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Article

Model Experimental Investigation on the Mechanical Properties of Recycled Aggregate Concrete Curbs by Incorporating Metakaolin and Basalt Fibre

by
Mengyao Wang
1,
Xueyuan Zhang
1,*,
Biao Zhang
1,
Daoming Zhang
1,2,
Dandan Wang
1 and
Yu Zhang
1
1
School of Architecture and Civil Engineering, Qiqihar University, Qiqihar 161006, China
2
Heilongjiang Xinyu Cement Products Co., Ltd., Qiqihar 161099, China
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(17), 3059; https://doi.org/10.3390/buildings15173059
Submission received: 18 July 2025 / Revised: 19 August 2025 / Accepted: 25 August 2025 / Published: 27 August 2025
(This article belongs to the Section Building Materials, and Repair & Renovation)
Browse Figures
Versions Notes

Abstract

To investigate the potential of metakaolin (MK) (5%, 10%, 15%, and 20% substitution of cement mass) and basalt fibre (volume contents of 0.1%, 0.2%, and 0.3%) in recycled aggregate concrete (RAC) products, RAC’s mechanical properties were first assessed with a singular incorporation of MK. The findings demonstrated that adding 15% MK optimised the compressive strength and flexural strength of RAC (at the recycled aggregate replacement levels of 30%, 45%, and 60% by weight). An orthogonal test was conducted to investigate the synergistic effect of MK and basalt fibre (BF), with the recycled coarse aggregate (RCA) replacement rate (mass ratio of RCA to natural coarse aggregates), MK content (cement mass substitution percentage), and BF content (volume dosage) identified as the influencing parameters. The variance analysis reveals that the influence of the replacement ratio of RCA on compressive strength surpasses that of MK content, which in turn exceeds that of BF content. Conversely, as for the flexural strength, BF is substantially more effective than that of MK. A model test of RAC curbs was performed based on the ideal mix ratio suggested by the single mixing of MK and MK and BF compound mixing inside the orthogonal test. The results demonstrate that the RAC curbs, with an RCA replacement rate of 30%, display optimal mechanical properties when 15% MK and 0.2% BF are incorporated. This surpasses the performance of 15% MK alone and illustrates that the mix incorporation of MK and BF is superior to that of MK alone.

1. Introduction

Concrete, as the primary and most widely utilised construction material, produces 30 billion tonnes per year worldwide [1]. Concurrently, the amount of waste concrete produced by the demolition of old buildings and structures annually has reached an extraordinary billion tonnes [2,3]. The rational and efficient utilisation of waste concrete to reduce its adverse environmental effects is a significant topic.
Currently, recycled aggregate obtained from the pulverisation of discarded concrete provides an innovative resolution. However, due to the weakened strength of recycled aggregate, recycled aggregate concrete’s mechanical performance is usually inferior to that of natural aggregate concrete [4,5], hence limiting its use in structural engineering. Unlike structural members, non-structural members (e.g., concrete curbs, concrete pavement bricks) have fewer bearing capacity requirements, hence permitting a broader spectrum of applications for recycled aggregate concrete (RAC). Figure 1 illustrates the manufacture of RAC products.
The concrete curb, as a non-structural component, is widely employed in municipal road engineering. The market size has exceeded 300 million USD annually on a global scale [6], and the annual output is substantial. During the manufacturing of concrete curbs, replacing natural aggregate with recycled aggregate can diminish resource consumption and construction expenses. In contrast to natural aggregate concrete curbs, recycled aggregate concrete curbs exhibit issues such as reduced compressive strength, increased cracking, and inadequate durability, requiring immediate intervention by new technology or new material [7].
Currently, several studies suggest that the strength of RAC curbs can be improved by integrating copper slag, glass fibre-reinforced polymers, rubber, or polyvinyl alcohol fibre [8,9,10,11,12,13,14]. Nonetheless, considering cost, environmental sustainability, and performance stability, the execution of these techniques necessitates empirical evaluation.
A plethora of experimental investigations have been carried out to reinforce the performance of RAC. The findings indicate that strengthening the intensity of the interfacial transition zone (ITZ) around recycled aggregate is the most common and practical approach. The integration of fibrous materials (e.g., basalt fibre, steel fibre, and polypropylene fibre) and supplementary cementitious materials (e.g., nano-silica, silica fume, fly ash, granulated blast furnace slag powder, and metakaolin) has attracted heightened attention from researchers [2,15,16,17,18,19,20,21,22,23,24,25]. Multiple studies have demonstrated that nanomaterials have the potential to enhance the property of RAC; nevertheless, their elevated cost and challenges in dispersion limit their extensive use in engineering [26,27]. Conversely, granulated blast furnace slag powder or fly ash yield only slight enhancements in mechanical performance [28,29,30]. Metakaolin (MK), manufactured by calcining kaolin at temperatures of 650–800 °C and noted for its strong pozzolanic activity, affordability, widespread availability, and low carbon emissions, has recently attracted considerable attention compared to these cementitious materials mentioned above [31,32].
Experimental studies have demonstrated that silica in MK will undergo a secondary hydration reaction with CH to generate C-S-H gel, which can reduce the quantity of internal micropores, lower porosity, improve the ITZ, and fill the internal voids and fissures of RAC [31,33,34,35,36]. The incorporation of 10% to 20% MK can strengthen the mechanical properties of RAC by 10% to 30%, indicating a considerable enhancement impact [35,37,38,39,40]. The RAC strength prediction model utilising the experimental–statistical method effectively forecasts and validates the RAC’s mechanical characteristics, which is crucial for the research and application of RAC [41,42]. Despite a number of experimental research on the mechanical characteristics of MK-RAC, investigations into the strength prediction model for MK-RAC based on these results are scarce.
The aforementioned investigations are crucial for elucidating the enhancing mechanism by which MK enhances RAC’s mechanical properties. It may be inferred that most of studies concentrate on the strengthening effects of the sole mixing of MK or its combination with other cementitious materials, whereas fewer investigations address the integration of MK with fibres. Moreover, the recycled aggregates examined in the majority of studies are primarily single-sourced, such as waste concrete derived from the demolition of a particular structure or fabricated in a laboratory environment. However, there exists a scarcity of study about recycled aggregates obtained from construction solid waste recycling companies, characterised by varied origins and significant variations in aggregate strength.
Basalt fibre (BF) is an innovative inorganic fibre with the characteristics of superior tensile strength, remarkable silicate compatibility, great corrosion resistance, and environmental sustainability [4,43,44]. It can markedly improve RAC’s properties. Research shows that adding BF to RAC (at a volumetric proportion of 0.1–0.3%) can increase its mechanical strength by 5–15%. Excessive addition of BF may lead to agglomeration, hence diminishing the strength of RAC [44,45,46,47,48,49]. Orthogonal tests have the benefits of minimising the number of tests and enhancing economic efficiency. They can evaluate multiple variables and have been extensively utilised in the examination of RAC’s mechanical properties. However, limited orthogonal test studies were conducted to investigate the addition of BF to RAC or MK to RAC [50,51,52,53,54].
The feasibility of a solitary MK admixture and the combination of MK and BF in RAC products is investigated in this study, utilising recycled aggregate provided by a construction solid waste treatment enterprise in Heilongjiang Province, China. The mechanical properties of RAC with recycled coarse aggregate (RCA) replacement rates of 30%, 45%, and 60% were evaluated at various MK contents (5%, 10%, 15%, and 20% cement substitution by mass). An orthogonal test was performed to investigate the mechanical behaviour of RAC incorporating MK and BF, concentrating on the variables: the RCA replacement rate, the MK content, and the BF volume content (0.1%, 0.2%, and 0.3%). A model test of metakaolin recycled aggregate concrete curbs was undertaken to validate the feasibility of the optimal mix ratio scheme proposed by the single mixing of MK and the combination of MK and BF in the orthogonal test.

