Aerogel-based renders with lightweight aggregates: Correlation between molecular/pore structure and performance
Graphical abstract
Introduction
The construction industry has undergone profound changes triggered by the growing concerns with sustainability, coupled with more demanding energy and environment European directives [1]. This has encouraged the search for new products and construction systems with a better thermo-hygric-mechanical performance and reduced environmental impact during their life cycle.
In this regard, thermal mortars are among the most promising materials, due to thermal conductivities lower than 0.1 or 0.2 W.m−1.K−1, classified as T1 or T2, respectively [2]. The relevance of silica-based aerogels as aggregates in thermal rendering and plastering mortars has been proven and thoroughly reviewed [3], [4], [5]. Namely, Achard et al. patented an insulating plaster with thermal conductivities between 0.095 and 0.034 W.m−1.K−1, depending on the hydrophobic aerogel content [6]. The hygrothermal performance of exterior walls covered with this rendering was examined and reduced heat losses and moisture risks were observed [7]. The long-term behaviour of aerogel-based plasters applied on historical façades has been recently analysed [8]. Stahl et al. developed a high-performance thermal rendering material (bulk density of 200 kg.m−3 and thermal conductivity as low as 0.025 W.m−1.K−1) by incorporating hydrophobic silica aerogel granules (60 to 90 vol% relative to the total mixture) in a purely mineral and cement-free binder [9]. By mixing natural lime plasters with higher contents of granular hydrophobic aerogel (96–99 vol%), even lower values of thermal conductivity (0.016–0.014 W.m−1.K−1) and bulk density (125–115 kg.m−3) were achieved [10]. A solution of this product with 80–90 vol% of aerogel is commercially competitive, presenting a thermal conductivity of 0.045 W.m−1.K−1 [4]. Currently, there is one mainly used aerogel-based thermal insulating plaster in the European market. It uses a combination of hydraulic lime, calcium hydroxide and white cement as binder matrix, and has been commercialized as the insulating layer of an external thermal insulation composite system (ETICS), as well as a one-layer plaster [11]. The performance characteristics of this product, available from the technical datasheet, are: density of 220 kg.m−3, thermal conductivity of 0.0261 W.m−1.K−1 and water vapour diffusion resistance coefficient of 4–5.
According to EN 998-1, low thermal conductivity and density should be accompanied by a minimum compressive strength of 0.4 MPa, maximum capillary water absorption of 0.4 kg.m−2.min−1/2 and maximum water vapour permeability coefficient of 15. Thermal insulation is usually enhanced at the expense of mechanical strength, and the compromise between excellent thermo-hygric behaviour and mechanical strength is extremely difficult to achieve [6]. This challenge has been addressed by formulating mortar composites that incorporate aerogel in ultra-high performance concrete granules, achieving either a compressive strength of 20 MPa but a thermal conductivity of 0.55 W.m.−1.K−1 (for 50 vol% aerogel) or a thermal conductivity of 0.31 W.m−1.K−1 but a negligible value of compressive strength (for 80 vol% aerogel) [12]. The authors attempted to correlate the low mechanical strength with the gaps at the aerogel/binder interface transition zones observed by SEM.
All the above mentioned studies used supercritical aerogel from well-established suppliers. We recently approached this problem for totally cementitious binders, using both supercritical and subcritical aerogels as aggregates: the capillary water absorption (1.6 kg.m−2.min−1/2) and water vapour permeability coefficient (14) obtained using 24 vol% of a subcritical silica aerogel have shown that higher contents of hybrid aerogel should be preferred [13]. The thermal conductivities of the hybrid aerogel-based renders with more than 60 vol% aerogel were correlated with the molecular and pore structure of the aggregate and the advantages of the subcritical aerogel were established. For total replacement of sand by hybrid aerogel, synthesized by design, a thermal conductivity of 0.085 W.m−1.K−1 and an envelope density of 410 kg.m−3 were obtained [14]. The very low mechanical strength of these renders encouraged further improvements towards selecting secondary aggregates and modified binders. Expanded cork, expanded clay and perlite appeared as the most promising aggregates. The effect of expanded cork in improving the thermal performance and water resistance of mortars, with a reduction in the mechanical strength, has been documented [15]. Expanded clay is known for not affecting considerably the mechanical strength of traditional mortars, when used in low contents, while improving the thermal insulation and reducing the capillary water absorption [16]. Perlite is a lightweight filler that decreases the thermal conductivity of cementitious materials and is an excellent additive to produce workable mortars for rendering applications [17].
