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Article

Effects of Multi-Walled Carbon Nanotubes and Recycled Fine Aggregates on the Multi-Generational Cycle Properties of Reactive Powder Concrete

by
Heng Wu
1,
Xibin Liu
1,
Xirui Ma
1 and
Guifeng Liu
2,*
1
School of Environmental Science and Engineering, Changzhou University, Changzhou 213164, China
2
School of Urban Construction, Changzhou University, Changzhou 213164, China
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(5), 2084; https://doi.org/10.3390/su16052084
Submission received: 26 January 2024 / Revised: 21 February 2024 / Accepted: 27 February 2024 / Published: 2 March 2024
(This article belongs to the Special Issue Advances in Sustainable Construction and Building Materials)
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Abstract

:
In order to investigate the effect of multi-walled carbon nanotubes (MWCNTs) on the recyclable properties of multi-generation recycled concrete, the physical properties of multi-generation recycled fine aggregate and the mechanical properties of multi-generation recycled concrete with different dosages of MWCNTs were tested, and the enhancement mechanism was analyzed by scanning electron microscopy (SEM). The results showed that the apparent density of multi-generation recycled fine aggregate with 0.05 wt% MWCNTs was increased by 1.04~2.03%, the crushing value was decreased by 38.21~49.45%, the compressive strength of the concrete prepared by it was increased by 11.11~18.96%, the splitting tensile strength was increased by 10~43.94%, the flexural strength was increased by 13.62~22.23%, and the mechanical properties were analyzed by scanning electron microscopy (SEM). Combined with the scanning electron microscope image analysis, the MWCNTs can fill the pores inside the specimen, bridge the cracks, and retard the decrease in concrete strength after multi-generation recycling.

1. Introduction

With the immediate development of the economy, the construction industry has gradually become the most critical resource-consuming industry [1]. China constantly develops new land and renovates old grounds to meet urban housing needs, resulting in much construction waste. According to statistics, the total amount of waste generated by the construction industry in China each year is close to 4 billion tons, close to 40% of the total amount of municipal waste, and this growth trend is still growing year by year [2]. By overexploitation, the resulting environmental degradation and ecological damage not only cause impairment to the Earth’s ecosystem but also pose severe obstacles and threats to the sustainable evolution of humankind and the economy. The methods of dealing with construction waste in China can be roughly categorized into two types [3]: the first is to transport the construction waste to the suburbs or rural areas; that way, it will form a source of garbage, and a large amount of farmland will be occupied. The second method, whereby construction waste is processed and then used as backfill, does not fully and efficiently use renewable resources. When new buildings need to be constructed, large amounts of raw materials are required. Gustavo Henrique Nalon et al. [4] concluded that various types of wastes, when properly processed and combined, are prepared into sustainable self-sensing composites, which can reduce the amount of cement used and reduce the impact on the environment. Therefore, reusing construction waste as recycled concrete not only helps reduce the amount of waste but also reduces the consumption of natural materials [5,6].
Today, the overexploitation of natural sand has caused environmental and economic problems; using recycled fine aggregates (RFAs) prepared from construction waste instead of natural sand can significantly contribute to manufacturing new types of concrete/mortar for the building and construction industry [7,8]. However, the physical properties of RFAs are generally insufficient for natural sand. RFAs have high water and early water absorption characteristics compared to natural sand. Some studies on recycled concrete aggregates (RCAs) [9,10,11] have noted that adsorption mortars on old, recycled aggregates exhibited high porosity and water absorption properties. Evangelista L et al. [12] counted the saturated surface dry water absorption of 38 RFA samples attributed to the higher open pore content and rough surface texture of RFAs, with values ranging from 4.28% to 13.1%, with a mean value of 8.4%. In addition, Poon, C. S et al. [13] compared and analyzed recycled aggregate from normal concrete and recycled aggregate from high-performance concrete and found that the strength of the RAC made from recycled aggregate recycled from high-performance concrete was higher than that of the RAC made from recycled aggregate recycled from normal concrete. Therefore, the performance of RFAs can be improved by selecting better-quality masterbatch materials, and the better the quality of the RFA, the better the performance of the prepared RAC [14]. In addition, modification techniques can also be used to change the properties of RACs, including adding geopolymers to fill in the original defects of recycled aggregates [15], adding fibers to reinforce the tensile possessions of RACs [16], increasing the proportion of cement to compensate for the loss of concrete strength [17], or reducing the water–cement ratio by using highly efficient water reducers [18]. Several investigators have tried to enhance the mechanical properties performance of RACs by adding reactive powder, i.e., reactive powder concrete (RPC). It is a particular variety of concrete. Among them, the coarse aggregate is replaced by fine aggregate. Its main characteristics are a high proportion of Portland cement, adding reactive powder, and a low water–binder ratio [19]. Compared to traditional concrete, the main characteristic of RPC is a denser microstructure, particle size uniformity, and minor porosity [20]. Considering the above points, the reactive powder recycled concrete prepared by adding the reactive powder to the RFA can compensate for the effects of the original defects of the RFA. Simultaneously, incorporating nanomaterials in concrete is also one of the effective means to modify it [21], using different nanomaterials to limit the shortcomings of the RFA to achieve the sustainable development of cementitious composites [22,23]. A carbon series of nanomaterials, such as carbon nanotubes (CNTs), are distinguished from traditional carbon fibers due to their excellent physical and mechanical properties [24]. Studies have shown that multi-walled carbon nanotubes (MWCNTs) have a small scale, light weight, high stiffness, high strength, high modulus of elasticity, excellent elasticity, good toughness, excellent electrical properties, ultra-high aspect ratio, super acid, and alkali resistant ability [25,26,27]. Ordinary fibers can only be used as fillers to limit the crack expansion. CNTs can fill micropores, optimize the microstructure, and reduce internal defects in cementitious materials [28,29,30].
To date, abundant studies have aimed at improving the performance of one-generation RFAs and RACs. However, there are still relatively few studies on the performance of recycling RFAs and preparing multi-generational RACs. The repetition of RFAs in an RAC will determine the sustainability of the RAC and its recyclability. Repeating this process can be referred to as “multi-generational” recycling [31]. RFA is an increasingly important material in construction manufacturing, and research into its use in RAC is an area of growing interest. The production of multi-generational RAC still requires further research, posing a significant challenge to the industry. Therefore, there is a need for further research into the use of RFAs in building construction materials to improve the recyclability of RAC products and reduce their environmental impact. The research of multi-generational RACs done well and effectively promoted and used will not only realize multiple environmental protection and the reuse of construction waste but also alleviate the problem of the scarcity of stone resources nowadays and reduce the mining and destruction of mountain stones from the source of construction waste and the collection of raw materials for newly formulated concrete to achieve multiple protection of the environment. However, the research on the application of MWCNTs in the field of multi-generational RACs is not yet mature and is still in the development stage. Scientific issues such as the influence law of MWCNTs on the properties of multi-generational RFAs, the properties of multi-generational RACs, and the mechanism of action are urgently needed to conduct the corresponding experimental research.
In this study, MWCNTs were incorporated into RPC to improve the microstructure and mechanical properties of RPC under multi-generational recycling, MWCNT-enhanced reactive powder concrete (MWRPC) was prepared, and then, discarded MWRPC was collected and processed to obtain RFA (MWRFA)-containing MWCNTs, which served as the next-generation RFA to continue to prepare multi-generational recycled RPC until the mechanical property enhancement effect of MWCNTs on multi-generational cycling RPCs decreased significantly. Through conducting the apparent density and crushing value tests of the MWRFA and the mechanical property tests of the MWRPC under multi-generational cycling, the effect of the MWCNTs on compensating the defects of the RFA under multi-generational cycling, as well as the influence law on the static strength characteristics of concrete, were investigated in terms of the mechanical property indices, such as the compressive strength, the splitting tensile strength, the flexural strength, and the flexure–compression ratio. In order to strengthen the contrast, the corresponding experimental study was carried out on the RPC not mixed with the MWCNTs and combined with the results of the scanning electron microscope (SEM) experiment to dissect the superiority of the effect of the MWRPCs on the multi-generational recyclable performance gain of concrete compared to the ordinary RPCs to provide specific references and lessons to optimize the preparation of the carbon fiber-modified multi-generational recycled cementitious materials and to promote their application in engineering practice. This investigation seeks to provide a reference for optimizing the preparation of MWCNT-modified multi-generational recycled cementitious materials and promoting their application in engineering practice.