2. Materials and Test Scheme

2.1. Materials Used in This Study

Cementitious materials: consisting of cement and MK. The cement utilised was composite silicate cement (P. C. 42.5), comprising 13.3% slag powder and 26.8% fly ash, manufactured by a local company in Qiqihar [7]. According to GB/T 18736-2017 [55], the MK parameters were quantified as follows: The activity index is 115, and the fineness is 10 μm. The chemical compositions of PC and MK are displayed in Table 1. Figure 2 displays the images of MK by scanning electron microscopy (SEM) and mineral composition by X-ray diffraction (XRD). The SEM morphology scan reveals that the MK particles are fine, while the XRD examination shows the existence of quartz, exhibiting the hump characteristic at 15–35°, indicative of amorphous components [56,57].
Aggregates: natural fine aggregate (NFA, with a fineness modulus of 2.85), natural coarse aggregate (NCA) and RCA (provided by a construction solid waste treatment enterprise in Heilongjiang). The RCA and NCA’s physical properties are illustrated in Table 2 [7]. The particle size distributions of NCA, RCA, and NFA are presented in Table 3 and Table 4. Both the RCA and NCA adhered to the Chinese standard GB/T 14685-2022 [58].
Fibres: BF with a length of 6 mm was utilised. The physical parameters of BF were measured in conformance with the standard GB/T 23265-2023, as presented in Table 5 [59]. The standard deviations of the measured length and density are 0.2 mm and 0.04 g/cm3, respectively, both conforming to the standard’ requirements for standard deviations.
Water and admixture: Tap water and water-reducing admixture with a minimum water reduction rate of 25% are utilised in this study.

2.2. Experimental Program

The compressive performance (at 3, 7, and 28 d) and 28 d flexural performance (at 28 d) of RAC with differing MK content to determine the impact of MK on the mechanical performance of RAC were evaluated, whose experiment program is displayed in Table 6. An orthogonal test, shown in Table 7 and Table 8, was performed to investigate the influence of certain factors—recycled aggregate replacement rate, MK content, and BF volume content—on the mechanical properties of RAC incorporating MK and BF.
According to references [35,39,43,46], the recycled coarse aggregate replacement ratio of 30%, 45%, and 60%; MK content levels of, 5%, 10%, 15%, and 20%; and BF volume concentrations of 0.1%, 0.2%, and 0.3% were selected (refer to Table 6 and Table 7). The mix ratios of RAC, metakaolinite RAC (MK-RAC), and metakaolinite–basalt fibre RAC (MK-BF RAC) are presented in Table 6 and Table 9. The quantity of aggregates (NCA and NFA) was determined following the quality method proposed in JGJ55-2011 [60].

