Processing, properties and applications of highly

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Review article

Processing, properties and applications of highly porous geopolymers: A review ⁎

Chengying Baia, , Paolo Colomboa,b a b

Department of Industrial Engineering, University of Padova, via Marzolo, 9, Padova 35131, Italy Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, United States

A R T I C LE I N FO

A B S T R A C T

Keywords: Geopolymers Foams Porosity Thermal conductivity Strength

Geopolymers, possessing a semi-crystalline three-dimensional inorganic network generated by the dissolution and reaction of a solid alumino-silicate source with an activating solution, have attracted increasing attention from both academia and industry because of their unique and favorable characteristics. This review deals with the synthesis, characterization and potential applications of porous geopolymers, realized through different processing routes. Firstly, the processing approaches are divided into five categories: (i) Direct foaming, (ii) Replica method, (iii) Sacrificial filler method, (iv) Additive manufacturing, and (v) Other methods. Their microstructure, porosity and properties are compared and discussed in relation also to the different processing routes. This review highlights the fact that porous geopolymers are promising low-cost candidates for technologically significant applications such as catalyst supports or membranes, filtration of liquid or gases, adsorption and insulation. This review aims at summarizing the main published results and fostering further investigations into developing innovative ways to generate components with improved properties.

1. Introduction

porous geopolymers [17,37,38]. Because of the large number of articles in the field, and the fact that a significant amount of micro- and mesoporosity could be obtained simply by regulating the formula and processing of bulk components [39–42], this review focuses on the manufacturing and properties of highly porous geopolymers (porosity ≥ 50 vol% or bulk density ≤ 0.7 g/cm3), in which macro-porosity is introduced on purpose into the micro- and meso-porous geopolymer matrix. The interest in these components has been recently rising, as it can be seen from the increase in the number of publications on this topic over the past few years. Fig. 1 shows the number of peer-reviewed journal papers (journals based on science citation index, conference proceedings are excluded) obtained searching Web of Science for publications since 2009 (no papers before this date were found in the literature). The processing methods used for the fabrication of porous geopolymer can be divided into five approaches, as schematically illustrated in Fig. 1(b): (i) Direct foaming (DF), (ii) Replica method (RM) (iii) Sacrificial filler method (SFM), (iv) Additive manufacturing (AM), and (v) Other methods (OM). The processing features of each of these approaches will be discussed and compared, as well as their influence on the bulk density, porosity, morphology, mechanical properties and thermal conductivity of porous geopolymers. It should be noted that a distinct advantage of using geopolymers for the fabrication of porous

In the 1970s, Davidovits [1,2] initially reported on geopolymers as semi-crystalline 3D aluminosilicate materials, which can be fabricated from natural/synthetic aluminosilicate minerals or industrial aluminosilicate byproducts/wastes (such as: metakaolin, fly ash, slag, red mud, glass, perlite, sand, rice husk ash, clay, or a combination of them) mixed with an aqueous solution containing reactive ingredients (potassium/ sodium hydroxide, phosphoric acid, potassium/sodium silicate, etc.) [1,3–11]. Today, porous geopolymer (PGs) or geopolymer foams (GFs, total porosity > 70 vol%) have been the focus of promising research in the field of porous inorganic materials because of their unique combination of good physical properties associated with good thermal and chemical stability, excellent mechanical properties [3,12–15], low CO2 emission and low energy use in their manufacture [16,17]. They have been tested for a variety of applications, including membrane and membrane supports [18,19], adsorbents and filters [20–24], catalysts [25,26], and acoustic and thermal insulators [12,17,27,28]. These applications cannot be pursued using their conventional dense counterparts. There has been a series of reviews related to geopolymers [8,29–35], geopolymer cements [36] or geopolymer concrete [17,37,38]. However, only few of these contained information on

⁎

Corresponding author. E-mail address: [email protected] (C. Bai).

https://doi.org/10.1016/j.ceramint.2018.05.219 Received 24 April 2018; Received in revised form 23 May 2018; Accepted 25 May 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Bai, C., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.05.219


C. Bai, P. Colombo

Fig. 1. Number of publications in the last decade on porous geopolymers, extracted from Journal Citation Reports (JCR): (a) with porosity ≥ 50 vol% or bulk density ≤ 0.7 g/cm3; (b) divided according to the different processing route. (DF= Direct foaming, RM= Replica method, SFM= Sacrificial filler method, AM= Additive manufacturing, OM= Other methods)

M = K or Na) are as follows:

inorganic components, over traditional ceramic processing routes, is that no sintering or high temperature heat treatment steps (unless high temperature applications are envisioned) are required. Furthermore, no (organic) gelling agents need to be added to the formulation, since the geopolymerization reaction can be used to set the wet part, retaining its shape.

2Al + 2MOH + 2H2O → 2MAlO2 + 3H2(g)

(1)

Si + 2MOH + H2O → M2SiO3 + 2H2(g)

(2)

2NaOCl → 2NaCl + O2(g)

(3)

2H2O2 → 2H2O + O2(g)

(4)

4NaBO3 + H2O → 2NaOH + Na2B4O7 + 2O2(g)

(5)

2.1. Direct foaming

AlN + MOH + H2O → MAlO2 + NH3(g)

(6)

The direct foaming (DF) method is the most used technique for producing porous geopolymers (PGs), particularly geopolymer foams (GFs), via processing of a suspension or liquid system, without sintering. In the direct foaming method, wet geopolymer foams are produced by incorporating air or gas into a homogeneous slurry, which is subsequently set by curing at certain temperatures to obtain consolidated porous bodies. During curing, the geopolymerization reactions go to completion, generating a continuous, three-dimensional inorganic network. Foaming is a thermodynamically unstable process, as the gas bubbles in the wet foams are likely to undergo spontaneous drainage, continuous Ostwald ripening, and coalescence for minimizing the overall Gibbs free energy of the liquid/slurry-air system. Due to the instability of the wet foams, large pores (hundreds of microns) and a wide range of pore sizes are typically present in the final porous geopolymer body. In order to avoid this phenomenon, the most frequently used approach is adding stabilizing agents (such as surfactants, particles, fibers) to the suspension or liquid media. The generation of gas into the homogeneous liquid or a slurry medium can be realized by adding blowing agents (BAs), which can be classified either as chemical or physical blowing agents. Chemical blowing agents form gaseous products (such as O2 and H2) and other byproducts by thermal decomposition or chemical reactions. Physical blowing agents generate porosity by a phase change, e.g., a liquid is volatilized or a gas dissolved in the system under high pressure may be desorbed by decompression. No specific examples of this second approach have been yet reported in the literature for geopolymer systems. In any case, the slurry needs to have a suitable rheology to enable gas retention for a sufficient time to allow for the stabilization of the wet foam by gelling and/or drying. The chemical blowing agents that have been used the most for the fabrication of geopolymer foams are metal powders, such as aluminum [40,43,44], silicon [45] powders or Si-containing compounds such as silica fume (SF), SiC, FeSi alloy [44,46–48], or hydrogen peroxide (H2O2) [7,19,23], NaOCl [49], sodium perborate [50], AlN and FeSO3 [51,52]. The reactions leading to gas release in alkaline solutions (here,

