Production and characterization of lightweight vermiculite/geopolymer ...

Materials and Design 85 (2015) 266–274

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Production and characterization of lightweight vermiculite/ geopolymer-based panels V. Medri a,⁎, E. Papa a, M. Mazzocchi a, L. Laghi b, M. Morganti b, J. Francisconi b, E. Landi a a b

National Research Council of Italy, Institute of Science and Technology for Ceramics (CNR-ISTEC), Via Granarolo 64, 48018 Faenza, RA, Italy CertiMaC, Via Granarolo 62, 48018 Faenza, RA, Italy

a r t i c l e

i n f o

Article history: Received 8 April 2015 Received in revised form 15 June 2015 Accepted 26 June 2015 Available online 4 July 2015 Keywords: Lightweight inorganic composites Geopolymer Expanded vermiculite Thermal properties Thermal conductivity

a b s t r a c t The production and the properties of lightweight composite panels, with expanded vermiculite as lightweight aggregate and geopolymer as binder, were investigated. Different compositions of the geopolymer binders (metakaolin or alumina-based) and two sizes of expanded vermiculite were tested. The produced composites were subjected to microstructural analyses, as well as to thermal and mechanical tests. Densities ranged between 700 and 900 kg/m3, while the average strength and thermal conductivity were about 2 MPa and 0.2 W/mK, respectively. Results show that lightweight composites can be produced with satisfactory density and mechanical and thermal properties compared with other materials used in building sector, such as plasterboard or cellular concrete. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction In the last few years, numerous studies were made on cellular or lightweight concrete materials [1–14], with thermal conductivity in the range 0.1–1 W/mK. In accordance with the sustainable development policies in buildings and constructions [15–17], the use of lightweight materials allows to reduce both the weight of the structure (i.e., the dead load [12]) diminishing the use of raw materials and wastes, and heat transfer preserving operational energy [18] and fostering better indoor thermo-hygrometric comfort conditions. The use of lightweight (expanded) aggregates, namely vermiculite, perlite, pumice, etc. with different kinds of inorganic binders, such as cement, gypsum or geopolymers, allows the production of composites masonry blocks, walls and panels with reduced apparent density, good mechanical performances and improved thermal properties (insulation, refractoriness and fire resistance) [6–14]. In particular, vermiculite is a mineral of the group of hydromicas, whose chemical composition consists of a complex hydrated aluminum and magnesium silicate. It can expand to 8–20 times its original thickness (exfoliation) upon heating to above 300 °C. Expanded vermiculite aggregate is formed by thin plates separated by air gaps, becoming a highly effective heat-insulating material. Expanded vermiculite can be used as filler for high temperature-resistant insulating materials thanks to high thermal stability, owing to its ability to relax temperature stress during heating [6,19]. ⁎ Corresponding author. E-mail address: [email protected] (V. Medri).

http://dx.doi.org/10.1016/j.matdes.2015.06.145 0264-1275/© 2015 Elsevier Ltd. All rights reserved.

Among possible binders, geopolymers are a class of environmentally friendly and sustainable inorganic aluminosilicate polymers, firstly introduced by Davidovits [20]. Geopolymers are produced by reacting an aluminosilicate powder (metakaolin, fly ashes, slags or any source of silica and alumina) with a highly concentrated alkali solution [20]. Geopolymer nanoprecipitates [21–23] act as a binder for fillers and aggregates producing composite materials that may meet many of the ideal characteristics of lightweight materials for building sector [5,21], such as non-flammability and high temperature resistance without changing physical properties or releasing smoke, corrosion resistance to organic solvents, acids and alkalis, durability, safety for human health and low carbon foot print. All these features are considered a fundamental improvement with respect to traditional cement and concrete technology [3,15–17,20,21]. The aim of the present research is to study geopolymer-based composites with expanded vermiculite as lightweight aggregate to produce precast panels for fire and thermal insulation. Due to the completely inorganic nature [5], expanded vermiculite–geopolymer composites can be included in the Fire Class A1, as totally noncombustible materials in accordance with the European standard EN 13501-1. Contrary to the hydraulic cement, both geopolymer binder and expanded vermiculite do not contain water in their framework, thus preventing degradation at high temperature (e.g., spalling concrete) due to conversion of the structural water into steam [24]. Zuda et al. [13] suggested vermiculite–geopolymer-composite as fire-protecting layers for concrete that overcome the intrinsic limit of concrete structure exposed at fire and thermo-mechanical stress scenarios.

