Foaming of Recyclable Clays into Energy-Efficient Low-Cost Thermal

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Foaming of Recyclable Clays into Energy-Efficient Low-Cost Thermal Insulators Clara Minas,$ Julia Carpenter,†,$ Jonas Freitag,† Gnanli Landrou,‡ Elena Tervoort,† Guillaume Habert,‡ and Andre ́ R. Studart*,† †

Complex Materials, Department of Materials, ETH Zürich, Vladimir-Prelog-Weg 5, 8093 Zürich, Switzerland Institute of Construction and Infrastructure Management, Chair of Sustainable Construction, ETH Zürich, Stefano-Franscini-Platz 5, 8093 Zürich, Switzerland

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‡

S Supporting Information *

ABSTRACT: Thermal insulators are crucial to reduce the high energy demands and greenhouse emissions in the construction sector. However, the fabrication of insulating materials that are cost-effective, fire resistant, and environmental-friendly remains a major challenge. In this work, we present a room-temperature processing route to fabricate porous insulators using foams made from recyclable clays that can be locally resourced at very low costs. Foams containing either pure Kaolin or a Kaolin-based clay mixture are produced through mechanical frothing or an in-situ gasgenerating reaction. Surface modification of the clay particles using a cationic amphiphilic molecule leads to particle-stabilized foams that are sufficiently strong to withstand the high capillary stresses developed during water evaporation. Self-supporting insulators with up to 90% porosity and thermal conductivities as low as 0.13 W/mK can thus be obtained by simple casting and drying at ambient temperature in an ultralow energy process. Such materials can be recycled by crushing, redispersion in water, and subsequent foaming. Porous structures with higher compressive strength are optionally created by sintering the dried foams at 1000 °C. The obtained porous materials perform comparably well with existing fire-resistant insulators while offering the possibility of closed-loop processing and wide availability from local resources as well as ultralow cost and embodied energy. KEYWORDS: Porous materials, Insulators, Kaolinite, Sustainable, Ceramics, Buildings

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hazardous to the human health and the environment.3,4 In response to these issues, several types of alternative insulating materials with low thermal conductivity combined with fire resistance and low environmental impact have been investigated in the last decades. Aerogels have gained increasing attention as an alternative to conventional insulators due to their ultralow thermal conductivity and fire-resistance in comparison to polymerbased foams.5 The high insulating properties of aerogels stem from their nanoscale porosity, which decreases the transport of gas molecules below the so-called Knudsen limit that dictates conductivity in coarser porous materials. Despite this superinsulating behavior, the high cost and the multistep processes required for their fabrication has so far limited a wider applicability of aerogels. Although promising aerogel/calcium sulfate composites have been proposed as insulating mortars, these materials are not expected to provide sufficient thermal performance at acceptable cost.6 More recently, inorganic cement foams have also been proposed as substitutes for existing insulators, since they achieve fire resistance without the hazardous issues of mineral wools and fibers. Such

INTRODUCTION Materials for the thermal insulation of buildings offer significant potential for energy savings and reduction in greenhouse emissions at relatively low cost. Because they account for up to 40% of the global and European energy consumption, buildings have become a top priority to achieve the target of 80% reduction in global emissions by 2050 set by the International Energy Agency (IEA). To reach this goal, the Energy Performance of Buildings Directive requires all new buildings to be nearly zero-energy by the end of 2020. Measures have been proposed to encourage best practices for cost-effective transformations of existing buildings into nearly zero-energy systems.1 This requires a deep energy retrofit of the building stock, which demands a large amount of insulation materials. Recent modeling of the building stock evolution shows that the energy embodied in the production of conventional insulation materials will control the environmental impact of the buildings.2 Furthermore, recycling challenges are expected to create a real waste management problem at the end of their lives.2 Although they could solve the immediate problem of high operation energy in badly insulated building stock, current thermal insulating materials, such as polyurethane foams and mineral wools, cause other serious problems. Polymer-based insulators are highly flammable and difficult to recycle, whereas mineral wools are © XXXX American Chemical Society