Stabilizing agents (SAs), such as surfactants [19,23], or particles that can be added to the slurry, decrease the surface tension of the air/ slurry system and stabilize the wet foam, by reducing Ostwald ripening (coalescence of bubbles), liquid film rupture and drainage [53]. Furthermore, their presence enables a better control of the cell size and size distribution and of the ratio between open and closed cells. The surfactants can be divided into nonionic (e.g. Triton X 100, Tween 80), anionic (e.g. sodium dodecyl sulfate, soap), cationic, proteinic (vegetable or animal). Their characteristics influence the morphology and pore architecture of the foams. For instance, alkyl ether sulfate (Micro Air 210), oleic acid and sodium lauryl sulfate (SLS) were used as SAs in the same system [54]. It was shown that samples produced using SLS had a more homogeneous porous structure and higher strength than samples obtained using oleic acid and Micro air 210. Cilla et al. [55] tested various amounts of two non-ionic SAs, showing that they led to foams with different average cell size and open porosity values. Further work to test different types and concentrations of SAs, while keeping other processing parameters constant, are necessary in order to fully assess their influence on the morphological development of geopolymer foams. Besides SAs, other additives such as particles or fibers can be used as thickening agents to modify the rheological properties of the slurries, influencing the final pore architecture and total pore volume. For instance, it was demonstrated that even a small amount of particles [44] or fibers [47] will increase the viscosity of the mixtures, leading to a different final microstructure. Samples produced using rice starch showed better inhibition of bubble coalescence than those obtained with fibers [47]. If no blowing agent is present, a large volume of porosity can be generated by gas entrapment produced by mechanical frothing or direct gas insertion [6,19,23]. It should be noted that, with this approach, the aim of mixing is to introduce a large volume of air from the environment into the slurry and therefore, besides a suitable rheology for the slurry, specific equipment (e.g. an air pressure foam generator [27,56,57], a foam generating tank [58,59] or special mixing blades)

2. Processing routes

2


C. Bai, P. Colombo

Fig. 2. The direct foaming fabrication route of porous geopolymers.

shown in Fig. 3 [19,71,94]. When using only a blowing agent, the produced cellular structures possessed a limited number of closed cells with a very inhomogeneous cell size distribution (Fig. 3(a)-(c)). The average pore size was about 770 µm, 250 µm, 2000 µm, respectively, depending on the type of blowing agent employed according to the corresponding chemical reactions (4), (2) and (1). In comparison to using only a blowing agent, the presence of only a stabilizing agent reduced the pore size, when comparing samples produced using H2O2 (Fig. 3(a),(d)) or Al (Fig. 3(c),(f)) as blowing agent, while the trend for the average pore size of samples obtained using Si (Fig. 3(b),(e)) only as blowing agent was the opposite. A possible explanation of this observation is that the chemical blowing agents or stabilizing agents are not the only factors affecting the pore size, since the viscosity of the system (and therefore the composition of the geopolymer which affects as well the rate of the geopolymerization reaction) also plays a significant role in determining the amount of gas retention or entrapment. The use of both a blowing agent and a stabilizing agent led to a decrease of the average pore size, a narrower pore size distribution, an increased amount of porosity and the predominance of open, interconnected pores (Fig. 3(g)-(i)). In order to generate some gas inside a geopolymer slurry, rather than using frothing Haq et al. [107] used a novel microwave foaming technique. Mixing mixed bottom ash, sodium silicate and NaOH, they produced high porosity (73 vol%) bodies with high compressive strength (3.55 MPa) and low thermal conductivity (0.075 W/m K). The amount of sodium silicate in the system controlled the gelling behavior which, in turn, regulated the amount of entrapped air generated by the formed steam. Consequently, by varying the bottom ash to sodium silicate ratio it was possible to modulate the physical and insulating properties of the samples. The influence of the alkali activation on the foaming ability of the system, and the physical and mechanical properties of the porous geopolymers were also investigated by the same group [108]. In the 2000s, Wagh [109] suggested that inorganic polymers having [PO4]3− in place of [SiO4]4−should be considered as a new class of geopolymers, as also mentioned by Davidovits [1]. Liu et al [110] prepared a porous phosphorus-based geopolymer at 80 °C using MK, H3PO4, alumina, and Al powder as a blowing agent. The pore size and porosity amount (40–83 vol%) could be controlled by the content of Al powder and/or water, and the porous samples possessed a high compressive strength (6–14 MPa). Gualtieri et al. [111] used natural limestone as blowing agent to obtain phosphate-based geopolymers with irregular cell morphology, high porosity (69–76 vol%), and low effective thermal conductivity (0.07–0.09 W/m K). Open cell phosphatebased PGs with a homogeneous microstructure were fabricated by frothing using TritonX-100 as stabilizing agent [6]. The direct foaming

and mixing rates [55] should be employed. In analogy with the foamed (aerated) concrete industry, geopolymer foams can be produced either by a pre-foaming method, or by mixing a foam concentrate with a geopolymer slurry adding some surface active agents (surfactants) [17,60]. Besides the physical mixing step, the pre-foaming method uses an air pressure foam generator firstly to form a foam. After weighing, the pre-foam is immediately transferred into the homogeneous geopolymer slurry for final mixing for a certain time at low speed rate [27,58]. Fig. 2 shows the schematic fabrication route of porous geopolymers by the direct foaming method. Unlike traditional concrete foams [60], porous geopolymers can be obtained using chemical BAs with or without SAs. In the case of chemical blowing, mixing should be carried out without the entrapment of additional air into the slurry, in order to minimize the pore size distribution. In the case of mechanical frothing, a porous geopolymer is produced either by mixing (simple mechanical mixing using surfactants)[6,55] or pre-foaming (foam prepared using an air pressure generator, with the addition of surfactants) [27,58]. Numerous processing routes have been investigated to produce porous geopolymers via the direct foaming technique. Tables 1–3 list the examples reported in the literature of the fabrication of porous geopolymers using only BAs, using both a BA and a SA, or using mechanical frothing, respectively. The main raw materials, the curing conditions, the type of alkaline activator (potassium-based or sodiumbased) and the bulk density of the components are also reported. As it can be seen from Tables 1–3, porous geopolymers were produced by using only a blowing agent, or by using both a blowing agent and a stabilizing agent, or only a stabilizing agent. The most widely used raw materials were fly ash and metakaolin, while different compounds were employed as blowing agents (mainly H2O2, Si, Al) and surfactants (ionic, anionic, protein-based). Different curing steps were adopted, but the most common choice was to keep the samples in an oven at 60–80 °C for 24 h. It should be noted that the pore structure and corresponding properties of porous geopolymers is not only determined by the type and amount of the blowing agents (BAs) or stabilizing agents (SAs), but is also strongly affected by additional parameters such as the viscosity of the slurry, the composition of the geopolymers, the mixing speed (for frothing), etc. Unfortunately, no comparative studies have been carried out so far, for instance, to assess the influence of the different processing approaches while maintaining constant the composition of the geopolymer slurry, and therefore a direct and definitive comparison between the data is not possible. Nevertheless, some general trends can be ascertained. The typical microstructure of porous geopolymers obtained by using only a blowing agent, or only a stabilizing agent or by using both are 3


C. Bai, P. Colombo

Table 1 Processing of highly porous geopolymers using the direct foaming method (chemical blowing: BAs without stabilizing agents). Blowing Agents (BAs)

