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Article

Effect of Recycled Concrete Aggregate Utilization Ratio on Thermal Properties of Self-Cleaning Lightweight Concrete Facades

by
Hatice Elif Beytekin
1,
Hatice Gizem Şahin
2 and
Ali Mardani
2,*
1
Department of Architecture, Faculty of Architecture, Bursa Uludag University, Bursa 16059, Turkey
2
Department of Civil Engineering, Faculty of Engineering, Bursa Uludag University, Bursa 16059, Turkey
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(14), 6056; https://doi.org/10.3390/su16146056 (registering DOI)
Submission received: 17 June 2024 / Revised: 4 July 2024 / Accepted: 9 July 2024 / Published: 16 July 2024
Browse Figures
Versions Notes

Abstract

:
In today’s environment, where energy is desired to be used more efficiently, it has been understood that the interest in the use of lightweight concrete with superior performance in terms of thermal insulation properties has increased. On the other hand, it has been stated that construction waste increases rapidly, especially after severe earthquakes. In this context, encouraging the use of recycled concrete waste and efficient disposal of construction and demolition waste is of great importance for the European Green Deal. It is also known that pollutants such as COx and NOx stick to facades over time, causing environmental pollution and visual deterioration. It has been reported that materials with photocatalytic properties are used in lightweight concrete facade elements to prevent such problems. This study examines the effect of using recycled concrete aggregates on the thermal properties of self-cleaning lightweight concrete mixtures (SCLWC). For this purpose, an SCLWC containing 1% TiO2 and 100% pumice aggregate was prepared. By replacing pumice aggregate with recycled concrete aggregate at the rates of 15%, 25%, 35%, 45% and 50%, four different SCLWCs with self-cleaning properties were produced. High-temperature resistance, thermal conductivity performance, microstructure analysis and photocatalytic properties of the produced mixtures were examined. It has been understood that the unit volume weight loss of SCLWC mixtures exposed to high temperatures generally decreases due to the increase in the recycled concrete-aggregate substitution rate. However, it was determined that the loss of compressive strength increased with the increase in the amount of recycled concrete-aggregate replacement. Additionally, it was determined that the thermal-conductivity coefficient values of the mixtures decreased with the use of pumice. After SCLWC mixtures were exposed to 900 °C, small round-shaped crystals formed instead of C–S–H crystals.

