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Article

Impact of Rice Husk Ash on the Mechanical Characteristics and Freeze–Thaw Resistance of Recycled Aggregate Concrete

by
Wei Zhang
,
Huawei Liu
and
Chao Liu
*
College of Civil Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2022, 12(23), 12238; https://doi.org/10.3390/app122312238
Submission received: 9 November 2022 / Revised: 25 November 2022 / Accepted: 27 November 2022 / Published: 29 November 2022
(This article belongs to the Section Materials Science and Engineering)
Browse Figures
Versions Notes

Abstract

:
With the accelerating growth of infrastructure construction, carbon emission and environmental pollution problems have become increasingly severe. In order to promote the sustainable development of the construction industry, using rice husk ash (RHA) in recycled aggregate concrete has aroused extensive interest. This study aims to investigate the impact of the partial replacement (0%, 10%, 20%, 30% of binder) of ordinary Portland cement (OPC) with RHA by equal mass on recycled concrete’s mechanical characteristics and freeze–thaw resistance. The workability, compressive strength, mass loss and dynamic elastic modulus of recycled concrete were tested, and the hydration products and microstructure were analyzed using scanning electron microscope (SEM) tests. The mechanism of the freeze–thaw damage deterioration of RHA recycled aggregate concrete was revealed. The results indicate that the incorporation of RHA has an adverse effect on the workability of fresh concrete. Its high specific surface area will provide a large number of nucleation sites for the hydration reaction, refining the pore structure in the paste and improving the weak bonding of the interfacial transition zone (ITZ) by enhancing the matrix’s pozzolanic reaction effect and filling effect, thus improving the compressive strength of concrete. Furthermore, the porous structure of the recycled aggregate attached mortar and mesoporous RHA will absorb a lot of water during the freeze–thaw cycles. With the continuous accumulation of expansion pressure, the interior pores and cracks will gradually expand and extend, leading to more severe damage to the concrete, and the degree of freeze–thaw damage deterioration grows as the RHA replacement ratios increase.