Alumino-silicate sources

Alkali ions

Curing

Bulk density (g/cm3)

Refs

Al Al Al Al Al Al Al, RAF Al Al Al+AlN Al Si Si Si Si SF SF SF SF SF SiC Silicon sludge NaOCl Sodium perborate AlN+FeSO3 AlN+FeSO3 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 + KMnO4

MK FA FA MK, RHA, VA MK, FA, RHA FA FCC MK, FA FA MK, Glass, ASRW, SPW. Red mud, MK MK MK, FA MK MK, (Na2SiF6) MK MK, Clay MK MK MK FA Slag, Silicon sludge FA FA, Sand, Calcium hydroxide NMP, LSG, MK Clay Perlite MK MK, FA FA, Sand, Calcium hydroxide FA MK, Biomass Ash Soil Slag FA MK, Polystyrene FA, Iron ore tailing MK, Waste glass

Na Na Na Na K Na Na Na Na Na Na K Na K Na K K K/Na K K/Na Na Na Na Na Na Na Na Na/K Na Na Na/K K Na Na Na Na Na Na

RT (S-14d) RT (S-24 h), 60 °C (24 h), RT (24 h) 22 °C (2 h), 80 °C (12 h) RT RT, 50 °C (S-(1–28d)) 60 °C (S-24 h) 23–65 °C (4 h-6d) 20, 75 °C (24 h), RT (1–180d) 60–90 °C (24 h) 70–87 °C (6 h or S-2 h) 40 °C (S-7d), RT (1–28d) RT-80 °C (24 h) RT (28d), 60 °C (24 h) RT(24 h), 80 °C (S-24 h, 24 h) 40 °C (24 h),60 °C (72 h) 70 °C (4 h) 70 °C (4 h) 70 °C (4–72 h) RT-70 °C (0.5 h−20d) 70 °C (24 h), RT (24 h) RT (30d) 70 °C (S−24 h), RT (3d) 30–90 °C (S-4d) RT (28d) 40–100 °C (24 h) 80 °C (24 h) 35 °C (2 h), 65 °C (24 h) 35 °C (2 h), 65 °C (24 h) 40 °C (S-8d), RT (21d) RT (28d) RT (24 h), 55–85 °C (24 h) 60 °C (S-24 h;12 h) 60 °C (S-24 h), 220 °C (4 h) 25 °C (S-48 h); 85 °C(hydrothermal- 24 h); 85 °C (24 h) RT (24 h), 50 °C (S-10 h), RT (7d) RT (24 h), 50 °C (S-10 h), RT (7d) RT (S-24 h) 30 °C (24 h), RT (1–60d)

– 0.4–1.3 0.4–08 – – 0.6–0.9 0.6–0.8 0.6 0.7–1.0 0.4–0.5 0.5–1.3 0.3–0.9 0.7–0.8 0.6–0.9 1.0 0.5 – – 0.3–0.9 0.4–0.6 – 0.1–0.4 0.8 0.7–1.3 0.5–0.6 0.5–0.7 0.3–0.7 0.3–0.6 0.6–1.2 0.7–1.4 0.2–0.4 0.3 0.9–1.1 0.9–1.0 0.5–1.6 0.1–0.4 – 0.5–1.4

[40] [43] [14] [61] [62] [63] [64] [65] [66] [67] [68] [45] [69] [70] [71] [48] [72] [73–75] [76–79] [80] [46] [81] [49] [50] [51] [52] [7] [12] [13,21,82,83] [50] [84] [85] [86] [87] [88] [89] [90] [91]

(References = Refs, Metakaolin = MK, Fly ash = FA, Room temperature = RT, Sealed = S, Rice husk ash = RHA, Volcanic ash = VA, Fluid catalytic cracking catalyst residue = FCC, Recycled aluminum foil = RAF, Sodium lauryl sulfate = SLS, Aluminum scrap recycling waste = ASRW, Steel-plant waste = SPW, Nonmetallic product = NMP, Lead–silica glass = LSG, Sodium dodecyl sulfate = SDS, Sodium dodecyl benzene sulfonate = SDBS).

2.3. Sacrificial filler method

(frothing) route enabled the production of phosphate geopolymer foams with a total porosity of 78.3 vol% (open porosity 76.8 vol%), average cell size about 280 µm, and possessing a compressive strength of 0.64 MPa.

The sacrificial filler method (SFM), leading to cellular materials with a structure that is the negative replica of the original template, is opposed to the positive porous structure obtained by the replica method. The porosity is generated by extracting the fillers, either by dissolution, melting or thermal decomposition, from a dense biphasic composite, comprising a continuous geopolymer matrix and a dispersed sacrificial phase, that could be interconnected or not. The way that the sacrificial material is extracted from the consolidated composite depends primarily on the type of pore former used and its interconnectivity. A wide variety of sacrificial materials could be employed as templates. Papa and coworkers [113,114] produced porous geopolymers with mesoporous matrices and lamellar macro-porosities by an ice-templating (freeze-casting) method. Fig. 5 shows the highly porous monoliths possessing lamellar pores with a thickness of about 50 µm, randomly oriented parallel to the freezing direction. The final components possessed a hierarchical pore structure with 53–83 vol% total porosity, depending on the water content. Franchin et al [115] developed a new processing method, based on 3D printing, for fabricating macroporous geopolymers with controlled and designed porosity. In the process, PLA (polylactic acid) sacrificial templates (molds) with different patterns were firstly produced by a 3D printer, a homogeneous geopolymer slurry was then poured into the molds under vacuum (~0.1 Pa) conditions and, after the curing step (48–72 h at room temperature), the PLA/geopolymer composites were

2.2. Replica method The replica method, dating back to the early 1960s, allows to retain the cellular structure of the polymeric template employed, and is one of the most used approaches for the industrial fabrication of ceramic foams (especially molten metal filters), because it is cheap and enables the production of components with a wide range of pore sizes. However, only a few reports exist concerning the use of this method for the fabrication of porous geopolymers. Kovářík et al [112] used a 10 pores-per-inch polyurethane foam as a template, which was infiltrated with an aqueous potassium-based geopolymer slurry, obtaining a geopolymer/polyurethane porous composite after the drying step. Cellular geopolymers with a large amount of open porosity (~ 79 to ~ 88 vol%) and reasonably good compressive strength (~ 0.15 to ~ 0.85 MPa) were produced after sintering at 1100–1300 °C for 4 h. The schematic fabrication route is presented in Fig. 4. It should be noted that the final samples (after the sintering step) cannot be classified as geopolymers, but should rather be considered as porous ceramics.