1. Introduction

It has been reported that various methods have been applied to ensure the sustainability of lightweight concrete facades, along with the new approaches adopted after the Paris climate agreement. In this context, methods include reducing energy consumption in the production and use process [1,2,3], using recycled materials [4,5], reducing the amount of waste [2,3], increasing the strength-to-weight ratio [6], increasing durability [7], ensuring ease of service and installation [8]. It has been understood that requirements such as [2] play an important role in the context of sustainability.
It has been reported that the use of recycled materials can reduce production (embodied energy density) and energy consumption during use [9,10,11]. In this regard, the use of concrete wastes generated after natural disasters such as earthquakes and urban transformation as recycled concrete aggregate (RCA) and the effective disposal of construction and demolition wastes are of great importance for the understanding of the green economy [3].
On the other hand, it is known that the increase in environmental pollution, one of the negative effects of urbanization, has an impact on air quality. Nath et al. [12] found that air quality is directly affected by the amount of volatile organic compounds, C, N and S oxides. This situation, in addition to health problems [13], causes acceleration of global warming [14], increased risk of pollution [15], formation of acid rain [16], and change in the color of the concrete surface [17]. It has been understood that most of these negative effects can be reduced by adding photocatalysis features to building facades [18]. In addition, the use of photocatalytic materials provides a self-cleaning feature on the facade and protects the interior from sunlight reflections. Improved air quality has been reported [19]. The photocatalytic degradation process in question is summarized in Figure 1.
Nowadays, with the development of concrete technology, it has been understood that the energy efficiency performance of buildings and their thermal properties such as high-temperature resistance and thermal insulation capacity have become important [20]. In this context, it has been reported that lightweight reinforced concrete systems containing recycled concrete aggregate with a porous structure exhibit superior performance in terms of thermal properties compared to the traditional reinforced concrete system [21]. A summary of some studies on the subject is summarized in Table 1.
It is understood from Table 1 that by using recycled materials, the production (embodied energy intensity) and energy consumption in the usage phase [1,9,10,22] can be reduced. It has been emphasized by many researchers that the compressive strength of concrete mixtures decreases by 20–30% as a result of substituting 100% of recycled concrete aggregate (RCA) with limestone aggregate [11]. It has been reported that by applying different improvement methods such as using mineral additives [23], impregnating with bacteria [24,25] or impregnating with polymer [26], this decrease in mechanical properties can be seriously eliminated by using RCA at the optimum rate. It has been understood that the production of concrete mixtures is possible without compromising too much with regard to mechanical properties [27]. However, due to the heterogeneous structure of RCA and the variability of its sources, it has been understood from the literature that there are conflicting statements about its effect on the mechanical and durability properties of concrete mixtures. It has been found that this situation becomes even more obvious when it comes to the thermal properties of lightweight concretes [2,28,29].
It has been understood that nano-TiO2 (nT) is generally used as a photocatalyst in concrete mixtures to provide photocatalytic properties [19]. Among the nT phases, it has been reported that the anatase phase is generally preferred due to its rapid photocatalytic effect [30]. However, it is understood from the literature that a limited number of studies have been conducted on the fresh- and hardened-state properties of cementitious systems containing photocatalysts [31].
Although there are many standards in force, it has been determined that there is no definitive procedure for determining the optimum mixture ratio for lightweight concrete facade panels due to the large number of effective parameters. For this reason, it has been understood that new research is necessary on the effect of aggregate type and usage rate on the properties of mixtures. In this study, the effect of using recycled concrete aggregate on the thermal performance of lightweight concrete mixtures containing photocatalysts was investigated.
Table 1. Various studies on photocatalytic, sustainability, energy efficiency and thermal performance of LW.
Table 1. Various studies on photocatalytic, sustainability, energy efficiency and thermal performance of LW.
Ref.AggregateDmax
(mm)		w/bUnit Weight (UW) (kg/m3)Compressive Strength (CS)
(MPa)		Thermal Conductivity Coefficient (TCC)
(W/mK)		Highlights
[31]Waste glass14 mm0.33–0.55 21–4621.8–46.7The goal is to create a translucent, air-purifying concrete mixture that contains waste glass aggregate, both fine and coarse, up to 60% of the volume. Three, five, and seven percent of the weight of the cement was utilized as the photocatalyst in TiO2.
[19]Cenosphere300 µm0.421400–230067–691.98–0.39A separation layer, a white base layer, and a photocatalytic top layer could all be combined to form a photocatalytic coating. TiO2 served as the photocatalyst material. It was found that photocatalytic coating reduces heat gain by 54% by increasing the solar reflectance of LW samples from 0.41 to 0.78.
[9]Pumice,
recycled
plastic
aggregate		4 mm0.53–0.672400–1720 1.74–0.74LW mixtures containing different proportions of recycled plastic aggregate were evaluated on a sample residential building within the scope of the building’s life cycle. It was found that a reduction of up to 15%, 13% and 21% can be achieved in terms of annual energy consumption, carbon emissions and total embodied energy intensity, respectively.
[32]Expanded clay-0.481244–104622.4–14.4-LW experiences a considerable decrease in porosity between 20 °C and 150 °C. The microcracks in lightweight aggregates could be the cause of this increase in porosity.
[33]Pumice40 µm0.35–0.401691–189222.14–7.9 It was emphasized that in LW, the resistance to disintegration at high temperatures is generally lower than that of normal-weight concretes, due to the ability of lightweight aggregates to serve as storage for evaporated water.
[34]RCA4.75 mm0.53 RCA increases the void volume of concrete due to its microcracks and porous structure, thus improving its thermal conductivity performance.
[35]RCA20 mm0.4–0.5 29.3–52.8 The CS performance of concrete mixtures does not change significantly when RCA is replaced with up to 25% natural aggregate.
[36]River sand,
expanded perlite (EP)		4 mm0.281703- 2267 1.8–0.68It was found that the TCC of concrete mixtures decreased by 60% as a result of substituting 100% of EP aggregate with natural aggregate.
[1]Limestone,
EP,
vermiculite,
Scoria,
EPS		9 mm0.49 Facade panels containing different lightweight aggregates were prepared. The mixture that contained the most Scoria aggregate performed the best in terms of thermal characteristics. It was reported that the heat resistance increased by 1.44 in the Scoria mixture compared to the control mixture containing limestone aggregate. In this mixture, a 150% and 2.4-fold reduction was measured in terms of energy consumption and CO2 emissions, respectively.
[37]Recycled wood aggregate-0.47,
1		 3.7–5.60.2–0.12Reductions of up to 99% in TCC values were observed with the use of recycled wood aggregate at different rates instead of limestone aggregate.
[28]EP
RCA		86.40 μm0.6722–5521.5–20.09–0.12It was found that as the percentage of recycled concrete aggregate in foam concrete grows, so does its porosity and maximum pore size. It was found that adding 30% by weight of recycled powder reduced the thermal conductivity of foam concrete by 22%.

2. Material and Method

2.1. Materials

In this study, cement (CEM I 42.5 R type) complying with EN 197-1 [38] Standard was used as the binder. The chemical composition and some mechanical and physical characteristics of cements provided by the manufacturer are shown in Table 2.
Pumice (P) and recycled concrete aggregate (RCA) with Dmax 2 mm were used in the preparation of photocatalytic self-cleaning LW mixtures. Table 3 shows the aggregates’ specific gravity, water absorption capacity, and Los Angeles wear loss values as calculated by ASTM C131 [39] and EN 1097-6 [40]. All aggregates were sieved and used in a similar gradation curve.
Nano-TiO2 (nT) was used at the rate of 1% of the cement weight in order to provide self-cleaning properties for concrete mixtures. Some features of nT provided are shown in Table 4.
A single kind of high-rate water-reducing admixture (WRA) was utilized to give concrete mixtures the required spreading value. Table 5 provides some technical details on the additive used, as provided by the manufacturer.