1. Introduction

With the continuous development of the construction industry, cement production has grown dramatically, resulting in increasingly aggravated carbon emissions and environmental pollution [1]. The main sources of carbon emissions in cement production are the chemical reaction of clinker components and the combustion of fossil fuels [2]. Based on the above two points, it is estimated that the total emissions from the cement industry are about 8% of global carbon emissions [3]. The partial replacement of cement with low-energy industrial and agricultural solid waste can reverse the rising carbon emissions trend, thus achieving energy efficiency and environmental protection [4,5,6]. Although using industrial solid wastes such as silica fume (SF), fly ash (FA) and ground granulated blast furnace slag (GGBFS) in concrete has the potential to reduce carbon dioxide emissions and contribute to the idea of sustainable development in the cement sector, their sources are limited by geographical conditions, technological conditions and economic costs [7,8]. Therefore, using agricultural solid wastes such as rice husk ash (RHA) in concrete has aroused extensive interest [9,10].
The rice husk is a broad source of agricultural waste collected during the grinding process from the surface layer of rice grains. In the field of green technology, rice husks are mainly used as fuel for processing rice grains, as boiler steam or as fuel for producing electricity in biomass power plants [11,12]. RHA is an agricultural residue generated by the incineration of rice husk [13]. Because of its high amorphous silica concentration, specific surface area and pozzolanic activity, it is regarded by many researchers as a cementitious material for increasing the properties of concrete [14,15,16].
Recycled concrete aggregate is an environmentally friendly material prepared from construction and demolition waste (CDW) by sorting, crushing, sieving and cleaning, which can be used in the construction sector to alleviate the gradually decreasing natural aggregate and solve the problem of the piling and disposal of construction solid waste, which is conducive to sustainable development [17,18]. Currently, the use of supplementary cementitious materials and recycled concrete aggregate for concrete preparation in the construction industry has attracted widespread attention.
Therefore, many researchers have studied the mechanical characteristics and durability of concrete prepared synergistically from RHA and recycled aggregate. Qureshi et al. [19] studied the coupling effect of hook-ended steel fibers (HSF) and 15% RHA on the mechanical properties and durability of RAC. The results revealed that the joint action of RHA and fibers significantly enhanced the compressive strength, splitting tensile strength, impermeability and acid attack resistance (AAR) of RAC. Rattanachu et al. [20] evaluated recycled concrete’s mechanical properties, chloride penetration depth and steel corrosion using ground RHA. The findings revealed that using RHA had adversely affected the compressive strength of RAC, while greatly improving steel corrosion and chloride resistance. Alyousef et al. [5] studied the effect of waste mineral admixtures and glass fibers on the mechanical and permeability properties of recycled aggregate concrete. The results showed that the combined action of RHA and glass fibers resulted in recycled aggregate concrete exhibiting improved mechanical and durability properties compared to natural aggregate concrete. Nuaklong et al. [21] investigated the influence of RHA replacing silica nanoparticles on the mechanical and fire resistance properties of recycled concrete. The results showed that the addition of rice husk ash could lead to enhanced mechanical properties of recycled concrete by improving the microstructure of the paste, but RHA will adversely affect the post-fire residual strength of recycled concrete. Padhi et al. [22] investigated the engineering application effect of 0–35% RHA and recycled aggregate to prepare concrete. The findings revealed that incorporating RHA and recycled aggregate greatly enhanced the performance of concrete. When the replacement ratio of RHA and recycled coarse aggregate was 10–15% and 100%, respectively, the concrete mix ratio satisfied the engineering application requirements. Koushkbaghi et al. [23] investigated the impact of RHA and fibers on the mechanical properties and acid attack performance of recycled concrete. The results revealed that RHA could enhance the inferior properties of concrete by enhancing the bond between concrete and fibers, and the improvement effect increased with the rise in the RHA replacement ratio. Salahuddin et al. [24] studied the influence of temperature increases on RHA recycled concrete performance. The results showed that a 20% replacement ratio of RHA and a 100% substitution rate of recycled concrete aggregate resulted in a reduction of the compressive strength of concrete. Alnahhal et al. [25] investigated the influence of the partial replacement of cement by RHA on the engineering performance and environmental effects of recycled concrete. The results indicated that the partial replacement cement with RHA not only improved the performance of recycled concrete but also reduced carbon dioxide emissions during the cement production.
Although previous studies have investigated the impact of RHA on various mechanical characteristics as well as the durability of recycled concrete, a mechanism analysis of the impact of RHA on the freeze–thaw resistance performance of recycled concrete has not been reported yet. Therefore, it is of great significance to carry out research on the frost resistance of RHA recycled concrete. In view of this, this research aims to investigate the impact of partially replacing Portland cement with RHA by equal mass on recycled concrete’s mechanical characteristics and freeze–thaw resistance. The workability, compressive strength, mass loss and dynamic elastic modulus of recycled concrete were determined. In addition, the paste’s hydration products and microscopic morphology were analyzed using scanning electron microscope (SEM) tests. The deterioration mechanism of the freeze–thaw damage of RHA recycled concrete was revealed. The outcome of this research is expected to promote the application of RHA recycled concrete in severe cold regions.

2. Materials and Methods

2.1. Raw Materials

In this paper, the Shieldstone P.O 42.5 ordinary Portland cement (OPC) and untreated residual RHA were used as the cementitious materials, and the RHA was sourced from a biomass power factory in Nanjing. The microstructure of RHA is shown in Figure 1, which shows a honeycomb structure. The physicochemical characteristics of the cementitious materials are shown in Table 1, and it can be seen that the specific surface area of RHA is approximately 5–6 times that of cement, and its internal silica content is as high as 92.1%. The particle size distributions of the cement and RHA are shown in Figure 2, which are 14.77 μm and 14.81 μm, respectively.
The fine aggregate was river sand from the Shaanxi region, with a water absorption of 3%, a maximum particle size of 3 mm and an apparent density of 2640 kg/m3. The coarse aggregate was prepared from crushed and manufactured recycled concrete. The basic performance indexes were measured following GB/T 14685-2011 [26], as shown in Table 2. The mixing water was tap water, and the superplasticizer was the PCA polycarboxylic superplasticizer produced by Subot New Materials Co., Ltd. (Nanjing, China).

