Aerogel-enhanced insulation for building applications

Aerogel-enhanced insulation for building applications

17

U. Berardi Department of Architectural Science, Faculty of Engineering and Architectural Science, Ryerson University, Toronto, Canada

17.1

Introduction

The awareness about the large environmental impacts of current resource depredation behaviors has raised the concerns about the urgency for promoting drastic building energy savings. In an effort of setting stringent energy-saving targets, the building sector has received increasing attention, as buildings are responsible for consuming up to 40% of the total energy in several developed countries, and contribute 40% to the total greenhouse gas (GHG) emissions (IEA, 2013). Previous data justify the high attention toward high building insulation standards, and consequently, the need to create highperformance materials to meet stringent building energy saving targets. The urgency of this need is also justified by the significant increase in the building energy consumption that has been recorded in many countries between 1990 and 2010 (Berardi, 2017). The concerns for the rising building-related GHG emissions are pushing the research and development of new insulating materials and systems, especially available for the retrofit of existing buildings. In fact, several countries have already promoted stringent policies for the building retrofits to reduce the heat losses through the building envelopes. In this context, super insulating materials, whose thermal conductivity is well below that of commonly adopted products, have received increasing attention in the last few years (Gillott and Spataru, 2010; Jelle, 2011; Cuce et al., 2014a,b). While common insulating materials have typical thermal conductivity values around 0.05 W/mK, materials with a thermal conductivity well below 0.02 W/mK have been consistently presented recently. However, many other properties are crucial for assessing an insulating material, including the building site adaptability, the mechanical strength, the fire reaction and protection behavior, the durability and resilience to weathering, the water resistance, the environmental and human health hazard impacts, and last, but probably the most important in day-to-day decisions, the material cost. For this reason, insulating material companies and researchers are struggling to create products with the lowest possible thermal conductivity at a reasonable price in order to promote high-performance building envelopes within thin layers. In this context, aerogels have received an

Nanotechnology in Eco-efficient Construction. https://doi.org/10.1016/B978-0-08-102641-0.00017-7 Copyright © 2019 Elsevier Ltd. All rights reserved.

396

Nanotechnology in Eco-efficient Construction

increasing attention over the last decade, since they show one of the lowest possible thermal conductivity. Aerogels are synthetic and highly porous nanostructured materials, inspired by a 1931 patent of Steven Kistler (1931). The term “aerogel” comes from the fact that they are produced from gels in which the liquid component of the gel is replaced with a gas, giving to the final product a solid and dry smoked aspect. Aerogels can be obtained by using silica, carbon, or alumina. However, silica aerogels are the most common and promising insulation materials for building applications. Silica aerogels are formed by a cross-linked internal structure of SiO2 with many small air-filled pores with diameter between 5 and 70 nm (Jelle, 2011). Compared to any other known material, aerogel has the highest porosity, the highest specific surface area, and the lowest density, typically between 70 and 150 kg/m3. The low thermal conductivity of aerogels is a result of the high porosity and the nanodimensional size of their pores. On the other hand, aerogels are very brittle due to their low tensile strength and are particularly expensive. Consequently, instead of being used alone, they are typically embedded in other materials. Many efforts are being carried out in order to develop new aerogel-enhanced products, and many products have already emerged in the market worldwide. Opaque aerogel-enhanced blankets have been developed as insulation layers, while aerogel granules are currently introduced in porous materials such as plasters, mortars, and other renderings to reduce the thermal conductivity. This chapter is dedicated to present the current state-of-the-art about aerogelenhanced opaque insulations. After a general overview of aerogel synthesis and market size, the performance assessment of aerogel-enhanced renders and blankets are analyzed and some in-situ applications of these materials are described. This chapter will limit the description to opaque systems only, as aerogel glazing systems are covered in Chapter 20.

17.2

Aerogel synthesis and market

The synthesis of aerogels was discovered almost a century ago, although only recently new mass production techniques have been developed (Bhuiya et al., 2016). In the late 1940s, the first commercial aerogel was produced by the Monsanto Chemical Corp. That production line was discontinued in the sixties and seventies and resumed in the eighties. Nowadays, North American industries such as Cabot Corporation and Aspen Aerogels (both in Northborough, MA) are the largest worldwide manufacturers of aerogels, with Nano Hi-Tech from China and EM-Power from Korea representing the main Asiatic sol-gel based suppliers. Aerogel granules are the only commercial pure products currently sold, since monolithic pieces of aerogel have not yet found a market, given their fragility. Beyond pure aerogel material, aerogel-enhanced plasters and blankets with embedded aerogel granules are also commercialized by several companies.

Aerogel-enhanced insulation for building applications

397

17.2.1 Aerogel synthesis Aerogels can be prepared using different materials such as silica, carbon, alumina, or less frequently chromium and tin oxide. However, silica aerogels are preferred for insulation purposes because their production is easier and more cost-effective (Najjar, 2012). In fact, the main chemical compounds for the production of silica aerogels are silicon alkoxides. Silica aerogels are synthesized by low-temperature sol-gel chemistry. Dorcheh and Abbasi (2008) presented a detailed review of the synthesis of silica aerogels. The synthesis of silica aerogels is carried out in three phases: gel preparation, gel aging, and gel drying (Fig. 17.1). The first step consists of hydrolyzing and condensing alkoxides; afterward, successive steps remove the alcohol to form aerogels by using methods which permit to preserve the porous texture of the wet phase (Baetens et al., 2011). The sol-gel process permits to obtain a solid material, the alcogel, by dispersing nanoparticles in a solution, the alcosol. The solution acts as the precursor that leads to an integrated product structure. The sol becomes a gel when the nanoparticles dispersed in it stick together and form a continuous three-dimensional structure throughout the liquid. The most used silicon alkoxides are Si (OCH3)4 (tetramethyl orthosilicate or TMOS), Si(OC2H5)4 (tetraethyl orthosilicate or TEOS), and SiOn(OC2H5)4-2n (polyethoxydisiloxane or PEDS-Px). PEDS and TMOS lead to uniform pores and higher surface area than TEOS (Dorcheh and Abbasi, 2008). Consequently, the thermal conductivity of aerogels obtained by using TMOS and PEDS is below that of aerogels obtained by using TEOS. On the other hand, TEOS is used to obtain a higher visible light transmittance (Baetens, 2011). The PEDS-Px can be obtained by reacting TEOS with a substoichiometric quantity of water in an alcoholic acid medium as follows: Si(OC2H5)4 þ nH2O 4 SiOn(OC2H5)42n þ 2nC2H5OH, for n  2 Si(OCH3)4 þ 2H2O 4 SiO2 þ 4CH3OH OH

A

B

Mixing of the precursors OH

OH

OH

OH

OH Start of hydrolysis and condensation

Solid

Sol Solid

Aging Liquid

"Wet" gel Liquid

Gelation

Solid Drying

Aerogel Gas

Figure 17.1 General scheme for preparing aerogels using sol-gel processing.

