Introduction

The cement industry of any country is the backbone for economic development. However, its production is a major source of greenhouse gas emissions. According to the latest research1,2, cement production contributes approximately 5.5% and 7% of the total carbon dioxide (CO2) produced in Pakistan and worldwide, respectively. Moreover, one ton of cement production consumes approximately 1.6 MWh of energy; hence, strong sustainable measures need to be taken to reduce its considerable energy demands and environmental damage. The three effective approaches for sustainable development are cleaner production, recycling and alternative cements3. This research investigated the viability of using LCB clay as a partial cement substitute for recycled aggregate concrete (RAC).

Bentonite is a naturally occurring clay comprising mostly the mineral montmorillonite. Montmorillonite is a subcategory of smectite minerals. The mineral exists as microscopic, platy grains that are up to 1 μm in length but < 1 nm in thickness. Furthermore, the basal surface of these grains is negatively charged, whereas the edges are positively charged, facilitating hydrophilic interactions. The swelling is incumbent on the type of bentonite: individual sodium bentonite particle sheets can be as far as 100 Å apart (the sheet thickness is approximately 10 Å), whereas calcium bentonite is more prone to clumping, and the interlayer spacing only reaches 10 Å4. It is suspected that the swelling of LCB is due to divalent ion interlocking of the sheets by Ca2+ ionsOnce the water molecules have settled between the individual bentonite platelets, ion exchange between the sheets and the solution can be facilitated, which accounts for the ability of bentonite to act as a fining agent for fermented beverages5,6. Typically, the swelling behavior does not make high-quality sodium bentonite a conducive material for construction; thus, this research uses a low-calcium bentonite that does not significantly expand upon contact with water. Notably, bentonite suspensions typically remain stable at pH >67, which should include the alkaline environment of fresh hardened cement.

The use of LCB as a partial replacement of cement in NAC has been widely investigated8,9,10,11,12,13. Memon et al.12 incorporated various levels (0–21%) of bentonite into concrete and reported that early strength decreases with increasing levels of bentonite, but the strength of the bentonite-concrete mixture and control mixture (CM) was approximately equal.

The demand for aggregates for the construction industry was approximately 26 billion tons14 globally in 2010, and this demand further increased to 40.2 billion tons in 2014, as reported by Freedonia15. It is predicted that this demand will increase to 52 billion metric tons by the end of 2019, assuming a 5.2% increase per annum. This has led to large-scale geomorphological changes, increasing global environmental pollution and ecological degradation16. To curtail this high demand, recycled concrete aggregates (RCAs) can be salvaged from construction and demolition waste and reused for future construction projects.

RCAs are typically manufactured by crushing various old concrete materials and reusing them as fine or coarse aggregates, or both. As such, these RCAs can vary widely depending on the source material. However, an early study by Buck17 reported no correlation between the parent concrete of RCAs and the new recycled aggregate concrete (RAC) strength. This does not imply that the parent concrete’s strength did not matter; rather, a review by Nixon18 revealed significant variance in the early strength and later strength of RAC depending on the source of the RCA. Scanning electron microscopy (SEM) of RACs by Malhotra19 confirmed that cracks in the cement paste ranging from 2 to 4 μm in length were suspected to be the source of water absorption18.

Despite its shortcomings, Buck17 was able to design higher strength RAC than NAC by using a w/c ratio of 0.45 with the addition of plasticizers, keeping the cement content at 385 kg/m3. Limbachiya et al.20 designed high-strength concrete with 30% RCA, which displayed strengths, workability, and durability comparable to those of NAC. However, their recorded shrinkage and creep strains were much greater than those of NAC, prompting slight concern as to the viability of RAC in the construction sector. As a mitigative measure, several studies have proposed the use of RA with supplementary cementitious materials (SCMs), such as mineral admixtures21,22,23 (i.e., fly ash, silica fume, metakaolin, etc.), or with fiber reinforcement (glass, steel, polypropylene fibers, etc.)24,25, and water-reducing admixtures26,27,28 (low–medium–high range plasticizers), nano-reinforcement29,30,31,32 etc., for improved mechanical and durability performance. Kou et al.33 suggested the use of silica fume (10% by weight of cement) and metakaolin (15% by weight of cement) to improve the mechanical performance of RAC. To improve the durability properties of RAC, they also proposed the use of fly ash (35% by weight of cement) and GBBS (55% by weight of cement).