2.2. Mixture Proportions and Specimen Preparation

The concrete design mix is shown in Table 3, in which RHA replaced cement with 0%, 10%, 20% and 30% replacement ratios, respectively. The designation of the specimens was sourced from the replacement ratios of RHA. An effective water-to-binder ratio of 0.4 was maintained by adjusting the amount of additional water to the system owing to the high water absorption of the recycled coarse aggregate. A target slump of 100–150 mm was kept for the fresh concrete by adjusting the superplasticizer amount due to the RHA’s high specific surface area [20].
In order to prepare concrete specimens, the dry solid components were taken following the mass in the mixture proportion, which was subsequently dumped into a blender and mixed evenly for 2 min. Then, superplasticizer and water were used, and the mixture was cast into cubic specimens for mechanical characteristics testing after mixing uniformly. Finally, the fresh concrete mixtures were loaded into the mold and vibrated on a table for compacting. The demolding and curing of the specimens were conducted following GB/T 50081-2019 [27].

2.3. Experimental Method

In this paper, the impact of RHA on recycled concrete’s mechanical characteristics and freeze–thaw resistance was investigated using experiments, and the hydration products and microstructure were analyzed in combination with scanning electron microscopy (SEM) experiments. The experimental procedure is presented in Figure 3, and the details of the experiments are given below.

2.3.1. Workability

According to GB/T 50080-2016 [28], the slump cone experiment was performed to evaluate the workability of fresh concrete. The slump value is measured in mm, the measurements were accurate to 1 mm and the test results were revised to 5 mm [29].

2.3.2. Mechanical Characteristics

According to GB/T 50081-2019 [27], the compressive strength of concrete specimens with dimensions of 100 mm × 100 mm × 100 mm cubic specimens was tested, and the YAW-3000 universal loading machine was used. The test ages for strength were 7d, 28d, 90d and 360d, and the experimental value is the average of three specimens.

2.3.3. Freeze–Thaw Cycling

According to GB/T 50082-2009 [30], the concrete specimens were put into the TDR-28 concrete freeze–thaw cycle testing machine for a rapid freeze–thaw cycle experiment. The freeze–thaw medium was tap water supplied by the laboratory, and the control system was suspended after reaching 50, 100, 150, 200, 250 and 300 cycles. The surface water of the specimens was dried, followed by mass and dynamic elastic modulus tests being carried out. The mass loss and relative dynamic elastic modulus were also used to assess the degree of the macroscopic performance deterioration of concrete [31], which was calculated as follows:
Δ m n = m 0 − m n m 0 × 100 %
where Δ m n is the mass loss; Δ m n and m n are the masses before and after n freeze–thaw cycles, respectively.
E r , n = E d n E d 0 × 100 %
where E r , n is the relative dynamic elastic modulus; E d 0 and E d n are the dynamic elastic moduli of concrete before and after n freeze–thaw cycles, respectively, and the measuring instrument is the NELD-DTV dynamic elastic modulus testing machine.

2.3.4. SEM Test

The small pieces of paste samples with different RHA replacement ratios were immersed in ethanol solution to prevent further hydration [32], dried in a GZX-9030MBE drying oven, coated with conductive tape and pasted on the specimens, and they were subsequently subjected to spray gold treatment using a Cressington 108 coater. The specimens were analyzed by a Gemini SEM 500 field emission scanning electron microscope with a resolution of 0.5 nm, an acceleration pressure of 0.02 KV to 30 KV, a probe-current of 3 pA to 20 nA and a magnification of 50 X to 20,000 KX.

3. Results and Discussions

3.1. Workability of RHA Recycled Aggregate Concrete

The workability of fresh concrete mixtures can be reflected by the slump value [33,34]. The impact of RHA with different replacement ratios on the workability of recycled aggregate concrete was investigated by means of a slump cone test, and the results are shown in Figure 4. It is noteworthy that 0% RHA and 10% RHA showed the same slump value, which may be due to the increase in the superplasticizer (Table 3), resulting in improved concrete workability. However, the improvement degree of the superplasticizer is similar to the reduction degree caused by the increase in RHA content. At the same time, it is due to the modification of the slump value, thus resulting in the same workability [35].
With the gradual increase in the RHA substitution rate, the slump value of 20% RHA and 30% RHA decreased by 15.38% and 23.08%, respectively, compared with 0% RHA. This may be due to the increase in the RHA replacement ratio, leading to an increase in the porosity and total specific surface area of the cementitious material (Table 1). The system absorbs more water, thus causing a reduction in the free water within the paste, which will increase the mixtures’ friction and discourage the flow, which in turn leads to the decrease in the concrete slump and reduced workability. This finding is similar to that of Amin et al. [15], who reported that the slump of fresh concrete mixtures increased with increasing RHA content, and the mixtures with 30% RHA content had the worst workability.