In the present paper, besides a totally cementitious binder, a 50 wt% cement-fly ash matrix was explored, because fly ash is lighter than cement and is a by-product of pulverized coal combustion in thermal power plants, thus not expensive [18]. Silica sand was replaced by subcritical hybrid aerogel, granular expanded cork and expanded clay, and also by a combination of these lightweight aggregates. Based on the analysis of the role of each aggregate (reviewed above), and since the priority in the present work was the renders’ thermal behaviour, a mixture of aggregates consisting mainly of subcritical hybrid aerogel (60 vol%) followed by granular expanded cork (20 vol%) was preferred [19]. A lower content in expanded clay (15 vol%) was added for mechanical resistance and an even lower content in perlite (5 vol%) was used for improving workability.
The performance of the aerogel-based renders (density, thermal conductivity, mechanical properties, capillary water absorption and water vapour permeability) was correlated with the molecular and porous structure induced by the different lightweight aggregates and with the binder matrices. A balance between the properties induced by the paste and the aggregates is made, in order to reach the best compromise between the render’s hygrothermal and mechanical performance and its production by safer and potentially more economical processes (e.g. based on subcritical aerogel), using sustainable materials (natural aggregates and by-products in cement replacement).
Section snippets
Synthesis of the hybrid silica-based aerogel
The hybrid silica-based aerogel, tailored to be used as aggregate in cement-based renders (Fig. 1), was synthesized by a two-step sol-gel process. The first step consists of the acid-catalysed hydrolysis of tetraethoxysilane (TEOS) and the second one of the polycondensation of the hydrolysed species, in neutral medium. TEOS (98%) was previously diluted in 2-propanol (2-PrOH, p.a.), and distilled water was added, dropwise, under orbital stirring. The reaction mixture was acidified with HCl 1 M (
Molecular structure
The DRIFT spectrum of the synthesized hybrid aerogel is shown in Fig. 3A, and those of pure granular expanded cork (GEC) and pure expanded clay (EC) are shown in Fig. 3B.
In Fig. 3A, the strong band centred at 1095 cm−1 is assigned to the νasSiOSi mode, where the shoulder at ∼1200 cm−1 correlates with the longitudinal optical (LO) component [27]. The corresponding νsSiOSi mode is much weaker, at ∼800 cm−1, and the poorly defined band at 950 cm−1 is assigned to the νSiO−/νSiOH mode. The organic
Conclusions
A careful combination of lightweight aggregates as total replacement of conventional silica sand proved to be valuable in enhancing the performance of cementitious aerogel-based renders. Using a cement-fly ash (50 wt%) binder matrix, subcritical hybrid aerogel as the main aggregate (60 vol%) and other lightweight aggregates (20 vol% granular expanded cork, 15 vol% expanded clay and 5 vol% perlite) yielded a lightweight (652 kg.m−3), porous (60%), with high surface area (196.9 m2.g−1), and class T1
Acknowledgments
The authors acknowledge CERIS and CQFM research centres and the financial support by FCT (Fundação para a Ciência e a Tecnologia): Projects PTDC/ECM/118262/2010, NANORENDER (2012-2015), PEst-OE/CTM/LA0024 and UID/NAN/50024/2013. António Soares acknowledges PhD Grant SFRH/BD/97182/2013. The authors also acknowledge the following manufacturers and companies: Weber, Amorim, Argex, Secil and EDP.
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