The authors of this research believe that a combination of pozzolanic materials such as bentonite with RAC could alleviate some of the bonding issues of adhered mortar and potentially develop strong late strength concrete. Figure S1 of the supplementary information (SI) illustrates the difference between normal aggregate concrete (NAC) and RCA.

A brief review of the present literature indicates that studies investigating the behavior of RAC with LCB are not available. Therefore, this study investigated the effect of LCB on the strength development of RAC. For this purpose, 0, 50, and 100% RCA is used for volume replacement of NCA, whereas 0, 5, 10, 15 and 20% RCA is used for weight replacement of cement. The results encouraged the use of LCB in RAC and were further verified through design of experiments (DoE), specifically via response surface methodology (RSM) design and analysis.

Experimental program

Materials

The general purpose of OPC type I cement used in this research is ASTM C150. The physical and chemical properties of this cement are given in Table S1 of the supplementary information (SI).

Bentonite

The bentonite used in this research was collected from Jahangira, Pakhtunkhwa Province, Pakistan. This LCB was wet ground to a passing size of 325 μm. The physical and chemical properties of this LCB are included in Table S1 of the SI. The chemical properties of LCB from Table S1 of the SI were determined via X-ray fluorescence (XRF) analysis, which confirmed that it meets the requirements of ASTM C618. An SEM image of LCB is shown in Figure S2 of the SI. The lateral size of the LCB sheets appears to vary from 2 to 8 μm, with the sheets stacked to show clump sizes of approximately 500–1 μm. The high degree of stacking is indicative of no intercalation of ions between the LCB sheets. Additionally, a laser particle size analyzer was used to estimate the particle size distribution of the LCB, and the analysis confirmed that at 25%, 48%, 88%, and 95% passing, the particle sizes were 5 μm, 10 μm, 45 μm, and 75 μm, respectively, as shown in Table S2 of the SI. The grain diameters at different percentages are listed in Table S3 of the SI.

Aggregates

The siliceous sand of the Lawrance Pur quarry is used as natural fine aggregate in this study. For NCA, crushed marble stone from Margalla Hills is used. The properties of these aggregates are presented in Table 1. To prepare RCAs, cubical concrete samples with compressive strengths ranging from 30 to 35 MPa (4.35–5.07 ksi) are manually crushed and sieved to obtain RCAs with gradations approximately similar to those of the NCA. The properties of the RCAs are also given in Table 1. The granulometry of the aggregates is shown in Figure S3 of the SI.

Table 1 General properties of the aggregates.
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Admixture

Sikament 520, a high-range water-reducing admixture, is used to achieve the desired workability. This admixture has a specific weight of 1.12 g/cm3 (70 lb/ft3).

Composition of the concrete mixtures

The ACI 211 was used in selecting proportions for concrete in this research. A total of fifteen concrete mixtures were produced in this research. The RCA is 0%, 50%, or 100% by volume replacement of NCA, whereas the LCB is 0%, 5%, 10%, 15%, or 20% by mass replacement of cement. The mix proportions are given in Table 2. To satisfy the water absorption requirements of RCA, 40 kg/m3 (2.5 lb/ft3) and 80 kg/m3 (5.0 lb/ft3) of additional water are used for 50% and 100% replacement of NCA, respectively. A superplasticizer is used to sustain the slump in the range of 60–80 mm for all mixes. The reduction in workability can be attributed to the high specific surface area of the LCB particles.