OH

OH OH

(1–1000 nm)

Condensation of the sol particles

OH

OH EOx

OH Solution of precursors

OH

398

Nanotechnology in Eco-efficient Construction

Figure 17.2 Aerogel granules (left) and aerogel-enhanced blankets obtained embedding a fiberglass panel with precursor aerogel before the drying process (right), both created at Ryerson University.

The hydrolysis of the gel is performed with a catalyst. There are three types of catalysis: acid catalysis, base catalysis, and two-step catalysis. They all lead to a wider distribution of larger pores and a lower thermal conductivity (Cuce et al., 2014a,b). Acid hydrolysis usually requires longer times (up to 3 days) before the catalyst addition (Rao and Bhagat, 2004; Wei et al., 2011). Once a sol reaches the gel point, it may still contain unreacted alkoxide groups and hydrolysis may continue. For this reason, the gel is aged in its mother solution in order to complete the hydrolysis and to prevent the gel shrink during the drying process (Dorcheh and Abba, 2008). The aging procedure often requires adding ethanol-siloxane to the gel, in order to increase its stiffness and strength. The mechanical and permeability properties of the gel depend on the aging time, the temperature, and the pH. The aging time is a function of reprecipitation of silica dissolved from the particle surfaces onto the necks between particles and of small dissolved silica particles onto larger ones. After the aging of the gel, all the water inside the pores is removed by washing the gel with ethanol and heptane before the drying process. The water that is not removed from the gel will not be removed through the (supercritical) drying process and will make the gel more opaque and dense (Hæreid et al., 1996). Aerogels are essentially the three-dimensional networks of the gel isolated through drying from the mother solution. As the gel mother liquid is removed from the network by using a liquid-to-gas phase change process, the drying of the gel is critical and possible shrinkages of the gel during drying are determined by the capillary pressure. Three different methods for drying the aerogels are used: supercritical drying (SCD), ambient pressure drying (APD), and freezing drying. The SCD permits to avoid capillary tension but comports higher costs. On the other

Aerogel-enhanced insulation for building applications

399

hand, the APD is more cost-effective but involves capillary tension which can lead to shrinking and possible fractures, so SCD is still generally used for silica aerogels (Baetens et al., 2011). The liquid comes out of the pores above the critical temperature and the critical pressure. When the liquid reaches the critical point, it is transformed into a gas without having two phases present at the same time. For this reason, SCD method allows to avoid capillary tension (Cuce et al., 2014a,b). The SCD process can be done at high temperatures (HTSCD) or at low temperatures (LTSCD). In 1931, Kistler presented a detailed description of the HTSCD method, whereas LTSCD was presented by Tewari et al. LTSCD is used for building applications more commonly. Silica aerogels can be either hydrophobic or hydrophilic due to their synthesis process. The silanol polar group in the aerogel network (SieOH) causes the absorption of water and leads to a hydrophilic behavior. However, a hydrophobic behavior can be obtained adding to the pore surface nonpolar side functions, such as a silylating agent. Usually, aerogels dried by HTSCD are hydrophobic and those dried through LTSCD by using CO2 are hydrophilic. Hydrophobic silica aerogels are produced by using two methods: the co-precursor method and the derivatization method. In the co-precursor method, a hydrophobic reagent containing the organic group is added to the sol during the sol-gel step and afterward, the gel is supercritically dried from methanol. In the derivatization method, the gel is immersed in a chemical bath containing the hydrophobic reagent and a solvent. Readers interested in more details about the aerogel production process are encouraged to look at the paper of Dorcheh and Abbasi (2008) or at the Aerogel Handbook by Aegerter et al. (2011).

17.2.2 Aerogel market The high cost of aerogels compared to traditional materials is a significant limiting factor for the diffusion of aerogel-enhanced products. However, in the current market, the cost of aerogels is very inhomogenous, and it has decreased significantly over the last few years. As a result, the global market of aerogels is tenfold increased since 2003. For example, these days (in 2018), a ballpark pricing of aerogel-based blanket 13 mm thick costs around US$ 7/m2, a price just a few times higher than that of panels of traditional insulating materials having the same thermal resistance. According to Koebel et al. (2012), the price of aerogel could drop below US$1500/m3 by 2020. Other studies reveal that the cost of a meter cube of aerogel will achieve 50% cost reduction in production within the next few years and will decrease to US$ 660/m3 by 2050 (Cuce et al., 2014a,b). The high price of aerogels is due to the low production volume and the high costs involved in the synthesis and the drying processes. Shukla et al. (2014) proposed some expedients to reduce the cost of production of aerogels. Firstly, the material cost can be reduced by using cheaper raw materials with amorphous silica, instead of silicon precursors, such as rice husk, clay, oil shale ash, and recycling process materials and by using cheaper processing solvents; for example, water glass is a cheaper silica source to reduce the cost of raw materials, while the synthesis of TEOS is preferred also because it is several times cheaper than

400

Nanotechnology in Eco-efficient Construction

TMOS (Rao and Bhagat, 2004). Secondly, the price of aerogel can be decreased by using low-vapor-pressure solvents such as an ionic liquid that does not evaporate during the aging process; for example, recent studies have been made about the use of water as a solvent and have found also an increased mechanical strength of the aerogel. Thirdly, the possibility to dry the gel at atmospheric pressure, using the APD method instead of the SCD one, is the most promising option for more market-competitive aerogels. The global market for silica-based aerogels was estimated to be USD 307.5 million in 2014, USD 427 million in 2016, and it is expected to reach USD 1.92 billion by 2022 (GVR, 2016), growing annually over 10%. Nowadays, the primary market sector of aerogel products is represented by the oil and gas field, but the building and construction aerogel market sectors are supposed to increase more than the others in the coming years. The main reasoning for the use of aerogel insulation is related to the possible space-saving resulting from exceptional thermal properties and to their high fire resistance. In particular, aerogel-enhanced products during building retrofits from the inside guarantee the advantage of significant space-saving, providing a high thermal resistance in thin layers (Ibrahim et al., 2014a,b; Cha et al., 2014; Ghazi Wakili et al., 2014; Galliano et al., 2016). In fact, already a few years ago, the benefit of a wall interior retrofit with 1 cm thick aerogel blankets was proved to be economically feasible by Shukla et al. (2014), although the material cost was high at that time. Beyond space saving, Koebel et al. (2012) highlighted the importance to consider a life cycle cost analysis (LCCA) in order to compare aerogel with traditional materials. Similarly, Cuce et al. (2014a,b) compared a brick wall insulated by using glass wool with a brick wall insulated with aerogel using an LCCA. The results showed that the required thickness of glass wool to achieve the U-value of 0.3 W/m2K was three times thicker than using aerogel (over 100 mm vs. only 37 mm, respectively), generating significant savings with the aerogel if the real estate value of the indoor space was taken into account. Ibrahim et al. (2015) carried out an assessment of an aerogel-based mortar and found a payback period of only 3.5 years. However, more climate sensitiveness analysis has revealed that the payback period of aerogel-enhanced products depends mainly on the climate; in fact, as the climate gets colder, the higher thickness of aerogel at the current cost of the aerogel-enhanced products is preferred (Berardi, 2018).

