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Article

The Effect of RHA as a Supplementary Cementitious Material on the Performance of PCM Aggregate Concrete

by
Bo Liu
1,
Sheliang Wang
2,
Wurong Jia
1,
Honghao Ying
2,*,
Zhe Lu
2 and
Zhilong Hong
3
1
China Railway 20th Bureau Group Co., Ltd., Xi’an 710016, China
2
School of Civil Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China
3
School of Civil Engineering, Xijing University, Xi’an 710123, China
*
Author to whom correspondence should be addressed.
Buildings 2024, 14(7), 2150; https://doi.org/10.3390/buildings14072150
Submission received: 14 June 2024 / Revised: 1 July 2024 / Accepted: 5 July 2024 / Published: 12 July 2024
(This article belongs to the Section Building Materials, and Repair & Renovation)
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Versions Notes

Abstract

The thermal insulated cement matrix further enhances the thermal insulation of PCM aggregate concrete, consequently reducing energy consumption. In this paper, 0–15% rice husk ash (RHA) was used as a replacement for cement. The effect of the substitution amount of RHA on the workability, mechanical properties, thermal properties, and pore structure of concrete was investigated. The results showed that the density of concrete decreased after replacing cement with RHA. The workability of concrete decreased with the increase in RHA content. The filling effect and pozzolanic effect of RHA resulted in an initial increase and subsequent decrease in the mechanical properties of the concrete. After the cement was replaced by 10% RHA, the concrete exhibited the highest compressive strength and splitting tensile strength. The high porosity of RHA reduced the thermal conductivity of concrete by 12.29%. The temperature response indicated that the temperature difference between 15% RHA concrete and the reference concrete can reach up to 1.2 °C, potentially reducing the energy demand. The NMR results showed that the total pore volume was minimal with a 10% RHA admixture. The capillary pore volume increased slightly with the increase in RHA substitution due to the presence of numerous micron-sized pores within the RHA. The micropore and macropore volumes exhibited a decreasing and then increasing trend.