Table 2 Concrete mix proportions.
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Mixing of the concrete mixtures

The mixing of the concrete mixture consists of two stages. In the first stage, the aggregates are mixed with 2/3 of the total water for approximately 4 min. In the second stage, the binders and admixture are added to the remaining 1/3 of the total water, and mixing continues for approximately 6 min. Mixing the aggregates with 2/3 of the water allows the RCA sufficient time to absorb the water28,34 so that these aggregates do not absorb significant quantities of water from the cement matrix postcasting of the samples. The speed of the mechanical mixer was maintained at 34 revolutions per minute (rpm) throughout the mixing process.

Preparation and testing of samples

The mechanical performance of each mixture is evaluated on the basis of compressive strength, splitting tensile strength (STS), and flexural strength. For compressive strength, 150 mm cubes are tested at the ages of 3, 28, and 90 days following EN:12,390 guidelines. For STS, cylinders 150 mm in diameter and 300 mm in height are tested after 28 and 90 days of curing following ASTM C496. The modulus of rupture or flexural strength of each mixture is determined after 28 and 90 days of curing for 100 × 100 × 500 mm prisms under three-point loading according to ASTM C78. For each strength parameter, three samples are cast and tested, and their average values are reported with standard deviations in this study.

RSM analysis

RSM analysis is widely used to determine optimum mix designs, especially for novel concrete mixes35,36,37,38,39. Design Expert 12 was used to develop an appropriate statistical model for further analysis of the strength development via LCB and RCA. An RSM analysis using Central Composite Design (CCD) was incorporated, using a rotatable Alpha (k < 6). A total of 135 compressive strength runs and 90 STS and flexural strength runs were performed. Three numeric factors (LCB% bwoc, RCA% by vol, and days cured) were used as input variables, while 3 responses (each of the strength types) were considered as outputs. A Type III partial sum of squares ANOVA was performed on the design, which is further detailed in the Results section of this article.

Results and discussion

Workability

The workability of fresh concrete was determined by conducting a slump test according to ASTM C143, and the results are shown in Figure S4 of the SI. As anticipated, adding LCB reduces the slump of concrete for both NAC and RAC. This can be attributed to the greater fineness of the LCB particles than the cement particles, as the specific surface area of the LCB particles is more than twice that of the cement particles (Table S1 (from SI)). Conversely, replacement of NCA with RCA results in improved workability, especially for concrete with 10% or less LCB replacement. At 5% and 10% LCB, the workability remains within the desired range, whereas at higher percentages, the slump decreases to values lower than 60 mm (2.36 in. ), as shown in Figure S4 (SI). At a given percentage of RCA, a very small amount of plasticizer (0.8–1% bwoc) is utilized to achieve the target slump at 10, 15, and 20% levels of LCB. Since additional water is used to compensate for the water absorption of RCA, no loss in workability is observed with increasing incorporation level of RCA. Similar studies12,36,37 have reported a reduction in the workability upon the addition of LCB as a partial cement replacement material.

Compressive strength

The compressive strengths of all the concrete mixtures at 3, 28, and 90 days are presented in Fig. 1a and b. The general trend in the results shows that both RACs have inferior compressive strengths with respect to CM. Increasing the amount of RA considerably reduces the compressive strength, and this gap only widens with increasing sample age. LCB replacement results in a worse 3-day compressive strength, whereas the compressive strengths of 5–15% LCB are similar for all the concrete samples at 28 and 90 days (Fig. 1a). Hence, it can be concluded that the addition of LCB can impact the compressive strength of concrete, especially at early stages, whereas the pozzolanic effect of the LCB particles results in similar concrete strengths at later stages. This can be attributed to the slow nature of the pozzolanic reaction between the aluminosilicate particles of LCB and free Ca(OH)2 reported in previous studies40. At 28 and 90 days, for a constant percentage of RCA, the maximum value of compressive strength is observed at 5–10% of the LCB.