2.3. Specimen Preparation and Test Method

Figure 3 depicts the preparation and curing of specimens. First, the aggregates entered a horizontal shaft mixer for 30 s, then a mixed solution of water and water-reducing agent was added (50% of the required mass) and mixed for 60 s. Thereafter, the cementitious material and BF were introduced, ensuring even distribution by hand to avoid clumping, and mixing for an additional 60 s. Finally, the residual water and the water-reducing agent solution were added, mixing for 90 s.
Upon completion of stirring, the slump test was conducted in accordance with GB/T 50080-2016, a Chinese standard [61]. According to the specification JGJ55-2011, the concrete produced by this study is classified as plastic concrete [60]. Upon testing, all of the concrete combinations in Table 6 and Table 9 have slump ranges between 10 and 95 mm, which complies with the specification. The law of slump is delineated as follows: RAC exceeds MK-RAC, which in turn surpasses MK-BF RAC. Following completion of the slump test, the concrete specimens are prepared. The concrete was poured into moulds (100 × 100 × 100 mm and 100 × 100 × 400 mm for the compression test and flexural test, respectively). Subject the moulds to vibration on a vibrating table at a frequency of 50 ± 2 Hz for 30 s, followed by indoor curing for 24 h prior to demoulding. The concrete was cured in a standard environment (temperature of 20 ± 2 °C and relative humidity of more than 95%) in accordance with the standard GB/T 50081-2019 [62].
The compressive strength test utilised a pressure testing machine (Model YAW-2000) at a loading speed of 0.5 Mpa/s, while the flexural strength test was performed using a hydraulic universal testing machine (Model WDW-1000B) at a loading speed of 0.05 Mpa/s. Figure 4 displays the representative specimens for the mechanical properties test.

3. Results and Discussion

The specimens for the compressive performance test comprised 57 groups, whereas those for the flexural strength consisted of 25 groups, each including 3 blocks. The procedures for the mechanical performance test and the test results calculation are in accordance with the standard GB/T 50081-2019 [62].

3.1. Impact of MK on RAC’s Mechanical Properties

3.1.1. The Compressive Strength of RAC with Single Addition of MK

Figure 5 depicts the compressive strength (at 3 d) and strength growth rates of RAC with MK substitution levels of 0%, 5%, 10%, 15%, and 20%. The 3 d compressive strength of RAC (at replacement levels of 30%, 45%, and 60%) was 15.0 Mpa, 14.1 Mpa, and 12.9 Mpa, respectively, which exhibited a gradual decrease with the increasing of the RCA replacement rate. Adding MK can enhance the RAC’s compressive strength. At RCA replacement rates of 30% and 60%, MK-RAC’s compressive strength first increases, subsequently declining as the MK admixture content rises. For MK-RAC with an RCA substitution rate of 45%, the compressive strength constantly improves with the incorporation of MK admixture. This law of intensity variation corresponds with the findings of the studies performed by Younis [37] and Muduli [63].
Specifically, at RCA replacement rates of 30%, 45%, and 60%, the compressive strength of RAC incorporating 15% MK is 18.0, 16.7, and 15.1 Mpa, respectively, which is increased by 20.00%, 18.44%, and 17.05%, respectively, compared to that of RAC without MK addition. In the scenario where MK content is 20%, the RAC with RCA replacement rates of 30% and 60% yields lower compressive strength values of 17.2 Mpa and 14.5 Mpa, respectively.
However, a further improvement in compressive strength was observed for the RAC with an RCA replacement rate of 45%, which resulted in 17.3 MPa for compressive strength, indicating a strength increase of 22.70%. The observed strength enhancement is attributed to the secondary hydration of silica in MK and the cement hydrated product calcium hydroxide (CH), which generates extra calcium silicate hydrate (C-S-H gel), increasing the matrix’s density, diminishing internal porosity, and thus improving the strength of the RAC [39].
Figure 6 and Figure 7 illustrate MK-RAC’s compressive strengths (at 7 and 28 d) along with the strength growth rates. These compressive strengths exhibited an increase relative to that of 3 d due to the longer curing time. As the substitution rate of RCA increased in RAC, the compressive strength gradually declined. At RCA substitution rates of 30%, 45%, and 60%, the compressive strengths were recorded as 23.6, 22.4, and 20.7 MPa at 7 days, and 32.7, 30.6, and 28.4 MPa at 28 days, respectively, which represent reductions of 13.24%, 17.65%, and 23.90% at 7 days and 11.14%, 16.85%, and 22.83% at 28 days, respectively, compared to that of NAC. Additionally, RAC’s compressive strengths (at 3 d and 7 d) can reach 45.87%, 46.08%, and 45.42% at 3 days and 72.17%, 73.20%, and 72.89% at 7 days, in relation to that of 28 d, respectively.
The variation rule for 7-day compressive strength in MK-RAC differs little from that for 3-day compressive strength. At the RCA replacement rate of 30%, the compressive performance of RAC improves as the dose of MK increases. As the content of MK grew, the RAC’s compressive properties initially rose and subsequently declined at recycled aggregate substitution rates of 45% and 60%. The compressive strength of RAC with a 15% MK admixture was 27.4, 26.6, and 24.7 MPa for RCA substitution rates of 30%, 45%, and 60%, respectively. This enhancement was achieved by 16.10%, 18.75%, and 19.32%, respectively, in comparison to RAC without MK admixture.
The 28-day compressive strength of MK-RAC with RCA substitution ratios of 30%, 45%, and 60% exhibited an initial rise followed by a decrease. As the MK content rose from 5% to 10% to 15%, the RAC’s compressive strength consistently improved. As the amount of MK reached 20%, the RAC’s compressive strength decreased. With 15% MK substitution, the compressive strengths at 28 d were 38.6, 35.9, and 33.1 MPa, respectively, reflecting increases of 18.04%, 17.32%, and 16.55%, respectively, compared to the scenario without MK substitution. With 20% MK substitution, the RAC’s compressive strength increased by 12.54%, 11.44%, and 9.86%, respectively. This can be attributed to the dilutive effect of excessive MK substitution, which diminishes RAC compressive strength by decreasing cement clinker content. These findings were also confirmed by the studies of Muduli [35].