2.2. Preparation of Mixtures

In all mixtures, the amounts of cement, water/cement (w/c) ratio, and dispersion values were maintained at 545 kg/m3, 0.46, and 240 ± 20 mm, respectively. In concrete mixtures, nT was added to the mixture at the rate of 1% of the cement weight to ensure the photocatalytic self-cleaning feature. In selecting this ratio, the results of studies in the literature were taken into account. In addition to the 100 P mixture containing 100% pumice aggregate, a total of 5 different LW mixtures were prepared by substituting RCA with 15, 25, 35 and 50% of the total aggregate volume. Table 6 shows the quantities of materials used to produce 1 m3 of concrete that was designed in accordance with ACI 211-2 Standard [41]. The naming of mixtures was made according to aggregate type and substitution rate. In this case, the mixture containing 50% pumice and RCA is called 50 P-50 RCA. All productions were carried out in a room where the relative humidity was kept constant at 60–65% and the temperature at 20 ± 2 °C. The produced samples were taken from the mold and allowed to cure in standard water at 20 ± 2 °C until the test day, following the standard of 24 h in the mold.

2.3. Test Procedure

The study’s focus is on how using RCA affected the self-cleaning LW mixes’ flow performance, the unit volume weight (UW), High Temperature Resistance (HTR), and TCC values. Additionally, the photocatalytic properties of concrete mixtures were examined by performing spectrophotometer analysis. In compliance with EN 12350-5 [42] and EN 12350-6 Standards [43], the mixtures’ flow value and fresh-state UW values were ascertained. The HTR of the mixtures was examined by taking into account the changes in UW and CS of the samples exposed to different temperatures before and after the temperature. For this purpose, 50 mm cube 5 different LW mixtures, which were subjected to a 28-day water cure, were first dried in an oven at 105 °C for 24 h until they reached a constant weight. These mixtures were then subjected to 3 different temperature regimes, as shown in Figure 2: 300, 600, and 900 °C. To reach the target temperatures, the temperature increase was kept constant at 5 °C/min. To achieve a thermal steady state, the oven temperature was maintained at that level for 180 min after the goal temperature was attained. Without opening the oven door, the mixtures were thereafter allowed to cool to room temperature. In accordance with EN 12390-7 [44], the mixtures’ 28-day hardened-state UW was measured both before and after high temperature. The CS values of the mixtures were determined on 50 mm cube samples in a four-column compressive-strength test press with a capacity of 2000 kN, in accordance with the EN 12390-3 [45] “Concrete—Hardened Concrete Tests—Part 3: Determination of CS in Test Samples” Standard. Each result obtained is given as the average of 3 samples. Additionally, the microstructures of the samples were analyzed using a ZEISS EVO 40 model (Carl Zeiss NTS, Peabody, MA, USA) scanning electron microscope (SEM). The samples were coated with argon gas and then subjected to electron vacuum bombardment to obtain clear SEM images. Images were taken at 500× and 1000× zoom.
TCC values of the mixtures were determined at 20 °C at the temperature difference between the hot and cold plate using the HFM-100 Heat Flow Meter (Thermtest Instruments Inc., Fredericton, NB, Canada), according to ASTM C518 [46] and ISO 8301 Standards [47] (Figure 3). During measurements, the temperatures of the upper and lower plates were set at 10 °C and 30 °C, respectively. Within the scope of the experiment, when the temperature difference between two surfaces is 1 °C under equilibrium conditions, it is measured by the amount of heat passing through unit area (1 m2) and unit thickness (1 m) perpendicular to this area in unit time (1 h).
In order to determine the self-cleaning properties of the mixtures, 50 mm cube samples were removed from the mold 24 h after casting and subjected to water cure for 28 days. Then, the samples were immersed in methylene blue solution, soluble in distilled water, at a concentration of 0.05 g/L, and kept for 5 min. At the end of 5 min, color measurements (∆E values, L*, a*, b*) of the samples were carried out with a Konica Minolta CM3600D (Konica Minolta, Tokyo, Japan) model reflectance spectrophotometer (Figure 4) [48].
Here,
  • L*: Lightness/darkness value (+ lighter, − darker)
  • a*: Redness/greenness value (+ redder, − greener)
  • b*: Refers to the yellowness/blueness value (+ more yellow, − more blue).
K/S values of the samples were measured. K/S values were calculated according to the Kubelka–Munk Equation (1) [48].
K S = 1 − R 2 2 R
Then, the samples were placed in the UV cabinet and exposed to light for 24 h. After this period, 24 h colorimetric readings were made using a spectrophotometer color-measuring device. Since methylene blue dye was used in this experiment, the b* value, which indicates the yellow–blue color change, was taken into account as the effective colorimetric reading value. There are UV lamps on both sides and on top of the UV cabin used for experiments, and 16 lamps with a total power of 470 Watt. The wavelength of the UV lamps was 254 nm. The size of the UV cabin was 100 × 70 × 138 cm [48].