Fig. 1
figure 1

Compressive strengths of each mixture with respect to (a) bentonite % by weight of cement and (b) development of strength at 3, 28, and 90 days.

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As already mentioned, the replacement of NCA with RCA reduces the compressive strength. Notably, compared with CM, RAC has greater strength development between 28 and 90 days. For example, between 28 and 90 days, mixing with 0% RCA results in a net increase in compressive strength of approximately 7.9%, whereas mixing with 100% RCA results in a net increase of more than 10%. This can be attributed to the hydration of Ca(OH)2 present in the RCA, which increases the strength of the concrete28. For a constant percentage of RCA, the compressive strength decreases when the LCB is increased beyond 5% at 3 days. At 28 and 90 days, mixes incorporating 5–10% LCB presented maximum compressive strength values at a constant percentage of RCA in the concrete. As the compressive strength depends on the microstructure development and pore volume of the concrete, the improvement in the compressive strength of the concrete by LCB replacement can be attributed to two reasons: first, the finer particle size of the LCB particles relative to the cement acts as a filler, reducing the pore volume in the concrete; second, the pozzolanic reaction of the LCB particles with Ca(OH)2 is present (1) in the new mortar of the NAC and (2) in the new and old mortar of the RAC. Ultimately, there may be a reduction in pore volume, improving the microstructure and thus the compressive strength of the concrete. Compared with the corresponding CM mixture (0% LCB), the LLCB mixture had 3–6% greater compressive strength at 90 days. However, at 28 days, the test results indicate that the compressive strength of the CM mixtures barely exceeded that of the corresponding CM.

The later strength development improvements by LCB for all the concrete samples can be clearly observed in Fig. 1b. Compared with the LCB mixtures of NAC, the LCB mixtures of RAC resulted in a high net increase in compressive strength. For example, at 20% LCB and 100% RCA, the mix shows a greater than 20% increase in strength between 28 and 90 days, whereas the mix with 0% RCA and 20% LCB shows a net increase of approximately 16%. This might be ascribed to the greater Ca(OH)2 content in the RAC than in the NAC; therefore, the addition of the LCB mixture resulted in a greater net increase in the compressive strength of the RAC. A study conducted by Kou et al.33 revealed that SCM (i.e., silica fume, GGBS, fly ash, metakaolin, etc.) owing to the presence of a high amount of Ca(OH)2 in an old mortar, is attributed more strongly to the strength of RAC. The performance of the concrete containing 50% RCA and 10% LCB is nearly equal to that of the CM; therefore, these levels of RCA and LCB can be considered for optimum results considering both compressive strength and sustainability.

Fig. 2
figure 2

Splitting tensile strengths of each mixture with respect to (a) bentonite % by weight of cement and (b) development of strength at 28 and 90 days.

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Splitting tensile strength

The STS is an indirect measure of the true tensile strength of concrete. Since the failure plane passes through both the coarse aggregates and the cement matrix, the tensile strength can be estimated fairly accurately via STS. The STSs of each mixture at 28 and 90 days are shown in Fig. 2a and b. The pozzolanic effects of LCB are clearly evident, as higher LCB replacement resulted in significantly better STS at 90 days. Conversely, all the concretes with 10% or higher LCB by weight of cement possessed worse STS at 28 days (Fig. 2a). Unlike compressive strength, STS does not significantly decrease when RCA partially or fully replaces the NCA. Since RCAs are more porous and angular than NCAs are, they can form better bonds with other aggregates and new cement mortar41, leading to an improvement in the tensile strength of concrete at both the 50% and 100% RCA levels. Figure 2b shows uniform improvements in all the concretes with increasing LCB, and the pozzolanic effects seem to improve the STS significantly more for 100% RAC than for 50% RAC. The rate of strength development from 28 to 90 days also increases in proportion to the amount of cement replaced by LCB, which is most pronounced for 100% RCA and 50% RCA.