17.3

Aerogel properties

The aerogels are dried gels with an exceptionally high porosity, which permits them to have a lower thermal conductivity than air (Jelle et al., 2012). The extremely low thermal conductivity of aerogels, w0.01e0.02 W/mK, results from a well-balanced relationship among the low solid skeleton conductivity, and the low gaseous conductivity (Baetens et al., 2011; Cuce et al., 2014a,b). Nanopores with diameters of a few tens nanometers occupy from the 85% up to the 99.9% of the total volume of the aerogel, whose bulk density often ranges between 70 and 150 kg/m3. The

Aerogel-enhanced insulation for building applications

401

high porosity and small pores lead to unique physical, thermal, optical, and acoustic properties, but they lead to low mechanical properties too (Baetens et al., 2011). Meanwhile, researchers are trying to reduce the thermal conductivity of aerogel even further to reach values below 0.01 W/(mK). For example, Neugebauer et al. (2014) described a technique for compacting a bed of granular aerogel P100 by Cabot. Different degrees of compression were applied to a bed of aerogel with a density of 68 kg/m3 and a thermal conductivity of 0.024 W/(mK). The corresponding thermal conductivity reached a minimum value of 0.013 W/(mK) at a bed density of about 165 kg/m3. The thermal conductivity of a porous material is made up of six contributions: solid state thermal conductivity, gas state thermal conductivity, radiation infrared thermal conductivity, gas state convection thermal conductivity, leakage thermal conductivity, and second-order thermal conductivity. As these heat transfer modes add together, each one must be minimized to obtain a low overall thermal conductivity. Aerogel unique structures well minimize all these modes: •

•

•

• • •

The solid state thermal conductivity has a great impact on the overall thermal conductivity, and increases with the bulk density of the material. It involves the heat transfer between atoms due to the lattice vibrations through the chemical bonds. The low solid state thermal conductivity of silica aerogels is due to the fact that, although the intrinsic solid thermal conductivity of silica is relatively high, aerogels have a small fraction of solid silica. Moreover, the skeleton structure has many “dead-ends” which lead to an ineffective and long tortuous path of heat flow. The gas state thermal conductivity is linked to the collision of the molecules which transfer the energy from one to the other. A way to reduce the gas state thermal conductivity is to reduce the pore size of the material below certain dimension to lead to the so-called Knudsen effect. This correlates the gas thermal conductivity to the characteristic pore diameter and the gas pressure in the pores. The thermal conduction through the gas is also inversely proportional to the pore diameter. The pore sizes of air pockets in aerogels about 20 nm are less than the mean free path of air molecules (w68 nm), which leads to virtually no gaseous conduction (Ebert, 2011). The radiation (infrared) thermal conductivity is related to the emittance of electromagnetic radiation from the material. The radiation effect is relevant for insulation materials with a small amount of solid, with a low bulk density, and can be neglected at room temperature. Heat transfer through radiation is minimized by the addition of opacifiers such as iron oxide or carbon black to the aerogel network during the synthesis to ensure that all the energy in the infrared range is absorbed or scattered, and the heat transfer through radiation is minimized. The convection thermal conductivity of the gas phase involves the movement of air and moisture. The small pore size of the aerogel particles (10e100 nm) prevents the Brownian motion of the gas molecules, and as a result, the convective heat transport can be neglected. The leakage thermal conductivity is given by air and water vapor leakage driven by pressure differences. This component is normally neglected when the materials are dry, poorly air permeable, and without holes. The second order thermal conductivity takes into account the second order (coupling) effects between previously discussed various thermal conductivities, and it is normally negligible.

Table 17.1 reports the main properties of silica aerogels, the most common aerogel nowadays in building applications.

402

Nanotechnology in Eco-efficient Construction

Table 17.1 Main physical properties of silica aerogels Property

Value

Density Pore diameter span Pore particle diameter Average pore diameter Porosity Thermal conductivity Primary particle diameter Surface area Tensile strength Compression strength Coefficient of linear expansion

3e350 kg/m3 (typical 70e150 kg/m3) 1e100 nm (w20 nm on average) 2e5 nm 20e40 nm 85%e99.9% (typical w95%) 0.01e0.02 W/m K 2e4 nm 600e1000 m2/g 16 kPa 300 kPa 2 to 4  106

As reported in Table 17.1, silica aerogels have a relatively good compressive strength of up to 300 kPa. On the other hand, they have very low tensile strength, around 16 kPa, which makes them very fragile. For this reason, aerogels are typically incorporated in a stronger fiber matrix, such as in blankets, to improve the overall tensile strength. The superior properties of aerogels suggested their use in buildings in insulating layers such as blankets, or in renderings such as mortars and plasters. Meanwhile, thanks to the translucent characteristics of the aerogels, their introduction in glazing systems has been proposed both using monolithic aerogels and granular ones for enhancing high thermal resistance and still high visible transmittance (Gao et al., 2014; Berardi, 2015). The good sound absorption properties of commercially available aerogels, ranging from small granules (0.01e1.2 mm) to large granules (1e4 mm), promise also new uses of aerogels. Finally, when compared to other insulation materials, aerogels show excellent fire resistance, and reduced aging effects (Berardi and Nosrati, 2018).