Received: June 25, 2019 Revised: August 14, 2019

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DOI: 10.1021/acssuschemeng.9b03617 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. Foaming of clays into highly porous structures. Processing schematic (A) used for the preparation of porous clay-based foams. Porosity is introduced through mechanical or in-situ foaming of aqueous clay suspensions. Particle-stabilized air bubbles result in a stable wet foam, which can be directly dried, and possibly fired, into a solid porous body. Dried foams can be redispersed in water and foamed again, thus demonstrating the recyclability potential of this raw material. SEM images show the pore walls (B) and overall pore geometry (C) of closed cell clay foams. Images of a dried body and a sintered foam are shown in (D) and (E), respectively. Air bubbles are stabilized by hydrophobized particles, as illustrated in the schematic (F). In this example, hexylamine is used as surface modifier of the clay particles.

While clay-based materials are commonly used as load bearing structural elements, foamed clay or mixtures containing clay have also been proposed as insulation materials for residential and industrial applications.13−15 However, most of these materials rely on complex processing such as freeze-drying or intricate material mixtures, which makes them unsuitable as low-cost building materials. Moreover, current research on porous clay materials is based on purified clay powders with well-defined chemical- and size distributions, which are not representative of locally sourced clays. The fabrication of porous insulators using robust processing technologies directly applicable to local clay resources is crucial for the development of the next generation of low-cost and closed-loop alternatives to current insulating materials. Here, we design and investigate a particle-based foaming technology for the preparation of ultralow-cost porous insulators using abundant and recyclable clays. The foaming approach is based on the surface functionalization of clay particles using short amphiphilic molecules, similar to previously reported approaches.16−20 Adsorption of such

inorganic materials are much cheaper than aerogel-based insulators, but their cementitious nature causes unavoidable CO2 emissions during fabrication. Although new public policies may facilitate a broader use of aerogel-based insulators for the restoration of historic buildings,7 these technologies will not be sufficiently cheap and abundant to fulfill the growing building demands in emerging countries. Natural fibers such as straw, hemp, or typha can be a low cost and low environmental impact solution,8 but their thermal performance requires relatively thick walls, which are not suitable in urban areas where the cost of land exceeds the cost of building materials. Recent research has also proposed the use of clay as an abundant and low-emission natural resources to cope with the booming construction industry.9,10 Clays have been traditionally used as a building material due to their abundance, low cost, formability, and robustness.11,12 The increasing demand for environmentally friendly construction materials has given new impulse for the use of clay in construction, as it can be sourced and processed locally and typically does not require expensive machinery or know-how. B


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Figure 2. (A) ζ potential of clay particles obtained from Kaolin and TN suspensions containing 0, 10, and 20 μL/g HA with respect to the clay powder. (B) Effect of HA concentration on the porosity of dried Kaolin and TN foams prepared by mechanical frothing of suspensions containing 40 wt % powder. (C.1, D.1) SEM images of Kaolin and TN powders and of the pore walls of structures obtained upon drying of Kaolin (C.2, C.3) and TN (D.2, D.3) foams.

interface, thus providing a very effective mechanism for foam stabilization (Figure 1F). Besides the formation of particlecoated air bubbles, foam stabilization is also promoted through the buildup of a network of percolating particles throughout the continuous phase. The wet foams are subsequently dried into stable self-supporting bodies of any specific geometry (Figure 1D), which may be sintered into fired foams to reach higher strengths (Figure 1E) or crushed and redispersed for a new foaming cycle. This method leads to porous monoliths with predominantly closed pores in the size range 40−400 μm and porosity levels between 53 and 90% (Figure 1B,C). The porosity and pore sizes of the clay bodies can be tuned by adjusting the concentration of particles and surface modifiers added to the initial clay suspension. To illustrate this microstructural control, we first investigate the foaming behavior of Kaolin (K) and Terranova (TN) powders suspended in water containing different surface modifier concentrations (Figure 2). Following our earlier work,21 positively charged hexylamine (HA) molecules were used in these experiments to surface-hydrophobize the negatively charged surface of the aluminosilicate particles present in the clays. SEM images of the two powders show that the Kaolin batch (Figure 2C.1) contains solely thin aluminosilicate sheets with a specific surface area (SSA) of 9 m2/g, while the TN batch (Figure 2D.1) is a mixture of different clays whose broad size and shape distributions result in a much higher SSA of 47 m2/g. With such a high specific surface area, the TN powder is also expected to release a higher concentration of cations into the aqueous phase of the suspension, which increases the ionic strength of this phase compared to the pure Kaolin. The size distribution and chemical composition of the two powders can be found in the Supporting Information. Such distinct physical