4


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Table 2 Processing of highly porous geopolymers using the direct foaming method (chemical blowing: BAs with stabilizing agents or thickening agents). Blowing Agents (BAs)

Stabilizing agents or thickening agents


Alkali ions

Curing


Refs

Al, SF, SiC, FeSi Al Al Al

FA MK,FA FA FA

Na Na Na Na

80 °C 70 °C 70 °C 70 °C

0.5–1.0 0.8–1.1 0.8–1.4 0.6–0.8

[44] [92,93] [94] [95]

Al

Portland cement, Lime VMPFs Sika Lightcrete 02 Commercial additives (no composition given) Micro Air 210, oleic acid, SLS

Slag

Na

0.7–1.2

[54]

Si Si

Protein Protein

Na Na

0.29 0.3–0.4

[71] [96]

Si SiC

Epojet®,Globasil AL20 Carbon fibers, Rice starch, Cellulose fibers Protein SDS Sika® Lightcrete 02 Commercial additives (no composition given) SDBS+Triethanolamine Oleic acid SDS Tween 80 SDBS+ 0.8% Triethanolamine A commercialSurfactant (no composition given) A commercial surfactant (no composition given)

MK, (Na2SiF6) MK, Diatomite, (Na2SiF6) MK MK

25 °C (2 h or S-2 h); 25 °C to 70–87 °C (3 h or S2 h); 70–87 °C (6 h or S-2 h); RT (3 h or S-2 h) 40 °C (24 h),60 °C (72 h) 40 °C (24 h)

Na K

RT (24 h), 60 °C (24 h), RT (27d) 70 °C (S-72 h)

0.3–0.9 0.3–1.1

[97] [47]

MK MK FA FA

K Na Na Na

RT (~24 h), 75 °C (S-24 h) 60 °C (24 h) 70 °C (S-24 h) 70 °C (24 h)

0.4–0.8 0.8 0.7–1.4 0.6–1.0

[19] [23] [94] [95]

FA FA FA MK FA MK, Slag, Soda

Na Na Na K Na Na

70 °C 80 °C 70 °C 40 °C 70 °C 20 °C

0.3–1.6 0.4 0.6–1.3 0.3–0.8 0.2–0.3 0.3–0.5

[98,99] [100] [101] [102] [103] [104]

MK, FA, Soda

Na

20, 40 °C (S-(15 min, 3 h, 24 h)), 20 °C (1–28d)

0.2–0.5

[105]

H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2

(12 h) (S-24 h) (S-24 h) (24 h)

(S-24 h) (10 h) (24 h), 20 °C (3d) (~24 h), 75 °C (S-24 h) (S-24 h), RT (7d, 28d) (S-24 h), (1–28d)

(References = Refs, Fly ash = FA, Metakaolin = MK, Room temperature = RT, Sealed = S, Virgin monofilament polypropylene fibers = VMPFs, Sikas Lightcrete02 = Surfactant with 40 wt% solution of fatty acid, amide and sodium salt of C14–C16 sulphonic acid in water, Globasil AL20 = Oligomeric dimethylsiloxane mixture, Epojet® = epoxy resin, Sodium dodecyl sulfate = SDS, Sodium dodecyl benzene sulfonate = SDBS).

which cannot be obtained by traditional processing technologies, with complex, non stochastic porosity and precise control on pore dimension, shape, and amount [116]. Franchin et al [117] used Direct Ink Writing (DIW) to fabricate porous Na-based geopolymer scaffolds. Fig. 6 shows images of a geopolymer lattice with a gradient in spacing between the struts. As it can be seen, no cracks or surface defects were visible in the structure. Scaffolds with high porosity, ranging from 50 to 71 vol%, and high compressive strength (2–12 MPa) were obtained. Kbased porous geopolymer scaffolds were produced as well, by the same group, using DIW [102]. Porous geopolymer nanocomposite structures containing graphene oxide were also produced by the DIW technique [118]. A significant difficulty with this approach(DIW) is that the paste

immersed in 15 M KOH solution (72 °C for 24 h) to dissolve and extract the PLA material. The elimination of the template was completed by a low temperature heat-treatment (330 °C for 24 h), resulting in porous geopolymers with interconnected macro-porosity in the range ~66–71 vol%. 2.4. Additive manufacturing Recently, additive manufacturing (AM), often called 3D printing, has successfully been employed to fabricate a wide range of porous components (such as scaffolds, filters, lightweight materials), taking advantage of the fact that AM technologies can produce structures

Table 3 Processing of highly porous geopolymers using the direct foaming method (mechanical frothing). Surfactants


Alkali ions

Curing


Refs

Diluted aqueous surfactant (no composition given) Synthetic organic surfactant (no composition given) SDS Diluted aqueous surfactant (no composition given) Diluted aqueous surfactant (no composition given) Tween 80 (Triton x100; Tween 80) with polyacrylic acid Protein Protein Protein with EAC; SLS

FA, Slag

Na

40 °C (S-24 h), RT (27d)

0.7–1.6

[27]

FA, Slag

Na

40 °C (S-24 h), RT (90d)

–

[56]

Slag MK, Slag

Na Na

60 °C (S-24 h), RT (S-(7–28d)) RT (28d)

0.4–0.8 0.4–1.0

[57] [58]

Reservoir sludge, Blast furnace, slag MK MK, FA

Na

RT(7–91d)

0.7–1.0

[59]

K K

40 °C (~24 h), 75 °C (S-24 h) 80 °C ((S-1 h), (4 h))

0.7 0.5–0.9

[102] [55,106]

MK, (Na2SiF6) MK Slag

Na K Na

0.5 0.9 1.0

[71] [19] [87]

FA

Na

40 °C (24 h), 60 °C (72 h) RT (~24 h), 75 °C (S-24 h) 25 °C (S-48 h); 85 °C (hydrothermal−24 h); 85 °C (24 h) 70 °C (S-24 h)

1.0–1.2

[94]

Sika Lightcrete 02

(References = Refs, Fly ash = FA, Metakaolin = MK, Sodium lauryl sulfate = SLS, Enzymatic active components = EAC, Sodium dodecyl sulfate solution = SDS, Surfactant with 40 wt% solution of fatty acid, amide and sodium salt of C14–C16 sulphonic acid in water = Sikas Lightcrete02). 5


C. Bai, P. Colombo

Fig. 3. Typical microstructure of porous geopolymers produced by using only a chemical blowing agent ((a) H2O2, [19]; (b) Si, [71] (c) Al, [94]); by using only a stabilizing agent (surfactant) ((d) egg white [19], (e) vegetable protein [71], (f) Sikas Lightcrete02 [64]); by using both a chemical blowing agent and a stabilizing agent ((g) H2O2 + egg white [19], (h) Si+vegetable protein [71], (i) Al+Sikas Lightcrete02, [94]). (Reprinted from Bai et al [19], copyright (2017), with permission from Elsevier; Reprinted from Verdolotti et al [71], copyright (2014), with permission from Springer; Reprinted from Masi et al [94], copyright (2014), with permission from Elsevier).

Fig. 4. The schematic diagram of the fabrication process of porous geopolymers by templating (with final conversion to ceramic foams) [112]. (Reprinted from Kovářík et al [112], copyright (2017), with permission from Elsevier).

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Fig. 5. Freeze-cast geopolymer samples (50 vol% water). (a) General morphology (samples were 25 mm in height and 10 mm in diameter); (b, c) SEM micrographs of the lamellar porosity (top surface) [113]. (Reprinted from Kovářík et al [112], copyright (2017), with permission from Elsevier).