2. Materials and Methods

2.1. Multi-Generational Recycled Fine Aggregates and Concretes Preparation

The experimental test consists of four phases using RFAs from three sources. The 1st type of recycled fine aggregate (RFA1) is obtained from construction waste after demolishing abandoned buildings. The 2nd type of recycled fine aggregate (RFA2, RFA3, and RFA4) is obtained from the crushing and screening of waste reactive powder concrete without MWCNTs, and the 3rd type of recycled fine aggregate (MWRFA2, MWRFA3, and MWRFA4) is obtained from the crushing and screening of waste reactive powder concrete containing MWCNTs.
The preparation process of multi-generational RFAs and corresponding concrete is as follows (Figure 1): The construction waste is crushed and screened (RFA1). The first-generation recycled concrete (RPC1 and MWRPC1) with MWCNT dosages of 0 wt% and 0.05 wt% are prepared, respectively, and then, RPC1 and MWRPC1 are crushed and screened to obtain the second-generation RFA (RFA2 and MWRFA2). The second generation of recycled concrete (RPC2 and MWRPC2) is prepared from RFA2 and MWRFA2, and then, RPC2 and MWRPC2 are crushed and sieved to obtain the third generation of re-zero generated fine aggregate (RFA3 and MWRFA3). The third generation of recycled concrete (RPC3 and MWRPC3) is prepared from RFA3 and MWRFA3, and RPC3 and MWRPC3 are crushed and sieved to obtain the fourth generation of RFAs (RFA4 and MWRFA4). Finally, the fourth-generation recycled concrete (RPC4 and MWRPC4) is prepared from RFA4 and MWRFA4. The RFAs without MWCNTs are obtained from construction waste and concrete with different recycling times (RPC1, RPC2, and RPC3) after crushing and screening, and the particle size distribution is shown in Figure 2a. The RFAs mixed with MWCNTs are obtained from concrete with different recycling times (MWRPC1, MWRPC2, and MWRPC3) after crushing and screening, and the particle size distribution is shown in Figure 2b. The three types of RFAs are all Zone II medium sands.