V. Medri et al. / Materials and Design 85 (2015) 266–274

267

Table 1 Compositions and characteristics of the raw materials (chemical composition and mean grain size D50 from technical data sheets provided by the suppliers). *Average values are subject to error due to the great chemical–morphological–dimensional variability of the expanded vermiculite aggregates. Material

Grade

Supplier

Chemical composition, % Al2O3

SiO2

Fe2O3

TiO2

K2O

Na2O

CaO

MgO

Metakaolin

M1200S

AGS Minéraux [24]

39.71

53.55

1.50

1.40

0.92

–

0.09

0.15

Alumina

CT 3000 SG

Almatis [26]

99.8

0.015

0.015

–

–

0.03

0.015

0.040

*Expanded vermiculite

Type 2 Type 4

Pull Rhenen [27]

6.5–10.0

35.0–41.0

6.0–9.5

0.6–1.5

4.0–7.0

–

2.0–5.0

20.0–24.0

Different compositions of the geopolymer binders (metakaolin or alumina-based) and two sizes of expanded vermiculite were tested, in order to change the final density and verify the processability of the composite materials. Macro and microstructure, porosity, mechanical strength, thermal behavior and thermal conductivity were also studied and discussed. The knowledge acquired about lightweight geopolymer– vermiculite composites would be beneficial for future applications (walls, partitions, protecting layers) in buildings and constructions. 2. Experimental procedure 2.1. Raw materials The geopolymer binders were produced by using ultrafine powders of metakaolin (grade M1200S, AGS Minéraux, Clérac, France [25], specific surface area 19 m2/g, more details are reported in ref. [26]) or alumina (Alumina Almatis CT 3000 SG [27], specific surface area 7.8 m2/g). The composition and characteristics of the raw powders are reported in Table 1. A commercial potassium poly-silicate solution was employed as activator (KSil 35-35, S.r.l. Ingessil Industria Silicati, 33.80 wt.% of potassium silicate, molar ratio SiO2/K2O = 3.22, pH = 11). Expanded vermiculite was used as lightweight aggregate. The characteristics and composition of the selected sizes (type 2 and 4, Pull Rhenen [28], The Netherlands, with a density of 95 and 85 ± 20% kg·m−3, respectively) are displayed in Table 1. 2.2. Preparation of geopolymer composite materials

Crystalline phases

D50

Quartz Muscovite (traces) Corundum Bauxite (traces) Micas/phlogopite Micas/phlogopite Hydrated vermiculite

1.7 μm 0.5 μm 3 mm 10 mm

alumina powder were cured for 72 h at RT in closed molds, then for 48 h at 80 °C in closed molds and finally 24 h at 80 °C in open molds. At least 4 panels of 55 cm × 47 cm × 3 cm were produced for each composition. 2.3. Characterization and analytical techniques The morphological and micro-structural features of the produced panels were examined by an environmental scanning electron microscope (FEI Quanta200 ESEM™) and by high-resolution photos (scanner Sharp JX330, Japan). Total open porosity in the range 0.0058–100 μm was determined by Hg intrusion porosimetry (ThermoFinnigan 240). The stability in distilled water of the samples was checked by complete immersion of cubic specimens (10 mm side) in distilled water at 25 °C for 11 days. Samples were preventively dried in a heater at 100 °C and, after cooling, their mass was measured. Samples were held by thin supports in order to avoid any contact with the bottom of the closed vessel. The mass of wet specimens was measured to calculate the maximum percentage of absorbed water (WS) reached after water saturation, while the weight loss percentage was calculated on the mass of the tested specimens after drying at 100 °C. The mineralogical composition was evaluated through X-ray diffraction (XRD) (Bruker D8 Advance diffractometer with Cu Ka radiation, k = 0.15406 nm) before and after thermal treatments at 600 °C, 800 °C, 1000 °C and 1200 °C in an electrical furnace in static air. The mineralogical composition was evaluated through X-ray powder diffraction (XRD) (Bruker D8 Advance in theta–theta configuration; scanning: 4–80 2θ; Cu-Kα rad., λ = 0.15406 nm; 40 kV; 40 mA) before