molecules on the surface of initially hydrophilic clay suspended in water makes the particles hydrophobic and not fully wetted by the aqueous phase. The partially hydrophobic particles slightly agglomerate in the aqueous phase and adsorb readily at the air−water interface if air bubbles are incorporated in the clay suspension. The resulting foams are remarkably stable against coarsening and sufficiently strong to withstand the capillary stresses developed during drying. We demonstrate this ultralow-cost processing route using purified Kaolin as a benchmark and a clay mixture (TerraNova, TN) with a powdery composition that is comparable to what can be sourced from local soils. The effect of processing parameters, such as surface modifier concentration and particle content, on the foaming behavior and porosity of dried foams are systematically investigated. This is followed by a comparison between mechanical frothing and in-situ gas generation as two possible foaming techniques. Finally, the porous materials obtained upon drying, and possibly sintering, of the clay foams are compared with state-of-the-art insulating materials in terms of mechanical strength and thermal conductivity.

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RESULTS AND DISCUSSION The preparation of ultralow-cost insulators using natural clay follows a series of very simple processing steps (Figure 1). Suspensions of the modified powder are first prepared by mixing the clay into water containing previously dissolved surface modifiers. Electrostatic interactions between the ionized surface modifier and charged sites on the surface of the clay enable hydrophobization of the particles in the aqueous phase. Foams are obtained by either mechanical frothing or in-situ generation of gas via a chemical reaction. The introduction of air into the clay suspension leads to the adsorption of hydrophobized particles at the air−water C


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Figure 3. Effect of clay concentration (A) on the viscosity of suspensions containing Kaolin (blue triangles) or TN (green circles) and (B) on the porosity and average pore size of the respective dried bodies. The SEM images shown in (C) and (D) were obtained from bodies prepared from suspensions containing 30 wt % (C.1, D.1), 40 wt % (C.2, D.2), and 50 wt % Kaolin or TN and 10 μL/g hexylamine. The apparent viscosity shown in (A) was measured at a shear rate of 100 s−1.

the aqueous phase, thus decreasing the particle ζ potential.23 Because the development of negative charges is a common feature of aluminosilicate sheets,24 our surface modification approach should also be applicable to other types of clays. The hydrophobic nature of the clay particles modified with HA enables the preparation of porous materials with porosity levels ranging from 53 to nearly 90%, depending on the concentration of HA used (Figure 2B). For both clay powders, highly porous foams are produced at a given window of modifier concentrations. Below the lower limit of this window, the hydrophobization of the particles with HA is not sufficient to induce strong adsorption of the modified particles at the air−water interface. As a result, the air bubbles introduced during foaming do not resist the high capillary forces developed in the structure during drying. By contrast, HA contents above the upper limit of the window result in particles that are too hydrophobic and insufficiently charged, thus leading to agglomeration and low adsorption on the surface of the air bubbles.25 The agglomeration of particles at these high HA concentrations leads to an excessive increase in the viscosity of the clay suspension, which impedes the incorporation of air during mechanical frothing. Interestingly, the HA concentration window leading to maximum foaming is much broader for the TN powder as compared to the reference Kaolin. This probably results from the higher concentration of impurities and the larger specific surface area of the TN powder. With a higher level of impurities and associated lower