2.2. Materials

The materials used in the research are composite Portland cement with the label P.O.52.5, fly ash, slag powder, and silica fume. The aggregates used in this study are the above three types of RFAs, and the comparison of the leading performance indicators is shown in Table 1. The MWCNTs are GT-20Y (Shandong, Dazhan Nano Materials Co., Ltd., Zouping, China). The appearance is illustrated in Figure 3, and the physical properties are exhibited in Table 2. Tap water is used as the mixing water, and polycarbonate super-plasticizer is used as the admixture. According to a previous experimental study [32], the effect of MWCNTs content on the concrete performance was compared by the mechanical property test, and it was found that 0.1 wt% was the optimal mixing amount. However, there was little difference in the strength of concrete between 0.05 wt% and 0.1 wt%, so considering the cost of multi-generation recycled concrete, the lower cost of 0.05 wt% MWCNTs was chosen.

2.3. Mix Ratio Design

As a result, the high water absorption of multi-generational RFA reduces the free water and effective water–cement ratio in the concrete mix, reducing the fluidity and increasing the viscosity of the recycled concrete mix [33,34]. Therefore, the designed water consumption needs to include the net water consumption (Water-m) calculated according to the standard concrete proportion design method and the additional water absorbed by the recycled sand in the concrete mix (Water-a) [35,36], i.e., the extra water consumption. Referring to previous studies, the flowability of recycled concrete and natural concrete with the same characteristics is similar only when the RFA is made in saturated face-dried conditions [37], but the slump and mechanical properties of concrete made from RFA in soggy face-dried conditions are lower than that of concrete made in air-dried conditions [38]. In addition, during the mixing process, the water absorption of the RFA could not achieve the saturated surface-dried (SSD) condition within 10 min. Therefore, the calculation approach was not applicable (as shown in Equation (1)). As a result, the additional water consumption compensation coefficient is used to regulate the additional water consumption. According to Li et al. [39], when the substitution rate of recycled fine aggregates was 100%, the workability of the concrete decreased. The mechanical properties increased at a low additional water amount (<0.6) and medium additional water amount (0.6–0.75). At a medium additional water consumption (0.6–0.75), some mechanical properties of recycled sand concrete increased, and their values were higher than those of natural sand concrete, while at a high additional water consumption (0.9–1.0), the mechanical properties of recycled sand concrete decreased. Therefore, an additional water consumption of 0.7 was selected. In this research, the water consumption adjustment coefficient (K𝑤𝑎) is set at 0.7, i.e., the extra water consumption of the multi-generational recycled concrete is 70% of the water absorption of the recycled sand when it absorbs water to reach the saturated face-dried state (as shown in Equation (2)). The mixing composition is shown in Table 3.
m w a - 100 % = ( w R F A · s s d − w R F A ) × m F A
m w a = ( K w a w R F A · s s d − w R F A ) × m F A
where m w a - 100 % is the mass (kg) of additional water consumption per unit volume of recycled concrete in the saturated surface-dried state; m w a is the mass (kg) of additional water consumption per unit volume of recycled concrete in the air-dried state; m F A is the calculated amount of fine aggregate used per unit volume of recycled concrete in dry form (kg); K w a is the adjustment coefficient of additional water consumption; w R F A · s s d is the water absorption rate of saturated surface dry recycled sand (%); w R F A is the air-dried recycled sand moisture content (%).

2.4. Methods

Owing to the small size and large specific surface area of the MWCNTs and the van der Waals forces interaction between them, causing the MWCNTs to be highly susceptible to aggregation, the extent of dispersion becomes one of the essential factors affecting their enhancement effect [40]. Therefore, MWCNTs with a mass ratio of 1:4 are mixed with a surface activator (polyvinylpyrrolidone) and water, stirred with a magnetic heating stirrer at 30 °C for 15 min. Then, the resolution is arranged in an ultrasonic cleaner for 40 min, and the prepared MWCNT dispersion is used instead of the mixing water of the concrete.
Each generation of a recycled concrete mixture is mixed according to the secondary mixing process. The steps are as follows: In the first step, put all the fine aggregate and half of the total water (including the mixing water and additional water) into the cement concrete mixer and mix for 120 s. In the second step, all the gelling materials and reactive powders are added to the blender to continue stirring for 90 s. The third step is to add the remaining water and water-reducing agent into the mixer, mix for 120 s, then load into the mold. All concrete test blocks are placed in an environment of 20 ± 5 °C for 24 h and removed from the mold. After the test blocks are released from the mold, they are positioned in a high-temperature water tank at 85 °C for seven days of curing. Referring to Chinese standards JGJ/T 70-2009 [41] and JTG3420-2020 [42], a cube test block of 70.7 mm × 70.7 mm × 70.7 mm is used for the compression resistance strength and split tensile test. The flexural strength test is founded on the Chinese standard (ISO Method) GB/T 17671-2021 [43] to prepare a 40 mm × 40 mm × 160 mm prism test block.