1. Introduction

As energy issues are becoming increasingly prominent, energy conservation and improving energy efficiency are topics that require attention. Globally, buildings consume about 40% of the total energy consumption [1]. A significant amount of the energy consumed by buildings is used to regulate the indoor temperature to improve thermal comfort [2]. To maintain thermal comfort, heating, ventilating, and air conditioning systems consume more than half of the total energy [3]. In winter, buildings located in cold regions consume large amounts of energy to enhance indoor comfort. The building materials for the external envelope significantly affect the thermal insulation performance. Therefore, improving the thermal insulation performance of building materials can decrease energy consumption.
The performance of phase change material (PCM) concrete for energy storage, in terms of thermal insulation, has attracted extensive attention both domestically and internationally. Due to the properties of absorbing latent heat by PCM, PCM aggregate concrete improves temperature damping, reduces the magnitude of temperature variations, and lowers energy demand. Plytaria et al. [4] incorporated PCM into the radiant walls of a building and showed that the indoor temperature was lowered by 0.6 °C, and auxiliary energy consumption was reduced by 30%. Similarly, the latent heat released by the PCM reduces indoor temperature drop in winter. Wang et al. [5] developed a PCM wall adapted for both summer and winter, which demonstrated that the heating load can be reduced by 10–30% in winter. Xie et al. [6] used Na2HPO4-12H2O composite PCM for a radiant floor heating system, and the results showed an average temperature increase of 3.1 °C. However, the transition temperature of these PCMs is positive, which is difficult to adapt to winter temperatures. Moreover, despite the incorporation of PCM, the thermal properties of the cement matrix still affect the thermal properties of the concrete envelope. Improving the thermal properties of the cement matrix to enhance the thermal insulation characteristics of concrete containing PCM aggregates is an interesting topic.
Rice husk is a major by-product of rice production. However, it is an agricultural waste with little nutrition and lacks economic value [7]. Currently, the most common disposal method is open burning, which has a detrimental impact on the air quality. Another disposal method involves burying the rice husks in the soil. These husks take up to 5 years to decompose and release significant amounts of methane [8,9]. In recent years, many power plants have started to use rice husk as a biomass fuel for electricity and heating. However, the rice husk ash (RHA) after incineration may still pollute natural resources such as water and soil if not treated. And it occupies a large amount of land resources. Therefore, the use of RHA as a substitute for cement material can be considered an advancement in green building materials [10]. When rice husk is burned, a large amount of amorphous SiO2 is produced. Thus, RHA is a highly effective pozzolanic material [11]. RHA is added to the concrete as a supplementary cementitious material due to this advantage. Most of the current research focused on utilizing the pozzolanic activity of RHA to enhance the mechanical properties and durability of concrete [12,13]. However, RHA has a high internal porosity and a high specific surface area. These indicators suggest that RHA possesses good thermal insulation properties.
RHA can be used as a supplementary cementitious material to improve the thermal inertia and reduce the thermal conductivity of concrete. Scholars have been focused on the topic of using RHA to enhance the thermal properties of concrete. Srikanth et al. [14] developed a plastering mortar using 15% RHA and 15% bagasse, which reduced the thermal conductivity by 31% compared to conventional mortar. Mortars with low thermal conductivity were used as wall plasters, which enhanced thermal comfort in indoor spaces and could reduce the building’s operational energy costs (i.e., heating and cooling expenses). Jofrishal et al. [15] developed a foam board with the incorporation of RHA, which served as an indoor building material. A foam board containing 20 wt% of RHA reduced the thermal conductivity and blocked the IR radiation, which was a suitable interior solar heat blockade material for use in tropical regions. Onyenokporo et al. [16] developed cement-based masonry blocks containing RHA. The results showed that the thermal conductivity was reduced by 17%, which reduced the need for mechanical cooling systems and improved the thermal comfort of the occupants. The study of Mobaraki et al. [17,18] showed that the addition of 12.5% natural pozzolan and 4% micro silica was most beneficial for the durability and strength of concrete.
RHA has good thermal insulation properties. However, its application in PCM aggregate concrete is less significant. In PCM aggregate concrete, the latent heat properties of PCM reduce the magnitude of temperature variation. The matrix of PCM aggregate concrete remains an ordinary cement matrix.
In this paper, RHA was incorporated into PCM aggregate concrete, further reducing the thermal conductivity of PCM aggregate concrete by enhancing the thermal insulation properties of the cement matrix. Additionally, the study examined the impact of RHA incorporation on mechanical properties, thermal response, and pore structure within the concrete. Firstly, cement was replaced by mass with 2.5%, 5%, 7.5%, 10%, 12.5%, and 15% of RHA. However, the addition of RHA reduced the workability properties, so fly ash (FA) was added to obtain good flowability properties. The effect of RHA on the quality of PCM aggregate concrete was subsequently evaluated through apparent density. The workability of concrete was evaluated using slump and slump flow tests. The mechanical properties of PCM aggregate concrete containing RHA were evaluated using a mechanical testing machine. The thermal properties of the concrete were evaluated using thermal conductivity. Furthermore, the time–temperature curve of the concrete was determined to visually assess the temperature control capability of RHA for PCM aggregate concrete. Finally, the variation in internal pores of concrete with RHA content was tested using the NMR method.