The composition of the mixtures (Table 2) and the processing conditions were initially set up by a trial and error approach. Metakaolin or alumina was mechanically mixed with the potassium poly-silicate aqueous solution for about 10 min. Expanded vermiculite and water (if needed) were subsequently added and thoroughly mixed until uniform mixtures were obtained. Expanded vermiculite was used as received. It is known that vermiculite is generally stable and it doesn't exhibit any potential alkali silica reaction [29] The resulting slurries were cast in silicon rubber molds. A customized curing method was set up for each different mixture in order to achieve gradual water removal and, hence, avoiding excessive shrinkages, planar deformations and cracks formation. In detail, materials prepared with metakaolin were cured for 24 h at RT in closed molds, then for 24 h at 80 °C in closed molds and finally for 48 h in open molds. Materials prepared with

Table 2 Compositions of the starting mixtures for the production of the composite panels. Sample

Raw powder wt.% Metakaolin

Alumina

V2–Mk V4–Mk V4–Al

24 26 –

– – 26

K-silicate wt.%

H2O wt.%

47 53 53

9 – –

Vermiculite Type

wt.%

2 4 4

20 21 21

Fig. 1. Example of geopolymer–vermiculite panels of 55 cm × 47 cm × 3 cm.

268


and after thermal treatments at 600 °C, 800 °C, 1000 °C and 1200 °C in an electrical furnace in static air. Shreds of all samples were selected and milled to obtain powders (b100 μm) to optimize the analysis conditions: the concept was to maintain as much as possible – in a small amount of powder – the reasonably real character of the whole material starting from the panels with an extremely randomized structure and texture, in which coexist large vermiculite platelets and fine geopolymer matrix, down to its average matrix/inclusions volume ratio. Dilatometric characterization was performed with a dilatometer (DIL402E Netzsch) up to 1200 °C in static air (heating rate 10 °C/min) and the recorded data were elaborated by Proteus Analysis Software. Due to the dimensional constraints of the dilatometer specimen, which can make the results not representative of the composite materials, shrinkage and weight loss were also measured for samples with dimensions 10 cm × 10 cm × 3 cm after heating at 1200 °C. The thermal conductivity was measured with a heat flow meter DTC 300 (TA Instruments, New Castle, USA) according to ASTM E1530 and UNI EN 12664 standards [30,31]. Cylindrical specimens (50.8 mm diameter and 6 mm thickness) were obtained by coring and machining the panels. Samples were tested in dry conditions (oven-dried at 50 °C until constant mass conditions were reached, conventionally reached when the percentage reduction in mass is of about 0.2%) and at a mean temperature of about 10 °C. At least three specimens for each composition were tested. The circular surfaces of the samples were grinded in order to reach a high degree of smoothness, and thus reducing the contact resistance at the interface with the measuring apparatus. The use of siliconic thermal compound and of a reproducible pneumatic load between the machine plates and the samples, further helped reaching a perfect thermal contact and ensuring a higher reproducibility of results. A preliminary evaluation of flexural and compressive strength was performed. Flexural strength was measured on 100 mm × 20 mm × 20 mm prisms, using a three-point jig with a span of 80 mm on a universal screw-type testing machine Zwick mod. Z050 (Zwick GmbH, Ulm, Germany) using a cross-head speed of 3 mm min− 1 . At least 5 prisms for each material were tested. The flexural strength was calculated with the formula (1): 2 σ ¼ 3 F l= 2 b h