Fig. 3
figure 3

Flexural strengths of each mixture with respect to (a) bentonite % by weight of cement and (b) development of strength at 28 and 90 days.

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Flexural strength

Flexural strength is another important strength parameter of concrete and is considered in the design of flexural members such as slabs, rigid pavements, and beams. As shown in Fig. 3a and b, unlike compressive strength and tensile strength, flexural strength uniformly increases with increasing incorporation level of RCA in concrete.

The flexural strength of the mixtures with added LCB was comparable to that of the corresponding CM at the age of 28 days. These mixtures also presented maximum flexural strength values at 90 days for a constant percentage of RCA. Similar to the STS results, the contribution of LCB to flexural strength improvement is greater for RAC than for NAC. Compared with the corresponding CM, the mixtures with added LCB had 3–4% greater flexural strength at 90 days.

LCB mixtures of RAC at both ages, i.e., 28 and 90 days, outperform CM by a fair margin. This can be ascribed primarily to the improvement in flexural strength due to RCA incorporation, whereas LCB causes minor increases at 90 days owing to its pozzolanic nature. Compared with CM, 100% RCA concrete with 10–20% LCB has 10% greater flexural strength.

Linear correlations between compressive, splitting tensile, and flexural strengths

Linear correlations are drawn between the experimental values of compressive strength, flexural strength and STS in Figure S5a, b, and c (from SI). The strengths at the ages of 28 and 90 days are considered cumulatively.Figure S5a shows that the compressive strength and STS strongly correlate with each other for NAC (R2 = 0.934), 50% RAC (R2 = 0.987), and 100% RAC (R2 = 0.867). A relationship, when derived for all mixes, is not as strong as that drawn for each incorporation level of RCA. This is because mixes involving RCA as coarse aggregate have a greater tensile-to-compressive strength ratio than does NAC. When RCA partially or fully replaces NCA, the compressive strength significantly decreases, unlike the tensile strength. Therefore, the STS-to-compressive strength ratio of RAC is greater than that of NAC.

As with compressive strength and STS, the flexural: compressive strength ratio of RAC is greater than that of NAC. The correlation also increases with the amount of RA, with R2 values of 0.983, 0.852, and 0.647, respectively (Figure S5b). These results follow a clear trend, which is logical, as the flexural strength is also limited by the tensile strength of the concrete.

Finally, Figure S5c highlights the flexural strength to STS ratios, where 100% RAC again has the highest ratio (R2 value of 0.868), closely followed by 50% RAC (R2 = 0.859) and NAC (R2 = 0.733). As previously mentioned in  “Flexural strength” section, standard errors were also generally lower for the RAC mixtures than for the NAC mixtures for the flexural and STS mixes. This behavior can be attributed to the better bonding of RCA with cement mortar than with NCA. NCA has comparatively rounded shapes with smooth surfaces, whereas RCA is angular with rough surfaces, which allows better bonding of the cement mortar with the aggregates. The flexural strength increases by 2.5% and 7.0% at 50% and 100% RCA, respectively. Therefore, RCA is not detrimental to the flexural or tensile strength of concrete. Ahmadi et al.42 and Das et al.43 reported that the flexural strength of normal-strength RAC is greater than that of NAC. Ali et al.41 reported that the flexural strength of RAC is not harmful among all basic strength parameters. This can also be related to the different deformability of RAC under compressive and splitting tensile loads.

Table 3 Response surface design modeli—inputs and results
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Strength analysis and design via response surface methodology (RSM) analysis

The input parameters and results of design optimization via the RSM are summarized in Table 3. The LCB% replacement, RCA quantity, and day of testing were the input parameters. As the model does not extrapolate, limits were prescribed on the basis of the concrete mixes (0–20% LCB, 0–100% RCA, and 3–90 days of curing). Strength results were provided (135 measurements for compressive strengths due to 3-day testing, 90 for remaining flexural and STS tests) for the model analysis. Using Type III partial sum of squares ANOVA, a quadratic model was the best fit for compressive strength prediction, and 2 factor interactions (2FIs) were the best fit for STS and flexural strength. All 3 models show significant p values (< 0.05) and good agreement between the adjusted and predicted R2 values (difference of less than 0.2).