Fig. 6. Images of a geopolymer graded lattice structure fabricated using Direct Ink Writing: (a) general view; (b) top view; (c, d) side views [117]. (Reprinted from Franchin et al [117], copyright (2017), with permission from Elsevier). 7


C. Bai, P. Colombo

Fig. 7. The mechanism for the generation of hierarchical porosity by direct foaming/reactive emulsion templating combined route.

stability. Medpelli et al. [122] developed an innovative reactive emulsion templating method to produce porous geopolymers with high specific surface area (SSA) up to 124 m2/g. The alkaline geopolymer slurry was first mixed with oil to form a homogeneous viscous emulsion that, after casting in molds, was cured at 60 °C. During the curing step, the saponification reaction between the oil and the alkaline solution occurred, with the production of soap molecules and glycerol. These soluble reaction products were then extracted by hot water, leading to components possessing variable specific surface area values, depending on the type of oil employed in the process. Based on this work, Cilla et al. [123,124] proposed a novel combined route by exploiting the formation of surfactant molecules in the saponification reaction to generate macro-pores. In their work, geopolymer foams with a hierarchical total porosity of ~ 85 vol%, open porosity as high as ~ 70 vol %, average cell size (D50) of 318 µm and SSA up to 60 m2/g, were obtained by a saponification/peroxide combined route, with H2O2 as the blowing agent. In comparison to components produced using only peroxide or only frothing, the combination of frothing and the in situ formation of a surfactant (soap molecules) via the saponification reaction led to foams possessing a larger amount of interconnected porosity and higher permeability to gas. Fig. 7 shows the multiscale pore generation mechanisms for the saponification/peroxide combined route [123–125]. Therefore, developing macro-porosity in geopolymer systems allows for the fabrication of components with hierarchical porosity, with pores ranging from the nm to the mm size and high specific surface area, simultaneously possessing high permeability, high capillarity, high water and gas adsorption ability making them suitable for a wide range of applications, including evaporative cooling, catalysis, gas adsorption, water purification, drug delivery, etc. Another approach that has been proposed for producing porous components is using hollow spheres as pore formers. Li et al. [126,127] developed parts using fly ash hollow spheres and a phosphate geopolymer as a binder, with a total porosity of 75 vol%, open porosity as high as 48 vol%, and possessing a compressive strength of 5.8 MPa. Besides of the fabrication of monoliths, geopolymer formulations can be used to produce other forms of highly porous components. For instance, porous spheres were obtained by a suspension and solidification method [23,128]. A homogeneous foamed geopolymer slurry

(ink) used for the fabrication of the structures is based on a reactive system, and therefore its rheological characteristics vary with time. Despite being able to develop formulations capable of maintaining an appropriately constant value for the viscosity for a period of time as long as 1 h [102], more effort should be devoted to finding appropriate retardants for the geopolymerization reaction, to achieve a longer printing time window. Finally, Xia and Sanjayan [119] reported on the fabrication of porous Na-based geopolymer components, for potential building applications, using a powder-based AM technique. Porous structures, with an apparent porosity of about 57 vol%, were produced by selectively joining geopolymer particles with an organic binder sprayed by a printing head. The porosity was randomly distributed throughout the body, and resulted from the voids existing between adjacent geopolymer particles. The resulting compressive strength was quite limited (about 0.9 MPa), but could be increased by post-fabrication infiltration with a sodium silicate solution.

2.5. Other methods As mentioned before, the microstructure of a fully reacted, bulk geopolymer body is intrinsically micro- and meso-porous (with typical pore size ranging from ~ 3 to ~ 20 nm), since it is based on the aggregation of alumino-silicate particles with size ranging from 5 to 40 nm [70,120]. The amount of this intrinsic porosity depends on the composition of the system, mainly the SiO2/Al2O3 or H2O/MO2 (M = K, Na) molar ratios, and can range from ~ 30 to ~ 60 vol%. Ge et al [18] used a designed molar ratio (SiO2/Al2O3 = 2.96, Na2O/Al2O3 = 0.8 and H2O/Na2O = 18–23) to obtain porous geopolymers without any addition of pore-forming agents. The samples possessed a total porosity of ~ 63 vol%, with pore sizes ranging from 10 to 1000 nm, and compression strength was ~ 18 MPa when H2O/ Na2O = 19. Glad and Kriven [121] developed a templating and surface interaction-based method for producing geopolymers with tailored porosity (> 70 vol%) and pore size (0.2–10 µm). A hydrophobic film was firstly formed on the surface of pores using alkyl-alkoxysilanes, which allowed for drainage without pore collapse; the porosity and pore size were tuned by manipulating the initial water content, the amount of hydrophobic phase, the drying humidity and the emulsion 8


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Table 4 Porosity, compression strength and thermal conductivity of porous geopolymers produced using different routes. Foaming methods

Pore size (μm)

Total porosity (vol%)

Thermal Conductivity (Wm−1 K−1)

Compression strength (MPa)

Flexural strength (MPa)

Refs

DF DF DF DF DF DF DF DF DF DF DF DF DF DF DF DF DF DF DF DF DF DF DF DF DF DF DF DF DF DF SFM AM OM OM OM OM OM DF-P DF-P DF-P OM-P

≤3000 440–920 ≤2000 ≤4000 50–150 ≤150 ≤300 ≤100 – – ≤2690 ≤3500 ~595; ~890; ~373 – – – 100–1600 ≤2100 ≤3000 ≤3000 – – ≤2000 ≤800 – – ≤150 ≤1000 – 4–33 – – 0.3 – 318 ~247-~760 130–490 280 – – 99

74–89 56–75 51–73 62 58 30–70 32–52 35–62 80–83 72–79 60–90 56–66 75;60;69 83–86 60–86 82–85 65–85 75–85 48–81 41–75 51–82 74–81 55 74–87 87 54–63 68 61–78; 60–74 42–73 66–76 66 50–71 63 50 84 84–88 67–89 78 40–83 69–75 75

0.03–0.06 0.15–0.17 0.10–0.40 0.15 – 0.15–0.60 0.42–0.67 – 0.14–0.15 – 0.10–0.25 – 0.62;0.70;0.58 0.14–0.15 – – 0.12–0.35 0.12–0.17 0.08–0.2 – 0.43–0.48 0.07–0.09 – 0.09–0.15 0.07 0.27–0.35 – – 0.08–0.09 0.07–0.09 – – – – – – – – – 0.07–0.09 –

0.2–0.8 1.8–5.2 1.2–6.6 6.0 21 – 1.2–4.1 3.1–3.3 1.1–2.3 1.4–3.8 – 0.4–1.6 1.8;3.6;3.5 1.1–2.0 0.7–5.8 0.6–1.5 – – 0.3–21 0.2–9 3.1–5.3 0.4–1.4 2.6 0.3–4.4 0.6 0.86–1.1 11.0 0.4–2.8; 1.0–3.4 3.0–6.2 1.2–3.5 8.5 2.0–12 18 6.0 0.5 0.2–0.4 0.4–8.8 0.6 6.8–11 – 5.8

– 0.5–1.4 – 1.0 – – – – 0.1 0.6–2.1 – – – 0.2–0.9 – 0.2–0.6 – – – – 0.4–0.5 – – – – – – – – – – – – – – – – – – – –

[7] [12] [13] [14] [19] [40] [46] [49] [51] [52] [61] [63] [64] [67] [68] [96] [77–79] [80] [82] [83] [91] [84] [101] [102] [85] [86] [102] [55,106] [107] [108] [115] [117] [18] [39] [123] [124] [125] [6] [110] [111] [127]

(References = Refs, Direct foaming = DF, Sacrificial filler method = SFM, Additive manufacturing = AM, Other methods = OM, Phosphate-based geopolymers using direct foaming = DF-P, Phosphate-based geopolymers using other method = OM-P).

was firstly obtained using 30% hydrogen peroxide and sodium dodecyl sulfate; secondly, the slurry was dripped into a warm polyethyleneglycol liquid medium to obtain porous geopolymeric spheres; the spheres were then collected, washed, and cured in controlled conditions. Geopolymeric spheres with bulk density about 0.8 g/cm3 and with total porosity about 60 vol% were produced [23]. Granulated metakaolin geopolymers with porosity about 58 vol% and particle sizes about 1–6 mm were obtained by a high shear granulator [129]. Geopolymer porous aggregates, with a dimension ranging from ~ 50 nm to ~ 1 µm, were obtained based on the aggregation of aluminosilicate nanoparticles with an average size ranging from about 5 nm to about 60 nm. The pores between the nanoparticles in the porous geopolymer aggregates had a size width between about 2 nm and about 100 nm, giving materials with a SSA of a few hundred m2/g [130].