ð1Þ

where: σ = fracture stress (MPa), F = peak force at fracture (N), l = jig span (mm), b = test piece width (mm), h = test piece thickness (mm). Preliminary compressive strength tests were performed on 5 cubic specimens (20 mm side) for each composition. The load was applied both perpendicular and parallel to the casting direction, using a testing machine (Zwick Z050, GmbH, Ulm, Germany) and a cross-head speed of 2 mm min−1. 3. Results and discussion 3.1. Macro- and micro-structure of the produced panels Fig. 1 shows an example of the produced panels (55 cm × 47 cm × 3 cm) after de-molding. Panels have a final shrinkage lower

a

b

c

1 cm Fig. 2. High resolution images of the macrostructure of machined cross sections parallel to the casting direction of V2–Mk (a), V4–Mk (b) and V4–Al (c) samples (Table 2).

than 2%, while the weight is about 6 kg for each compositions (Table 3). It should be noted that the handling characteristics, such as the moderate size and weight of each module, may favor the mounting and demounting operations. The weight's standard deviation of V4–Al composite, prepared from alumina and vermiculite type 4 (Table 2) is the highest. An explanation can be found in the higher density (3.99 g/cm2) and grain size (Table 1), and lower reactivity [32] of alumina raw powder in respect with metakaolin. The average geometric densities range between 700 and 900 kg/m3 (Table 3). The variation in geometric density could be mainly ascribed to the vermiculite type (Table 1). Both the higher density of type 2 (95 kg·m−3 versus 85 kg·m−3 of type 4) and mean size (3 mm instead of 10 mm in type 4) contribute to increase the density (and the particle packing) in V2–Mk respect to V4–Mk and V4–Al. In details, the high resolution images in Fig. 2 show the macrostructure of machined cross sections of the panels. Composite V2–Mk (Fig. 2a), containing the smaller-sized vermiculite (Table 2), has a more homogeneous distribution of the aggregate. The expanded vermiculite of bigger dimensions appears, within the composite V4–Mk (Fig. 2b) and V4–Al (Fig. 2c), as an elongated and compressed aggregate with high aspect ratio, in part piled and aligned perpendicularly to the casting direction. Although the shrinkage after consolidation was very low (2%), the loss of the

Table 3 Average weight and geometric density of the panels; total porosity, modal pore diameter, median pore diameter and total pore volume measured by mercury intrusion in the size range 0.0058–100 μm; the maximum percentage of absorbed water (WS) reached after water saturation and mass loss (Δwt.%) after drying. Sample

Average weight kg

Geometric density kg/m3

Total porosity %

Modal pore diameter μm

Median pore diameter μm

Total pore volume mm3/g

WS wt.%

Δwt.%


6.23 ± 0.02 6.07 ± 0.05 6.06 ± 0.20

841 ± 25 723 ± 25 737 ± 25

61 73 65

0.4594 0.4097 0.2988

0.8240 1.0293 2.1052

684 912 701

49 48 54

−11 −10 −10


water medium absorbed between the plates of the aggregates during the preparation of the slurry, caused a partial compression of the expanded vermiculite. It follows that vermiculite type 4 confer an anisotropic macrostructure to V4–Mk and V4–Al. The microstructural characterization performed by SEM on fracture surfaces (Fig. 3) shows the presence of geopolymer precipitates both in metakaolin-based (a, c, d) and alumina-based (b, e) composites. The precipitates belonging to alumina based binder are coarser (around 100 nm, Fig. 3b) than the metakaolin based ones (70–60 nm, Fig. 3a). Due to the high chemical inertia of the α-Al2O3 bulk, only the alumina particles' surface and/or hydrated species can take part to the geopolymerization process [32]. 3.2. Porosity The pore size distributions measured by Hg intrusion porosimetry of the composite materials are displayed in Fig. 4. In Table 3 total porosity, modal pore diameter and median pore diameter are reported. The results account mostly for the intrinsic porosity of the geopolymer matrix in the range 0.0058–1 μm [33]. The pore size distributions for the metakaolin-based materials V2–Mk and V4–Mk are similar being about the 50% of accessible volume in both cases due to pore size ≤1 μm, with an evident most frequent size peak located at ≈0.46 and 0.41 μm (modal pore diameter). However sample V4–Mk presents the higher total pore volume due to the use of the coarser type of vermiculite. Sample V4–Al shows a different pore size distribution, which is continuous in the range 0.2–100 μm. Three different pores size ranges, b1 μm, 1–10 μm and 10–100 μm, contribute similarly to the total pore volume and more peaks of frequency are evidenced beside the main one detected at ≈0.30 μm. The wide range of pore sizes detected and in particular the high presence of big pores present in V4–Al composite is due to the different microstructure of the alumina-based geopolymer binder, where alumina particle cores remain as unreacted filler, and its