17.4 17.4.1

Aerogel-enhanced opaque systems Aerogel-enhanced mortars and concretes

Although cement is responsible for a significant portion of the GHG emissions of the building sector and a reduction of its use is often advocated, there is no doubt about its dominance in building sector still nowadays. In fact, even when the material mechanical strength is not challenging, lightweight concretes find many applications due to their porosity which leads to fewer loads on the bearing structure. For instance, lightweight concrete is used as a screed for floors and roof slabs, as covering for architectural purposes, to realize partition walls, panel walls in framed structures, and precast elements. Lightweight concrete is prepared by substituting partially the traditional aggregates with lightweight materials such as pumice, diatomite, volcanic cinders,

Aerogel-enhanced insulation for building applications

403

perlite, light expanded clay, or expanded polystyrene. Concrete has a very high thermal conductivity ranging between 1.7 and 2.5 W/mK; therefore, it requires to include thermal insulation layers in order to reach adequate thermal insulating properties. For example, an expanded polystyrene incorporated concrete has usually a density from 95 to 750 kg/m3, a compressive strength between 2.9 and 5.8 MPa, and a thermal conductivity between 0.23 and 0.26 W/mK, depending on the quantity of EPS employed (Ng et al., 2015). Ongoing researches on aerogels-incorporated mortars and concrete aim to design new mixtures which guarantee both adequate compressive strength and low thermal insulating performance. In 2013, Kim et al. studied the insulation properties of aerogel-enhanced renders. They prepared various samples of aerogel cement by using aerogel powder and cement and also by using aerogel, cement, and pozzolan. As known, pozzolan reacts with the calcium hydroxide Ca(OH)2 formed during the hydration of the cement and results in a water-resistant product. The author prepared different samples with the percentage of aerogels ranging from 0.5 wt% to 2 wt% (in weight) and a water/cement ratio about 0.5 (Kim et al., 2013). Also, in some samples, 20% of pozzolan substituted the cement. The methanol was used to mix the hydrophobic aerogel with cement paste, to reduce pores between hydration particles, and thus, to maintain compressive and flexural strength. TGA analysis showed that the aerogel-enhanced mortar was very stable up to 1150 C; thus, it could be used as a fire-resistance insulation material without emitting toxic gases and deforming. The FT-IR spectroscopy showed that the treatment with methanol reacted very well and helped the formation of cement composite. The SEM photographs showed that aerogel particles were stably settled in the cured mix. The thermal conductivity decreased with increasing the aerogel contents by 75% of regular concrete when 2 wt% of aerogel was added. New studies about aerogel-incorporating mortars and concretes have been proposed by Gao et al. (2014), Ng et al. (2015), and Fickler et al. (2015). These authors highlighted the importance to improve the manufacturing procedure since the alkali-silica reaction during the hydration of the cement may destroy the aerogel particles. Previous studies obtained mixes which had a thermal conductivity and compressive strength of 0.26 W/(mK) and 8.3 MPa, 0.55 W/(mK) and 20 MPa, and 0.17 W/(mK) and 2.7 MPa, respectively. Fig. 17.3 shows that aerogel volume content to lower the thermal conductivity below 0.30 W/(mK) had to be particularly high, and above 60% in volume. In 2015, Serina et al. presented an experimental investigation of ultra-high performance concrete (UHPC) modified adding aerogel. The aim was to improve the mechanical properties while maintaining constant the thermal ones. When 20 vol.% aerogel was added, the compressive strength decreased from 120 to 70 MPa. An aerogel loading of 50 vol.% led to a compressive strength of about 20 MPa and to a thermal conductivity of about 0.55 W/mK. However, UHPC modified aerogel-enhanced samples presented a thermal conductivity of 0.74 W/mK, almost twice that of the aerogel-enhanced counterparts, which was about 0.47 W/mK (Gao et al., 2014; Ng et al., 2015).

404

Nanotechnology in Eco-efficient Construction

Thermal conductivity (W/mK)

2,5 2 1,5 1 0,5 0 0

10

20

30

40

50

60

Aerogel (vol%)

Figure 17.3 Aerogel volume content vs. thermal conductivity in mortars.

In 2015, Fickler et al. carried out a research in which silica aerogels were mixed with high strength cement matrix. They tested many samples of aerogel incorporating concrete with a distribution of aerogel between 60 and 70 vol%. The AIC obtained with the aerogel content of 70 vol% had a density between 400 and 570 kg/m3 and values of thermal conductivity between 0.06 and 0.1 W/mK. However, the compressive strength was in the range of 1.4 to 2.5 MPa for the mixtures with a density of 500e620 kg/m3. Samples with compressing strength between 3 and 23.6 MPa had corresponding thermal conductivity values between 0.16 and 0.37 W/mK (Fickler et al., 2015). More recently, Berardi’s research group at Ryerson University investigated new aerogel-enhanced mix concrete designs (Berardi et al., 2017). General use Portland cement was used to realize many samples of mortars using natural aggregates with a maximum diameter of 4.75 mm together with superplasticizers to maintain consistency and workability of the mix (Figs. 17.4 and 17.5). An alternative approach was the introduction of an air-entraining admixture (Grace Darex AEA EH). This permitted a reduction in mixing water with no loss of slump. Superplasticizers were also used to reduce the amount of water while maintaining a certain level of consistency and workability (Berardi et al., 2017). The compressive strength of the samples of standard mortar without aerogel was around 50 MPa and halved by adding the air-entraining admixture as shown in Fig.17.6. The compressive strength of the samples of mortar with 36 vol% of aerogel

Figure 17.4 Aerogel-enhanced lightweight concrete preparation.

Aerogel-enhanced insulation for building applications

405

15 cm

Standard mortar

Mortar + air entraining

Mortar + 30% aerogel

Mortar + 33% aerogel

Mortar + 36% aerogel

Figure 17.5 Samples of aerogel-enhanced lightweight concretes (Berardi et al., 2017).

decreased by a factor of 10 compared to the standard mortar. The mortar with 30 vol% of aerogel had a compressive strength of 12 MPa, a value generally suitable for lightweight mortars (Berardi et al., 2017). The results of thermal conductivity tests showed that the thermal conductivity decreased linearly by increasing the quantity of aerogel (Fig.17.6). The samples of mortar almost halved the thermal conductivity by adding 36 vol% of aerogel. The thermal conductivity of mortar with 30 vol% aerogel was 0.236 W/(mK) and that of mortar with 36% aerogel was 0.149 W/(mK). The investigation with aerogel-enhanced concrete showed that a volumetric increase of the aerogel percentage of aerogel beyond 30 vol% results in a significant reduction of the compression strength, which would compromise the possibility of using the mix unless additional strengthening would have added.