Ryshkewitch [134] and further developed and perfected by Rice [136,137], was extensively applied to study the strength-porosity behavior. The model, also known as the minimum solid area (MSA) model, can be approximated as follows:

σ = σ0 exp(−bp)

(7)

where σ and σ0 are the strength of a material with total porosity p and of the same material without porosity, respectively, and b is a constant that is dependent on pore characteristics. A collection of data for the compression strength as a function of total porosity, for porous geopolymers with various compositions and obtained via different processing routes, is shown in Fig. 8. While we can state that several works have confirmed that the compression strength of porous geopolymers can be appropriately expressed according to an exponential increase with the decrease of the total porosity, at a fixed composition and under the same processing conditions (see Figs. 9) [12,19,68,102,117,125], we should observe that no discernible trend can be directly extracted from Fig. 8. The fact that the samples had various compositions [51,52,67,86,91,96], different degrees of geopolymerization [7], different NaOH/Na2SiO3 ratios [63], different water contents and NaOH molarity [82], different curing temperatures [84], or were fabricated using different processing parameters [55,56,106,107], makes the data not directly comparable and, unfortunately, prevents from reaching definite conclusions concerning, for instance, the effect of using

3. Mechanical and thermal insulating properties In Table 4 are reported the mechanical strength(compression and flexural strength) and thermal conductivity of porous geopolymer monoliths produced using different routes. The relationship between strength and porosity has been investigated by several authors, with the proposal of several models [131–137]. A simple equation (Eq. (7)), firstly proposed by 9


C. Bai, P. Colombo

Fig. 10. Flexural strength vs. total porosity for PGs produced by direct foaming. Data points are labeled with the corresponding reference numbers. Fig. 8. Compression strength vs. total porosity for PGs produced using different fabrication routes. Data points are labeled with the corresponding reference numbers.

In Fig. 9 we report the fitting of experimental data, using Eq. (7), for PGs fabricated using different processing routes (direct foaming [12,19,68,102], AM [117], other methods [125]). A very good correlation coefficient for the fitting can be observed. It should again be noted that the samples had the same composition only within the same dataset, so a direct comparison of the influence of the different foaming approaches cannot be made. A collection of data for the flexural strength as a function of total porosity, for porous geopolymers with various compositions, is shown in Fig. 10. Literature data exist only for PGs produced using direct foaming. For samples having the same composition, the relationship between flexural strength and porosity can be correlated using the MSA model (b = 4.62; R2 = 0.83) [12]. Data for samples of different composition cannot be directly fitted using a single formula [51,52,67,91,96]. The thermal conductivity also decreases with increasing porosity (see Fig. 11), as expected in general for porous materials of different nature (i.e. polymers, metals and ceramics). Different analytical relations have been proposed to account for this trend, but the choice of the proper model depends on several morphological and microstructural factors [141]. There are not sufficient data in the literature to comment on the effect of different processing methods on the relationship between thermal conductivity and porosity, at least for geopolymers. We can note, however, that no significant differences between acid-based geopolymers and alkali-based porous geopolymers can be observed. In summary, the mechanical strength and thermal conductivity of

different foaming approaches. In any case, the data indicate that by varying several parameters, it is possible to fabricate geopolymers possessing a wide range of porosity and compression strength values. Phosphate-based porous geopolymers possessed a higher strength than alkali-based geopolymers at a given porosity (except for samples obtained by frothing using Triton X-100 as stabilizing agent) [6], in accordance with their different network architecture and composition [138]. It should be also noted that the pore size has a significant effect on the compression strength of porous PGs, similarly to what observed for porous ceramics and attributed to the change in the critical flaw population, with a linear increase in strength with decreasing pore size for a given total porosity [139,140]. The data reported in the literature, confirm a similar trend; PGs with ~ 61% porosity had a compression strength of 3.7 MPa when the average macro-pore size was ~ 609 µm, while the strength increased to 7.8 MPa when the average macro-pore size was ~ 266 µm [54]. Samples possessing a total porosity ~ 82 vol% produced using canola (~ 339 µm, ~ 2 MPa) or sunflower oil (~ 391 µm, ~ 2 MPa) had lower compression strength than those produced using olive oil (247 µm, ~ 3 MPa) [125]. The different oils used, however, could have affected the size of the micropores in the cell walls.

Fig. 11. Thermal conductivity vs. total porosity for PGs produced using different fabrication routes. Data points are labeled with the corresponding reference numbers.

Fig. 9. Compression strength vs. total porosity for PGs produced using different fabrication routes [12,19,68,102,117,125]. 10


C. Bai, P. Colombo

porous geopolymers are strongly dependent on the amount of total (open and closed) porosity as well as on the amount of micro- and meso-pores in the matrix (struts and cell walls), the average pore size and size distribution and the composition of the material. The maximum values reported in the literature for the compression and flexural strength were of ~ 26 MPa (~ 62 vol% porosity, reactive emulsion templating [125]) and ~ 2.6 MPa (~ 63 vol% porosity, H2O2 as chemical blowing agent [91]), while the lowest thermal conductivity was 0.03 W/(m K) (~ 63 vol% porosity, H2O2 as chemical blowing agent,) [7]. However, there are not sufficient data to evaluate the influence, for a given porosity, of the processing method used to fabricate the porous components, considering that too many other factors contribute to these properties. More detailed investigations should be carried out to further elucidate the role of the different parameters on the relationship between strength and thermal conductivity and fabrication methods. Fig. 13. Linear shrinkage of different types of porous geopolymers after thermal treatments at 600 °C, 800 °C, 1000 °C, and 1200 °C: (a, b,c-d) metakaolin-based PGs ( [6,19,70]); (e) fly ash-based PGs ([103]); (g) metakaolin/blend biomass ashes-based PGs ( [85]); (h-k) metakaolin/fly ash-based PGs ([106]); (l) metakaolin/Al2O3-based PGs ([110]).