and chemical characteristics directly affect the adsorption of the HA molecules on the surface of the clay particles. The adsorption of the modifier on the clay platelet surface was indirectly quantified with the help of ζ potential measurements. The faces of clay platelets hold permanent negative charges resulting from the dissolution of cations. By contrast, the charges on the edges of the particles are pH dependent and are associated with the reversible (de)protonation of hydroxyl groups. Such surface charge heterogeneities are inherent to clay materials due to their crystal structure. Since the magnitude of the ζ potential is proportional to the electrical charges on the particle surface, the change of ζ potential with changing pH is only effected by the edge charges.22 The surface modifier, hexylamine, adsorbs to the oppositely charged face of the clay platelets. The neutralization of negative surface sites is reflected by a decrease in the absolute value of the ζ potential. Our results indicate a successful modification of the particles from both powders, as evidenced by the shift of the ζ potential curves to lower magnitudes with increasing concentration of HA. A direct comparison between the absolute ζ potential values of the two powders is not straightforward due to the presence of various types of oxide particles of broad size distribution in the TN clay (see Supporting Information). The consistently lower absolute ζ potential of the TN powder might reflect the higher concentration of impurities in this clay. Water-soluble impurities are known to increase the concentration of ions in D


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ACS Sustainable Chemistry & Engineering absolute ζ potential (Figure 2A), a lower concentration of HA is needed to induce bulk agglomeration and adsorption of the modified clay particles at the air−water interface. This decreases the lower HA limit required for strong foaming. In turn, the higher surface area of the TN powder increases the adsorption capacity of the clay particles, which can accommodate larger concentrations of HA on their surface before the hydrophobicity levels become too high, increasing the viscosity of the aqueous phase and preventing further air incorporation. Further insights into the stabilizing effect of the clay particles are gained by examining the microstructure of the porous materials obtained after drying of the foams (Figure 2C,D). SEM images of the pore walls of structures obtained at a HA concentration of 10 μL/g show that modified Kaolin platelets clearly align along the walls of the pores to form thin layers with a thickness of a couple of particles (Figure 2C.2,C.3). Conversely, the pore walls of TN foams tend to be thicker while still showing some alignment of clay platelets at the surface of the pore. Although further work is required to fully elucidate the stabilization mechanisms at play, the heterogeneous nature of the pore walls in TN foams reflect the wide distribution of sizes (Figure S1), morphologies, and chemistries (Table S1) of the particles that form this clay. This heterogeneity probably leads to the preferential adsorption of specific particle types and morphologies at the interface, while others remain within the continuous phase between the air bubbles. By contrast, Kaolin exhibits well-defined plateletshaped particles that are homogeneously adsorbed at the interface to create more faceted pore walls consistent with the morphology of the individual clay particles. In addition to the modifier content, the concentration of particles in the initial clay suspension is another effective parameter to control the microstructure of porous materials obtained from the clay foams. We study the effect of this parameter on the porosity and pore size of foam-derived structures by preparing formulations containing particle contents in the range between 10 and 50 wt % (Figure 3). Increasing the particle concentration up to 50 wt % was found to significantly reduce the porosity of the dried bodies. The porosity decreases from 98 to 72% and from 95 to 81% for compositions prepared with Kaolin and TN, respectively. This is explained by the fact that a higher powder concentration in the suspension increases the fraction of solid phase that remains after drying. Such lower porosity may also result from the difficulty to incorporate air into highly viscous suspensions. This effect is particularly pronounced for the suspension containing 50 wt % Kaolin. Along with the decrease in porosity, the pore sizes of the structure also reduce by as much as 5 and 6.5 times when the contents of Kaolin and TN powder, respectively, are increased from 30 to 50 wt %. The effect of the particle concentration on the pore size of the foamed structures can be rationalized in terms of the viscosity of the clay suspensions used in the foaming process.26,27 As expected, the viscosity of the clay suspensions increases by orders of magnitude when the particle concentration changes from 10 to 50 wt % (Figure 3A). Due to the anisotropic shape of the Kaolin particles, the increase in viscosity for the suspension containing this clay is sharper than that of the TN suspension. For foams produced by mechanical frothing, higher viscosities of the continuous phase result in increased shear forces applied to the incorporated air bubbles. This facilitates the rupture of air bubbles into smaller sizes,