3. Experimental Results and Discussion

3.1. Fresh-State Properties

The values of the freshly measured unit volume weight (UW) of the mixtures and the WRA requirement to ensure the target flow value are given in Table 6. It was understood that the UW values of the mixtures increased with the use of RCA and the increase in the usage rate. As a result of substituting 50% pumice aggregate with RCA, the UW value of the mixture increased by 160%. This is due to the more porous structure of pumice aggregate and the fact that the specific gravity value of RCA aggregate is 2 times greater (Table 3). Other researchers observed similar findings [6,49].
It is understood from Table 6 that with the increase in RCA usage and usage rate, the need for WRA decreases in order to achieve the desired spread value. It was found that with 15, 25, 35 and 50% RCA replacement, the WRA requirement decreased by 1, 2, 6, and 12%, respectively. This is because pumice aggregate possesses a structure that is more porous and rough. It is thought that in mixtures containing pumice aggregate with a rougher surface, the flow performance is negatively affected by the higher intergranular friction force. Similar statements were also emphasized by other researchers [50,51]. Additionally, the water absorption capacity of pumice aggregate is 3.5 times higher than that of RCA (Table 3).

3.2. High-Temperature Resistance (HTR)

As emphasized before, the HTR of the mixtures was examined by taking into account the change in UW and CS. UW values of the samples before and after exposure to high temperature are shown in Figure 5a. Additionally, the relative weight changes of samples exposed to different temperatures compared to 25 °C are given in Figure 5b. It was determined that the UW values of all mixtures produced within the scope of the study were lower than 2000 kg/m3, which is the limit value of LW.
It was understood that the UW values of LW mixtures increased as the RCA usage rate increased after looking at the UW values of samples that had not been exposed to high temperatures. It was acknowledged that the UW values of the mixtures increased by 1%, 6%, 12%, and 19%, respectively, as a result of replacing RCA aggregate at the rates of 15%, 25%, 35%, and 50%, with pumice. As emphasized before, this is due to the denser structure and higher specific-gravity value of RCA compared to pumice aggregate.
The UW values of the mixtures decreased after exposure to high temperature, regardless of the temperature intensity and aggregate type. This situation is caused by microcracks which formed as a result of the evaporation of free water in the concrete (osmotic pressure) due to the effect of temperature [50,51]. Furthermore, it was recognized that as temperature rises, the binding property is lost because of the dehydration-induced breakdown of the bound water between the C-S-H layers [52,53]. Figure 6a,b summarize the mechanism of damage and thermal crack formation in reinforced concrete systems caused by temperature increases.
In a study by El-Diadamony et al., [54], it was shown that the amorphous part of C-S-H increased to 100 °C. It was stated that the crystalline part decomposes between 150 and 160 °C. However, it was emphasized by some researchers that when concrete mixtures are exposed to 150–180 °C, a very small increase in CS is observed as a result of rehydration of unhydrated cements due to the formation of microcracks [55,56]. It was reported that weight losses occurring around 450 °C are due to the decomposition of calcium hydroxide (CH) crystals [57]. It was stated by Bao et al. [58] that, at 900 °C, C-S-H completely loses its binding properties as it decomposes. It is understood that similar results were obtained in this study.
Sustainability 16 06056 g006
Figure 6. Damage mechanism of reinforced concrete systems (a) due to temperature increase. (b) Thermal crack-formation mechanism [58].
Figure 6. Damage mechanism of reinforced concrete systems (a) due to temperature increase. (b) Thermal crack-formation mechanism [58].
Sustainability 16 06056 g006
After a 300 °C high temperature, UW values of mixtures containing 100%, 85%, 75%, 65% and 50% pumice decreased by 10%, 9%, 8%, 7% and 6%, respectively (Figure 5b). It was determined that this decrease was 18%, 13%, 10%, 12% and 16%, respectively, at 600 °C, while it was 19%, 17%, 18%, 20% and 23%, respectively, at 900 °C.
It was emphasized by many researchers that the large number of independent voids in the matrix of cementitious systems creates an extra area for the osmotic pressure created by the high-temperature effect, resulting in a decrease in the severity of damage [59]. In the study conducted by Pettman et al. [32], it was reported that lightweight aggregate concretes have a closed porous structure and, with the increase in temperature, microcracks caused by water or vapor pressure make the initially closed porosity accessible. Pumice aggregate was found to have a higher void volume and more porous structure than RCA [60]. It was known going into this study that pumice aggregate is more porous than RCA. The weight loss of concrete mixtures after exposure to high temperatures, however, generally decreases with the use of RCA aggregate and the increase in its usage rate, regardless of the intensity of the temperature, according to the results. This is thought to be due to the fact that pumice aggregate has a lower strength compared to RCA. It is also understood from Table 3 that the Los Angeles wear resistance of pumice aggregate is 10% lower compared to that of RCA. It is thought that crack formation and weight loss may be greater in aggregates with lower strength performance due to the thermal expansion/contraction effect.
