Keywords

1 Introduction

Energy use in buildings is increasing due to population growth, changes in lifestyle, and urbanization, and it accounts for 30–40% of the total energy use in many countries [1]. In buildings, the heating, ventilation, and air conditioning (HVAC) system is responsible for 42–68% of the total energy use in building to maintain acceptable levels of thermal comfort [2, 3]. Energy consumption by the HVAC system depends on internal heat gain due occupancy, lighting and appliances, and heat transfer through the building envelope (50–60% of HVAC system energy consumption). The latter is reduced by using various insulating materials to reduce building energy consumption by 20–80% [4]. However, inappropriate use of insulation materials accounts for 30% of heat loss in buildings [5]. Traditional insulation materials such as extruded polystyrene boards, expanded polystyrene panels, glass felt, and polyurethane foam have lower resistance, compressive strength, and poor hydrophobicity [4].

Lightweight concrete (LWC) was developed using silica aerogel (SA) as an alternative insulating material with comparable thermal conductivity (0.026–0.37 W/m–K) to foam and autoclave aerated concrete, high compressive strength, and high fire resistance [6]. Ibrahim et al. [7] reported that the thermal performance of SA-based LWC materials was better than that of insulation. Gao et al. [8] developed an aerogel-incorporated concrete by replacing 10–60 vol% of sand with SA, having thermal conductivity and compressive strength of 0.26 W/m–K and 8.3 MPa, respectively. Fickler et al. [9] used SA granules instead of SA particles to develop LWC with minimum thermal conductivity and compressive strength of 0.16 W/m–K and 3.0 MPa, respectively. Suman et al. [10] developed ultra-LWC using expanded glass particles, SA particles and prefabricated plastic bubbles to replace the weight percentage of cement. They found a 42% decrease in compressive strength at 15% decrease in cement content with SA particles. The thermal conductivity of the cement mortar was reduced by 60, 53 and 48% by replacing 20 wt% of fine aggregates with SA, expanded perlite and vermiculite, respectively. They achieved the same thermal performance to the reference coating material by reducing its thickness by 60% with the addition of SA [11]. Li et al. [12] used alkali-activated binder instead of cement because of its low thermal conductivity. They developed an ultra-LWC with thermal conductivity equivalent to insulating materials (0.0434 W/m–K) with minimum compressive strength of 0.38 MPa at 30 °C. The volumetric heat capacity and thermal conductivity of the SA-LWC was lower than those of the reference concrete by 0.17–0.227 J/m3-K and 1.22–1.27 W/m–K, respectively. The thermal mass of LWC is lower than that of normal concrete because of its low density and high porosity [13].

Some researchers have integrated form-stable phase change material (FSPCM) into expanded perlite [14], expanded clay [15] and hollow ceramsite [16]-based LWCs to develop low thermal conductivity and high thermal storage composites. They first developed LWC composites by volumetrically replacing sand with lightweight fillers, the mass of which was calculated using a particle density approach. The FSPCM composites were added as a mass fraction of dry mixture [14] and total aggregate [15, 16]. In these mixing and trial methods [17], the mass of binder and of aggregate was changed to those of the integrated FSPCM composite, resulting in a significant loss of materials to achieve desirable thermophysical properties. Han et al. [18] collected data regarding concrete laboratory waste for the period of 2011–17. They found that a cylindrical specimen was annually producing 50 m3 (61%) concrete waste, followed by 18% of tensile specimen, 15% cubes and remaining pavement block and mortar specimen. Annually, the concrete and cement waste was 80 m3 and 20 m3, respectively, with CO2 emissions >15 m3. Therefore, there is need to develop a scientific methodology to design concrete mixtures for achieving desirable thermophysical properties and performance without wasting materials.

In this study we aimed to develop a novel concrete mix methodology using the particle density of lightweight filler and FSPCM (Fig. 1). This study considered SA granules as the filler. The FSPCM is made up of capric acid and hydrophobic expanded perlite [19]. We also investigated the thermophysical properties of the LWC and FSPCM-LWC composites by considering bulk density, compressive strength, thermal conductivity, and latent heat storage.