3.1.2. The Flexural Strength of RAC (Single Mixing with MK)

Figure 8 depicts the 28-day flexural strengths and growth rates of NAC and RAC with replacement rates of 30%, 45%, and 60%, in addition to RAC incorporating MK at proportions of 5%, 10%, 15%, and 20%. The RAC’s flexural performance is inferior to that of NAC, which exhibits a flexural strength of 4.1 MPa. As the replacement ratio of RCA rises, RAC’s flexural strength decreases. At replacement rates of 30%, 45%, and 60%, the flexural strengths of RAC are 3.6, 3.4, and 3.1 MPa, respectively, reflecting reductions of 12.20%, 17.07%, and 24.39% compared to NAC. This pattern of strength variation aligns with that of Muduli et al. [35].
Incorporating MK can initially increase RAC’s flexural strength, which subsequently declined with higher MK concentrations. At MK dosages of 5, 10, 15, and 20%, the flexural strength of RAC (replacement rate of RCA at 30%) was measured at 3.8, 4.0, 4.3, and 4.1 MPa, respectively, reflecting increases of 5.56%, 11.11%, 19.44%, and 13.89% compared to specimen R30M0 (without MK incorporation). Flexural strength reached its peak at 15% MK, similar to compressive strength. In both instances of 45% and 60% recycled aggregate substitution rates, RAC’s flexural strength initially increases and then falls with rising MK substitution rates.
The greatest notable improvement in flexural strength was observed at a 15% MK substitution rate, yielding increases of 17.65% and 16.13% compared to that of specimens R45M0 and R60M0, respectively. Nonetheless, as the MK substitution rate increased to 20%, the flexural strength decreased, although it continued to exceed the flexural strength of RAC without MK incorporation (R45M0, R60M0). The consistency of this alteration is due to the dilutive impact of the cement clinker, which decreases the availability of CH for reaction with the silica in MK, consequently producing a diminished quantity of C-S-H gel and ultimately leading to a decrease in strength [63].

3.2. Impact of MK and BF on RAC’s Mechanical Properties

3.2.1. The Compressive Strength of MK-BF RAC

Table 10 shows MK-BF RAC’s compressive strength (corresponding to the scheme in Table 8), as well as the range analysis based on the orthogonal test. MK-BF RAC’s compressive strength is affected by factor A (RCA replacement rate), factor B (MK content), and factor C (BF volume content) in the following order of significance: A surpasses B, and B surpasses C. The optimal formulation for enhancing compressive strength is A1B2C2, indicating that compressive strength is maximised when the RCA substitution rate is 30%, the content of MK is 15%, and the volume dosage of BF is 0.2%.
Figure 9 depicts the relationship between the levels of factors and the compressive strength for RAC. A reduction in compressive performance is presented as an increase in the replacement rate. Specifically, as the RCA replacement rate increases from 30% to 45%, the compressive strength decreases by 2.72%; then, when it is increased to 60%, the compressive strength decreases by 7.98%. As for the relationship between MK content and RAC’s compressive strength, it is shown that the RAC’s compressive strength initially rises and subsequently declines as the MK content increases. Increasing the content of MK from 10% to 15% results in a 7.53% improvement in the compressive strength. Nonetheless, as it rises to 20%, the compressive strength falls by 3.26%, which indicates that the MK content of 15% exhibits the highest compressive strength. Excessive replacement of MK will lead to a dilution of cement clinker that surpasses the pozzolanic benefits of MK, hence reducing the RAC’s strength. The impact of BF volume dosage on the compressive strength of RAC is marked by a strength boost at first, followed by a decline as the BF concentration increases, with the ideal volume concentration determined to be 0.2%.
Range analysis can illustrate the effect of variables on the compressive strength of RAC via the R value; however, it neglects the role of experimental error on the results. Variance analysis is conducted to assess the experimental results with greater scientific rigour. Table 11 presents the findings of the variance analysis. The influence of each factor on the compressive strength of RAC is as follows: the substitution rate of recycled aggregate surpasses the quantity of MK, which, in turn, exceeds the quantity of BF, in accordance with the findings of the range analysis. Table 11 reveals that the significance of the recycled aggregate replacement is less than 0.01, demonstrating a notably significant impact on compressive strength. The importance of MK content is between 0.01 and 0.05, indicating that it moderately significantly influences compressive strength. BF can be categorised as a mistake due to its significantly less impact on compressive strength in comparison to the error margin.

3.2.2. The Flexural Strength of RAC (Compounding of MK and BF)

Table 12 displays the range analysis on MK-BF RAC’s flexural strength. The effect of these aforementioned factors on flexural strength is as follows: The RCA replacement rate exceeds BF content, which in turn surpasses MK content. The optimal formulation for enhancing flexural strength is A1B2C2, that is, with a recycled aggregate substitution rate of 30%, an MK content of 15%, and a BF volume dosage of 0.2%, yielding the maximum flexural strength in RAC.
The influence of factors A, B, and C on MK-BF RAC’s flexural strength is shown in Figure 10. MK-BF RAC’s flexural strength is gradually reduced as the replacement rate of RCA increases. For example, the flexural strength decreases by 7.28% as the RCA replacement rate increases from 30 to 45 percent. While the RCA replacement rate increases from 45% to 60%, the flexural strength decreases by 9.24%. However, the flexural strength initially rises and subsequently declines as the MK substitution rate and BF concentration increase. The optimal flexural strength is achieved with an MK content of 15% and a BF content of 0.2%. For MK-BF RAC, the influence of BF on the flexural strength is characterised by an initial increase in flexural strength with rising BF content, followed by a decrease, with the optimal volume content level at 0.2%.
The variance analysis on MK-BF RAC’s flexural strength is displayed in Table 13, indicating that the significant hierarchy of each factor corresponds with the range analysis’s conclusions. The replacement rate of RCA and BF volume content exhibits a significance level below 0.05, demonstrating a moderate impact on MK-BF RAC’s flexural strength. Conversely, the significance of MK content falls between 0.05 and 0.1, suggesting a general impact on the flexural strength.