The CS values of 28-day-old samples before and after exposure to high temperature are shown in Figure 7a. Additionally, the relative CS change in samples exposed to temperatures of 300 °C, 600 °C and 900 °C compared to 25 °C is given in Figure 7b.
It was determined that when high temperature was not applied, the CS values of mixtures with 15%, 25%, 35% and 50% RCA replacement increased by 3%, 12%, 15% and 18%, respectively. It was found that when RCA was used more frequently and its CS values rose, the mixtures’ CS values increased. This is because RCA aggregate has a denser structure, which increases its Los Angeles strength and wear resistance. Other researchers emphasized similar findings [10]. In addition, it is stated that RCA contains unhydrated cement grains and C-S-H gels can form as a result of the hydration of these grains [51,56,61,62], something which was also emphasized. This situation is thought to cause the aggregate-dough interface (ITZ) to become stronger. It was understood that the CS results obtained for the LW produced within the scope of the study were quite high compared to the LW mixtures available in the literature. It was thought that this situation may be due to the use of nT. It was reported that with the addition of nT, the void volume of physically cemented systems decreases, as well as the formation of C-S-H gel at an earlier age [63]. According to reports, the grain boundary zone is densely populated with nuclei, and nT particles may serve as heterogeneous nucleation sites for hydration products [64,65]. Thus, it was emphasized that cementitious systems contribute to strength development [66]. It is thought that this may enable lightweight concrete mixtures to lose less strength after exposure to high temperatures. The SEM image of matrices with nT is shown in Figure 8. The figure illustrates clearly that the matrix that contains nT has a dense structure.
It is understood from Figure 7b that when a temperature of 300 °C is applied, the CS value of the mixture containing 100% pumice increases by 14%. It was reported by Anwar Hossain [67] that this situation may be partly due to the increase in strength of the cement paste due to the increase in Van der Waal’s forces between the cement gel layers moving closer together, as a result of the evaporation of free water with increasing temperature. Additionally, it was emphasized that as the temperature increases, cements that do not undergo hydration can become hydrated, thus causing the formation of a fuller structure [68]. The superior performance of the LW mixture containing 100% pumice is due to the fact that it is a lightweight aggregate that has the ability to shrink at high temperatures, unlike normal aggregates that have expansion properties. In a study by Harmathy and Allen [69], it was determined that LW containing pumice exhibited significant shrinkage at temperatures above 315 °C. In another study conducted by Uygunoğlu and Topçu [70], it was emphasized that the strength performance was positively affected as the matrix became more compact due to the high shrinkage properties of pumice aggregate. After 15, 25, 35, and 50% RCA was substituted, it was found that the CS values of mixtures exposed to 300 °C temperature dropped by 15, 19, 23, and 24%, respectively.
It was reported that this situation is due to the increase in porosity that occurs at these temperatures. When the temperature reaches 300 °C, the water at the C-S-H interfaces, the loss of some of the chemical bond water from C-S-H, and sulfoaluminate, cause microcracks. Microcracks formed in 100 P and 50 P-50 RCA LW mixtures are shown in Figure 9.
It was understood that as the temperature increased to 600 °C, the loss rate in CS varied between 27 and 48%. This was assumed to be caused by the pore system in the hardened cement paste completely drying at this temperature, which broke down hydration products and caused C-S-H gels to disintegrate. When a temperature of 900 °C was applied, decreases of 67, 78, 84, 87 and 90% were measured in the CS values of the mixtures with 15, 25, 35 and 50% RCA replacement, respectively. This situation is thought to be due to the complete destruction of the C-S-H structure at temperatures of 900 °C and higher. Similarly, it was reported by [71] and Abeles and Bardhan-Roy [72] that the loss of CS in concrete containing lightweight aggregate increased by 60% as the temperature increased from 500 °C to 800 °C.
The relationship between weight loss and CS loss of mixtures exposed to different temperatures is shown in Figure 10. It was known that the losses in strength and weight have a strong linear relationship. Similar results were reported by Bao et al. [58].
Although the CS values of LW mixtures were positively affected by the use of RCA before exposure to high temperature, the opposite was observed with the application of high temperature and the increase in temperature value. It was understood that pumice aggregate is relatively successful in terms of resistance performance under high-temperature effects, although RCA has higher strength and contains non-hydrated cement grains. As mentioned before, this situation is thought to be due to the pumice aggregate (i) shrinking more under the influence of high temperature and (ii) having a more porous structure compared to RCA. Additionally, it was emphasized before that pumice aggregate has a rougher surface compared to RCA. Thus, ITZ is expected to be stronger in mixtures containing pumice aggregate. In a similar study conducted by Sancak et al. [73], the strength performance of LW under high temperatures was associated with the mineral structures of the aggregates. In another study conducted by Kumar et al. [74], it was reported that the increase in porosity and micro-void structure in a material causes the thermal conductivity of the material to decrease significantly.