2. Materials and Methods

2.1. Materials

The rice husk ash (RHA) used in this paper was produced by Henan Lize Environmental Protection Co., Ltd (Henan province, China). The incineration temperature of rice husk was 600 °C. The large particle size of incinerated RHA is not conducive to the pozzolanic effect. In this paper, RHA was ground for 90 min using A Los Angeles abrasion machine. The fly ash (FA) used in this study was produced by Henan Hengyuan New Material Company. The cement was produced by Shaanxi Conch Group, and its strength grade was 42.5R. In order to further characterize the material composition of the cementitious materials, the chemical compositions of RHA, FA, and cement were determined by X-ray fluorescence (XRF), as shown in Table 1. The particle size distribution and cumulative particle size of RHA, FA, and cement were determined using a laser particle sizer, as shown in Figure 1, and the specific particle sizes are presented in Table 2.
Direct incorporation of PCM into concrete reduced the strength of the concrete. Branko Šavija et al. [19] showed a 54% reduction in the strength of cement paste by incorporating 30% of Microencapsulated Phase Change Material. To mitigate the adverse impact of PCM incorporation on concrete strength, shale ceramsites were utilized as a carrier for PCM in this paper. The particle size of shale ceramsites ranges from 5 to 20 mm and conforms to the particle grading. The PCM was encapsulated within the shale ceramsite through vacuum impregnation. Subsequently, epoxy resin was used as the first layer to coat the aggregate, and cement was used as the second layer to further coat the aggregate. This preparation method enhanced the strength of PCM aggregate concrete.
The coarse aggregate in phase change material (PCM) aggregate concrete consists entirely of PCM aggregate. The schematics for developing PCM aggregate are shown in Figure 2. The detailed steps for the preparation of PCM aggregate can be referenced in the literature [20]. The PCM aggregate in this paper consisted of dodecane aggregate, tridecane aggregate, and tetradecane aggregate in the ratio of 4:3:3. Dodecane, tridecane, and tetradecane were purchased from China Chemical Co. and their phase change temperatures were determined by DSC, as shown in Table 3. The fine aggregate in the PCM aggregate concrete was sand with a maximum particle size of no more than 5 mm.

2.2. Preparation of Concrete

The coarse aggregate in PCM aggregate concrete consisted exclusively of PCM aggregate. It was a combination of three types of PCM aggregates: dodecane aggregate, tridecane aggregate, and tetradecane aggregate in a ratio of 4:3:3. Subsequently, the effect of 0–15% RHA on PCM aggregate concrete was studied due to the high porosity and high specific surface area of RHA. Therefore, the addition of RHA reduced the workability of concrete. FA improved the workability of concrete. In this study, FA was utilized to improve the workability of concrete. A fly ash content of 10–20% was recommended for enhancing the mechanical and durability properties of concrete. In this research, 15% fly ash was selected to substitute cement [21,22,23]. The preparation process of concrete is shown below and the specific concrete mixture is shown in Table 4.
(1)
Cement, RHA, FA, and sand were placed into a mixer and thoroughly mixed for 60 s.
(2)
Then 50% water and superplasticizer were poured and mixed for 120 s.
(3)
The remaining 50% water and PCM aggregate were mixed for 120 s.
(4)
The concrete specimens were placed into the mold and compacted by vibration. After 1 day, these specimens were demolded and placed in a curing room.

2.3. Test Techniques

The slump test was used to evaluate the workability of concrete. The test was based on the specification GB/T 50080-2016 [24]. The concrete was cast into the slump cone after mixing. The slump cone has two openings: the upper opening is 100 mm, the lower opening is 200 mm, and the height of the cone is 300 mm. The concrete was cast into the slump cone three times, and each time, the concrete specimens were compacted with a metal rod. After the cone was filled, it was lifted vertically when the concrete was not slumping or slumping after 30 s. The height difference between the cone and the concrete was recorded, which represented the slump value. For specimens with a slump higher than 160 mm, the slump did not reflect the workability accurately. Therefore, the slump flow test was conducted. The method of this test was similar to the slump test. The cone was filled with mixed fresh concrete. Then, the cone was lifted vertically. When the specimen no longer flowed or flowed for 50 s. The diameter of the specimen in two vertical directions was recorded, and the average value was taken as slump flow value.
The compressive strength and splitting tensile strength of concrete were tested using a 2000 kN microcomputer-controlled electro-hydraulic servo universal testing machine. The strengths were determined at different ages, including 7 d, 14 d, and 28 d. Each strength value was the average of at least three specimens [25,26].
The thermal conductivity of concrete was determined by Hotdisk equipment based on the transient heat source method. The surfaces of two concrete specimens, with a size of 100 mm × 100 mm × 100 mm, were polished before testing. The two flat surfaces of concrete were then stacked together at room temperature. The probe of the device was pressed in the center of the concrete specimens. The principle of testing thermal conductivity was that heat was generated by passing an electric current through the probe. The device then calculated thermal conductivity through the diffusion of heat.
The time–temperature curve was conducted in a freeze-thaw testing machine. The internal temperature of the concrete was measured by a thermocouple inserted into the concrete. One end of the thermocouple wire was embedded inside the concrete, while the other end was connected to the temperature inspection instrument to record the temperature variations over time. The minimum temperature of the freeze-thaw testing machine was set to −20 °C. The maximum temperature was set to 15 °C. Subsequently, the cubic specimens were placed in a rubber bucket in the freeze-thaw testing machine. The temperature was recorded every 60 s. Thermocouple wires were also placed on the surface of the concrete, which was considered to be the ambient temperature. Therefore, data on the internal temperature of concrete as a function of environmental temperature changes were obtained. After turning on the freeze-thaw machine, the refrigeration phase began, which lasted for about 300 min before entering the heating phase. The heating phase lasted for about 200 min, and the entire experiment took about 500 min, which was considered the end of a cycle.
Nuclear magnetic resonance (NMR) is a non-destructive testing method. Prior to testing, all concrete specimens were vacuum-saturated with water. Thus, the internal pores in the concrete specimens were filled with water. The principle of the test was that the transverse relaxation time was obtained through the spin of hydrogen atoms. The relationship between the transverse relaxation time and the pore size characteristics can be approximated by the following equation. From the formula, it can be seen that there is a specific relationship between the transverse relaxation time and the pore size.
1 T 2 = ρ 2 , s u r ( S V )
where T2 is the transverse relaxation time, ρ2,sur is the T2 surface relaxivity, which is related to the properties of the material itself, and S/V is the pore surface area-volume ratio.