3.2. Compressive Strength of RHA Recycled Aggregate Concrete

The impact of RHA with different replacement ratios on the compressive strength of recycled concrete is shown in Figure 5. As shown in the figure, the compressive strength of the recycled concrete gradually increased as the hydration reaction continued, and the growth rate shows a trend of first being rapid, then slowing down and finally gradually stabilizing. For instance, the compressive strength of 30% RHA at 7 days, 28 days, 90 days and 360 days was 33.8 MPa, 42.9 MPa, 46.4 MPa and 48.3 MPa, which increased by 69.98%, 18.84%, 7.25% and 3.94%, respectively. The variation in recycled concrete’s compressive strength may be because, in the initial stage of curing, the strength growth of the concrete is mainly due to the RHA particle’s filling action. The fine RHA particles can fill the pores and microcracks of the paste and interface transition zone, reducing the porosity to a certain extent and increasing the bulk density of the concrete, thereby significantly improving the early strength of concrete.
In contrast, the strength growth of recycled concrete in the later stages of curing is primarily due to the RHA particle’s pozzolanic effect [36]. Although the pozzolanic reaction between RHA and free calcium hydroxide (CH) led to a reduction in the paste porosity [37], the existence of the porous structure of the recycled aggregate attached mortar led to the absorption of large amounts of water by the system and a reduction in the free water content of the matrix, resulting in a weakening of the pozzolanic effect of the RHA particles, thus causing a reduction in the total volume of hydration products, which leads to a slow strength growth of the concrete in the later curing period. This is similar to the results of Salas et al. on the later strength variation of RHA natural aggregate concrete [38]. The strength growth of concrete at different curing ages is caused by a combination of the cement hydration reaction, the filling effect and the pozzolanic effect of RHA particles [39,40].
The compressive strength of recycled concrete shows a tendency to first increase and then decrease as the RHA replacement ratio increases. For instance, the compressive strength of 10% RHA, 20% RHA and 30% RHA at 360 days was 51.9 MPa, 52.8 MPa and 48.3 MPa and 102.17%, 103.94% and 95.08% of 0% RHA, respectively. This is probably because the high silica content and high specific surface area of RHA particles, which can contact and react with more CH and water, produce extra C-S-H gels to improve the matrix compactness, thus leading to the improvement of concrete compressive strength. However, when the strength loss due to the decrease in cement content was more significant than the compressive strength increment contributed by the RHA filling effect and pozzolanic effect, this will lead to a reduction in the compressive strength of concrete at a high RHA replacement ratio. According to the variation in the compressive strength of concrete with an RHA replacement ratio, it is clear that the optimum replacement ratio of RHA is 20%, which is similar to the report of RHA natural aggregate concrete. For instance, He et al. [32] and Chao-Lung et al. [41] found that the concrete specimens with a 20% RHA replacement ratio resulted in the highest paste compactness due to the joint action of the filling effect and pozzolanic effect, thus resulting in the optimum mechanical characteristics of concrete.

3.3. Mass Loss of RHA Recycled Aggregate Concrete

The mass loss is an essential indicator representing the damage degree of surface spalling of concrete specimens subjected to freeze–thaw cycles [42]. Figure 6 shows the variation in recycled concrete’s mass loss with different RHA replacement ratios during the freeze–thaw cycle. As shown in the figure, the mass loss tends to decrease with the increase in the number of cycles at the initial stage of the freeze–thaw cycle, and the reduction amount increases with the rise in the RHA substitution rate. For instance, when concrete specimens were subjected to 100 freeze–thaw cycles, the mass losses of concrete with 0% RHA, 10% RHA, 20% RHA and 30% RHA were −0.48%, −0.71%, −0.73% and −0.83%, respectively. This could be attributed to the existence of the porous structure of the recycled aggregate attached mortar, which resulted in more external water transfer to the internal pores and cracks. In addition, the high porosity of RHA particles will enhance the permeability of the concrete; thus, it will also cause a large amount of water absorption. The synergistic effect of both will reduce the mass loss of the concrete.
However, the water absorbed by the RHA particles will lead to concrete freeze–thaw damage due to icing pressure and expansion pressure with the increase in the number of freeze–thaw cycles. During the freeze–thaw cycle, the original pore structure continuously expands into visible pores and sprouts to generate microcracks. It then gradually develops into macrocracks under repeated expansion pressure, which eventually leads to spalling damage and aggregate loss on the concrete surface. When the mass loss of concrete caused by surface damage is greater than the mass increment generated by the absorbed water of the concrete specimens, the mass of concrete will decrease, and the mass loss will increase. According to GB/T 50082-2009 [30], if the mass loss exceeds 5% as the basis of concrete freeze–thaw resistance damage, the concrete specimens with 10% RHA and 20% RHA lose their freeze–thaw resistance after 300 cycles, while the concrete specimens with 30% RHA lose their freeze–thaw resistance after only 250 freeze–thaw cycles.