Fig. 1
An illustration includes material information, development, and properties. Material information includes binder and fillers. The development includes L W C and F S P C M-L W C. Properties include physical and thermal.

Research methodology. Note: FSPCM, form-stable phase change material; LWC, lightweight concrete

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2 Methods

2.1 Materials

Ordinary Portland Cement (OPC; purchased from Bunnings, Australia) was used as the binder in accordance with AS3972. Silica sand was selected as the aggregate for the reference concrete in compliance with ASTM C105. SA granules (SAG; purchased from Enersen, France) was the lightweight aggregate for the LWC composite. Finally, CAHEP was used to develop the FSPCM-LWC composite, as described previously [19, 20] (Table 1).

Table 1 Properties of different aggregates
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2.2 Development of LWC and FSPCM-LWC Composite

Our proposed novel concrete mix design methodology for developing LWC and FSPCM-LWC composites is shown in Fig. 2. The reference concrete was according to ASTM C105 regarding cement, water and sand as a binder, activator, and primary filler, respectively. At a given quantity of binder, the sand-to-cement and water-to-cement ratios were 2.75 and 0.485, respectively. LWC was developed by volumetrically replacing the sand particles (primary filler) with SAG (secondary filler). The complete replacement of sand with SAG makes SAG the primary filler due to the presence of only one filler. The primary filler is replaced by a secondary filler from top to bottom at a given mass of cement and sand. The secondary filler (SAG) is volumetrically replaced by a tertiary filler (CAHEP) to develop the FSPCM-LWC composite without changing the primary filler (sand) from left to right. When the secondary filler (SAG) is completely replaced by CAHEP, the developed concrete has sand as the primary filler and CAHEP as the secondary filler.

Fig. 2
A methodology includes cement, water, binder, fillers, primary silica sand A S T M C 105, binary sand and silica aerogel, tertiary silica sand and aerogel and F S P C M, binary silica sand and F S P C M, primary silica aerogel, binary filler silica aerogel and F S P C M, and primary F S P C M.

Novel concrete mix design methodology. PCM, phase change material

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The reference concrete composite was developed using a water to OPC ratio of 0.485 and silica sand to OPC ratio of 2.73, in accordance with ASTM C105. The LWC composite was developed by volumetrically replacing silica sand with SAG at 20–80%. The mass of sand and SAG was calculated using Eq. 1 and Eq. 2, respectively. To develop the CAHEP-LWC composite, the mass of SAG was volumetrically replaced by CAHEP at 20–80% without changing the mass of sand. The mass of CAHEP was calculated using Eq. 3.

$$m_{Sand} = \rho_{sand} V_{sand}$$
(1)
$$m_{SAG} = \frac{{ \rho_{SAG} m_{sand} (V_{sand} - V_{SAG} )}}{{\rho_{sand} V_{sand} }}$$
(2)
$$m_{CAHEP} = \frac{{ \rho_{CAHEP } m_{SAG} (V_{SAG} - V_{CAHEP} )}}{{\rho_{SAG} V_{SAG} }}$$
(3)

where, \(m\), \(\rho\) and V show the mass (kg), density (kg/m3) and volume (m3) of sand, SAG and CAHEP composites, respectively, to develop the reference concrete, LWC composite, and FSPCM-LWC composite. The workability of the LWC and FSPCM-LWC composites was kept the same as that of the reference concrete by using a superplasticiser. The mass of OPC, water, and the calculated mass of fillers are given in Table 2.

Table 2 Mix design recipes
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2.3 Properties of LWC and FSPCM-LWC Composites

2.3.1 Physical Properties

Cubic specimens were cast using our mix design recipes in 50 × 50 × 50 mm metallic molds, in accordance with ASTM C1009. Three specimens were cast for each mix design for precision and accuracy of results. They were demolded after 24-h curing in an environmental chamber at temperature and relative humidity of 23 °C and 90%, respectively. The demolded specimens were water cured until the test date. The mass of the cubes was measured by a simple balance with accuracy of 0.1 g. The techno-test machine with accuracy of 0.1 kN was used to measure the compressive strength of the test specimens, as shown in Fig. 3.