17.4.2 Aerogel-enhanced Plasters As seen in Section 17.4.1, a commonly adopted approach for using aerogel is represented by the possibility to embed aerogel granules in the mix of a porous mixed material. Aerogel-enhanced plasters have the benefit of being simple to implement and flexible with respect to unevenness of surfaces allowing to create a continuous thermal insulation layer by filling the gaps and joints in a building envelope (Buratti et al., 2014, 2016). The low density of aerogel-based renders allows the application of thick

406

Nanotechnology in Eco-efficient Construction Standard mortar

Mortar + air entraining Mortar + 30% aerogel

Thermal conductivity (W/mK)

Mortar + 33% aerogel Mortar + 36% aerogel 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00

3

0

6

9

15

12

18

21

24

27

30

33

36

39

Aerogel (vol%) Mortar + 30% aerogel

Mortar + 33% aerogel

Thermal conductivity (W/mK)

Gao et al. (18)

Mortar + 36% aerogel

Fickler et al. (36)

0.30 0.25 0.20 0.15 0.10 0.05 0.00 3

4

5

6

7

8

9

10

11

12

Compressive strength (MPa)

Figure 17.6 Properties of new aerogel-enhanced lightweight concrete, compared to existing literature (Berardi et al., 2017).

layers (up to 8 cm) with internal fiberglass mesh grids, creating adequate insulation, especially in those circumstances where other traditional insulating materials cannot be used, for example, in historical uneven surfaces or vaults (Stahl et al., 2012; Cuce et al., 2014a,b). Owing to the hydrophobic nature of the aerogel, aerogelenhanced plasters have also the advantage of being water repellent, which avoids water absorption, while they are water vapor- permeable and more breathable than conventional plasters. Kobel et al. (2012) developed an insulating aerogel-based render with mineral and organic binders with a density of 156 kg/m3 and a thermal conductivity of 0.027 W/(mK). Stahl et al. (2012, 2017) studied an insulating plaster based on granular silica aerogel with purely mineral and cement-free plaster and additives to enhance the workability of the rendering; the thermal conductivity of the plaster was 0.025 W/(mK), while its density was 200 kg/m3. In 2016, Buratti et al. carried out a research on aerogel incorporated plasters, mixing natural chalk with granular aerogel in different percentages. The first mixture had a percentage of aerogel between 80% and 90% in volume, the second one between 91% and 95%, and the last one between 96% and 99%. Thermal conductivity values between 0.014 and 0.05 W/mK were

Aerogel-enhanced insulation for building applications

407

found well below the thermal conductivity of traditional plasters, which vary between 0.29 and 0.70 W/mK. Meanwhile, Westgate et al. (2018) have recently presented a detailed laboratory characterization of both physical and mechanical properties of plasters incorporating aerogel granules. The most widely adopted aerogel-enhanced plaster was developed at the Swiss Federal Institute EMPA, and it is commercialized with the name of FIXIT 222. This material uses more than 50%vol. silica aerogel and has a thermal conductivity of 0.028 W/(mK). This product has been used in several thousands of m2 since 2013. Mmeanwhile, its price has significantly dropped (current price is around 100 to 150V for a 50 L bag, i.e., around 2 to 3 V per dry kilogram), making it a valid alternative among the thermal-insulating plasters (Ibrahim et al., 2014a,b). Garrido et al. (2017) recently reported a comprehensive assessment of the economic and energy life cycle assessment of aerogel-based thermal renders. Even more recently, Wu et al. (2018) proposed a novel low-cost method of silica aerogel fabrication using fly ash and trona ore with ambient pressure drying technique. Meanwhile, Dr. Berardi’s research group recently produced aerogel-enhanced plasters by mixing hydraulic limebased plaster with granular silica aerogels supplied by Cabot Corp in different percentages (varying from 25% to 95% in volume), as well as using self-created proprietary aerogel granules (shown in Fig. 17.2). Different types of hydraulic lime were utilized such as the hydraulic lime plaster NHL 3.5 by CHIRAEMA and Saint Astier NHL 3.5 by TransMineral Inc. Fig. 17.7 shows some samples of the aerogel-enhanced plasters (Nosrati and Berardi, 2018). The results of thermal conductivity tests of these products confirmed a linear relationship between the density of the plaster and the resulting thermal conductivity (Table 17.2). The thermal conductivity assumed values below 0.03 W/(mK) only mixing more than 70% vol. of aerogel granules, although this resulted in significant cost increase and lower

Table 17.2 Main properties of different aerogel-enhanced plasters (Nosrati and Berardi, 2018) Composition Mixture

Plaster (L)

Aerogel (L)

Water (L)

Aerogel (vol%)

Density (kg/m3)

Thermal Conductivity (W/(mK))

Hydraulic lime

6

0

2.174

0

1109.8

0.2032

Hydraulic lime þ25% aerogel

5

2.5

2.7

25

735.6

0.1151

Hydraulic lime þ50% aerogel

2

3.5

1.26

50

501.0

0.0687

Hydraulic lime þ70% aerogel

0.4

1.5

0.3

70

260.7

0.0311

408

Nanotechnology in Eco-efficient Construction 15

Hydraulic lime

Hydraulic lime + 25% aerogel Hydraulic lime + 50% aerogel Hydraulic lime + 70% aerogel

Figure 17.7 Samples of the different aerogel-enhanced plasters.

workability (Nosrati and Berardi, 2018). The comparable thermal conductivity values for the FIXIT and the hydraulic plasters with 70%vol. and the higher workability of the new aerogel-enhanced plaster suggested the capability to produce a mix of aerogel-enhanced plasters in-house. The aerogel-enhanced plasters were water vapor-permeable (permeability resistance factor around 5) and were stable to different aging factors (Berardi and Nosrati, 2018) (Fig. 17.8). Increasing the percentages of aerogel granules in the plaster content lowered the material density and thermal conductivity (Fig. 17.8). However, a trade-off between the aerogel content and the mechanical strength of the material emerged and a 70%vol. aerogel-enhanced plaster resulted in an optimal thermal performance and density (being 0.032 W/mK and 230 kg/m3, respectively). In fact, increasing aerogel volumetric content above 70%vol. marginally improved thermal characteristics while significantly lowering the mechanical strength. More recent studies about the long-term behaviors of aerogel-enhanced plasters and their weathering aging under different conditions confirmed that the moisture content had a much greater impact on the thermal degradation of the samples. The thermal conductivity elevation of aerogel-enhanced insulating materials due to extremely high levels of moisture and temperature was found to be statistically significant (Berardi and Nosrati, 2018). Negligible change in thermal performance of the materials was observed under moderate wetting condition (Berardi and Nosrati, 2018).

0.00 0%

Density (kg/m3) 1400 1200

0.113

1000 800

789

0.072 515

600

0.032 237

25%

50% 70% Aerogel content (vol%)

0.027

0.027

204

199

85%

95%

400

Density (Kg/ m3)

Thermal conductivity at 23°C (W/mK)

Thermal conductivity (W/mk) 0.18 0.16 1110 0.14 0.12 0.140 0.10 0.08 0.06 0.04 0.02

200 0

Figure 17.8 Thermal conductivity and density of aerogel-enhanced plasters as a function of aerogel content (Nosrati and Berardi, 2018).