3. Results and Discussion

3.1. Density

Figure 3 illustrates the change in the density of concrete after the incorporation of RHA. The density of concrete gradually decreased with the increase in RHA. The density of concrete without RHA was 2086 kg/m3. The density of concrete with 5% RHA was 2051 kg/m3, which represented a decrease of 1.6%. The density of concrete with 15% RHA was 1994 kg/m3, which represented a decrease of 6.8%. The decrease in density can be attributed to the high porosity and high specific surface area of RHA. These characteristics make the density of RHA lower than that of cement [27]. Therefore, the replacement of cement with RHA led to a decrease in the density of concrete. It is worth noting that the more RHA was added, the more pronounced the trend of density decrease. Concrete with a 7.5% addition of RHA showed a 2.8% decrease in density compared to the reference concrete, whereas concrete with a 15% addition of RHA showed a 6.8% decrease in density compared to the reference concrete. This phenomenon is attributed to the difference in particle sizes of RHA, FA, and cement. When lower amounts of RHA are added, the concrete becomes denser because the accumulation of different particle sizes helps to offset the density reduction caused by replacing cement with RHA. At higher RHA additions, the particles are no longer in the densest state; thus, the density of the concrete decreases more rapidly.

3.2. Fluidity

Figure 4 shows the effect of adding RHA on the workability of concrete. The slump without the addition of RHA was 250 mm, and the slump flow was 620 mm. As more RHA was added, a greater loss of slump and slump flow in concrete was observed. Compared to the control concrete, the workability of concrete with 7.5% RHA decreased by 22%, while the slump flow decreased by 20%. The workability of concrete was significantly reduced by 46% with a 15% addition of RHA, resulting in a complete loss of flowability in the concrete. The decrease in the workability of concrete after adding RHA can be attributed to the numerous pores within the RHA and its large specific surface area [28,29], which results in the absorption of significant amounts of free water by the RHA, consequently reducing the flowability of the concrete. On the other hand, the absorption of water by RHA increases the viscosity of the concrete, leading to a decrease in flowability [30,31]. Therefore, the adverse effect of RHA on workability needs to be considered when it is added to concrete. In this study, 15% of FA was added to the concrete. Fly ash, due to its spherical shape, enhanced the flowability of concrete through the roller ball effect [32]. So, even with the addition of 15% RHA to the concrete, the slump can be maintained at 135 mm. The acceptable workability of concrete was obtained with the combined effect of RHA and FA.