Fig. 3
2 photographs. They illustrate the techno-test machine and the compression of a cubic specimen on the techno-test machine.

Techno-test machine a and test specimen b

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2.3.2 Thermal Properties

Thermal properties include thermal conductivity and latent heat storage. Thermal conductivity of the developed cementitious composites was measured using a transient line source (TLS-100), complying with ASTM D5334, as shown in Fig. 4. A 50-mm diameter cylinder of length 120 mm was cast and demolded after 24-h curing in an environmental chamber. The samples were air dried in the environmental chamber at temperature and relative humidity of 23 °C and 50%, respectively, until the test date. The TLS-100 probe was inserted into the test specimen and kept for 15 min to achieve thermal equilibrium between the specimen and probe surface. Final measurements by executing the test were obtained using a digital display meter.

Fig. 4
2 photographs of the digital display thermal conductivity meter and the T L S-100 probe inserted into the test specimen.

Thermal conductivity meter a and sensor b

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Differential scanning calorimetry provided inaccurate data due to the smaller sample size of 5–20 mg. The mass fraction of each material in the developed composites could not be accurately balanced in such a small sample. Consequently, the latent heat storage of the FSPCM-LWC composite (\({h}_{FSPCM-LWC})\) was calculated using Eq. 4 [20].

$$h_{FSPCM - LWC} = \frac{{m_{FSPCM} h_{FSPCM} }}{{m_{w} + m_{OPC} + m_{S} + m_{FSPCM} }}{ }$$
(4)

where, \({m}_{w}, {m}_{OPC}, {m}_{S}\, and\, {m}_{FSPCM}\) are the mass of water, OPC, sand and FSPCM, respectively, and \({h}_{FSPCM}\) is the latent heat storage of the FSPCM composite.

3 Results and Discussion

Figure 5 shows the density of the LWC and CAHEP-LWC composites. The density of the reference concrete was 2226 kg/m3 and that of the composites was 9 and 40%, respectively, lower than the reference concrete due to replacement of 20 and 80% volume of silica sand with SAG, because the particle density of SAG is 20-fold lower than that of silica sand. The effect of CAHEP on density depends on the presence of SAG. For instance, the density of LWC with 20 vol% of SAG was only 3%, but the density of LWC with 80 vol% of SAG was maximally increased by 30% with the addition of CAHEP. Thus, the density of LWC increases dramatically at higher volume fractions of the lightweight filler. LWC must contain the highest proportion of lightweight fillers as given binding materials.

Fig. 5
A graph of density in kilograms per meter cube versus replacing S A G with C A H E P composite includes L W C, L W C-C A H E P 20, L W C-C A H E P 40, L W C-C A H E P 60, and L W C-C A H E P 80 for V 20 and V 80. The density of V 20 is higher than V 80 for all composites.

Effect of capric acid/hydrophobic expanded perlite (CAHEP) on bulk density of lightweight concrete (LWC)

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Figure 6 exhibits the compressive strength of LWC and CAHEP-LWC. The compressive strength of 31 and 46 MPa were measured for the reference concrete after 7 and 28 days, respectively, of water curing. The addition of 20 vol% and 80 vol% of SAG as a partial replacement of sand reduced compressive strength by 56% and 81%, respectively. The decrement percentage of density was smaller than the compressive strength even at the same proportion of SAG because SAG have smaller particle density and more open porous structure, making SAG more fragile and breaking at very small loading. Moreover, SAG have an amorphous shape, promoting heterogeneous porosity, and resulting in lower compressive strength. The addition of CAHEP increased the compressive strength of the LWC by filling the large pores of the LWC. The compressive strength hardly changed with the addition of CAHEP to the LWC with 20 vol% of SAG. At higher volume fractions of SAG, the compressive strength of LWC doubled with the addition of CAHEP. The developed LWC and LWC-CAHEP composites both had higher than the minimum compressive strength (4.14 MPa) required for nonload-bearing structural material as required by ASTM C129–17 [15].

Fig. 6
A graph of compressive strength in megapascals versus replacing S A G with C A H E P composite for 7 and 28 days C S of V 20 and V 80. The compressive strength of 28 days C S of V 20 is higher than others.