3. Results

3.1. Effect of MWCNTs on Multi-Generational Recycled Reactive Powder Fine Aggregate

To analyze the influence of MWCNTs on multi-generational RFAs, the apparent density and crush value of the RFA obtained after crushing recycled concrete with 0 wt% and 0.05 wt% of MWCNT doping in different numbers of cycles are tested, respectively. The experimental outcomes are exhibited in Figure 4.
Figure 4a–c exhibit that RFA1 crushed from construction waste has a more regular shape, less angularity, and the color is close to that of natural sand. RFA mixed with only reactive powder and recycled several times has angularity, irregular shape, and gray color. MWRFA mixed with reactive powder and MWCNTs and recycled several times has an uneven surface and gray-black color.
Figure 4d shows the apparent density of RFA obtained after crushing recycled concrete with MWCNT dosing of 0 and 0.05 wt% under different cycle generations. From Figure 4d, it is clear that, with the augmentation in cycle generations, the apparent density of the RFA tends to increase first and then decrease. After one generation of cycling, the pores of RFA1 and the cracks produced by crushing are filled with the addition of the reactive powder. Therefore, the apparent density of the second-generation RFA increased. Still, as the number of cycle generations increased, the adsorption mortar also increased, and the aggregate surface became rough, together with the increase in the cumulative number of cracks within the RFA after multiple crushing, resulting in a reduction in the apparent density of the third- and fourth-generation RFAs. The evident density decline slowed as the cycle generation was exceeded three times. Meanwhile, the apparent density of the MWRFA was more significant compared to the RFA with the same number of cycles, and decreased when the number of cycle generations increased, partly because of the filling effect of MWCNTs on the concrete interfacial transition zones, pores, and cracks [44], with improved densification of the matrix. Figure 4b also shows the crushing values of different aggregates. As shown in Figure 4b, RFA1 has the highest crushing value, followed by RFA4. It indicates that RFA1 has the worst load resistance. In contrast, the load resistance of RFA4, doped only with reactive powder, is close to that of RFA1 after four cycles, i.e., the reinforcing effect of the reactive powder is gradually weakened or even lost after four cycles. The MWRFA doped with 0.05 wt% MWCNTs has the lowest crush value after two cycles, indicating that MWRFA2 has the best load resistance. It is noteworthy that, after four times of cycling, MWRFA4 maintains a better load resistance, indicating that improving multi-generational RFA with MWCNTs is feasible.

3.2. Effect of MWCNTs on the Mechanical Properties of Multi-Generational Recycled Reactive Powder Concrete

3.2.1. Compressive Strength

The compressive strength of each generation of test blocks after 7 d of high-temperature hydration is illustrated in Figure 5. As illustrated in Figure 5, the compressive strength of RPC1 prepared from RFA crushed from construction waste is the lowest when the amount of MWCNTs incorporated is 0 wt%. The compressive strength of RPC2 increases after one cycle, indicating that the adsorption mortar on the RFA after one generation of recycling improves the quality of the RFA. Consequently, the compressive strength of RPC2 is higher than that of RPC1. As the number of cycles exceeds two, the compressive strength of the concrete shows a gradual decline, owing to the increase in the surface adsorption mortar of the RFA after multiple crushing, which affected the interfacial reaction between the RFA, and the mortar weakened the interfacial transition zone between the two and led to a decline in the compressive strength. As the digit of cycles exceeds two, the concrete’s compressive strength gradually decreases. A contributing factor to this trend is the increase in concrete adherence to the surface of the RFA after multiple crushing. It affects the interfacial reaction between the RFA and the mortar. The interfacial transition zone between the two weakens, leading to a reduction in the compressive strength.
The compressive strength of MWRPC1 is more significant than that of RPC1 by increasing the dosage of the MWCNTs from 0 wt% to 0.05 wt%, indicating that suitable MWCNTs can enhance the compressive strength of concrete. Furthermore, the MWRPC2, MWRPC3, and MWRPC4 prepared from the previous generation of RFAs containing MWCNTs are not mixed with MWCNTs during the preparation process. Still, after the second-generation cycle, the decrease in compressive strength of the MWRPC is less than the RPC. As can be noticed, MWCNTs can still improve the compressive strength of concrete after multiple generations of cycles. Specifically, the following results are obtained:
(1)
RPC1 has the lowest compressive strength of 49.93 MPa, failing to reach the expected strength value of cement. It shows that directly using RFA crushed from construction waste to prepare fully recycled concrete with a 100% substitution rate, even with reactive powder, is insufficient to bridge the gap between recycled and natural aggregates’ material properties.
(2)
The compressive strength of RPC2, prepared from recycled aggregate after one generation of recycling, reaches a maximum value of 56.97 MPa, 14.1% higher than RPC1. Evidently, after one generation of recycling, the mortar attached to the RFA can improve the total deficiency of the aggregate. The compressive strengths of RPC3 and RPC4 grew by 10.15% and 3.54%, respectively, corresponding with that of RPC1. The enhancement effect of the adsorption mortar shows a decreasing trend, indicating that the adsorption mortar could repair the microcracks of the RFA to a certain extent and fill in the old pores of the RFA, but the reinforcing effect is limited.
(3)
The compressive strength change pattern of the MWRPC is analogous to that of RPC when the cycle is not completed more than three times. Among them, the compressive strength of MWRPC1 is the lowest at 58.03 MPa. Compared to RPC1, the number of cycle generations is the same. The compressive strength increased by 16.22%. It can be seen that a dosage of 0.05 wt% of the MWCNTs can improve the compressive strength of the concrete. Meanwhile, when cycling to the fourth generation, the compressive strength of MWRPC4 is 61.50 MPa, similar to the compressive strength of the previous generation, with no significant decreasing trend. In comparison, the decrease of RPC4 of the prior generation is 6.00%. This indicated that MWCNTs could still enhance the compressive strength of concrete under multiple cycles.
The reasons for the above results are analyzed as follows: The compressive strength is related to the aggregate quality, and the generation of RFA crushed from construction waste has poor physical properties, loose surface mortar, and more cracks. After one generation of recycling, the second-generation RFA has a reactive powder attached to it, filling in some of the cracks and pores and increasing its quality. Therefore, the compressive strength of second-generation recycled concrete is higher than that of first-generation recycled concrete. Meanwhile, 0.05 wt% MWCNTs can continue to refill the interface transition zone, pores, and cracks inside the concrete, making the inside denser and further improving the concrete’s compressive strength. With the cycle growth, the adsorption mortar on the RFA increases after repeated rolling, affecting the interfacial reaction between the RFA and the cement. It weakens the interfacial transition zone between the material and the cement, so it results in a reduction in compressive strength.