3.4. Relative Dynamic Elastic Modulus of RHA Recycled Aggregate Concrete

The stiffness of concrete specimens subjected to the freeze–thaw cycles can be characterized by the variation in the relative dynamic elastic modulus [43]. Figure 7 shows the variation in the recycled concrete’s relative dynamic elastic modulus with different RHA replacement ratios during the freeze–thaw cycles. As shown in the figure, the relative dynamic elastic modulus of concrete decreases continuously with an increase in the number of freeze–thaw cycles. Specifically, the relative dynamic elastic modulus decreases slowly in the initial stage of the cycle, while it decreases rapidly in the later stage of the cycle, indicating that the internal damage of the concrete gradually increases, subjected to the freeze–thaw cycle. The variation in the relative dynamic elastic modulus is related to the number of cracks and pores in the concrete itself, as there are many unfavorable interfacial transition zones within the recycled concrete aggregate, thus resulting in a lower initial dynamic modulus of elasticity for the concrete. The higher the initial damage degree, the easier it is for the specimens to form new cracks [44]. Under the freeze–thaw cycling, the expansion and icing pressure of pore water inside the specimen will lead to numerous new pores and new microcracks in the unfavorable interfacial transition zone. The presence of these discontinuities in the matrix increases the propagation time of the ultrasonic pulse velocity, which leads to a decrease in the relative dynamic elastic modulus.
In addition, the decreasing degree in the relative dynamic elastic modulus increased with the increase in the RHA replacement ratio. This is probably because the addition of RHA can enhance the pore structure within the concrete, reducing the freezing point and pore connectivity, retarding the crack generation and water migration between the pores inside the concrete. However, the introduction of the RHA mesoporous structure will lead to the expansion and extension of the pore structure under the action of expansion pressure, resulting in the deterioration of the pore structure, which will adversely affect the concrete’s freeze–thaw resistance, thus resulting in the freeze–thaw damage of concrete specimens with 20% RHA after 300 freeze–thaw cycles. In addition, the reduced cement content at a high RHA replacement rate resulted in the poor filling of paste pores, thus leading to the damage of concrete specimens with 30% RHA after only 250 freeze–thaw cycles.
In conclusion, introducing RHA will reduce the concrete freeze–thaw resistance compared to ordinary concrete (without RHA), and the decreasing degree increases with the increase in the RHA replacement rate. The influence mechanism of the RHA substitution rate on the freeze–thaw damage deterioration of recycled concrete will be analyzed in detail in Section 3.6.