269

particle packing within the lightweight vermiculite aggregates. The detection of big pores of size close to the upper limit of detection of the mercury porosimeter, explains the discrepancy which arises from a measured ‘low’ geometric density combined with a ‘low’ pore volume, differently from the trend found for Mk based samples. 3.3. Stability in water The stability in water of the expanded vermiculite–geopolymer panels was tested, after immersion in distilled water for 11 days, by measuring absorbed water and the weight loss after drying. Despite the soaking in water, all samples remained undamaged. The use of different compositions and grades of vermiculite does not appear to influence significantly the quantity of absorbed water and the weight loss. As reported in Table 3, the maximum percentage of absorbed water (WS) after water saturation is about 50% and the weight loss is about 10% for all the samples tested. The high WS is due to the mesoporosity of the geopolymer binder [34–36] and to the expanded structure of the vermiculite that is able to absorb water between its plates [37]. Although water absorption can cause an overload of the structure, the water retention properties (capillary water absorption and water release) of geopolymer binder [34–36] and expanded vermiculite [37] can be exploited for passive cooling by water evaporation. The weight loss is mainly due to the dissolution of soluble phases [34], such as unreacted potassium silicate or alkali carbonates, and material loss (vermiculite layers breakage). 3.4. Thermal properties 3.4.1. Phase modification in temperature To assess phase modification in temperature, the mineralogical composition of V2–Mk, V4–Mk and V4–Al panels was evaluated

a

b

2 µm

c

10 µm

2 µm

d

10 µm

e

10 µm

Fig. 3. SEM micrographs of metakaolin based (a) and alumina based (b) binders and fracture surfaces of V2–Mk (c), V4–Mk (d) and V4–Al (e) composites.

270


800

a

700 24 600 20

500 400

16

300

12

200

8

100

4

0 0.001

Relative pore volume (%)

Cumulative pore volume (mm³/g)

28

0.01

0.1

1

10

100

0 1000

Pore diameter (µm) 20 18

900

16

800

14

700 12 600 10

500

8

400 300

6

200

4

100

2

0 0.001

0.01

0.1

1

10

100

b Relative pore volume (%)


1000

0 1000

Pore diameter (µm) 10 9

700

8

600

7

500

6 5

400

4

300

3 200 2 100

1

0 0.001

c Relative pore volume (%)


800

0.01

0.1

1

10

100

0 1000

Pore diameter (µm) Fig. 4. Pore size distributions by Hg intrusion of samples V2–Mk (a), V4–Mk (b) and V4–Al (c).

through X-ray powder diffraction analysis (XRD) before and after thermal treatments at 600 °C, 800 °C, 1000 °C and 1200 °C (Fig. 5). In sample V2–Mk (Fig. 5a), the presence of vermiculite reduced to a micaceous structure (~10 Å) is evident already in the not heat-treated sample, as vermiculite is completely dehydrated (Table 1). Quartz is present likely as impurity of metakaolin and/or vermiculite. Two micaceous phases without interlayer or coordinated water (phlogopite and