Aerogel-enhanced insulation for building applications

409

17.4.3 Aerogel-enhanced Blankets As extensively discussed in Section 17.2, silica aerogels have extraordinary small pores, which result in remarkable thermal properties, but they have low mechanical strength and stability. In order to strengthen the tensile properties of the silica aerogels to be used as an insulating material, it has been recently proposed to reinforce the aerogels with mechanically stronger materials and nonwoven fiber matrixes such as glass, mineral, or carbon fibers. When the fibers or fibrous matrix are added to the pregel mixture which contains the gel precursors, the resulting dried composite is an aerogel blanket (Aegerter et al., 2011; Baetens et al., 2011). Aerogel blankets composed of synthetic amorphous silica dioxide have been proposed in retrofitting projects or whenever space and weight constraints exist (Ghazi Wakili et al., 2014; Cuce and Cuce, 2016; Galliano et al., 2016). Aerogel blankets are flexible, highly porous, and have a remarkably high thermal resistance (Hoseini et al., 2016; Lakatos, 2017). In fact, they have started to be produced and commercialized by several manufacturers worldwide. The thermal conductivity of aerogel blankets on the market is around 0.015 W/mK. For example, Spaceloft developed by Aspen Aerogels, Inc. (MA, US) is a flexible fiber-reinforced blanket with a declared thermal conductivity of 0.013 W/(mK) at 0 C; other aerogel blankets are CryogelZ by Aspen, available in 5 and 10 mm thickness and suitable for industrial application below ambient temperature, and Thermal-Wrap, available in 5 and 8 mm thickness by Cabot Corporation. The thermal conductivity of these last two products is 0.014 and 0.023 W/(mK), respectively. Finally, among the aerogel-based commercial blanket products, Proloft by Advanced Technologies has attracted some attention as a thermal barrier strip to provide thermal bridging protection around door and window frames, thanks to its thermal conductivity of 0.014 W/(mK). In 2015, Russia had the highest share regarding both volume and value in the market of aerogel blankets, as oil and gas are the leading fields of application for these products. The leading manufacturers worldwide of aerogel blankets are Aspen Aerogel, Inc., Cabot Corporation, Svenska Aerogel AB, Acoustiblok UK Ltd., Active Space Technologies, and Airglass AB. The advantages of aerogel-enhanced blankets are that the final panel does not show any granularity of the aerogel, since the aerogel particles are chemically attached to the fiberglass matrix. Most of the commercially available aerogel blankets are made with amorphous silica, and they usually suffer from dust production. However, several health organizations, including the International Agency for Research on Cancer and the Occupational Safety and Health Organization have declared aerogel blanket particles not hazardous for human health. The nontoxicity combined with the excellent fire protection make aerogel blankets ideal for internal adoption; although being hydrophobic, they can also be used on the external side of the wall. Aerogel blankets are fully recyclable, have zero ozone depleting potential, and a Global Warming Potential below 5 (Berardi, 2018). To lower the price of the blankets and avoid supercritical drying, common fiberglass has been immersed in aerogel precursor chemicals in the gel form and is dried

410

Nanotechnology in Eco-efficient Construction

in a vacuum oven for 24 h to obtain the aerogel-hybrid fiberglass panels without SCD, as shown in Fig. 17.2 (Berardi, 2018). The thermal conductivity of this product resulted to be 0.021 W/(mK), generally above that of commercial products, although the price of this bat would be extremely competitive. Table 17.3 reports main properties of the different aerogel-enhanced blankets available on the market currently, while Fig. 17.9 shows two aerogel-enhanced blankets. Hoseini et al. (2016) studied the thermal resistance and mechanical deformation of aerogel blankets under compressive mechanical loading. By analyzing the samples at a unit cell level, the bending of the fibers was also considered theoretically. The predicted rational behavior was then verified by studying the thickness change after compression and how the decrease in thickness influenced the thermal behavior of the aerogel blankets. It was observed that the maximum decrease in thickness was of about 20% for 8 mm ThermalWrap blankets while the maximum change in thermal conductivity value due to compression was around 10% for a 10 mm sample of Cryogel. Even after 20 compression-decompression loading cycles, the reduction of thermal conductivity was <5% while the permanent deformation was <6%.

Table 17.3 Main properties of the different aerogel-enhanced blankets available on the market Fiber Composition

Density (kg/m3)

l (W/mK)

Cabot

Polyester and PET

w70

0.023

Cryogel x201

Aspen aerogel

Polyester/fiber glass

w130

0.014

Cryogel Z

Aspen aerogel

PET/fibrous glass

w160

0.014

Dow Corning HPI 1000

Dow Corning

Silicon



0.015

Pyrogel HPS

Aspen aerogel

Fiber glass

w200

0. 014

Pyrogel XTE

Aspen aerogel

Fiber glass

w200

0. 014

Pyrogel XTF

Aspen aerogel

Fiber glass

w200

0. 014

Spaceloft

Aspen aerogel

Polyester/fiber glass

w151

0.015

Silica aerogel fiberglass blanket, SACB-0-X

Joda

Fiber glass

<300

0.016

Silica aerogel ceramic fiber blanket, SACTT-X

Joda

Ceramic fiber

<301

0.016

Commercial Name

Provider

Thermal Wrap

Aerogel-enhanced insulation for building applications

411

Figure 17.9 Cryogelx201 (left) and Dow Corning HPI 1000 (right).

Fig. 17.10 shows the declared manufacturer values of thermal conductivity across temperatures for different commercially available aerogel blankets. To help comparing products and after the exposure to same moisture content conditions, Fig. 17.11 reports the thermal conductivity values for different aerogel blankets obtained in the authors’ laboratory using an HFM device in the temperature range from 20 to þ50 C. Lakatos (2017) and Nosrati and Berardi (2018) assessed how the thermal conductivity of aerogel blankets increases as a function of the moisture content. In particular, Lakatos (2017) studied the application of the aerogel blanket to a brick-based wall. Firstly, sorption isotherms were investigated to understand the temperature sensitivity of the moisture uptaking. Each aerogel slab was tested individually and after wetting the dried samples for 0, 4, 8, 12, 16, 20 h at 293 K at 93% of relative humidity. A comprehensive investigation of the thermal conductivity of aerogel blankets in humid conditions in transient and steady-state regimes were also made by Hoseini et al. (2016) looking both at Cryogel by Aspen aerogel Inc. and Thermal Wrap by Cabot corp. The moisture build up in the two aerogel blanket samples was measured

0,1

Cryogelx201

Cryogelx201

PyrogelHPS

PyrogeLXTF

Spaceloft

PyrogelXTE

Thermalwrap

Joda

0,09

Thermal conductivity w/mk

0,08 0,07 0,06 0,05 0,04 0,03 0,02 0,01 0 –250 –200 –150 –100 –50

0

50

100

150 200 250 300 350 400 450 500 550 600

650 700

Temperature ºC

Figure 17.10 Thermal conductivity values across temperature for different aerogel blankets (figure drawn using values declared by the manufacturers).