The resulting coefficients of the respective design equations are listed in Table S4. A reasonable variance inflation factor (VIF) of 1 ± 0.2 is displayed, indicating low collinearity between coefficients. From these design equations, the strengths are interpolated between the 90 days of testing and the amounts of LCB/RCA to verify the impact of LCB on the late-stage development of both NAC and RAC.

Compressive strength model

Contour plots of the compressive strength development are modeled and shown in Fig. 4a–f. The 7-day, 14-day and 50-day strengths were interpolated from the RSM design model. The changes in the contour plot lines clearly indicate the pozzolanic activity of LCB over time. The transition point, i.e., where increasing effects of LCB do not reduce the compressive strength of either NAC or RACs, occurs at approximately 28 days. This is evidenced by the small dome-shaped contour of 29 MPa (Fig. 4d) in the 0–5% LCB replacement range, implying that some replacement of cement with LCB may result in a higher compressive strength. At later stages (Fig. 4e and f), this contour region continues to expand; hence, the pozzolanic activity of the LCB is apparent.

Fig. 4
figure 4

Contour plots modeling the compressive strength development of the concrete mixtures at (a) 3 days, (b) 7 days, (c) 14 days, (d) 28 days, (e) 50 days, and (f) 90 days. (a) 3 days, (b) 7 days, (interpolated), (c) 14 days, (interpolated), (d) 28 days, (e) 50 days, (interpolated), (f) 90 days.

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Moreover, the inferior compressive strength of RAC is also noted. The uniform narrowing of contour regions by 90 days has no effect on the pozzolanicity of LCB on RAC. However, this may not be an issue of microstructural development but rather an inefficient translation of compressive forces to the RCA, possibly due to the microcracks in the old RCA mortar. Essentially, there are three types of interfacial transition zones (ITZs) in RAC: the ITZ between aggregates and new mortar (ITZa), that between new mortar and old mortar (ITZb), and that within RAC between old mortar and old NA (ITZc), which is the weakest44,45,46. These ITZ zones are illustrated in Figure S6a (from SI). Moreover, the overall water content of RAC is higher than that of NAC, potentially increasing the pore volume of the concrete and subsequently reducing the compressive strength.

Splitting tensile strength and flexural strength models

STS and flexural strength showed similar improvements with RCA and LCB, as shown in Fig. 5a–f. At 28 days, the STS is highest for NAC and/or low RAC with < 5% of LCB (Fig. 5a). However, by 90 days, the situation is reversed, with the maximum amounts of RAC and LCB showing the highest tensile strength regions (Fig. 5e). The contours of the surface plots are mostly aligned parallel to the RCA axis), indicating that the tensile strength does not vary between the RCA amounts. Overall, it can be inferred that the addition of LCB increases the late-stage tensile strength of the concrete regardless of the presence of RCA.

Fig. 5
figure 5

Surface plots modeling the splitting tensile strength development at (a) 28 days, (b) 58 days, and (c) 90 days and the flexural strength development at (d) 28 days, (e) 60 days, and (f) 90 days for all the concrete mixtures. (a) STS – 28 days, (b) Flexural – 28 days, (c) STS – 58 days, (interpolated), (d) Flexural – 60 days, (interpolated), (e) STS – 90 days, (f) Flexural – 90 days.