3.5. Mechanistic Analysis of the Influence of RHA on the Microstructure of Recycled Aggregate Concrete

Figure 8 shows the influence of different RHA replacement ratios on paste samples’ microstructure and hydration products before the freeze–thaw cycles. As shown in Figure 8a, the 0% RHA paste is characterized by significant amounts of rod-shaped CH crystals and numerous pores in the matrix, and the surface microstructure is relatively loose. This is probably because the insufficient hydration of cement particles leads to a decrease in the degree of paste compactness, which agrees with previous research results [45,46]. The 10% RHA paste is characterized by a dense paste without obvious pores in the matrix, and cracks of significant width were found in the interfacial transition zone. The 20% RHA paste is characterized by dense paste at different locations in the matrix, small microcracks between the interfacial transition zone of the aggregate and mortar and no significant CH crystals in the paste. This may be due to the smaller particle size of RHA particles, which can promote cement hydration due to their high specific surface, resulting in a pozzolanic reaction between the CH generated by the cement hydration and the amorphous silica of the RHA to reach a threshold value, leading to the optimum compacting effect of the generated C-S-H gels on the paste pores and enhancing the bonding of the interfacial transition zone, reducing the formation of cracks, which will contribute to the mechanical characteristics of concrete enhancement [47,48]. As shown in Figure 8d, the 30% RHA paste is characterized by large pores and a large number of randomly distributed needle-shaped AFTs in the matrix. This may be due to the lower cement content, which leads to a reduced CH content generated by cement hydration, which in turn leads to a weaker degree of pozzolanic reaction, which is not conducive to pore compactness. In conclusion, the compactness degree of the paste samples showed an increase followed by a decrease with an increasing RHA replacement ratio, which corresponds to its compressive strength magnitude.
The microscopic morphology of paste samples with different RHA replacement ratios after undergoing freeze–thaw cycles is presented in Figure 9. As shown in Figure 9a, the 0% RHA paste shows a relatively dense microstructure with a small number of pores. It can be seen from Figure 9b that a penetrating crack was found on the surface of the 10% RHA paste sample, which may be caused by human factors during the sample preparation process [40,49]. Figure 9c shows that 20% RHA paste is characterized by a noticeable increase in the number of pores as well as a large number of staggered cracks in the cement paste matrix, which may be because, although the low content of RHA can improve the paste compactness and the bonding of the interfacial transition zone, it cannot compensate for the lack of its porous properties, which will cause a reduction in the freeze–thaw resistance of the concrete.
As shown in Figure 9d, the surface of the 30% RHA paste sample showed significantly porous characteristics, and the mortar matrix appeared brittle, indicating that the addition of a large number of RHA would further deteriorate the compactness degree of the cement paste. This is probably because of the recycled aggregate attached mortar’s porous structure, which increases the water absorption of the concrete. In addition, a reduction in cement content leads to a weakening of the paste densification. This leads to a more significant icing pressure generated inside the concrete subjected to the freeze–thaw cycles, an increase in the number of pores and cracks in the paste due to the expansion pressure and further damage to the internal structure, which is macroscopically manifested by the increase in mass loss and the decrease in dynamic elastic modulus.

3.6. Mechanistic Analysis of the Influence of RHA on the Mechanical Characteristics and Freeze–Thaw Resistance of Recycled Aggregate Concrete

During the hydration process of cement paste, the RHA particle’s high specific surface area will promote the hydration reaction of cement. At the same time, the amorphous silica within the RHA particles will react with the CH generated by the hydration of the cement to produce C-S-H gel. The extra C-S-H gels and the incompletely hydrated cement and RHA particles will refine the pore structure in the paste and improve the bonding of the interfacial transition zone, which leads to a densified microstructure and consequently has a positive contribution to the development of the compressive strength of the concrete. However, under the condition of a high RHA replacement rate, the reduced free water content leads to a weaker hydration reaction, less CH generation and a reduced pozzolanic reaction between CH and amorphous silica, leading to a reduction in C-S-H gel production, which is not conducive to the improved compactness of the paste pore and the interfacial transition zone, thus leading to a loose microstructure of the cement paste and a weaker bonding of the interfacial transition zone, which will have a negative impact on the load-bearing capacity of the concrete.
When recycled coarse aggregate is added to concrete, two types of interfaces are formed inside the concrete: the attached mortar–old aggregate interface and the new mortar–attached mortar interface, as shown in Figure 10a. The recycled aggregate attached mortar’s porous structure will make the concrete absorb a large amount of water during the process of the freeze–thaw cycle. The RHA particle’s high specific surface area will cause water enrichment to occur on its surface and enter the interface transition zone of the new mortar–attached mortar interface of the recycled aggregate through cracks and pores. When the pore water inside the concrete is in the process of repeated freeze–thaw, it causes the continuous accumulation of the expansion pressure of the pores within the concrete, which consequently leads to the gradual transformation of the tiny pores in the interface transition zone into larger pores under the action of tensile stress and the gradual development of microcracks into macrocracks, which will provide a channel for the migration of water. The more water retained in the pores and cracks inside the concrete, the more significant the volume increase when freezing, which leads to a higher expansion pressure inside the concrete and accelerates the formation of cracks [50]. The interface transition zone, as the strength-limiting phase of concrete, makes the concrete reach an ultimate tensile strength under a slight expansion pressure, which results in the freeze–thaw damage of concrete due to stress concentration [51].
Meanwhile, as the bond is the weakest at the interface transition zone between the new mortar and the attached mortar, the transverse tensile stress continues to increase, which will cause the rapid expansion and extension of cracks in the new mortar–attached mortar interface transition zone, and the attached mortar–old aggregate interface will also generate cracks until the concrete specimen is destroyed. The effect mechanism of recycled concrete freeze–thaw damage deterioration by RHA is shown in Figure 10b. In the process of freeze–thaw cycles, with the increase in the RHA replacement ratio, the degree of cement paste compactness decreases. In addition, the introduction of a large amount of the RHA mesoporous structure causes a significant increase in the porosity and the number of cracks inside the concrete, and the increase in pore connectivity leads to the enhancement of specimen permeability, which makes the concrete more sensitive to freeze–thaw cycles. Overall, the freeze–thaw resistance of recycled concrete shows a decreasing trend with the increase in the RHA replacement ratio.