3.2.2. Splitting Tensile Strength

The splitting tensile strength of each generation of test blocks is shown in Figure 6. The results of Figure 6 demonstrate that the splitting tensile strength of the four types of RPCs tested varies significantly. RPC4 had the lowest value of 3.96 MPa, followed by RPC1 with 4.8 MPa. RPC2 had the highest value with 6.06 MPa. These results provide insight into the different strengths of the four RPCs. As with compressive strength, the change in law with the cycle growth algebra follows a pattern of first growing and then dropping. As the cycle algebra increases, the law initially increases in strength before eventually reaching a peak and diminishing. This pattern is a common trend in both compressive strength and cycle algebra. This is consistent with the compressive force, on the one hand, due to the introduction of a reactive powder to enhance the quality of the concrete. On the other hand, subjected to compressive force in the crushing process, many microcracks appear in the material’s interior, making it easier for the pulp to separate when subjected to tensile force. Therefore, it is essential to consider the effects of compressive and tensile forces when designing a method for crushing materials. In addition, the cumulative cracks inside the RPC gradually increase as the number of cycle generations increases, also leading to an increase in its quality instability.
As for the change rule of the MWRPC, the splitting tensile strength is consistent with that of RPC, and the splitting tensile strength of each generation of MWRPC is greater than that of RPC. The splitting tensile strength of MWRPC1 made by blending MWCNTs increased by 10.00% compared to that of RPC1. MWRPC2, MWRPC3, and MWRPC4, made by utilizing fine aggregates containing MWCNTs, increased by 15.68%, 18.84%, and 43.94% compared to the same generation of RPC2, RPC3, and RPC4. This is because MWCNTs can create a bridging role between cement-based composite pores, cracks, and matrix [30]. When concrete is exposed to an external force, the force is distributed from the matrix to the MWCNTs through the connecting interface. This interface links the two, enabling the transfer of energy and stress to the MWCNTs. As a result, the MWCNTs can absorb and dissipate the energy, thereby strengthening the concrete structure and increasing its ability to resist external forces. MWCNTs can impede the inception and progression of micro-fissures and augment the durability. Also, MWCNTs can improve the splitting tensile strength of concrete when added in the right portion. As an added benefit, it can be evenly dispersed in the concrete after multiple cycles, filling the pore and interface transition area and compensating for the negative impact of adding too much mortar. This demonstrates the potential for MWCNTs to be a helpful additive in concrete production, significantly improving the strength of the material.

3.2.3. Flexural Strength

Analyzing Figure 7, it can be found that the flexural strength indicates a decreasing trend with the cycle growth, and the highest flexural strength of 10.33 MPa in RPC1 is located at the doping amount of the MWCNTs of 0 wt%. After one cycle, the flexural strength of RPC2 decreased, indicating that the adsorption mortar and reactive powder on the RFA could not enhance the flexural performance of the concrete after one cycle. Therefore, after many cycles, the flexural strength of concrete tends to decrease. When the range of the MWCNTs improved from 0 wt% to 0.05 wt%, the concrete flexural strength peaked at 12.63 MPa, an increase of 22.23%. This shows that the incorporation of MWCNTs can effectively inhibit the generation and blossoming of cracks inside the concrete and enhance the flexural strength of the concrete. After two cycles, the relative growth rate of the concrete’s flexural strength gradually slowed to 21.48%. The growth rate was 14.71% in the third cycle and dropped to 13.62% in the fourth cycle, indicating that the enhancement effect of the MWCNTs gradually weakened with the increase in the cycle number. In short, with cycle growth, the flexural strength of the concrete shows a trend of reaching the peak point first and then beginning to decrease. The flexural strength of the specimens in each group of MWRPC is higher than that of the representatives of the RPC group, indicating that the MWCNTs can effectively enhance their flexural strength in the concrete.
The reasons for the above results are as follows: When the concrete is subjected to a bending load and new microcracks are initiated, the MWCNTs distributed randomly and across both sides of the crack can buffer part of the stress at the tip of the crack, thereby hindering the expansion and extension of the crack, benefitting the improvement of the flexural strength of the concrete. As the number of cycles grows, the RFA is damaged by external forces during the crushing preparation process, resulting in many microcracks. At the same time, the strength of the mortar attached to the surface of the RFA is low, and the content of adsorption mortar in RPC increases with the number of cycles, thus making the overall strength decline evident.