3.3. Compressive Strength

Figure 5 shows the compressive strength of concrete at 7 d, 14 d, and 28 d after incorporating RHA into concrete. The 7 d strength of concrete with 7.5% RHA was the highest, showing a 7.86% higher than that of concrete without RHA. The increase in strength is mainly due to the filling effect and pozzolanic effect. The average particle size of RHA is 72 μm, while the average particle size of cement is 27 μm, and the average particle size of FA is 12 μm. Therefore, replacing cement with RHA and FA is conducive to filling the voids in the concrete, which in turn improves its compactness. The pozzolanic effect of RHA promoted the generation of more hydration products and thus increased the strength of the concrete, whereas the strength of the concrete decreased by 13.46% compared to the control concrete when 15% RHA was added. This may be due to the excessive replacement of cement with supplementary cementitious materials in the concrete, which reduces the formation of early hydration products and impacts the development of early strength.
In 28 d strength, the highest compressive strength was achieved with a 10% incorporation of RHA, which was 10.26% higher compared to the concrete without RHA. The strength of concrete decreased when the amount of incorporated RHA exceeded 10%. The strength of concrete with a 15% addition of RHA was 5.35% lower than that of concrete without RHA. A comparison of 7 d strength and 28 d strength showed that the later-stage strength growth was greater with the addition of RHA. In concrete without the addition of RHA, the 28 d strength increased by 35.78% compared to the 7 d strength. However, in concrete with the addition of 5%, 10%, and 15% RHA, the 28 d strength increased by 38.29%, 41.06%, and 49.60% compared to the 7 d strength, respectively. This is due to the fact that as the amount of RHA increases, the amorphous silica in the RHA undergoes further hydration reactions with the cement hydration products, resulting in a higher strength of the concrete in the later stage. Thus, the increase in strength of concrete with RHA addition can be attributed to the densification effect of RHA and the pozzolanic effect [33,34].
Figure 6 shows the splitting tensile strength of concrete at 7 d, 14 d, and 28 d after incorporating RHA. The splitting tensile strength initially increased and then decreased with the increase in RHA content. Concrete with 10% RHA exhibited the highest splitting tensile strength. The 7 d and 28 d strength of concrete with 10% RHA increased by 8.98% and 10.73%, respectively, compared to the control concrete. Thereafter, the strength started decreasing with the increase in RHA content. Compared to the 10% RHA addition, the 7 d and 28 d strength decreased by 9.36% and 6.38%, respectively, after the 15% RHA addition. The decrease in strength is attributed to the increasing impact of replacing cement with RHA. When the beneficial effect of RHA on strength is insufficient to offset the adverse effect of cement replacement, the strength starts to decrease [35].

3.4. Thermal Conductivity

The thermal properties of concrete were evaluated by thermal conductivity. The thermal conductivity of concrete directly affects its heat transfer properties. Lower thermal conductivity indicates better thermal insulation performance. The impact of incorporating RHA in concrete on thermal conductivity is illustrated in Figure 7. As RHA replaced cement, the thermal conductivity of concrete demonstrated a decreasing trend. The more RHA added, the lower the thermal conductivity. The thermal conductivity of concrete with 15% RHA decreased by 12.29% compared to concrete without RHA. The decrease in thermal conductivity with the addition of RHA can be attributed to the fact that RHA contains a large number of pores, which contribute to its excellent thermal insulation properties [14,36]. When RHA replaces cement, the thermal conductivity of concrete decreases. The low thermal conductivity helps improve the temperature damping and thermal insulation properties of concrete.