eventually causing a decrease in the size of the pores obtained after drying the foam.26 These results illustrate the possibility of tailoring the porosity and pore size of the structures within a broad range by changing an easily tunable single control parameter. The mechanical frothing process described above is ideal for the fabrication of objects with predefined geometries that can be shaped by, for example, extrusion-based techniques. Such foams can be extruded using conventional equipment utilized for the manufacturing of bricks or can be deposited using more advanced 3D printing technologies (Figure 4A). Similar to

Figure 4. (A) Extrusion-based 3D printing of mechanically frothed TN foam. (B) Wet foam with grid-like structure obtained after printing. (C) Crack-free hierarchically porous TN structure after drying. Scale bars: 1 cm.

previously reported alumina-stabilized foams and emulsions,28 the yield stress and storage modulus of the wet foams are sufficiently high to allow for the creation of distortion-free filaments that can bridge over a few millimeters in grid-like structures (Figure 4B). Crack-free hierarchically porous clay structures can be obtained after drying the printed 3D grids at ambient conditions (Figure 4C). Drying into crack-free bodies is possible if the wet foam is allowed to shrink uniformly in the absence of physical constraints (Figure S2). The high stability of the foam is crucial to avoid the coalescence of air bubbles during shrinkage, whose linear value typically ranges from 30 to 20% for clay concentrations between 30 and 50 wt %. Besides extrusion and 3D printing, porous bodies can also be fabricated by simple casting of suspensions into complexshaped molds followed by in-situ foaming through an internal chemical reaction that generates gas bubbles on demand. A simple and low-cost chemical reaction to enable in-situ foaming of suspensions is the decomposition of hydrogen peroxide into oxygen and water at room temperature.29,30 This decomposition reaction is facilitated in the presence of catalyzing oxides, such as MnO, TiO2, and V2O5.31,32 Because natural clays like TN already contain such oxides as impurities, foaming of a TN suspension occurs spontaneously at room temperature by simply adding hydrogen peroxide to the initial mixture. Control experiments have shown that the catalyzing effect of the oxide impurities present in the TN clay is quite strong, since it masks any foaming effect that could be potentially achieved by adding MnO ex situ (Figure S3). To investigate the effect of the in-situ foaming process on the microstructure of dried foams, TN suspensions with different initial concentrations of clay particles and hydrogen peroxide were formulated and studied (Figure 5). In contrast to the behavior of mechanically frothed foams, the concentration of particles shows no significant effect on the porosity and pore size of in-situ foamed TN samples. Porosity varies between 88 and 92% and the average pore size varies between 290 and 410 μm for suspensions containing 40−60 wt % clay. This indicates E


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Figure 5. (A) Porosity and pore size of dried clay bodies obtained by in-situ foaming of suspensions containing 10 μL/g hexylamine, 1 wt % H2O2, and different concentrations of TN clay. SEM images of dried in-situ foams prepared with 50 wt % TN clay and H2O2 concentrations of (B) 0.3 wt % and (C) 1.1 wt %. (Scale bars: 500 μm) (D) Porosity and (E) droplet size of the same foams as a function of the concentration of pore forming agent H2O2. (F) The interdependence between the porosity and pore size of the in-situ foamed TN clay suspensions. The dashed lines indicate the theoretical dependences expected from the simple analytical relations derived in the SI.