3.3. Prediction Model and Validation—Flexural Strength of MK-RAC

Flexural strength, an essential strength parameter, is vital for concrete engineering. Currently, there are few prediction models available for MK-RAC’s flexural strength. This research utilised regression analysis to create a flexural strength prediction model for MK-RAC, as indicated in Equation (1).
f c b = − 0.85994 + 0.13646 f c ( 28 d )
where f c ( 28 d )   and   f c b are the 28 d compressive and flexural strengths of MK-RAC, respectively.
Figure 11a illustrates the correlation between the flexural strength value derived from Equation (1) and the flexural strength value obtained in this experiment. The goodness of fit (R2) between the two is 0.862, indicating that the model demonstrates an excellent predictive capability. To further validate the predictive capacity of this model, the flexural strength of MK-RAC was forecasted based on references [35,37,39,63,64], with the results illustrated in Figure 11b. The analysis shows that the difference percentage between the predicted value and measured value of flexural strength is less than ±10%, and the goodness of fit is 0.896, suggesting that the prediction model developed in this study demonstrates excellent predictive capability.

4. Microstructure Analysis

According to the test results presented in Section 3, concrete specimens (NAC, R30M0, R30M15, A-2) were chosen for microstructure analysis by SEM and energy dispersive spectroscopy (EDS), as illustrated in Figure 12, Figure 13 and Figure 14.
Figure 12 illustrates the dense microstructure in specimen NAC. Natural aggregates displayed an abundance of C-S-H gel, CH, and acicular AFt on their surfaces. Nevertheless, the specimen R30M0 has very substantial apertures and fissures. The recycled aggregate displays increased defects, such as cracks, and the cement hydration seems insufficient, resulting in less hydrated products being formed to fill the RAC’s pores and fractures, thus weakening its strength. In R30M15, the amount of reticulated C-S-H gel is abundant, whereas CH and AFt are significantly reduced, resulting in a denser structure that indicates the elevated pozzolanic activity of MK [65]. The incorporation of MK enhances the secondary hydration reaction, lowers the hydrated product’s CH, and yields a more cohesively interconnected C-S-H gel [31,39]. In specimen A-2, the adhesion between the BF and the matrix is enhanced by the presence of hydrated products on the BF surface. The fibres alleviate and redistribute energy when the RAC encounters external pressure, therefore limiting micro-cracks and enhancing RAC strength [3,7,39].
The EDS analysis was performed on specimens of NAC, R30M0, R30M15, and A-2 (A1B2C2D2), as illustrated in Figure 13 and Figure 14. Figure 13 demonstrates that R30M0 predominantly comprises the elements Ca, Si, Zr, Al, S, Au, and Mg. The ratios of Ca to Si for NAC, R30M0, R30M15, and A-2 are 2.20, 2.59, 2.25, and 2.17, respectively; the (Al + Fe)/Ca ratios are 0.32, 0.04, 0.11, and 0.16, respectively. Taylor et al. found that C-S-H gel is the primary hydrated product in the condition where the value of Ca/Si is between 0.8 and 2.5 and the value of (Al + Fe)/Ca is no more than 0.2 [66]. The quantity of C-S-H gel diminishes when the value of Ca/Si rises. In comparison to NAC, the Ca/Si ratio of R30M0 increased by 0.39, signifying that the internal microstructure of RAC became less dense after replacing 30% of NCA with RCA. Compared to R30M0, R30M15’s Ca/Si ratio fell by 0.34, while the Ca/Si ratio of A-2 decreased by 0.42, demonstrating that adding MK significantly enhances the formation of C-S-H gel [67,68]. Simultaneously, the microstructure of RAC becomes denser with the addition of BF.
The hydration degree of cement in concrete can be clarified through the distribution of Ca and Si elements observed in the EDS mapping analysis. The concentrations of Ca and Si in specimens R30M15 have increased, possibly related to the increased development of C-S-H gel [67]. This discovery aligns with the performance of macroscopic mechanical properties.