3.5. Time–Temperature Curve

In order to further investigate the effect of RHA on the temperature response of concrete, the study measured the temperature change in the concrete in the ambient temperature condition. The time–temperature curve of concrete with 0–15% RHA is shown in Figure 8, and the temperature difference between concrete with and without RHA is shown in Figure 9. As the ambient temperature decreased, the temperature of all concrete specimens began to decrease. But, the extent of decrease in each specimen was different, which was correlated with the content of RHA in the concrete. The higher the content of RHA, the slower the temperature of the concrete decreased during the cooling stage. This is because when the cement in the concrete is replaced by RHA, the thermal conductivity of the concrete decreases due to the high internal porosity of the RHA. As a result, the temperature of the concrete with RHA decreased more slowly. The temperature response results are consistent with the thermal constant results. When the ambient temperature was close to −15 °C, the temperature of the concrete without RHA was −10.2 °C, while the temperature of the concrete with 15% RHA was −9.8 °C. During the heating stage, the temperature of the concrete incorporating RHA increased at a slower rate, and the rate of temperature rise was also correlated with the amount of RHA added. When the ambient temperature was close to 5 °C, the temperature of the concrete without RHA was −3.4 °C, whereas the temperature of the concrete with 15% RHA was −3.8 °C.
The temperature difference between the concrete with 0% to 15% RHA and the reference concrete is shown in Figure 9. The data in Figure 9 were obtained by subtracting the temperature of PCM aggregate concrete without RHA from the temperature of PCM aggregate concrete with RHA. Therefore, the temperature of the RHA-incorporated concrete was higher than that of the reference concrete during the cooling phase, resulting in a positive temperature difference. Conversely, in the heating phase, the temperature of the RHA-incorporated concrete was lower than that of the reference concrete, leading to a negative temperature difference. In the first 200 min, the temperature difference increased with the addition of more RHA. Concrete with 5%, 10%, and 15% RHA was 0.4 °C, 0.8 °C, and 1.2 °C higher than the reference concrete, respectively. The point of maximum temperature difference was located at the fastest decrease in ambient temperature, after which the temperature difference gradually decreased. During the heating stage, the temperature of RHA concrete was lower than that of the reference concrete. The maximum temperature differences between the concrete with 5%, 10%, and 15% RHA and the reference concrete were 0.4 °C, 0.7 °C, and 1.4 °C, respectively. The results of the temperature difference clearly indicate that the addition of RHA is beneficial in reducing the temperature fluctuation of the concrete and improving the thermal inertia of the concrete.

3.6. Nuclear Magnetic Resonance

The horizontal coordinate of the T2 spectrum of concrete represents the transverse relaxation time, which is proportional to the pore diameter. The vertical coordinate of the T2 spectrum represents the signal intensity, whose area is proportional to the pore volume. The T2 spectrum curve comprises three peaks, which can be roughly classified into small pores with a size of ≤100 nm, capillary pores with a size of 100–1000 nm, and large pores with a size of >1000 nm [37]. The T2 spectrum of concrete with 0–15% RHA is shown in Figure 10. In all concrete specimens, the small pores had the largest signal intensity, indicating a high percentage of small pore volume. The signal intensity of small pores decreased and then increased with the increase in RHA content. Small pores are pores in C-S-H gels. The incorporation of RHA contributes to the densification of the gel, resulting in a decrease in the signal intensity of small pores following the substitution of cement with RHA. The signal intensity of small pores increased after replacing cement with 15% RHA, possibly due to the gel structure becoming sparse caused by the cement replacement. Capillary pores and large pores are pores formed by water that does not participate in hydration. The signal intensity of the capillary pores decreased and then increased with the increase in RHA content; the signal intensity of the large pores also decreased and then increased with the increase in RHA content. The capillary pores correspond to micron-sized pores. RHA has numerous micron-sized pores. The difference in particle size between RHA and cement facilitates the dense filling of the particle voids. And RHA exhibits high pozzolanic activity, which reactes with calcium hydroxide to generate more hydration products, thereby reducing the pore volume [29,38]. Thus, the change in capillary pore volume is a result of the combined effect of increased micron-sized pores, filling effect, and pozzolanic effect. When the RHA content was further increased, the filling effect and pozzolanic effect of RHA could not compensate for the increase in micron-sized pores of RHA. Consequently, the capillary pore signal intensity increased. The decrease in large pore signal intensity was also attributed to the filling effect and pozzolanic effect of RHA. Additionally, the large pore signal intensity increased with higher RHA content.
Figure 11 illustrates the changes in pore volume and pore proportion of concrete after the incorporation of RHA. Compared to the concrete without RHA, the total pore volume of concrete with 5% RHA decreased by 4.20%. Small pores decreased by 5.88%, capillary pores increased by 0.32%, and large pores decreased by 3.53%. The decrease in pore volume is due to the filling effect and the pozzolanic effect of RHA. The increase in the volume of capillary pores is attributed to the fact that while the filling effect and the pozzolanic effect of RHA reduce the pore volume, the increase in micron-sized pores within the RHA offsets the beneficial effect, ultimately leading to an increase in the volume of capillary pores. The concrete with 10% RHA had the least pore volume, which was 18.53% lower than the control concrete. It showed a 24.38% reduction in small pore volume, a 1.47% increase in capillary pore volume, and a 19.35% reduction in large pore volume. There was a significant change in the proportion of pore volume. The small pore volume proportion decreased by 4.16%, while the capillary pores proportion increased by 4.40%, and the large pore volume proportion decreased by 0.24%. When adding 15% RHA, the total pore volume increased by 14.80%, small pore volume increased by 14.17%, capillary pore volume increased by 11.64%, and large pore volume increased by 18.64%. The increase in pore volume after adding 15% RHA can be attributed to two factors. Firstly, the replacement of cement by RHA reduces the generation of hydration products. Secondly, the cement particles have a higher local water/cement ratio when supplementary cementitious materials are added to high content. Both of these effects increase the pore volume.