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Conversely, the flexural strength surface plot contours are relatively horizontally inclined at 28 (Fig. 5b) and 90 days (Fig. 5f). The RCA significantly increased the flexural strength from 28 days onward. Moreover, high-LCB NAC performed worst at 28 days, but pozzolanic LCB activity significantly improved by 90 days, whereas NAC with no LCB had the worst flexural strength. The interpolated 60-day flexural strengths (Fig. 5d) and STSs (Fig. 5c) are similar, albeit with greater improvement in performance at 100% RAC. Ultimately, the concrete with the highest 90-day flexural capacity possessed the greatest amounts of LCB and RCA.

Summary of the discussion of the effects of the LCB/RCA on concrete

In summary, the following inferences are made on the basis of the workability and strength results and subsequent RSM design analysis:

  1. 1.

    The addition of LCB significantly reduces the workability of fresh concrete, necessitating the use of superplasticizers. Conversely, replacement of NA with presoaked RCA slightly improved the workability.

  2. 2.

    The compressive strength is improved in later stages by LCB. However, the replacement of normal aggregate with RCA decreased the compressive strength for all days.

  3. 3.

    STS is considerably improved by LCB in later stages. The improvement by LCB is equal for both the NAC and RAC mixes. The RCA mixture only slightly reduced the tensile strength.

  4. 4.

    The flexural strength was improved by the addition of RCA and further increased at 90 days by the addition of LCB.

With respect to the above observations and the previous sections of this research, the following characteristics explain the changes in strength and workability. The strength results are also illustrated in Figure S6 (from SI).

  1. 1.

    The fineness of the LCB reduces the workability of the mixture. Owing to the addition of extra water to the RCA, the workability is improved despite the addition of LCB.

  2. 2.

    The soaking of RCA increases the pore water availability in RAC, reducing the compressive strength. Additionally, the adhered old mortar causes an inefficient translation of compressive forces from the cement matrix directly to the rough aggregate. (Figure S6a).

  3. 3.

    The angular shape of the RCA allows increased contact surface area with the cement; however, the poor ITZ zones are unable to provide superior tensile capabilities (Figure S6b).

  4. 4.

    In terms of flexural strength, the increased surface area of RAC and pozzolanic activity from LCB replacement both work in tandem to provide significantly higher flexural strengths (Figure S6c).

Conclusions

This study investigated the mechanical performance of concrete incorporating various amounts of LCB (0, 5, 10, 15, and 20% by mass of cement) and RCA (0, 50, and 100% by volume of RA). Workability, compressive strength, tensile strength, and flexural strength are investigated. The following conclusions can be drawn from this study:

  1. 1.

    No loss in workability is observed with the application of soaked RCA in the concrete.

  2. 2.

    LCB replacement reduces the mix workability because of its high degree of fineness.

  3. 3.

    Increasing the LCB percentage reduces the early-age compressive strength but improves the 90-day compressive strength. All the RAC mixtures had inferior strengths to those of the NAC mixtures regardless of the addition of LCB. However, RSM analysis revealed that late strength concrete can potentially favor RCA replacement.

  4. 4.

    STS is also improved in late stages by the addition of LCB. This improvement is equal for both the NAC and RAC mixtures.

  5. 5.

    The flexural strength of concrete is positively influenced by the incorporation of RCA, and a further improvement in flexural strength is observed owing to the pozzolanicity of the LCB.

  6. 6.

    A greater correlation between the three strengths was observed uniformly with greater RCA replacement, which was attributed to the rough angular surface of the recycled aggregates resulting in stronger microstructural bonds.

  7. 7.

    Compared with the NAC mixture, all the RAC mixtures presented less variance between the same mixed samples.

  8. 8.

    Considering overall mechanical performance and sustainability, concrete with 50% RCA and 10–20% LCB can produce better strength results than the control mixture.

Overall, the addition of pozzolanic LCB considerably improved the flexural strength of RAC, whereas the other strengths were similar (splitting tensile) or slightly reduced (compressive). As such, this concrete mix can be recommended for building scenarios where environmental concern is of higher priority. Additionally, the microstructural characterization of RA and LCB interaction can further reinforce the current field of study and can provide a valuable insight.