4. Conclusions

In this paper, the impact of RHA on the mechanical characteristics and freeze–thaw resistance of recycled concrete was investigated, and the mechanism of the freeze–thaw damage deterioration of recycled concrete by RHA was revealed by combining SEM experiments. The main conclusions are as follows:
(1)
The gradual increase in the RHA replacement ratio increased the cementitious material’s total specific surface area and porosity, which will increase the mixtures’ friction resistance and discourage the flow, which in turn leads to the decrease in the concrete slump and the reduced workability.
(2)
With the continuous hydration reaction process, the compressive strength of recycled concrete increased with the growth of the curing age, and the growth rate shows a trend of first being rapid, then slowing down and finally gradually stabilizing. In addition, the optimum compressive strength of recycled concrete is observed with a 20% RHA replacement ratio.
(3)
With the gradual increase in the number of freeze–thaw cycles, the mass loss of recycled concrete showed a tendency to first decrease and then increase, while the relative dynamic elastic modulus showed a continuous decreasing trend.
(4)
Compared with ordinary concrete (without RHA), the RHA particle’s high specific surface area will promote the cement hydration and the formation of C-S-H gel by providing nucleation positions before the freeze–thaw cycle. During the process of the freeze–thaw cycle, the existence of the recycled aggregate attached mortar’s porous structure will cause a large amount of water absorption. With the continuous accumulation of expansion pressure, the internal pores and microcracks will gradually expand and extend, which will provide a channel for water migration, thus resulting in recycled concrete undergoing more severe freeze–thaw damage, and the degree of freeze–thaw damage deterioration grows as the RHA replacement ratio increases.
(5)
In addition, more exploration and experimentation in other durability aspects are needed to expand the application of RHA in recycled aggregate concrete.

Author Contributions

Conceptualization, W.Z. and H.L.; methodology, W.Z. and H.L.; software, W.Z.; validation, W.Z.; investigation, W.Z.; resources, W.Z.; writing—original draft preparation, W.Z.; writing—review and editing, C.L.; visualization, W.Z.; supervision, W.Z.; project administration, W.Z. and C.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Grant NO. 51878546, Grant NO. 52178251), the Science Foundation Project for Outstanding Youth of Shaanxi Province (Grant NO. 2020JC-46), the Key Research and Development Projects of Shaanxi Province (Grant NO. 2020SF-367) and the Xi’an Science and Technology Plan Key Industrial Chain Core Technology Project (Grant NO. 2022JH-ZCZC-0026).

Data Availability Statement

Not applicable.

Acknowledgments

The authors thank the editor and reviewers very much for their comments and helpful suggestions.

Conflicts of Interest

The authors declare that they have no conflict of interest to report regarding the present study.