4. Conclusions

The incorporation of RHA into PCM aggregate concrete helps reduce the thermal conductivity of the cement matrix. In this paper, the effect of concrete after replacing cement with 0–15% RHA on the density, workability, mechanical properties, thermal conductivity, temperature response, and pore structure of concrete was evaluated. The following conclusions are obtained:
  • Since the density of RHA is lower than that of cement, the density of concrete decreases with an increase in the amount of RHA replacement. The high porosity and specific surface area of RHA cause a decrease in the workability of the concrete. Additionally, the slump flow of the concrete disappears with the addition of 15% RHA.
  • RHA exhibits pozzolanic activity and has a different particle size compared to cement particles. Consequently, the strength of concrete initially increases and then decreases with an increase in RHA substitution content. The strength increase is due to the filling effect and pozzolanic effect of RHA. When 10% RHA replaces cement in concrete, it exhibits the highest mechanical properties. However, beyond this percentage, the strength decreases.
  • The incorporation of RHA reduces the thermal conductivity of concrete, which helps improve the temperature damping of concrete. The decrease in thermal conductivity suggests that incorporating RHA is effective in increasing the thermal insulation properties of the cement matrix. The time–temperature curves at ambient temperature conditions show that adding more RHA during the cooling stage helps to slow down the temperature drop of the concrete; the maximum temperature difference can reach 1.2 °C and also delay the time to reach the peak temperature.
  • Ten percent RHA replacing cement facilitates pore refinement and reduces pore volume compared to the control concrete. The increase in pore volume is mainly attributed to the addition of RHA as its internal pores consist of micron-sized pores. Therefore, the amount of RHA used has a significant impact on the capillary pores. With the increase in RHA content, the volume of small and large pores initially decreases and then increases, which is consistent with the trend of strength change.
RHA can further improve the thermal performance of concrete. In future research, the effect of different biomass material properties on the thermal properties of PCM aggregate concrete should be investigated, specifically different biomass materials, different fineness of biomass materials, etc.

Author Contributions

Conceptualization, B.L. and S.W.; Data curation, W.J.; Formal analysis, B.L. and H.Y.; Funding acquisition, B.L.; Investigation, H.Y. and Z.L.; Methodology, B.L.; Project administration, B.L. and H.Y.; Resources, W.J.; Software, H.Y., Z.L., and Z.H.; Supervision, S.W.; Validation, Z.L.; Visualization, H.Y. and Z.H.; Writing—original draft, B.L. and H.Y.; Writing—review and editing, B.L. and H.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by China Postdoctoral Science Foundation (No. 2022M723683), Key Research and Development Program of Shaanxi, China (No. 2022SF-375), Research Project of China Railway 20th Bureau Group Co. Ltd. (No. YF2200LJ12B), Shaanxi Key Laboratory of Safety and Durability of Concrete Structures Open Fund project (No. SZ02307).

Data Availability Statement

The data presented in this study are available on request.