3.2.4. Flexure–Compression Ratio

The flexure–compression ratio is the flexural strength and compressive strength ratio. It can be used as an indicator to evaluate the material’s toughness. The larger the flexure–compression ratio, the better the material’s toughness.
Based on the consequences of the compressive and flexural strength tests of concrete in this research, the flexure–compression ratios of RPC and the MWRPC specimens with different cycle generations are calculated. Analyzing the relationship between the changes in RPC and the MWRPC specimens for different cycles (see Figure 8) shows that the trends of the concrete flexure–compression ratio differ from the trends of its strength properties with the number of cycles. Generally, the flexure–compression ratio of the MWRPC specimens of each group improves to a certain extent when the number of cycles does not exceed three, and the improvement ranges from 5.23% to 9.40%. Among them, the second generation shows the most significant improvement. Both types of concrete flexure–compression ratios show a trend of decreasing gradually with the growth in the number of cycles, and with the identical number of cycles, adding 0.05 wt% of MWCNTs causes the compressive and flexural strengths to increase to different degrees compared to concrete with only the reactive powder. The advancement in the compressive strength of concrete with the MWCNTs is significantly better than that of the flexural strength. As a result, the flexure–compression ratios of the concrete are increased to a certain extent, and then, the flexure–compression ratios tend to decrease with the cycle of growth. In the second generation of the cycle, the maximum flexure–compression ratio improvement rate (9.40 × 10−2) occurs in MWRPC2 (9.40%), even though the flexure–compression ratio of MWRPC2 decreases compared to that of MWRPC1 in the first generation of the cycle.
The reason is that the addition of well-dispersed MWCNTs and reactive powder significantly improves the compressive strength of the concrete. In contrast, the enhancement of the flexural strength of the concrete by the incorporation of MWCNTs and reactive powders is relatively smooth, leading to an addition in the flexure–compression ratio of the concrete, a weakening of brittleness, and an enhancement of the toughness of the concrete. It can be inferred that the modification of multi-generational recycled concrete with MWCNTs and reactive powder can strengthen the toughness of concrete.

4. Discussion

4.1. MWCNTs and Reactive Powder Mechanism Analysis

Adding MWCNTs combined with reactive powders could significantly enhance the microstructure of multi-generational recycled concrete. This would reduce the expansion of existing microcracks and constrain the formation of new, larger cracks. In this way, the MWCNTs and reactive powders could improve the recycled concrete’s long-term durability and structural integrity. Compared to the RFA obtained directly from the crushing and screening of construction waste and the RFA obtained from the crushing and screening of waste reactive powder concrete, MWRFA is composed of MWCNTs and active powder composite, has significantly higher surface roughness and reactivity, and can synergistically regenerate the recycled concrete on multiple scales. The mechanical properties of multi-generational recycled concrete can be strengthened effectively. Furthermore, it can resist the layer-by-layer cracking of the regenerated concrete during the damage process under loading, giving full play to the respective strengthening effects. In conclusion, MWCNTs and reactive powders combine and excite each other, leading to a step-by-step and multifaceted gain effect on the concrete, as shown in Figure 9.