Effect of capric acid/hydrophobic expanded perlite (CAHEP) on compressive strength of lightweight concrete (LWC)

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Thermal conductivity and theoretical latent heat storage of LWC and CAHEP-LWC are shown in Figs. 7 and 8, respectively. Thermal conductivity of the reference concrete was 2.27 W/m–K, which decreased by 60 and 93% by adding 20 vol% and 80 vol% of SAG, respectively. The effect of SAG on thermal conductivity was higher than on density and compressive strength because of the 100-fold lower thermal conductivity of SAG (0.01–0.02 W/m–K [5, 18]) compared with sand, and the creation of macro-porosity due to the heterogeneous porous structure of SAG. The addition of CAHEP almost increased thermal conductivity of the LWC from 0.92 to 1.87 W/m–K and from 0.16 to 0.532 W/m–K at 20 vol% and 80 vol% of SAG, respectively. The addition of CAHEP dramatically increased the thermal conductivity of the LWC due to the higher thermal conductivity of CAHEP (0.38 W/m–K) and its smaller particle size which filled the macro-porosity of the LWC. The latent heat storage increased linearly with CAHEP proportion. The reference concrete and LWC stored heat directly, whereas the CAHEP-LWC composite stored latent heat by changing the material’s phase. Heat storage increased by 3.3 kJ/kg and 28 kJ/kg at 20 vol% and 80 vol%, respectively, of SAG replaced by CAHEP (Fig. 8).

Fig. 7
A graph of thermal conductivity in watts per meter Kelvin versus replacing S A G with C A H E P composite includes L W C, L W C-C A H E P 20, L W C-C A H E P 40, L W C-C A H E P 60, and L W C-C A H E P 80 for V 20 and V 80. The thermal conductivity of V 20 is higher than V 80 for all composites.

Effect of capric acid/hydrophobic expanded perlite (CAHEP) on thermal conductivity of lightweight concrete (LWC)

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Fig. 8
A graph of latent heat storage in joules per grams versus replacing S A G with C A H E P composite includes L W C, L W C-C A H E P 20, L W C-C A H E P 40, L W C-C A H E P 60, and L W C-C A H E P 80 for V 20 and V 80. They trend in an increasing pattern.

Effect of capric acid/hydrophobic expanded perlite (CAHEP) on latent heat storage of lightweight concrete (LWC)

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4 Conclusions

We proposed a new concrete mix design methodology for developing tertiary filler cementitious composite to meet structural and thermal properties. We considered three fillers—silica sand, silica aerogel granules and CAHEP composites—with different particle densities. Silica sand was used for high compressive strength of cementitious composites, but the addition of SAG decreased the thermal conductivity of the reference concrete to an acceptable level for non-load bearing application in buildings. The developed LWC with the lowest thermal conductivity had low thermal mass due to the high porosity of SAG. To increase the thermal mass of the LWC, CAHEP partially replaced the volume fraction of SAG. The mass of the LWC-CAHEP composite was determined by a particle density approach instead of the hit and trail mixing methodology. The particle density-based mix design methodology revealed that the high fraction, denser filler increased compressive strength, thermal conductivity, and density, whereas the porous material reduced thermophysical properties. Thermophysical properties are a function of particle density, surface morphology and porous structure. The use of a denser filler increases compressive strength at given binding materials. A hydrophobic surface results in lower compressive strength due to less affinity with cement paste. Finally, an amorphous porous structure promotes macro-porosity, reducing the strength of the concrete. Future studies should consider the structure and surface morphology of fillers to investigate optimal mass fractions of silica sand, SAG and CAHEP in thermally enhanced LWC composites.

The selection of the CAHEP-LWC composite depends on thermal conductivity and latent heat storage. Both properties are essential for energy-efficient building design. We found that increasing the thermal storage of LWC and increasing its thermal conductivity was undesirable due to heat transfer. Therefore, there is a need to conduct a sensitivity analysis to investigate the optimum thermal conductivity and latent heat storage of heat resistive and storage panels to design energy efficient buildings.