4. Other properties and potential applications Figs. 8–11 showed that porous geopolymers generally possess low thermal conductivity and acceptable compressive strength; therefore, they have a potential for use as building materials, especially taking into consideration their other favorable characteristics, such as being fiber-free and retaining structural integrity during and after exposure to fire [17]. The problem of the formation of surface efflorescence reported by some authors, which is more aesthetic than functional, could be addressed by enhancing the stabilization of Na ions in the geopolymer network (e.g. by adding calcium aluminate species in the formulation) [17]. Fig. 12 reports the variation of compression strength (post-cooling mechanical strength tests) in some typical porous metakaolin-based [6,19], fly ash-based [14,44], metakaolin/Al2O3based [110], and metakaolin/fly ash-based [124] geopolymers after high temperature exposure. The mechanical strength of PGs increased after the high temperature treatment, which is an excellent feature for fireproof construction materials. Additionally, a relatively limited thermal shrinkage was observed up to at T < 600 °C, in agreement with the limited weight loss shown by thermogravimetric analysis [6,14,19,85,106], while a larger shrinkage during or after exposure to high temperature is always present (T > 600 °C), due to the removal of meso-porosity caused by viscous flow and crystallization of the matrix. However, the introduction of thermally-stable fillers could limit this effect. Fig. 13 shows the linear shrinkage in typical metakaolin-based [6,19,70], fly ash-based [103], metakaolin/blend biomass ashes-based [85], metakaolin/fly ash-based [106], PGs as a function of the heat treatment temperature. It

should be noted that acid-based porous geopolymers showed much lower shrinkage than alkali-based PGs between 600 and 1200 °C [6,110], and that the phase transformation temperature resulting in an abrupt volume decrease was much higher for acid-based geopolymers (~ 1500 °C)[110] than for akali-based geopolymers (~ 800–1200 °C) [19,70,103]. A maximum linear shrinkage of ~ 30% at 1200 °C was observed for samples produced using H2O2 as blowing agent combined with egg white as stabilizing agent. Besides thermal insulation applications, another promising application for porous geopolymers is as adsorbent components. The rationale for using geopolymers as sorbents, for instance as functional water treatment materials, is based on their cation exchange properties [142]. Compared with conventional powdered or packed-bed adsorbents, porous monoliths (such as foams, honeycombs or scaffolds) display enhanced performance as they reduce pressure drop, increase contact time and improve thermal/mass transfer, and they can be easily collected and recycled [143,144]. The removal of Ni2+ by a low cost self-supporting metakaolingeopolymer membrane, with a total porosity of 62 vol% and produced without blowing agent, was investigated by Ge et al.[18] Novel porous fly ash-containing porous geopolymer monoliths produced using H2O2 as blowing agent were tested for lead adsorption by Novais et al.[21], see Fig. 14. Floatable and permeable porous fly ash-based porous geopolymer blocks, obtained using H2O2 as chemical blowing agent combined with oleic acid as stabilizing agent (SA), were used for removing methylene blue [100]. Bai et al [102] tested high-porosity metakaolinbased porous geopolymers for adsorption of copper and ammonium ions (NH4+). Metakaolin-based geopolymer granules with total porosity about 58 vol% were successfully formed using a high shear granulator [129]. They also showed a promising potential application for removal and possible recovery of ammonium ions from municipal wastewaters. Porous geopolymers obtained using metakaolin and rice husk ash silica as raw materials, foamed using water vapor, were employed for the removal of cesium ions [22]. Porous geopolymer spheres were tested for adsorption of Pb, Cu, Ca ions by Cui and co-authors [23,128]. Porous geopolymers with fly ash and iron ore tailing as raw materials, foamed using only using H2O2 as blowing agent, were studied for removal of Cu ions by Duan et al. [90] Data show that the dosage of geopolymer, the initial concentration of Cu2+, pH, contact time and temperature were the main factors affecting the adsorption capacity. After ion exchange with copper ions, samples showed a good antimicrobial activity [145]. Iron oxide-modified, hierarchically porous geopolymers for arsenic removal were discussed by Medpelli et al. [24] The adsorption performance of porous geopolymers with respect to

Fig. 12. Compression strength of different types of porous geopolymers before (NH: non-heated samples) and after exposure to high temperature: (a, b) metakaolin-based PGs ([6,19]); (c, d) fly ash-based PGs ([14,44]); (e) metakaolin/ fly ash-based PGs produced using reactive emulsion templating method ([124]); (f) metakaolin/Al2O3-based PGs ([110]). 11


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Fig. 14. Porous geopolymer monolith produced using 1.2 wt% H2O2 as chemical blowing agent, and the EDS (Energy Dispersion Spectroscopy) map of its surface after lead adsorption [21]. (Reprinted from Novais et al [21], copyright (2016), with permission from Elsevier). Table 5 Adsorption capacity of porous geopolymers for different cations. Ion

C0 (ppm)

T(°C)

pH

Time(h)

Test method

R (%)

qmax (mg/g)

Refs

Ni2+ Pb2+ Pb2+ Cu2+ Cu2+ Cu2+ Cu2+ Cs+ As3+ Ca2+ NH4+ NH4+

300–700 50 100 3 120 100 100–200 50–500 0.12 100 3 ~36

10–50 RT 25 RT 25 25 20–40 30 – 25 RT –

6–13 5–10 5 – 5 5 1–6 7 7.6 5 – –

– 3–24 2–60 0.5–60 48 2–60 0.2–3.5 6 72 2–60 24 0–6

Filtration membrane Batch Batch Batch Batch Batch Batch Batch Batch Batch Batch Column

75–97 – – 13–87 – – 5–99 18–96 – – 95 25–65

45 6.3 45 0.5 53 35 113 51 0.9 25 0.6 –

[18] [21] [128] [102] [23] [128] [90] [22] [24] [128] [102] [129]

(References = Refs, Room temperature = RM).

monolithic filters, porous beads for packed beds or self-supported inorganic membranes that are easily collected and recycled when exhausted, which is a major advantage in comparison with the use of powdered adsorbents. The geopolymers could also be reactivated by putting them in contact with a solution containing Na or K ions, and in some cases (e.g. NH4+) the adsorbed ions could be recovered and used for some other purposes. Porous fly ash-containing geopolymers with high pH buffering capacity and tailored alkali leaching were produced by direct foaming using H2O2 as chemical blowing agent [83]. Data showed that the pH buffering ability was not strongly dependent on the porosity, but was affected by the solid/liquid ratio. Additionally, the same group produced porous geopolymeric spheres through a suspension and solidification method, showing a gradual and prolonged leaching which reduced the pH fluctuation over time [146]. The CO2 adsorption capacity and selectivity of porous GPs, produced using fumed silica as blowing agent, was investigated by Minelli et al.[20] The samples possessed a promising adsorption capacity of about 0.6 mmol/g at atmospheric pressure. Furthermore, the CO2/N2 (~ 200) and CO2/CH4 (~ 100) selectivity was excellent. Besides adsorption applications, the porous architecture of geopolymers lends itself for applications as catalyst support, and loading of the catalyst in the structure can be carried out either by directly adding the active material to the slurry before foaming, or by impregnation or ion exchange of the formed porous body. TiO2 as photocatalyst was added to GP formulations that were then foamed, and the resulting photocatalytic performance (NO degradation in air) showed a strong variation depending on both matrix constituents and the curing

selected ions is reported in Table 5. The table lists the major factors affecting the removal efficiency (R) and adsorption capacity (q), such as: initial concentration of ions (C0), temperature (T), pH, contact time. R and q values were calculated according to the following formulas [102]:

R(%) = (C0 − Ct)/C0

(8)

q(%) = ((C0 − Ct) × V)/m

(9)

where C0 is the initial concentration of the test ions in the solution and Ct is the concentration at time t (batch tests); V is the volume of solution (L) and m is the mass of porous adsorbent (g). It should be noted, however, that other factors exist that affect the adsorption capacity: porosity [21,102], dosage of PGs [90], composition [24]. The published data demonstrate that PGs are very effective sorbents for heavy metal ions, such as Ni, Pb, Cu, Cs, as well as ammonium and Ca cations. Data suggest that in the geopolymer structure there is availability of numerous active sites for binding with various ions and that, in the case of Cu2+, the results of a study on the adsorption kinetics suggest that the ion adsorption process consisted of chemical adsorption and physical adsorption [128]. The limited adsorption capacity (q) observed in some studies for certain ions, is probably attributable to their very low starting concentration in the initial solution (arsenic, 0.12 ppm [24], ammonium, 3 ppm [102], copper, 3 ppm [102],). In summary, porous geopolymers can be considered as a new type of low-cost adsorbing materials, exhibiting good removal efficiency and uptake capacity for different cations. The PGs can be employed as 12


C. Bai, P. Colombo

showed that oil waste can be immobilized in an alkali-based geopolymer, simultaneously generating a porous structure. Since porous geopolymers possess a high geometric surface area, they are obviously more prone to interaction with the surrounding environment than bulk components. The durability of porous geopolymers was investigated by several authors [14,52,75,92]. The change in mass and compressive strength of porous geopolymers after exposure to a corrosive environment was monitored, in order to characterize their resistance to sulfate attack. Furthermore, their freeze-thaw resistance was determined by freezing water-saturated samples at − 18 °C for 12 h and deicing them in water for 4 h. Reported data indicate that both the durability and freeze-thaw resistance depended on the selected raw materials, the alkali element used for the synthesis of the geopolymer and the porosity of the component [52]. The stability in solutions at different pH values (from 2 to 12) was shown to be strongly influenced by the alkali element (K or Na) used for the synthesis of the foam [75]. Finally, besides the above-mentioned potential applications, porous geopolymers have also been tested for oral drug delivery [39]. Casting a geopolymer solution into a mold produced pellets with a porosity of 18–50 vol%, as measured by helium pycnometry, and a compression strength of 1–100 MPa. The characteristics of the material were varied by adjusting the SiO2 and water contents, providing components with prolonged release dosage and a satisfying initial release of 10–85% of the active pharmaceutical ingredient (Zolpidem tartrate) within 10–24 h.

Fig. 15. Sound absorption coefficient of porous geopolymer samples as a function of frequency: samples with the same composition but different bulk density [58]. (Reprinted from Hung et al [58], copyright (2014), with permission from Elsevier).

temperature [69]. A hierarchically porous geopolymer was impregnated with Ca2+ ions and used for the catalytic transesterification of soybean oil, using methanol as a solvent, achieving almost 100% conversion into biodiesel in one hour under refluxing conditions [25]. For adsorption applications, a large amount of open, interconnected porosity is required, as well as low pressure drop. The permeability to gas, as a function of the amount of porosity and pore architecture were investigated by Cilla et al [123,124] and Boher et al, [147] showing a strong influence of the pore morphology and size. Sound absorption of porous geopolymers was investigated by Zhang et al,[27] Hung et al,[58] and Papa et al [80] using the impedance tube method, and the ability of absorbing sound was quantified using the sound absorption coefficient. The raw materials (fly ash and slag) and the thickness (20 or 25 mm) of the PGs had an effect on the sound absorption coefficient [27]. The PG specimens showed a pretty good acoustic absorption coefficient (0.7–1.0) in the low frequency region, ranging from 40 to 150 Hz. Fig. 15 shows the sound absorption coefficient for porous geopolymers with different bulk density as a function of the frequency (100–4000 Hz). It can be seen that the sound absorption coefficient increased with decreasing bulk density, regardless of frequency. Thin PG specimens with the density of 0.4 g/cm3 exhibited an impressive acoustic absorption coefficient (0.6–0.9) in a specific frequency region (300–4000 Hz) [58]. Furthermore, the effect of different alkali ions (Na or K) on the sound absorption coefficient was investigated [80]. The porous samples exhibited a good sound absorption coefficient (0.4–0.9) at frequency region (1000–1500 Hz) and (4200–5000 Hz). Based on the above-mentioned research, the factors affecting the sound absorption properties were: the starting raw mineral materials [27,80], the thickness [27], the different alkali ions [80], the pore structure (porosity of open and closed, pore size and distribution) [27,58,80]. Although more investigations should be carried out on the acoustic properties of porous geopolymers, to further validate and extend these observations, the current data indicate that PGs can be used as new type of sound absorbing materials, especially in defined frequency regions. Water absorption was investigated by Novais et al,[13], Bajare et al. [51], Bumanis et al. [52], Yang et al. [59], Kumar et al. [66], Dembovska et al. [67], Ascensão et al. [68], Masi et al. [94], Okada et al. [148] and Emdadi et al. [149] The experiments showed that porous geopolymers are promising components that can be used for evaporative cooling applications. The samples, of different composition and amount of porosity, showed a wide range of water absorption capabilities (from 5% to 90%) and acceptable mechanical strength (from 0.4 to 22 MPa) for water retention applications. Besides water, other liquids can be stored (stabilized) in a geopolymer; Cantarel et al. [150]

5. Summary Tremendous efforts have been recently devoted to developing varied and novel processing technologies for the fabrication of porous geopolymers possessing a wide range of hierarchical pore architectures, driven by the need for low-cost, eco-friendly engineering parts. The resulting components have been characterized in terms of amount of porosity, pore morphology and pore characteristics as well as of relevant properties such as mechanical strength, permeability, thermal conductivity, durability. The review of the different processing methods for the fabrication of PGs, and the analysis of their influence on the main characteristics of the porous bodies can serve as a basis for the development and selection of components with microstructure and properties tailored for a specific application. Direct foaming is the simplest way to produce porous geopolymers, and has been so far the most used, in its different forms. However, the resulting components can suffer from inhomogeneity in terms of the pore architecture generated (wide range of pore sizes, variable ratio of open/closed pores). Additive manufacturing approaches, therefore, could provide benefits in terms of the constancy of pore morphology, leading to more controlled and often improved characteristics, provided that advancements in the current understanding and control of rheology development in geopolymer pastes can be achieved. Furthermore, additive manufacturing enables the a priori and simultaneous optimization of different features, enabling the fabrication of components with enhanced properties. Unfortunately, however, the lack of specific data prevents from conducting a straightforward assessment of the influence of the fabrication method on the mechanical properties of the porous geopolymers, as a direct comparison between the data reported in the literature is not possible, since several important factors contributing to strength were not constant among different experiments. The combination of the intrinsic micro- and meso-porosity of geopolymer bodies with the introduction of macro-pores has led to the development of a components with hierarchical porosity and different morphology, possessing a variety of properties making them suitable for a wide range of applications. This review summarized the testing of porous geopolymers in applications such as ion or gas adsorption, water

13


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purification and retention, catalyst support, thermal insulation, lightweight parts, pH buffering and drug release, demonstrating that these components have the potential for being successfully applied in an continuously widening range of engineering applications.

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