absorption is observed at this early age compared to the mechanically activated kaolin samples.

5.9 Porosity test

Figure 11 provides insight into the porosity characteristics of UHPC samples, emphasizing the impact of using mechanically and thermally activated kaolin as a cement substitute. The derived data from the graph reveal captivating insights into the interaction between pore size and porosity, essentially showcasing how the material’s internal structure influences its porosity properties. The porosity test result for UHPC shows three curve lines representing the porosity (%) at different pore diameters (nm) for three different mixtures: the reference mixture, B3-TAK-20 (sample with 0.20% thermally activated nano-kaolin), and B2-MAK-20 (sample with 0.20% mechanically activated nano-kaolin). The dashed line, representing the reference UHPC sample, exhibits the highest porosity across most pore diameters. This indicates a relatively more porous structure in comparison to the other samples. The dotted line, representing the UHPC sample with 0.20% mechanically activated kaolin (B2-MAK-20) as a partial substitute for OPC, demonstrates a noticeable reduction in porosity, especially in the smaller pore diameter range. This suggests that mechanically activated nano-kaolin has a role in refining the pore structure, leading to lower porosity. Most prominently, the solid line representing the UHPC sample with 0.20% thermally activated nano-kaolin (B3-TAK-20) showcases the lowest porosity across almost all pore diameters.

Figure 11

Porosity test of UHPC.

Initially, all three curve lines start at 4 nm (24%) for the reference mixture, 4 nm (11%) for B3-TAK-20, and 4 nm (14.5%) for B2-MAK-20. This indicates that all three mixtures have some porosity at this pore diameter. However, an observation was made when the pore diameter increased to around 30 nm. At this point, all three curve lines exhibit a sudden decrease in porosity, with the reference mixture’s porosity dropping to 15%, B3-TAK-20 to 8%, and B2-MAK-20 to 11%. The reason behind this abrupt decrease in porosity can be attributed to the microstructure of the UHPC. At smaller pore sizes, the cementitious matrix and filler materials (such as kaolin) fill the void spaces effectively, resulting in higher porosity [64]. However, as the pore size increases, the microstructure of the UHPC starts to form a more compact and denser network, reducing the overall porosity. As the pore diameter continues to increase beyond 30 nm, the three curve lines show a gradual decrease in porosity. This indicates that the microstructure continues to become denser with larger pores, reducing the amount of void space within the UHPC [65]. The slight reduction in porosity observed just before the end of the curve lines at around 100,000 nm could be due to larger voids or air pockets within the concrete mix. These larger voids may be challenging to fill with the cementitious matrix, resulting in a slight increase in porosity [66].

The differences become more pronounced as the pore diameter increases towards 1,000 nm. The reference UHPC sample exhibits porosity approaching 0.20%, suggesting a more porous structure. In contrast, the B2-MAK-20 sample registers a porosity of about 15%. The most significant reduction is in the B3-TAM-20 sample, where the porosity is slightly above 10%. For larger pore diameters beyond 100,000 nm, the reference sample’s porosity appears to plateau, maintaining close to 0.20%. The mechanically activated nano-kaolin sample (dotted line) stabilizes around 15%. Notably, the thermally activated kaolin sample’s porosity (solid line) remains the lowest, further emphasizing its effectiveness in reducing porosity, settling just above 10%. Finally, the porosity drops again when reaching 100,000 nm. This could be because huge pores might be excluded from the analysis, leading to decreased apparent porosity. The observed variations in porosity in the porosity test result for UHPC can be attributed to the microstructure of the concrete mixtures. Factors such as the size and distribution of pores, the effectiveness of filler materials like kaolin, and the overall densification of the microstructure play crucial roles in determining the porosity at different pore diameters. Understanding and optimizing the microstructure is essential in designing UHPC with desired properties such as high strength and durability. While the reference UHPC sample consistently shows the highest porosity across varying pore diameters, incorporating mechanically activated kaolin leads to up to 5% reductions. The effect of thermally activated kaolin is even more pronounced, which reduces porosity by nearly 10% compared to the reference, especially in the mid-range pore diameters.