3.3. Superficial Damage and Microstructural Features

Color changes in LW mixtures were examined after exposure to high temperatures. Depending on how high a temperature the samples were exposed to, different color changes were seen. Since the color change in the samples due to high temperature is similar, regardless of the aggregate type, only the 100 P mixture is shown here as an example (Figure 11). It is understood from Figure 11 that as the temperature reaches 600 °C, the samples turn a whitish color, and as the temperature increases to 900 °C they turn a greenish color. Many researchers made statements that were similar [75,76]. It is observed that the number, width and length of cracks on their surfaces increase depending on the increase in temperature to which the samples are exposed. It was determined that serious superficial damage occurred in samples exposed to 900 °C. However, samples exposed to 900 °C were observed to decompose after being left in air conditions for 24 h (Figure 12). It was determined that this situation is caused by the volume increase that occurs as a result of calcium oxide, which forms at temperatures of 400 °C and above, reacting with water in the air and turning into calcium hydroxide [77]. SEM images of 100 P and 50 P-50 RCA mixtures after exposure to temperatures of 25, 300, 600 and 900 °C are shown in Figure 13 and Figure 14. Figure 13 focuses on the change in the aggregate phase of LW mixtures with high temperature. Regardless of the pumice-aggregate usage rate, no visible change was observed in the aggregate phase of LW mixtures at a high temperature. Similarly, a study by Aydin [78] ascertains how pumice changed with temperature; X-ray analyses were conducted on pumice that had not been heated (20 °C) and pumice that had been exposed to 900 °C for three hours.
As a result of the analysis, it was shown that pumice aggregate consists of quartz and feldspar at temperatures between 20 and 900 °C. As a result, no change was observed in pumice at 900 °C.
It is understood from Figure 14 that the characteristic crystal structures of all hydrated phases, including C-S-H and CH, of LW mixtures exposed to a temperature of 900 °C appear as amorphous structures. As seen in Figure 14a (red circles), small round-shaped crystals formed instead of C-S-H crystals after exposure to 900 °C temperature. It was reported [78,79] that crystals with a round shape could be b-C2S, which is a byproduct of the high-temperature breakdown of C-S-H.
Additionally, as seen in Figure 14b, voids were formed as a result of the separation of cement paste from the aggregate in LW mixtures exposed to 900 °C. This situation caused the formation of cracks. It is thought that the fact that ITZ voids are not clearly visible at lower temperatures may be due to the nucleation effect and filling effect of nT making the cement paste structure more compact and strengthening the ITZ.

3.4. Thermal Conductivity

The 28-day values of LW mixtures are shown in Table 7. Compared to the mixture containing 100% pumice, it was understood that the thermal conductivity values of the mixtures with 15, 25, 35 and 50% RCA substituted increased by 3%, 8%, 11% and 14%, respectively.
Since RCA has a denser structure compared to pumice, increasing the ratio of RCA in the mixture negatively affects the thermal conductivity performance. It was emphasized that air has less thermal conductivity than solids and liquids due to its molecular structure, causing lower thermal conductivity in porous concrete [80,81]. Additionally, it is predicted that the void volume created in the matrix by the use of pumice reduces the TCC value. Theoretical and fresh UW values of the mixtures also confirm this mechanism. In addition, it was reported that the TCC generally increases with the increase in concrete density [80,81].
On the other hand, depending on the void volume in LW, more severe drying–shrinkage problems may occur. It was reported by many authors that this situation may cause the TCC to increase [82]. However, it was emphasized by Ren et al. [83] that in mixtures containing nT, the void volume is lower and the products formed are relatively denser. For this reason, it was understood that the first mechanism is more effective for thermal conductivity performance.

3.5. Self-Cleaning Feature

Color measurements of the 100 P, 75 P-15 RCA and 50 P-50 RCA samples were carried out using a Konica Minolta CM3600D model reflectance spectrophotometer. Reference values in color measurements are the values measured immediately after the samples are stained with methylene blue. Figure 15 shows the color change of the samples. It was determined, as can be seen from Table 8, that the ∆E between the color values of the samples was above 20 after 24 h. When the L* values of the samples were examined, an increase was observed. This was interpreted as the color of the samples changing from dark to light [48].
K/S values of the samples are presented in Figure 16. While the K/S value of the 100 P sample at 360 nm was 3.43, the K/S value of the 100 P sample measured after 24 h decreased to 1.66. While the K/S value of the 75 P sample was 1.98, the K/S value decreased to 1.64 after 24 h. Similarly, while the K/S value of the 50 P sample was 1.09, the K/S value of the 50 P sample measured after 24 h decreased to 0.81. Accordingly, when the K/S graphs were examined, it was determined that the darkest stained samples were 100 P, 75 P-15 RCA and 50 P-50 RCA, respectively [48]. When the self-cleaning properties under UV light were examined, it was understood that the largest color change belonged to the 100 P sample, and the sample with the least color removal was the 75 P sample.