increases both the total porosity and the average pore size of the dried foams to 225 and 370 μm, respectively. Because partially hydrophobic clay particles adsorb on the surface of the generated gas bubbles, structures with closed pore walls were obtained for all the H2O2 concentrations tested (Figure 5B,C). Assuming that the porosity of bodies prepared via in situ foaming is determined by the concentration of oxygen gas generated by the decomposition of hydrogen peroxide, it is possible to establish a direct correlation between the microstructure obtained and the initial H2O2 concentration (see Supporting Information, Figures S5 and S6). One can show, for example, that the porosity (P) of dried bodies produced by in situ foaming should vary with the H2O2 concentration (wf) according to the simple relation: p = wf/ (a + wf), where a is a constant that depends on the gas generated and the density of the clay. Using the same argument, the porosity (P) is expected to correlate with the pore size (d) as follows: P = d3/(b + d3), where b is a constant that depends on the concentrations of particles and catalyst. Combining the above equations, one can also show that d = (bw f/a) 1/3 . A comparison of these simple theoretical predictions with the experimental data confirms the validity of our initial assumption (Figure 5D−F). The agreement between theory and experiments is a very interesting finding, since it allows for the prediction of the pore size and porosity of the dried bodies using simple analytical relations. The porosity of the dried foams strongly influences the mechanical and thermal performances of the final insulating

that the viscosity of the suspension does not affect the microstructure of porous bodies produced by the in-situ foaming process. The different air incorporation mechanisms involved in these two foaming routes explain this discrepancy. In the case of mechanical frothing, air is introduced through continuous shearing and folding of the initial suspension, which is strongly dependent on the viscosity and thus particle concentration of the suspension. Instead, the in-situ foaming process relies on the incorporation of air via simple expansion of gas bubbles that uniformly nucleate throughout the sample. This special feature of the in-situ foaming approach allows one to produce porous bodies from suspensions that are too viscous for mechanical frothing. Indeed, highly porous dried bodies were prepared by in-situ foaming of suspensions with TN concentrations up to 60 wt %, whereas mechanical foaming could only be performed up to a particle concentration of 50 wt %. in-situ foaming was carried out on suspensions without prior ball-milling in order to minimize the embodied energy of the resulting samples. For mechanically frothed suspensions, the ball-milling process was not found to affect the general dependence of the porosity and pore size on the initial clay concentration (Figure S4). While the particle concentration shows a negligible effect on the microstructure of porous bodies made by in-situ foaming, the concentration of the gas-forming agent was found to strongly influence the porosity and average pore sizes of structures produced using this approach (Figure 5B). Increasing the concentration of H2O2 from 0.3 to 1.1 wt % F


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Figure 6. Mechanical and thermal properties of the clay-based foams. (A) Compressive strength, (B) E-Modulus, (C) and thermal conductivity of green body (filled symbols) as well as sintered samples (open symbols) of TN foams produced with in situ foaming (ISF) and mechanical frothing (M). Scaling factors of the respective material properties are included in the plots. For the compressive strength, density exponents of 1 and 1.5 are expected for closed and open cellular structures, respectively. For the elastic modulus, density exponents of 1 and 2 should apply in case of closed and open cellular structures, respectively.35 (D) Compressive strength of the investigated earth foams and reference insulation materials15,36−38 as a function of the density. (E) Thermal conductivity and embodied energy of the presented clay foams compared to reference materials.4,9,10,15,36,38−43

for open-cell structures, suggesting that the mechanical properties of our porous materials are governed by bending of cell struts. The stretching-dominated mechanical properties expected for close-cell structures were probably not observed due to the relatively low thicknesses of the pore walls. Similar to the mechanical properties, the thermal conductivity of our porous materials (Figure 6C) lies within the same order of magnitude of that of reference insulation materials, such as soil/cork mixtures44 and porous clays.10 Compared to commercially available polymer foams and hollow bricks,45 our porous materials are nonflammable and are expected to show a reduced convective flow contribution to the thermal conductivity, respectively. The dependence of the thermal conductivity on the relative density shows the trend expected from percolation models proposed in the literature for highly porous materials.46 This predictable relation between thermal conductivity and relative density provides a convenient tool to tune the thermal behavior of our porous materials by adjusting processing parameters that control the relative density of the final structure (Figure 5). Most importantly, the thermal properties of the room-temperature dried porous bodies stand out when the energy required to produce the insulating materials are compared (Figure 6E).