4.2. Microscopic Mechanism Analysis

Combined with the scanning electron microscope (SEM) test results of the damaged section of the concrete material, the microscopic action mechanism of the reactive powder and MWCNTs in the multi-generational recycled concrete can be analyzed from the following aspects:
(1)
As can be noticed from Figure 10a, for the concrete without MWCNTs and the reactive powder, the RPC1 has apparent cracks. Moreover, in Figure 10b, the newly added reactive powder particles can fill in the cracks in RPC2, effectively reinforce the feeble parts of RPC, and reduce the generation and development of microcracks.
(2)
Figure 10c shows the pore morphology of concrete with only reactive powder. With the addition of reactive powder, some original pores in RFA1 are gradually covered by the reactive powder. Figure 10d shows that the pores in RFA2 are significantly reduced compared to RFA1. It indicates that the reactive powder can reduce the content of harmful macropores in the concrete, improve the microscopic pore structure of the concrete, and enhance the compactness of the matrix. This is consistent with the phenomenon of Wu, J.-D. et al. [45] in their study that slag powder and silica fume have a synergistic effect to repair the initial damage of RPC.
(3)
After four cycles, as the adsorbed mortar content increases, as can be noticed from Figure 11a,b, the cracks around the adsorption mortar and the internal primary pores increase, resulting in the gradual weakening or even loss of the gain effect of the reactive powder on the structural integrity of the concrete, and the overall strength of the concrete begins to decrease.
(4)
After blending MWCNTs, it can be seen from Figure 11c that MWCNTs can fill in the micropores, microcracks, and other defects inside the concrete to make up for the original defects of the recycled aggregates. Also, the disordered and densely packed MWCNTs can form a fiber mesh structure system, strengthen the connection between the concrete and the components of the reactive powder, improve the structure of the cement matrix, and slow down the tip stress concentration effect at the internal cracks of the concrete, thereby effectively enhancing its mechanical properties. This is similar to the conclusion reached by Hong, S.-H. et al. [46] that MWCNTs are able to bridge microcracks and retard damage progression. Moreover, there are polarization and adsorption effects between MWCNTs and cement particles. Therefore, MWCNTs attract cement particles and make them become attached. It means that MWCNTs provide more nucleation sites for cement hydration, increase the hydration rate, and strengthen the integrality of the hydration products of the cement, so they have a pronounced enhancement impact on the development of the strength properties of concrete. Zhang, Y. et al. [47] also found that MWCNTs were able to enhance the interfacial properties of reactive powders, such as fly ash, in concrete, thereby improving the hydration properties of reactive powders.
(5)
As shown in Figure 11d, there is still a tiny amount of MWCNTs with good compatibility between the interface of the MWCNTs and the matrix after four cycles, slowing down the magnitude of the concrete strength decline and giving full play to the reinforcing influence of the MWCNTs on the regenerated concrete with multi-generational recycling.
(6)
In Figure 12a, MWCNTs being pulled out of the MWRFA can be seen. When MWCNTs are evenly dispersed and effectively bonded with the matrix, they can take part of the load and reduce the stress concentration of the matrix, resulting in the weakening of the energy stored as cracks in the MWRFA, thus preventing the further expansion of those cracks. Figure 12b shows MWCNTs being pulled out at the cracks of the MWRPC. It is probable that, as MWCNTs lap the pores and cracks of the cementitious materials, they strengthen the weak parts of the MWRPC and reduce the generation and extension of microcracks, thereby reducing the degradation performance.

5. Conclusions

The mechanical property indices, such as compressive strength, split tensile strength, flexural strength, and flexure–compression ratio, were obtained by preparing multi-generational recycled concrete specimens and testing them in mechanical experiments, focusing on the influence law of MWCNTs towards the reactive powder multi-generational RFAs and consequently analyzing the action mechanism of MWCNTs on the reactive powder multi-generational recycled concrete. The main conclusions are as follows:
(1)
The addition of reactive powder reduced the water absorption of the RFA, increased the apparent density of the RFA, and optimized the load resistance of the RFA compared to fine aggregate recycled from construction waste. Adding MWCNTs further enhanced the physical properties of reactive powder RFA, with the best results in the second generation, when the water absorption and crushing values were reduced by 32% and 73%, respectively, and the apparent density increased by 5.7%. Multiple cycles weakened the enhancement effect, and MWCNTs slowed down the decrease.
(2)
The quality of the RFA directly determines the recyclability of concrete, and the use of MWRFA containing MWCNTs shows superior mechanical properties. MWCNTs can improve the microstructural morphology, such as microcracks and micropores inside the concrete, which is favorable to the practical synergistic bearing of reactive powder and MWCNTs so that the MWCNTs can increase the compressive strength by 11.1–19.0%, the splitting tensile strength by 10–43.9%, the flexural strength by 9.6–22.2%, and the fold-to-compression ratio by 5.2–9.4%.
(3)
When the number of cycles was exceeded twice, the compressive strength, split tensile strength, and flexural strength decreased. The effect of the reactive powder gain weakened, but the mechanical properties of the MWRPC containing MWCNTs were still higher than those of its control group. The residual MWCNTs inhibited the development of defects, and the mutual solid bond with the matrix made the concrete maintain good mechanical properties.
(4)
SEM images further demonstrated that the reactive powder could cover some original defects in the RFA. However, the increase of additional mortar would weaken the effect of reactive powder on the concrete gain. In the precycling stage, MWCNTs can form a fiber mesh structure system, which synergizes with the active powder to enhance the mechanical properties of concrete. In the late stage of cycling, the MWCNTs are dispersed in the cementitious matrix to form bridges in the matrix and enhance the recyclability of concrete.
Based on previous studies and cost considerations, a dosage level of 0.05 wt% MWCNTs was selected in this study to evaluate the multi-generation RPC recyclability. Since other dosage levels were not considered, it was impossible to comprehensively evaluate the effects of different dosage levels of MWCNTs on the multi-generation recyclability of concrete to maximize the advantages of MWCNTs. Future studies may consider comparing the effects of different admixtures of MWCNTs and additional water use on the mechanical properties of multi-generation recycled concrete to prepare sustainable MWCNT concrete with the best results using appropriate costs and maximizing the advantages of MWCNTs.

Author Contributions

Study conception and design: G.L. and H.W.; data collection: H.W.; analysis and interpretation of the results: H.W., X.L. and X.M.; draft manuscript preparation: H.W. and G.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the financial support of the experimental technology research project of Changzhou University (KYH21020340).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data are contained within the article.