4. Conclusions

Within the scope of the study, the following results were obtained:
  • The increasing use of recycled concrete aggregate has shown that the UW of lightweight concrete mixtures with self-cleaning properties has increased. It was found that if pumice aggregate was replaced with 50% recycled concrete aggregate, the UW of the mixture increased by 160%. In addition, it was understood that the need for the WRA required to obtain the desired spreading value decreased with the increase in the use and ratio of recycled concrete aggregate.
  • It was observed that in samples not exposed to high temperatures, compressive strength values increased with the increase in the use and rate of recycled concrete aggregate. It was determined that the compressive strength of the mixtures replaced with 15%, 25%, 35% and 50% recycled concrete aggregate increased by 3%, 12%, 15% and 18%, respectively.
  • When samples exposed to high temperatures are examined, the following is found:
    ○
    After 300 °C high-temperature exposure, it was observed that the UW values of mixtures containing 100%, 85%, 75%, 65% and 50% pumice decreased by 10%, 9%, 8%, 7% and 6%, respectively. It was determined that this decrease was 18%, 13%, 10%, 12% and 16%, respectively, at 600 °C, and 19%, 17%, 18%, 20% and 23%, respectively, at 900 °C.
    ○
    It was determined that at 300 °C, the compressive strength value of the mixture containing 100% pumice increased by 14%. It was observed that the compressive strength values of mixtures replaced by 15%, 25%, 35% and 50% recycled concrete aggregate decreased by 15%, 19%, 23% and 24%, respectively.
    ○
    It was determined that when the temperature increased to 600 °C, the loss rate in compressive strength varied between 27% and 48%. The highest compressive strength performance was observed in the mixture containing 100% pumice.
    ○
    At 900 °C, decreases of 67%, 78%, 84%, 87% and 90% in compressive strength values were measured in mixtures replaced with 15%, 25%, 35% and 50% recycled concrete aggregate, respectively.
    ○
    Although the compressive strength values of lightweight concrete mixtures were positively affected by the use of recycled concrete aggregate before exposure to high temperature, the opposite was observed with the application of high temperature and the increase in temperature value. It was understood that pumice aggregate is relatively successful in terms of resistance performance under high-temperature effects, although recycled concrete aggregate has higher strength and contains non-hydrated cement grains.
    ○
    It was understood that as the temperature reached 600 °C, the samples turned whitish, and as the temperature increased to 900 °C, they turned greenish. It was observed that the number, width and length of cracks on their surfaces increased depending on the increase in the temperature to which the samples were exposed. It was determined that serious superficial damage occurred in samples exposed to 900 °C. However, samples exposed to a temperature of 900 °C were observed to decompose after being left in air conditions for 24 h. Regardless of the pumice usage rate, no visible change was observed in the aggregate phase of lightweight concrete mixtures at a high temperature.
    ○
    It was determined that the characteristic crystal structures of all hydrated phases, including C-S-H and CH, appear as amorphous structures in the SEM images of lightweight concrete mixtures exposed to 900 °C. After 900 °C, small round-shaped crystals formed instead of C-S-H crystals. It was determined that voids form as a result of separation of cement paste from aggregate in lightweight concrete mixtures exposed to a 900 °C temperature.
    ○
    It was understood that at lower temperatures, the cement paste structure becomes more compact, thanks to the nucleation and filling effect of nT. ITZ voids could not be clearly observed at low temperatures.
  • When the self-cleaning properties of the mixtures were examined under UV light, it was understood that the largest color change belonged to the 100 P sample, and the sample with the least color removal was the 75 P sample.

Author Contributions

Conceptualization, H.E.B., H.G.Ş. and A.M. methodology, Conceptualization, H.E.B., H.G.Ş. and A.M., writing—original draft preparation, H.E.B., H.G.Ş. and A.M., writing—review and editing, A.M., All authors have read and agreed to the published version of the manuscript.

Funding

The Bursa Uludag University Science and Technology Center provided help to the authors, which they gratefully recognize, with grant number FOA-2022-1135 and FHIZ-2023-1619.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