materials. We assessed the performances of porous bodies produced by mechanical frothing and in-situ foaming by comparing their mechanical compressive properties and thermal conductivities with those of reference insulation materials (Figure 6). In terms of mechanical properties, our sintered porous bodies showed compressive strength and elastic modulus that are similar and superior, respectively, to the levels obtained for sintered Kaolin foams15 as well as other fire-resistant insulation materials (Figure 6D). The porous clay bodies made without any sintering showed sufficient mechanical strength to be handled and to form large selfstanding bricks (Figure S7). Considering that no chemical or cementitious reactions were used to consolidate the foams prepared at room temperature, it was remarkable to see that the compressive strength and elastic modulus of such material was on the same order of magnitude to those of commercial insulators. Further improvements of such mechanical properties are expected through the addition of optimized binders into the proposed formulation.33 Overall, the strength and elastic moduli of our porous materials follow a power law dependence on the relative density, as expected from established models for cellular materials.34 The exponents of such power law dependence are not far from those predicted G


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frothed foams without prior ball milling was also prepared for direct comparison with these in-situ foams. ζ potential measurements (DT1200, Dispersion Technologies Inc., Mount Cisco, NY) were carried out using 5 wt % clay suspensions containing 0, 10, and 20 μL/g hexylamine. The initial pH of the suspensions was set to 12 with a 2 M NaOH solution. During the measurement, the pH was gradually decreased through dropwise titration using a 1 M HCl solution. Room temperature viscosity measurements (Gemini Advanced Rheometer, Bohlin Instruments) were performed by stress-controlled steady-state rheology using increasing stresses from 5 to 300 Pa in a serrated plate (2.5 cm diameter) setup. Preparation of Porous Materials. Mechanical frothing: The clay suspensions were frothed mechanically with a common kitchen mixer (Kenwood, Major Classic) at full power (800 W) for 2−5 min. The foams were then poured/ladled into PVC molds and covered with tissue paper. In-situ foaming was performed as follows: the clay suspension and a H2O2 solution were mixed at a 10:1 volume ratio using a Sulzer-mixer. This resulted in homogeneous mixtures with the desired compositions of clay and surfactant. The mixtures were extruded into paper molds and left to foam. Wet-foams were dried at room temperature (RT) for at least 2 days and subsequently cut into cuboids for further firing and testing for mechanical properties and thermal conductivity. Firing was undertaken in an electrical furnace (Naberthem, Switzerland) at 1000 °C for 2h. This temperature was found to be enough to promote sintering between the clay particles, while minimizing shrinkage and the amount of energy needed in the firing process.47 Material Characterization. Scanning electron microscopy (SEM) images were taken with a LEO 1530 instrument (Zeiss GmbH, Germany). Pore size measurements were conducted on three different SEM images per sample including at least 60 pores in every image. Density and porosity were measured through weight and volume measurements of the cut cuboids. Compressive strength measurements were conducted on the cuboid samples using a universal mechanical testing machine (Shimaddzu AGS-X, Japan) equipped with a 100 N load cell. Tests were performed in displacement control mode at a loading rate of 0.1 mm/min until sample fracture was detected in the force−displacement plot. The elastic modulus was determined from the initial slope of the force−displacement experimental data. Thermal conductivity measurements were carried out using a thermal property analyzer (KD2 Pro) with a dual needle measurement head (sensitivity 0.01 W/mK) from Meter Environment. For this analysis, in-situ foams were prepared with varying concentrations of H2O2 and cut into cubic samples with side-lengths of about 5 cm.

Because no sintering is used for their consolidation, such porous structures require one order of magnitude less energy to be produced in comparison to other insulating clay-based products. This remarkable feature is combined with a minimum CO2 footprint, since high-emission cementitious materials are not used in our formulations. Finally, the recyclable nature of such nonsintered bodies adds another compelling characteristic to this simple clay-based system.