5. Model Test of RAC Curbs

Based on the optimal mix ratio derived from single-factor analysis and orthogonal test analysis, three mix ratios designated as R30M0, R30M15, and A-2 have been selected for the fabrication of RAC curbs, with external dimensions of 500 × 150 × 120 mm [69]. The RAC curb mould employs a custom mould with interior dimensions of 500 × 150 × 140 mm.
Figure 15 shows the manufacturing process of recycled concrete curbs. Initially, the mould assembly and the application of the internal release agent coating are completed first. Thereafter, the agitated filler is introduced into the mould and vibrated on the vibrating table for 30 s. An iron plate with a thickness of 20 mm is placed on top of the mould. The pressure testing machine (Model YAW–2000) is utilised to compact at a pressure of 15 MPa to enable moulding and perform indoor and standard condition curing [7]. The designated curing parameters correspond with those of the previously referenced RAC.
Mechanical property tests of RAC curb models (with a recycled aggregate replacement rate of 30%) were undertaken per the standards of JC/T 899-2016 and GB/T 50081-2019 [62,69]. The results show that the 28-day compressive and flexural strengths for the RAC curb models (per the R30M0 mixing ratio) are 36.4 and 5.61 MPa, respectively. When 15% MK was added (corresponding to the R30M15 mix ratio), the RAC curb model exhibits an increase in compressive strength at 23.08% and flexural strength at 10.52%, respectively, yielding values of 44.8 and 6.20 MPa. Adding 15% MK and 0.2% BF (according to the mix ratio of A-2), the RAC curb model performs better in compressive and flexural strength, achieving values of 49.6 and 6.65 MPa, respectively.
The integration of MK and BF can strengthen the mechanical qualities of RAC and RAC products. The primary cause is that the silica in MK and the hydrated product CH produce hydrated calcium silicate, hence augmenting the matrix’s compactness. The incorporation of BF can concurrently hinder fracture initiation and propagation while enhancing the recycled aggregate’s strength. The mechanical property growth law exhibited in the model test of the RAC curb aligns with the law outlined in the RAC specimens test in this study. An increase in RAC strength was observed when MK and BF were combined, and this composite mixing methodology is more effective than using MK alone.
Utilising the cost platform, a cost analysis of the raw materials for RAC curb was conducted, exemplified by Heilongjiang Province of China as illustrated in Table 14 [70,71]. It is evident that the cost of materials per cubic meter of conventional RAC curb (specimen R30M0) is elevated further when MK or a combination of MK and BF is incorporated. Nonetheless, taking into account the mechanical performance of RAC curbs, particularly compressive strength, the ratio of the material cost per cubic meter to compressive strength is employed for thorough assessment. The calculated material cost per cubic meter related to the RAC curbs’ unit compressive strength, based on the specimens of R30M0, R30M15, and A-2, is 9.74, 9.64, and 9.42 RMB/MPa, respectively. It is obvious that the RAC curb (mixed with MK and BF) demonstrates the most favourable economics, succeeded by the RAC curb mixed solely with MK, and lastly, the conventional RAC curb.

6. Conclusions

This study utilised single-factor analysis and orthogonal tests to investigate the mechanical performance of RAC alone and incorporated with MK, as well as the combined inclusion of MK and BF. The effect of RCA replacement rate, MK content, and BF volume content on RAC’s mechanical properties was discussed. Model tests for RAC curbs were conducted based on the optimal mix ratios determined by single-factor analysis and orthogonal test. The primary conclusions are as follows:
  • As the MK content increased, MK-RAC’s mechanical properties initially rose and then declined. The best concentration of MK is 15%. The compressive and flexural strength of MK-RAC with RCA replacement rates of 30%, 45%, and 60% at 28 d are 18.04%, 17.32%, and 16.55%, and 19.44%, 17.65%, and 16.13%, superior to those of RAC without MK incorporation, respectively.
  • The orthogonal test indicates that the MK content significantly influences the RAC’s compressive strength more than the volume of BF does. The influence of BF on flexural strength is more significant than that of MK content. Introducing BF notably enhances MK-RAC’s mechanical properties compared to studies that focus on a single variable. The mechanical performance of MK-BF RAC is optimised at an RCA replacement rate of 30%, an MK content of 15%, and a BF volume content of 0.2%. Compressive and flexural strength increase by 27.22% and 41.67%, compared to samples devoid of MK and BF, and by 7.77% and 18.60%, compared to specimens containing only 15% MK, respectively.
  • The model test of RAC curbs demonstrates that MK single mixing and MK combined with BF mixing increase the RAC curbs’ mechanical properties. From the perspective of engineering applications, it is recommended to utilise either 15% MK only or 15% MK in conjunction with 0.2% BF (length of 6 mm) to increase the RAC products’ mechanical properties. A predictive model for MK-RAC‘s flexural strength was established based on test results and thoroughly validated by literature data.
Most research on MK-RAC focuses on incorporating MK alone or in conjunction with other extra cementitious materials, with limited studies addressing the combination of MK and fibres. This work investigates the mechanical performance of RAC added with MK and BF. Additional research is recommended in the following areas: (1) The mechanical performance of RAC incorporating MK and BF at a high recycled aggregate substitution rate may encompass impact resistance, compressive strength, and flexural capabilities. (2) The investigation of the long-term performance and durability characteristics of RAC incorporated with MK and BF, including resistance to freezing, cracking, and surface erosion. (3) Investigate RAC products employing single-mixed MK and the synergistic incorporation of MK and BF, encompassing non-load-bearing components such as pavement bricks and concrete curbs, as well as structural components such as concrete beams, slabs, and columns. For instance, evaluate the durability performance of RAC curbs subjected to severe environmental conditions, such as freeze–thaw cycles.

Author Contributions

Investigation, conceptualisation, writing—original draft, and methodology, M.W.; resources, methodology, investigation, and writing—review and editing, X.Z.; conceptualisation, validation, and investigation, B.Z.; resources, project administration, and supervision, D.Z.; supervision and project administration, D.W.; investigation and methodology, Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding

The study presented in this paper was supported by the Fundamental Research Funds for Heilongjiang Provincial Universities (Grant Nos. 145309209 and 135509214). and the Central Guidance for Local Technology Development Projects (Grant No. ZY23QY12). This financial support is gratefully acknowledged.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