Acknowledgments

The authors greatly acknowledge Shandong Dazhan carbon nano technology Co., Ltd. (Shandong, China) for providing the MWCNTs.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Phase of the experimental study.
Figure 1. Phase of the experimental study.
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Figure 2. Particle size distribution of the RFAs. (a) RFAs without MWCNTs incorporated; (b) RFAs with MWCNTs incorporated.
Figure 2. Particle size distribution of the RFAs. (a) RFAs without MWCNTs incorporated; (b) RFAs with MWCNTs incorporated.
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Figure 3. Appearance of the MWCNTs.
Figure 3. Appearance of the MWCNTs.
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Figure 4. Morphology and physical properties of the RFAs: (a) morphology of RFA1; (b) morphology of RFA2; (c) morphology of MWRFA2; (d) apparent density and crushing value of the RFAs.
Figure 4. Morphology and physical properties of the RFAs: (a) morphology of RFA1; (b) morphology of RFA2; (c) morphology of MWRFA2; (d) apparent density and crushing value of the RFAs.
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Figure 5. Compressive strength of the RPC and intensity increase rate.
Figure 5. Compressive strength of the RPC and intensity increase rate.
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Figure 6. Splitting tensile strength of RPC and the intensity increase rate.
Figure 6. Splitting tensile strength of RPC and the intensity increase rate.
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Figure 7. Flexural strength of RPC and the intensity increase rate.
Figure 7. Flexural strength of RPC and the intensity increase rate.
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Figure 8. Flexure–compression ratio of RPC and the increase rate.
Figure 8. Flexure–compression ratio of RPC and the increase rate.
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Figure 9. Schematic diagram of the enhancement mechanism of MWCNTs and reactive powder on concrete.
Figure 9. Schematic diagram of the enhancement mechanism of MWCNTs and reactive powder on concrete.
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Figure 10. RPC crack morphology without MWCNTs: (a) RPC1; (b) RPC2. RFA pore morphology without MWCNTs: (c) RFA1; (d) RFA2.
Figure 10. RPC crack morphology without MWCNTs: (a) RPC1; (b) RPC2. RFA pore morphology without MWCNTs: (c) RFA1; (d) RFA2.
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Figure 11. Microscopic morphology of RPC without MWCNTs after four cycles: (a) adsorption mortar; (b) pores. Microscopic morphology of MWRPC-containing MWCNTs: (c) MWRPC2; (d) MWRPC4.
Figure 11. Microscopic morphology of RPC without MWCNTs after four cycles: (a) adsorption mortar; (b) pores. Microscopic morphology of MWRPC-containing MWCNTs: (c) MWRPC2; (d) MWRPC4.
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Figure 12. Effect of MWCNTs on the microscopic morphology of the RFA and RPC. (a) MWRFA. (b) MWRPC.
Figure 12. Effect of MWCNTs on the microscopic morphology of the RFA and RPC. (a) MWRFA. (b) MWRPC.
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Table 1. Physical properties of the RFAs.
Table 1. Physical properties of the RFAs.
Fine AggregateFineness ModulusMoisture Content/%Water Absorption/%
RFA12.52.8 7.9
RFA22.63.1 5.7
RFA32.82.9 6.2
RFA42.82.7 7.1
MWRFA22.92.8 5.4
MWRFA32.72.9 5.5
MWRFA42.92.7 6.1
Table 2. Physical properties of the MWCNTs.
Table 2. Physical properties of the MWCNTs.
Diameter/nmLength/μmPurity/%Ash Content/%Effective Density/(g/cm3)Specific Surface Area/(g/cm2)Young’s Modulus/(TPa)Bending Strength/(GPa)
10–205–50>85<20.06–0.1160–2101.814.2 ± 10.8
Table 3. Mix composition of the tested concretes.
Table 3. Mix composition of the tested concretes.
ConcreteMaterial/(kg·m−3)Polycarbonate Super-Plasticizer/wt%Total Water–Cement RatioMWCNTs/wt%
CementRFAFly AshSilica FumeSlag PowderWaterAdditional Water
RPC16301050210105105210292.30.23 0
RPC26301050210105105210102.30.21 0
RPC36301050210105105210152.30.21 0
RPC46301050210105105210242.30.22 0
MWRPC16301050210105105210292.30.23 0.05
MWRPC26301050210105105210102.30.21 0
MWRPC36301050210105105210102.30.21 0
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Wu, H.; Liu, X.; Ma, X.; Liu, G. Effects of Multi-Walled Carbon Nanotubes and Recycled Fine Aggregates on the Multi-Generational Cycle Properties of Reactive Powder Concrete. Sustainability 2024, 16, 2084. https://doi.org/10.3390/su16052084

AMA Style

Wu H, Liu X, Ma X, Liu G. Effects of Multi-Walled Carbon Nanotubes and Recycled Fine Aggregates on the Multi-Generational Cycle Properties of Reactive Powder Concrete. Sustainability. 2024; 16(5):2084. https://doi.org/10.3390/su16052084

Chicago/Turabian Style

Wu, Heng, Xibin Liu, Xirui Ma, and Guifeng Liu. 2024. "Effects of Multi-Walled Carbon Nanotubes and Recycled Fine Aggregates on the Multi-Generational Cycle Properties of Reactive Powder Concrete" Sustainability 16, no. 5: 2084. https://doi.org/10.3390/su16052084

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