The second author would like to thank TUBITAK for funding her doctoral studies under grant number 2211-A. The authors also thank Aliye Akarsu Özenç for her support in the experimental studies.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Photocatalytic degradation process [14].
Figure 1. Photocatalytic degradation process [14].
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Figure 2. High-temperature resistance test process.
Figure 2. High-temperature resistance test process.
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Figure 3. Thermal conductivity test device (a) and test procedure according (b) to ASTM C518 and ISO 8301 Standards.
Figure 3. Thermal conductivity test device (a) and test procedure according (b) to ASTM C518 and ISO 8301 Standards.
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Figure 4. Spectrophotometer analysis.
Figure 4. Spectrophotometer analysis.
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Figure 5. Weight value of mixtures due to high temperature (a) and relative weight change at 25 °C (b).
Figure 5. Weight value of mixtures due to high temperature (a) and relative weight change at 25 °C (b).
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Figure 7. CS values of the mixtures due to high temperature (a), and relative CS at 25 °C (b).
Figure 7. CS values of the mixtures due to high temperature (a), and relative CS at 25 °C (b).
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Figure 8. SEM image of the matrix (a) with and (b) without nT.
Figure 8. SEM image of the matrix (a) with and (b) without nT.
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Figure 9. Representation of microcracks in (a) 100 P and (b) 50 P-50 RCA mixtures.
Figure 9. Representation of microcracks in (a) 100 P and (b) 50 P-50 RCA mixtures.
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Figure 10. The relationship between the high-temperature effect of mixtures and weight and CS losses.
Figure 10. The relationship between the high-temperature effect of mixtures and weight and CS losses.
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Figure 11. Illustration of superficial damage of 100 P mixture after high temperature.
Figure 11. Illustration of superficial damage of 100 P mixture after high temperature.
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Figure 12. Decomposed sample after exposure to 900 °C.
Figure 12. Decomposed sample after exposure to 900 °C.
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Figure 13. SEM image of the aggregate phase of (a) 100 P and (b) 50 P-50 RCA mixtures.
Figure 13. SEM image of the aggregate phase of (a) 100 P and (b) 50 P-50 RCA mixtures.
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Figure 14. Microstructure images of (a) 100 P and (b) 50 P-50 RCA mixtures.
Figure 14. Microstructure images of (a) 100 P and (b) 50 P-50 RCA mixtures.
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Figure 15. Demonstration of color change of samples.
Figure 15. Demonstration of color change of samples.
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Figure 16. Wavelength of Samples.
Figure 16. Wavelength of Samples.
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Table 2. Characteristics of cement.
Table 2. Characteristics of cement.
Material(%)Physical
SiO218.86Specific gravity 3.15
CaO62.70MechanicalDay
Fe2O33.09Compressive Strength (MPa)114.7
Al2O35.71226.80
SO32.39749.80
Na2O + 0.658 K2O0.922858.5
Cl-0.01Fineness
MgO1.16Residual on 0.045 mm sieve (%)7.6
Loss on ignition (LOI)3.20Blaine-specific surface (cm2/g)3530
Free CaO1.26
Insoluble residue0.32
Table 3. Characteristics of the aggregates.
Table 3. Characteristics of the aggregates.
TypeSpecific WeightWater Absorption Capacity (%)Los Angeles Attrition Loss (%)
0–2 mm Pumice1.14436
0–2 mm RCA2.41226
Table 4. Characteristics of nT used in the study.
Table 4. Characteristics of nT used in the study.
ValueUnits28 nm nT
Purity%>99
Sizenm28
Specific Surface Aream2/g>60
Loss of Weight in Drying%2 max.
Loss of Weight in Ignition%5 max.
pH-5.5–7.0
Color-White
Table 5. Some properties of the WRA.
Table 5. Some properties of the WRA.
Density (g/cm3)Solids Content (%)pH ValueChloride Content (%)Alkaline Ratio, Na2O (%)
1.023–1.063325.8<0.1<10
Table 6. Amounts of materials used in the production of 1 m3 SCLWC (kg/m3).
Table 6. Amounts of materials used in the production of 1 m3 SCLWC (kg/m3).
MixturesCement (kg/m3)Aggregate (kg/m3)nTWRA (kg/m3)UW
PumiceRCA
100 P545605-5.455.501420
85 P-15 RCA5152245.201488
75 P-25 RCA4543745.011568
65 P-35 RCA3945234.951648
50 P-50 RCA3037484.901763
Table 7. The 28-day TCC values of LW mixtures.
Table 7. The 28-day TCC values of LW mixtures.
MixturesTCC (W/Mk)
100 P0.39
85 P-15RCA0.41
75 P-25RCA0.47
65 P-35RCA0.50
50 P-50RCA0.53
Table 8. Color measurements of samples.
Table 8. Color measurements of samples.
Sample DL*Da*Db* ∆E
100 P23.282−16.7419.10830.088
75 P-15 RCA16.046−9.25612.81722.526
50 P-50 RCA22.463−14.50419.53933.116
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Beytekin, H.E.; Şahin, H.G.; Mardani, A. Effect of Recycled Concrete Aggregate Utilization Ratio on Thermal Properties of Self-Cleaning Lightweight Concrete Facades. Sustainability 2024, 16, 6056. https://doi.org/10.3390/su16146056

AMA Style

Beytekin HE, Şahin HG, Mardani A. Effect of Recycled Concrete Aggregate Utilization Ratio on Thermal Properties of Self-Cleaning Lightweight Concrete Facades. Sustainability. 2024; 16(14):6056. https://doi.org/10.3390/su16146056

Chicago/Turabian Style

Beytekin, Hatice Elif, Hatice Gizem Şahin, and Ali Mardani. 2024. "Effect of Recycled Concrete Aggregate Utilization Ratio on Thermal Properties of Self-Cleaning Lightweight Concrete Facades" Sustainability 16, no. 14: 6056. https://doi.org/10.3390/su16146056

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