Author Daoming Zhang was employed by the company Heilongjiang Xinyu Cement Products Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. The manufacture of RAC products.
Figure 1. The manufacture of RAC products.
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Figure 2. Mineral composition and SEM images of MK: (a) mineral composition and (b) SEM images.
Figure 2. Mineral composition and SEM images of MK: (a) mineral composition and (b) SEM images.
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Figure 3. The preparation, curing, and loading of specimens.
Figure 3. The preparation, curing, and loading of specimens.
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Figure 4. Representative specimens: (a) specimens for compressive strength before loading (b) specimens under loading, and (c) specimens for flexural strength after loading.
Figure 4. Representative specimens: (a) specimens for compressive strength before loading (b) specimens under loading, and (c) specimens for flexural strength after loading.
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Figure 5. Compressive strength performance of MK-RAC at 3 d: (a) compressive strength and (b) strength growth rate.
Figure 5. Compressive strength performance of MK-RAC at 3 d: (a) compressive strength and (b) strength growth rate.
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Figure 6. Compressive strength performance for MK-RAC at 7 d: (a) compressive strength and (b) strength growth rate.
Figure 6. Compressive strength performance for MK-RAC at 7 d: (a) compressive strength and (b) strength growth rate.
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Figure 7. Compressive strength performance of MK-RAC at 28 d: (a) compressive strength and (b) strength growth rate.
Figure 7. Compressive strength performance of MK-RAC at 28 d: (a) compressive strength and (b) strength growth rate.
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Figure 8. Performance of flexural strength for MK RAC: (a) flexural strength and (b) strength growth rate.
Figure 8. Performance of flexural strength for MK RAC: (a) flexural strength and (b) strength growth rate.
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Figure 9. The relationship between compressive strength and levels of factors for MK-BF RAC.
Figure 9. The relationship between compressive strength and levels of factors for MK-BF RAC.
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Figure 10. The relationship between flexural strength and levels of factors for MK-BF RAC.
Figure 10. The relationship between flexural strength and levels of factors for MK-BF RAC.
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Figure 11. Prediction model and model validation of MK-RAC’s flexural strength: (a) flexural strength in this study and (b) model validation.
Figure 11. Prediction model and model validation of MK-RAC’s flexural strength: (a) flexural strength in this study and (b) model validation.
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Figure 12. SEM microscopic morphology: (a) NAC, (b) R30M0, (c) R30M15, and (d) A-2 (A1B2C2D2).
Figure 12. SEM microscopic morphology: (a) NAC, (b) R30M0, (c) R30M15, and (d) A-2 (A1B2C2D2).
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Figure 13. EDS energy spectrum: (a) NAC, (b) R30M0, (c) R30M15, and (d) A-2 (A1B2C2D2).
Figure 13. EDS energy spectrum: (a) NAC, (b) R30M0, (c) R30M15, and (d) A-2 (A1B2C2D2).
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Figure 14. EDS surface scanning: (a) NAC, (b) R30M0, (c) R30M15, and (d) A-2 (A1B2C2D2).
Figure 14. EDS surface scanning: (a) NAC, (b) R30M0, (c) R30M15, and (d) A-2 (A1B2C2D2).
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Figure 15. Manufacturing process of RAC curb.
Figure 15. Manufacturing process of RAC curb.
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Table 1. Chemical composition of PC and MK (%).
Table 1. Chemical composition of PC and MK (%).
Chemical CompositionAl2O3Na2OSiO2K2OMgOFe2O3SO3TiO2CaO
PC10.950.7231.601.192.633.853.670.6144.23
MK44.930.0552.210.100.180.930.080.490.02
Table 2. Physical properties of NCA and RCA.
Table 2. Physical properties of NCA and RCA.
Aggregate TypeApparent Density (kg/m3)Water Absorption (%)Crushing Value (%)
NCA27010.6217.8
RCA25526.2523.2
Table 3. Coarse aggregate particle size distribution.
Table 3. Coarse aggregate particle size distribution.
Sieve Hole (mm)199.54.752.36
NCA cumulative sieve residue (%)3.2351.0496.8598.97
RCA cumulative sieve residue (%)2.7749.8597.3899.10
Specification requirements (%)0–1040–8090–10095–100
Table 4. Particle size distribution of NFA.
Table 4. Particle size distribution of NFA.
Sieve Hole (mm)4.752.361.180.60.30.15
Cumulative sieve residue (%)5.717.2331.8963.1686.8597.96
Aggregate specification requirements (%)0–100–2510–5041–7070–9290–100
Table 5. Physical parameters of BF [7].
Table 5. Physical parameters of BF [7].
Calibre (μm)Density (g/cm3)Tensile Strength (MPa)Elastic Modulus (GPa)
172.62155035.8
Table 6. Types of RAC (concrete with RCA and MK additives) and its mix proportion (kg/m3).
Table 6. Types of RAC (concrete with RCA and MK additives) and its mix proportion (kg/m3).
SpecimensCementMKWaterRCANCANFAADSP
NAC380.000.00163.400.001206.79649.810.001.90
R30M0380.000.00163.40362.04844.75649.8122.631.90
R30M5361.0019.00163.40362.04844.75649.8122.631.90
R30M10342.0038.00163.40362.04844.75649.8122.631.90