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CONCLUSIONS

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EXPERIMENTAL SECTION

Clay-based porous materials exhibiting low thermal conductivity and sufficient mechanical properties for handling of self-standing bodies can be successfully produced through a low-cost and energy-efficient foaming process with minimal CO2 footprint. Foaming is achieved by modifying the surface of the clay particles with amphiphilic molecules to induce their adsorption on air bubbles incorporated into an initial aqueous suspension. Bubbles can be introduced either by mechanical frothing or the in-situ decomposition reaction of hydrogen peroxide in the presence of catalytic oxides already present in the clays. This simple process is directly applicable to widely available clay mixtures without special preprocessing of the raw material. The microstructures of the porous materials obtained after drying, and possibly sintering, of the clay-based foams depend on the formulation of the initial suspension. The porosity and pore size of foams prepared by mechanical frothing are mostly controlled by the concentration of clay in the initial suspension, whereas the hydrogen peroxide content determines the microstructural parameters in the case of in-situ foamed formulations. The mechanical properties and thermal conductivity of porous clay bodies produced by the proposed technique follow the expected trend with the relative density (porosity) of the structures. Porous bodies obtained via direct drying of in-situ foamed suspensions are particularly attractive due to the very low amount of energy consumed in this process. This allows one to fabricate self-supporting thermal insulators featuring a combination of low thermal conductivity and low embodied energy that is not accessible using other manufacturing techniques. With these unique features, such recyclable materials provide an enticing approach to meet the low-cost and sustainability demands of insulating materials in the building industry.

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ASSOCIATED CONTENT

S Supporting Information *

Clay Suspensions. Suspensions were prepared with deionized water and two clays: Kaolin FP 80 (Alberto Luisoni, CH) and TerraNova (TN, Stroba, CH). Hexylamine (Acros Organics, 99%) was used as the particle surface modifier. Powders were characterized using X-ray fluorescence (XRF), BET, and laser particle analysis. To prepare clay suspensions, 10 to 60 wt % Kaolin/TN powder was added to deionized water under continuous stirring. The pH values of Kaolin and TN suspensions were adjusted to 11.5 and 12, respectively, with an aqueous solution of 2 M NaOH. The resulting suspensions were ball-milled for 3 h in the case of Kaolin and 3 days in the case of TN in order to homogenize and break down powder agglomerates. Ball-milling was carried out in polyethylene milling pots using 10 mm alumina balls and a powder to ball ratio of 2:1. After ball-milling, the pH was readjusted to 11.5 (Kaolin) and 12 (TerraNova) with 2 M NaOH aqueous solutions. Hexylamine in concentrations ranging from 0 to 20 wt % was slowly added to the ball-milled suspensions under gentle stirring in order to avoid premature foaming. Suspensions that were used for in-situ foaming were prepared without ball-milling in order to demonstrate a low-tech process with a minimum embodied energy. A set of mechanically

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b03617.

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Characterization of clays; formation of crack-free monoliths; in-situ foaming in the presence of MnO powder; effect of ball-milling on the foaming behavior of TN clay suspensions; quantitative analysis of processingstructure relationships for in-situ foaming process; mechanical behavior of clay-based foams (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Guillaume Habert: 0000-0003-3533-7896 André R. Studart: 0000-0003-4205-8545 H


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ACS Sustainable Chemistry & Engineering Author Contributions $

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These authors contributed equally to the study.

Notes

The authors declare the following competing financial interest(s): Andr R. Studart and Elena Tervoort are cofounders of the company FenX, which develops insulating foams.

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ACKNOWLEDGMENTS Parts of this research project were funded by the National Research Program “Energy Turnaround” (NRP 70, www. nrp70.ch) of the Swiss National Science Foundation (SNSF). The authors would like to thank Dr. Patrick Ruch from the IBM research center Rüschlikon for fruitful discussions.

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