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Figure 1. Microstructure of RHA measured by scanning electron microscopy.
Figure 1. Microstructure of RHA measured by scanning electron microscopy.
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Figure 2. The particle size distributions of cement, RHA, river sand and RCA.
Figure 2. The particle size distributions of cement, RHA, river sand and RCA.
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Figure 3. Experimental procedure.
Figure 3. Experimental procedure.
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Figure 4. The impact of RHA replacement ratios on the workability of fresh concrete mixtures.
Figure 4. The impact of RHA replacement ratios on the workability of fresh concrete mixtures.
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Figure 5. The impact of RHA replacement ratios on the compressive strength of recycled concrete.
Figure 5. The impact of RHA replacement ratios on the compressive strength of recycled concrete.
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Figure 6. The relationship between the mass loss of recycled concrete and the number of freeze–thaw cycles with different RHA replacement ratios.
Figure 6. The relationship between the mass loss of recycled concrete and the number of freeze–thaw cycles with different RHA replacement ratios.
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Figure 7. The relationship between the relative dynamic elastic modulus of recycled concrete and the number of freeze–thaw cycles with different RHA replacement ratios.
Figure 7. The relationship between the relative dynamic elastic modulus of recycled concrete and the number of freeze–thaw cycles with different RHA replacement ratios.
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Figure 8. Microscopic morphology of the paste samples with different RHA replacement ratios before the freeze–thaw cycles.
Figure 8. Microscopic morphology of the paste samples with different RHA replacement ratios before the freeze–thaw cycles.
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Figure 9. Microscopic morphology of the paste samples with different RHA replacement ratios after the freeze–thaw cycles.
Figure 9. Microscopic morphology of the paste samples with different RHA replacement ratios after the freeze–thaw cycles.
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Figure 10. (a) Schematic diagram of the component phases of RCA; (b) Schematic of the deterioration mechanism of RHA on recycled aggregate concrete freeze–thaw damage.
Figure 10. (a) Schematic diagram of the component phases of RCA; (b) Schematic of the deterioration mechanism of RHA on recycled aggregate concrete freeze–thaw damage.
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Table
Table 1. The physicochemical characteristics of the cementitious materials used.
Table 1. The physicochemical characteristics of the cementitious materials used.
CharacteristicsCementitious Materials
CementRHA
Physical properties
Mean particle size (μm)14.7714.81
Specific surface area (m2/g)1.728.60
Setting time (min)
Initial time202-
Final time258-
Chemical composition (%)
Silicon dioxide (SiO2)21.0792.08
Ferric oxide (Fe2O3)3.780.05
Aluminium oxide (Al2O3)4.910.11
Potassium oxide (K2O)0.593.50
Magnesium oxide (MgO)1.430.49
Calcium oxide (CaO)65.360.78
Sodium oxide (Na2O)0.39-
Phosphorous oxide (P2O5)-1.35
Sulfur trioxide (SO3)2.450.92
Loss on ignition4.521.83
Table
Table 2. The basic properties indexes of recycled concrete aggregate (RCA).
Table 2. The basic properties indexes of recycled concrete aggregate (RCA).
Aggregate TypeGrading
(mm)		Water Absorption
(%)		Apparent Density
(kg/m3)		Bulk Density
(kg/m3)		Crush Index
(%)
RCA5–253.82458.01430.017.0
Table
Table 3. The mixture proportion of the RHA recycled concrete mixtures.
Table 3. The mixture proportion of the RHA recycled concrete mixtures.
SpecimenW/BMixture Proportion (kg/m3)
CementRHASandRCAWaterAWSP
0% RHA0.40410-8251085164228.5
10% RHA0.40369418251085164229.5
20% RHA0.403288282510851642211.5
30% RHA0.4028712382510851642212.0
Notes: W/B represents the water-to-binder ratio. AW represents the additional water amount. SP represents the superplasticizer amount.
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Zhang, W.; Liu, H.; Liu, C. Impact of Rice Husk Ash on the Mechanical Characteristics and Freeze–Thaw Resistance of Recycled Aggregate Concrete. Appl. Sci. 2022, 12, 12238. https://doi.org/10.3390/app122312238

AMA Style

Zhang W, Liu H, Liu C. Impact of Rice Husk Ash on the Mechanical Characteristics and Freeze–Thaw Resistance of Recycled Aggregate Concrete. Applied Sciences. 2022; 12(23):12238. https://doi.org/10.3390/app122312238

Chicago/Turabian Style

Zhang, Wei, Huawei Liu, and Chao Liu. 2022. "Impact of Rice Husk Ash on the Mechanical Characteristics and Freeze–Thaw Resistance of Recycled Aggregate Concrete" Applied Sciences 12, no. 23: 12238. https://doi.org/10.3390/app122312238

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