412

Nanotechnology in Eco-efficient Construction Joda

Dow Corning HPI 1000

Aspen Pyrogel

Aspen Spaceloft

0,022

Thermal conductivity W/mk

0,021 0,02 0,019 0,018 0,017 0,016 0,015 0,014 0,013 0,012 –15

–5

5 15 Temperature ºC

25

35

45

Figure 17.11 Thermal conductivity values across temperature for different aerogel blankets (values measured by the author after having normalized the sample hygrothermal conditions).

as a function of the relative humidity and temperature. Transient plane source tests revealed that the thermal conductivity of aerogel blankets could increase by approximately 15% as the ambient relative humidity increases from 0% to 90% at 25 C. However, when the aerogel blankets were placed in a humid environment, it took hours for moisture to diffuse from the surface of the material to its depth. The aging under elevated temperature and freeze-thaw cycles had no significant impact on the thermal performance of aerogel-enhanced blanket (1%); while weathering and humidity driven aging resulted in a higher increase in thermal conductivity by 10% and 8.4%, respectively. The results underlined once again some issues of the blankets with regard to wetting (Nosrati and Berardi, 2018). Gnahore (2010) did some experiments to test the maximum compressive strain under hydrostatic pressure of aerogel blankets. Spaceloft by Aspen Aerogels showed a maximum compressive strain of 0.48 mm/mm under a hydrostatic pressure of 1.2 MPa. The compressive strain of aerogel blanket was reduced from 50% to 100% of its value of decompression strain, during the compression period. During the decompression phase, the pressure of the prototype returned to the starting value, while the stress of the market product remained constant. Finally, several studies have started using aerogel blankets in real buildings and reported about the in-situ behavior of such products (Cuce et al., 2014a,b; Cuce and Cuce, 2016; Berardi, 2018).

17.5

Conclusions

The chapter has summarized several ongoing research activities, which have already led to manufacturing aerogel-enhanced products for building opaque systems. An overview of systems, also including products available on the market, has been

Aerogel-enhanced insulation for building applications

413

done with the intent of providing an updated review of current state-of-the-art and possible use of aerogel-enhanced products. Meanwhile, research conducted in both research centers and private companies is proposing new aerogel-enhanced products. For example, in Europe, international research projects have been financed within the Horizon 2020 scheme, to look at new formulations of aerogel-enhanced systems, by specifically focusing on the long-term performance of new products, their mechanical strength, and thermal conductivity. Moving forward, the following three aspects seem to represent the most valuable research challenges: •

• •

further development of aerogel-enhanced thin layers, such as coatings and paints, is needed to broaden the possibilities of using aerogel in building envelopes almost everywhere in order to improve thermal efficiency in existing buildings and new constructions. In this regard, water-based paints and pastes with aerogel have recently been proposed, but still, limited knowledge has been disseminated about them; lowering the price, for example, by looking at the potentialities of other than supercritical drying processes remains critical to guarantee more competitive advantages in adopting aerogel-enhanced products; finally, the adoption of new aerogel-enhanced products will need to be carefully monitored to further verify their long-term performance and provide the building stakeholders all the needed insurances about the resilience of these innovative building materials.

References Aegerter, M.A., Leventis, N., Koebel, M.M., 2011. Aerogels Handbook. Springer, New York. Baetens, R., Jelle, B.P., Gustavsen, A., 2011. Aerogel insulation for building applications: a state-of-the-art review. Energy and Buildings 43, 761e769. Berardi, U., Nosrati, R., 2018. Long-term performances of aerogel-enhanced insulating materials. Energy 147, 1188e1202. Berardi, U., Calisesi, M., Garai, M., 2017. Investigation of the Aerogel Inclusion in Cement Products. Proceeding SET 2017, Bologna, Italy. Berardi, U., 2015. The development of a monolithic aerogel glazed window for an energyretrofitting project. Applied Energy 154, 603e615. Berardi, U., 2017. A cross country comparison of building energy consumption and their trends. Resource Conservation and Recycling 123, 230e241. Berardi, U., 2018. Aerogel-enhanced solutions for building energy retrofits: insights from a case study. Energy and Buildings 159, 370e381. Bhuiya, M.H., Anderson, A.M., Carrol, M.K., Bruno, B.A., Ventrella, J.L., Silbermann, B., Keramati, B., 2016. Preparation of monolithic silica aerogel for fenestration applications: scaling up, reducing cycle time, and improving performance. Industrial & Engineering Chemistry Research 55, 6971e6981. Buratti, C., Moretti, E., Belloni, E., Agosti, F., 2014. Development of innovative aerogel based plasters: preliminary thermal acoustic performance evaluation. Sustainability 6 (9), 5839e5852. Buratti, C., Moretti, E., Belloni, E., 2016. Aerogel plasters for building energy efficiency. In: Torgal, P., et al. (Eds.), Nano Biotech Based Materials for Energy Building Efficiency, pp. 17e40.