Conflicts of Interest

Authors Bo Liu and Wurong Jia were employed by the company China Railway 20th Bureau Group Co., Ltd., the remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Buildings 14 02150 g001
Figure 1. (a) Particle size distribution and (b) cumulative particle size of RHA, FA, and cement.
Figure 1. (a) Particle size distribution and (b) cumulative particle size of RHA, FA, and cement.
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Figure 2. Macroscopic morphology of PCM aggregate.
Figure 2. Macroscopic morphology of PCM aggregate.
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Figure 3. Density of concrete with different content of RHA.
Figure 3. Density of concrete with different content of RHA.
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Figure 4. Workability of concrete with different contents of RHA.
Figure 4. Workability of concrete with different contents of RHA.
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Figure 5. Compressive strength of concrete with different content of RHA.
Figure 5. Compressive strength of concrete with different content of RHA.
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Figure 6. Splitting tensile strength of concrete with different content of RHA.
Figure 6. Splitting tensile strength of concrete with different content of RHA.
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Figure 7. Thermal conductivity of concrete with different contents of RHA.
Figure 7. Thermal conductivity of concrete with different contents of RHA.
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Figure 8. Time–temperature curve of concrete.
Figure 8. Time–temperature curve of concrete.
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Figure 9. Temperature difference between concrete with and without RHA.
Figure 9. Temperature difference between concrete with and without RHA.
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Figure 10. T2 spectrum of concrete.
Figure 10. T2 spectrum of concrete.
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Figure 11. Pore volume and pore proportion of concrete.
Figure 11. Pore volume and pore proportion of concrete.
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Table 1. Chemical composition of cementitious material (wt. %).
Table 1. Chemical composition of cementitious material (wt. %).
SiO2CaOAl2O3Fe2O3MgOSO3Na2OK2OTiO2MnOP2O5
RHA86.821.700.471.710.610.550.205.120.050.451.89
FA43.4610.0530.579.122.080.531.020.670.550.051.30
Cement18.6864.274.013.972.873.260.120.430.570.101.05
Table 2. Particle size distribution of cementitious material (μm).
Table 2. Particle size distribution of cementitious material (μm).
d10d50d90
RHA9.2672.86313.83
FA2.9512.2838.03
Cement3.7427.63118.66
Table 3. Thermal performance of PCMs.
Table 3. Thermal performance of PCMs.
PCMDodecaneTridecaneTetradecane
Density (g/mL)0.74870.75600.7628
Solidifying temperature (°C)−12.86−7.844.26
Melting temperature (°C)−10.96−7.264.85
Latent heat (J/g)200.5142.9206.3
Table 4. The mixture of concrete (kg/m3).
Table 4. The mixture of concrete (kg/m3).
MixtureCementRHAFASandPCM AggregateWaterSP
No. 1467.5082.56525601765.5
No. 2453.7513.7582.56525601765.5
No. 344027.582.56525601765.5
No. 4426.2541.2582.56525601765.5
No. 5412.55582.56525601765.5
No. 6398.7568.7582.56525601765.5
No. 738582.582.56525601765.5
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MDPI and ACS Style

Liu, B.; Wang, S.; Jia, W.; Ying, H.; Lu, Z.; Hong, Z. The Effect of RHA as a Supplementary Cementitious Material on the Performance of PCM Aggregate Concrete. Buildings 2024, 14, 2150. https://doi.org/10.3390/buildings14072150

AMA Style

Liu B, Wang S, Jia W, Ying H, Lu Z, Hong Z. The Effect of RHA as a Supplementary Cementitious Material on the Performance of PCM Aggregate Concrete. Buildings. 2024; 14(7):2150. https://doi.org/10.3390/buildings14072150

Chicago/Turabian Style

Liu, Bo, Sheliang Wang, Wurong Jia, Honghao Ying, Zhe Lu, and Zhilong Hong. 2024. "The Effect of RHA as a Supplementary Cementitious Material on the Performance of PCM Aggregate Concrete" Buildings 14, no. 7: 2150. https://doi.org/10.3390/buildings14072150

APA Style

Liu, B., Wang, S., Jia, W., Ying, H., Lu, Z., & Hong, Z. (2024). The Effect of RHA as a Supplementary Cementitious Material on the Performance of PCM Aggregate Concrete. Buildings, 14(7), 2150. https://doi.org/10.3390/buildings14072150

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