R30M15323.0057.00163.40362.04844.75649.8122.631.90
R30M20304.0076.00163.40362.04844.75649.8122.631.90
R45M0380.000.00163.40543.06663.73649.8133.941.90
R45M5361.0019.00163.40543.06663.73649.8133.941.90
R45M10342.0038.00163.40543.06663.73649.8133.941.90
R45M15323.0057.00163.40543.06663.73649.8133.941.90
R45M20304.0076.00163.40543.06663.73649.8133.941.90
R60M0380.000.00163.40724.07482.72649.8145.251.90
R60M5361.0019.00163.40724.07482.72649.8145.251.90
R60M10342.0038.00163.40724.07482.72649.8145.251.90
R60M15323.0057.00163.40724.07482.72649.8145.251.90
R60M20304.0076.00163.40724.07482.72649.8145.251.90
Note: NAC denotes natural aggregate concrete; R30M0 and R30M5 represent RAC with a replacement ratio of 30% (no MK addition) and RAC (replacement ratio of 30%) with MK addition of 5%, respectively. AD and SP are additional water and water-reducing agents based on polycarboxylic acid, respectively; NFA is natural fine aggregate.
Table 7. Factors and levels of orthogonal test.
Table 7. Factors and levels of orthogonal test.
Level NumberExperimental Factors
ABCD
130%10%0.1%1
245%15%0.2%2
360%20%0.3%3
Note: A, B, and C represent the recycled coarse aggregate substitution rate, the MK content (replacing cement quality percentage), and the volume content of BF, respectively. D stands for a blank column devoid of experimental factors, reflecting uncontrollable random errors in the experiment and providing freedom for variance analysis.
Table 8. Combination of factors and levels.
Table 8. Combination of factors and levels.
SpecimensFactors CombinationABCD
A-1A1B1C1D130%10%0.1%1
A-2A1B2C2D230%15%0.2%2
A-3A1B3C3D330%20%0.3%3
A-4A2B1C2D345%10%0.2%3
A-5A2B2C3D145%15%0.3%1
A-6A2B3C1D245%20%0.1%2
A-7A3B1C3D260%10%0.3%2
A-8A3B2C1D360%15%0.1%3
A-9A3B3C2D160%20%0.2%1
Table 9. Mix ratios of metakaolin–basalt fibre RAC (kg/m3).
Table 9. Mix ratios of metakaolin–basalt fibre RAC (kg/m3).
SpecimensCementMKBFRCANCAADWaterNFASP
A-1342.0038.002.62362.04844.7522.63163.40649.811.90
A-2323.0057.005.24
A-3304.0076.007.86
A-4342.0038.005.24543.06663.7333.94
A-5323.0057.007.86
A-6304.0076.002.62
A-7342.0038.007.86724.07482.7245.25
A-8323.0057.002.62
A-9304.0076.005.24
Table 10. Compressive strength of MK-BF RAC.
Table 10. Compressive strength of MK-BF RAC.
SpecimensFactors
ABCDCompressive Strength (MPa)
A-130%10%0.1%138.1
A-230%15%0.2%241.6
A-330%20%0.3%338.5
A-445%10%0.2%336.7
A-545%15%0.3%139.4
A-645%20%0.1%238.9
A-760%10%0.3%234.0
A-860%15%0.1%336.0
A-960%20%0.2%135.8
k139.4036.2737.6737.77
k238.3339.0038.0338.17
k335.2737.7337.3037.07
Range (R)4.132.730.731.10
Note: ki denotes the average value of a certain factor at the i-th level, e.g., k1 signifies the average value of compressive strength corresponding to the first level under a certain factor.
Table 11. Analysis of variance on compressive strength of MK-BF RAC.
Table 11. Analysis of variance on compressive strength of MK-BF RAC.
FactorsDegree of FreedomSum of Square of DeviationsMean SquareF-Valuep-Value
A227.62713.813320.720.008
B211.2275.61338.420.037
Error42.6670.6667
Total841.520
Table 12. Analysis of range on flexural strength of MK-BF RAC.
Table 12. Analysis of range on flexural strength of MK-BF RAC.
SpecimensFactors
ABCDFlexural Strength (MPa)
A-130%10%0.1%14.3
A-230%15%0.2%25.1
A-330%20%0.3%34.6
A-445%10%0.2%34.4
A-545%15%0.3%14.6
A-645%20%0.1%24.0
A-760%10%0.3%23.9
A-860%15%0.1%33.8
A-960%20%0.2%14.1
k14.674.204.034.33
k24.334.504.534.33
k33.934.234.374.27
Range (R)0.730.300.500.07
Table 13. Analysis of variance on flexural strength of MK-BF RAC.
Table 13. Analysis of variance on flexural strength of MK-BF RAC.
FactorsFreedomSum of Square of DeviationsMean SquareF-Valuep-Value
A20.808890.40444491.000.011
B20.162220.08111118.250.052
C20.388890.19444443.750.022
Error20.008890.004444
Total81.36889
Table 14. Material cost analysis of RAC curbs (RMB /m3).
Table 14. Material cost analysis of RAC curbs (RMB /m3).
NumberCementNCARCAMKBFSPNFAWaterMaterials Price
R30M0171.0099.688.690.000.009.6664.980.36354.37
R30M15145.3599.688.69103.060.009.6664.980.36431.78
A-2145.3599.688.69103.0635.639.6664.980.36467.41
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Wang, M.; Zhang, X.; Zhang, B.; Zhang, D.; Wang, D.; Zhang, Y. Model Experimental Investigation on the Mechanical Properties of Recycled Aggregate Concrete Curbs by Incorporating Metakaolin and Basalt Fibre. Buildings 2025, 15, 3059. https://doi.org/10.3390/buildings15173059

AMA Style

Wang M, Zhang X, Zhang B, Zhang D, Wang D, Zhang Y. Model Experimental Investigation on the Mechanical Properties of Recycled Aggregate Concrete Curbs by Incorporating Metakaolin and Basalt Fibre. Buildings. 2025; 15(17):3059. https://doi.org/10.3390/buildings15173059

Chicago/Turabian Style

Wang, Mengyao, Xueyuan Zhang, Biao Zhang, Daoming Zhang, Dandan Wang, and Yu Zhang. 2025. "Model Experimental Investigation on the Mechanical Properties of Recycled Aggregate Concrete Curbs by Incorporating Metakaolin and Basalt Fibre" Buildings 15, no. 17: 3059. https://doi.org/10.3390/buildings15173059

APA Style

Wang, M., Zhang, X., Zhang, B., Zhang, D., Wang, D., & Zhang, Y. (2025). Model Experimental Investigation on the Mechanical Properties of Recycled Aggregate Concrete Curbs by Incorporating Metakaolin and Basalt Fibre. Buildings, 15(17), 3059. https://doi.org/10.3390/buildings15173059

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