414

Nanotechnology in Eco-efficient Construction

Cha, J., Kim, S., Park, K.W., Lee, D.R., Jo, J.H., Kim, S., 2014. Improvement of window thermal performance using aerogel insulation film for building energy saving. Journal of Thermal Analysis and Calorimetry 116, 219e224. Cuce, E., Cuce, P.M., 2016. The impact of internal aerogel retrofitting on the thermal bridges of residential buildings: an experimental statistical research. Energy and Buildings 116, 449e454. Cuce, E., Cuce, P.M., Wood, C.J., Riffat, S.B., 2014a. Optimizing insulation thickness analysing environmental impacts of aerogel-based thermal superinsulation in buildings. Energy and Buildings 77, 28e39. Cuce, E., Cuce, P.M., Wood, C.J., Riffat, S.B., 2014b. Toward aerogel based thermal superinsulation in buildings: a comprehensive review. Renewable and Sustainable Energy Reviews 34, 273e299. Dorcheh, S.A., Abbasi, M.H., 2008. Silica aerogel: synthesis, properties characterization. Journal of Materials Processing Technology 199, 10e26. Ebert, H.P., 2011. Thermal properties of aerogels. In: Aerogels Handbook. Springer, New York, pp. 537e564. Fickler, S., Milow, B., Ratke, L., Schnellenbach-Held, M., Welsch, T., 2015. Development of high performance aerogel concrete. Energy Procedia 78, 406e411. Galliano, R., Ghazi Wakili, K., Stahl, T., Binder, B., Daniotti, B., 2016. Performance evaluation of aerogel-based perlite-based prototyped insulations for internal thermal retrofitting: HMT model validation by monitoring at demo scale. Energy and Buildings 126, 275e286. Gao, T., Jelle, B.P., Ihara, T., Gustavsen, A., 2014. Insulating glazing units with silica aerogel granules: the impact of particle size. Applied Energy 128 (1), 27e34. Garrido, R., Silvestre, J.D., Flores-Colen, I., 2017. Economic and energy life cycle assessment of aerogel-based thermal renders. Journal of Cleaner Production 151, 537e545. Ghazi Wakili, K., Binder, B., Zimmermann, M., Tanner, C., 2014. Efficiency verification of a combination of high performance conventional insulation layers in retrofitting a 130-year old building. Energy and Buildings 82, 237e242. Gillott, M., Spataru, C., 2010. In: Hall (Ed.), Materials for Energy Efficiency and Thermal Comfort in the Refurbishment of Existing Buildings, pp. 649e680 (Chapter 26). Gnahore, B.G., 2010. Mechanical Degradation of Thermal Properties of Flexible Aerogel Blankets. West Virginia University Libraries. GVR, G.V., 2016. Aerogel Market Size to Reach $1.92 Billion by 2022. Hæreid, S., Nilsen, E., Einarsrud, M.A., 1996. Properties of silica gels aged in TEOS. Journal of Non-Crystalline Solids 204, 228e234. Hoseini, A., Malekian, A., Bahrami, M., 2016. Deformation and thermal resistance study of aerogel blanket insulation material under uniaxial compression. Energy and Buildings 130, 228e237. Ibrahim, M., Biwole, P.H., Wurts, E., Achard, P., 2014a. A study on the thermal performance of exterior walls covered with a recently patented silica-aerogel-based insulating coating. Building and Environment 81, 112e122. Ibrahim, M., Wurtz, E., Biwole, P.H., Achard, P., Salle, H., 2014b. Hygrothermal performance of exterior walls covered with aerogel-based insulating rendering. Energy and Buildings 84, 241e251. Ibrahim, M., Biwole, P.H., Achard, P., Wurtz, E., Ansart, G., 2015. Building envelope with a new aerogel-based insulating rendering: experimental numerical study, cost analysis, and thickness optimization. Applied Energy 159, 490e501.

Aerogel-enhanced insulation for building applications

415

IEA, International Energy Agency, 2013. Transition to Sustainable Buildings: Strategies and Opportunities to 2050. Jelle, B.P., Hynd, A., Gustavsen, A., Arasteh, D., Goudey, H., Hart, R., 2012. Fenestration of today and tomorrow: a stateeofetheeart review and future research opportunities. Solar Energy Materials and Solar Cells 96, 1e28. Jelle, B.P., 2011. Traditional, stateeofetheeart future thermal building insulation materials solutions e properties, requirements possibilities. Energy and Buildings 43, 2549e2563. Kim, S., Seo, J., Cha, J., Kim, S., 2013. Chemical retreating for gel-typed aerogel insulation performance of cement containing aerogel. Construction and Building Materials 40, 501e505. Kistler, S.S., 1931. Coherent expanded aerogels. Journal of Physical Chemistry 52e64. Koebel, M., Rigacci, A., Achard, P., 2012. Aerogel-based thermal superinsulation: an overview. Journal of Sol-Gel Science and Technology 63 (3), 315e339.  2017. Investigation of the moisture induced degradation of the thermal properties of Lakatos, A., aerogel blankets: measurements, calculations, simulations. Energy and Buildings 139, 506e516. Najjar, I., 2012. Aerogel - a promising building material for sustainable buildings. American Journal of Chemical Science 2 (3), 4e10. Neugebauer, A., Chen, K., Tang, A., Allgeier, A., Glicksman, L.R., Gibson, L.J., 2014. Thermal conductivity characterization of compacted, granular silica aerogel. Energy and Buildings 79, 47e57. Ng, S., Jelle, B.P., Sandberg, L.I., Gao, T., Wallevik, O.H., 2015. Experimental investigations of aerogel-incorporated ultra-high performance concrete. Construction and Building Materials 77, 307e316. Nosrati, R., Berardi, U., 2018. Hygrothermal characteristics of aerogel-enhanced insulating materials under different humidity and temperature conditions. Energy and Buildings 158, 698e711. Rao, A.V., Bhagat, S.D., 2004. Synthesis and physical properties of TEOS-based silica aerogels prepared by two step (acid-base) sol-gel process. Solid State Sciences 6 (9), 945e952. Shukla, N., Fallahi, A., Kosny, J., 2014. Aerogel thermal insulation d technology review cost study. ASHRAE Transactions 120, 294e307. Stahl, T., Brunner, S., Zimmermann, M., Ghazi Wakili, K., 2012. Thermo-hygric properties of a newly developed aerogel based insulation rendering for both exterior and interior applications. Energy and Buildings 44, 114e117. Stahl, T., Ghazi Wakili, K., Hartmeier, S., Franov, E., Niederberger, W., Zimmermann, M., 2017. Temperature moisture evolution beneath an aerogel based rendering applied to a historic building. Journal of Building Engineering 12, 140e146. Wei, G., Liu, Y., Zhang, X., Yu, F., Du, X., 2011. Thermal conductivities study on silica aerogel and its composite insulation materials. International Journal of Heat and Mass Transfer 54 (11e12), 2355e2366. Westgate, P., Paine, K., Ball, R.J., 2018. Physical and mechanical properties of plasters incorporating aerogel granules and polypropylene monofilament fibres. Construction and Building Materials 158, 472e480. Wu, X., Fan, M., Mclaughlin, J.F., Shen, X., Tan, G., 2018. A novel low-cost method of silica aerogel fabrication using fly ash and trona ore with ambient pressure drying technique. Powder Technology 323, 310e322.

416

Nanotechnology in Eco-efficient Construction

Further reading Buratti, C., Merli, F., Moretti, E., 2017. Aerogel-based materials for building applications: influence of granule size on thermal acoustic performance. Energy and Buildings 152, 472e482. Gao, T., 2014. Aerogel-incorporated concrete: an experimental study. Construction and Building Materials 52, 130e136. Gl oria Gomes, M., Flores-Colen, I., da Silva, F., Pedroso, M., 2018. Thermal conductivity measurement of thermal insulating mortars with EPS and silica aerogel by steady-state and transient methods. Construction and Building Materials 172, 696e705.