chlorite–vermiculite–smectite type) are detectable up to 1000 °C. At 1000 °C the presence of enstatite is evident, while leucite just appears. At 1200 °C, enstatite, leucite and quartz are the main phases. V4–Mk shows the same phase evolution of V2–Mk, while it shows a lower degree of crystallinity. Vermiculite V4 (Table 1) still exhibits the baseline reflection (14 Å) as a characteristic of a hydrated mineral phase despite the industrial heat treatment of exfoliation. Reflections at 12 Å and 11 Å could be correlated to the reticular portions which have partially lost the water molecules [38]. However, the dehydrated vermiculite (10 Å) is preponderant. In V4–Al, the presence of alumina is revealed constant without changes up to 1200 °C, while quartz impurity seems to disappear in the sample treated at 1000 °C. At 600 °C, due to an almost total dehydration, vermiculite V4 is completely reduced to a phillosilicatic mica structure type. At 1000 °C mica is still present, while mullite, leucite and a magnesium silicate type Mg2SiO4/MgSiO3 appear. These phases form the final composition at 1200 °C. 3.4.2. Dilatometric analysis The thermal behavior of materials was monitored by dilatometric analysis up to 1200 °C in static air, while shrinkage and weight loss were also measured on panels (10 cm × 10 cm × 3 cm) treated at 1200 °C. In Table 4 the weight loss and the shrinkage of the composites, are reported. All the composites show a weight loss of about 9% in agreement with the values obtained from dilatometer specimens. The final shrinkages are greatly higher in samples obtained from metakaolin because alumina particles act also as refractory filler [39]. Anisotropic shrinkages (higher in the thickness than in the side of the panels) account for the elongated aggregates aligned perpendicularly to the casting direction (Fig. 2). The dilatometric curves (first and second runs) and the CTE curves (referred to an initial temperature of 30 °C) are displayed in Fig. 6a and b, respectively. In general, composites show a first zone of slight expansion up to 100 °C followed by a contraction zone up to about 250 °C mainly due to evaporation of the entrapped water. Starting from approximately 250 °C a new expansion occurs due to the expansion of vermiculite. All the samples reveal a thermal resistance up to 800 °C, after which the sintering for viscous flow starts [40]. Concerning Mk-based composites, the dilatometric curves of the first run account for the relative amount of the geopolymer matrix and vermiculite particles (the volumetric percentage of vermiculite in the composite panel can be estimated as 65 vol.% and 70 vol.% respectively for V2–Mk and V4–Mk). In fact, the initial contraction of the metakaolin based matrix is more pronounced, even if an expansion trend is detected starting from about 300 °C, while Mk composites experience in the whole range the dilatation of vermiculite. The dilatometric analyses highlight that the composites did not substantially underwent thermal shrinkage compared to the initial dimensions, up to about 1000 °C. The thermal expansion mismatch between the contracting geopolymer matrix and expanding aggregates resulted in very low linear dimensional change up to 1000 °C for the composites (b 1%). Looking at the dilatometric curves the addition of vermiculite results in the thermal stabilization of the composites which experience at higher temperature, compared to the geopolymer matrix, the same shrinkage (for example Mk based composite show a shrinkage of 4% at around 1100 °C, i.e., 100 °C later than geopolymer matrix). The dimensional stability in temperature is further increased in the alumina based composites, which experience 1% of shrinkage at temperature 60 °C higher than Mk based ones. The shrinkage of Mk composites exceeded the detection range of the dilatometer but the curve behaviors are in agreement with the trend of the whole shrinkages measured for the composites after heating at 1200 °C, as reported in Table 4: Mk based composites experience higher shrinkages than alumina based one. In Table 4 the experimental values of CTE in the range 300–800 °C are reported for the composites. The CTE are lower than the value


a

271

10000

20000

Counts

30000

A: Alumina M: Micas/phlogopite Q: Quartz V: Vermiculite : Leucite, low : Forsterite : Clino-Enstatite : Mullite

1200°C 1000°C 800°C Q

0

M 4

MM

10

MQ

20

Q Q M M MM M M 30

40

600°C

Q

Q M 50

M 60

Not treated

Q 70

80

b Q Q

Q

Q

1200°C

10000

Counts

20000

2 theta (°)

1000°C 800°C Q MM

0

V M M 4

10

M M

20

M

M M MM Q M Q

30

40

600°C Q

Q

M

50

M

Q

60

Not treated

70

80

2 theta (°) A

A

c

A

20000

A A

A

A A

Counts

A 1200°C

10000

1000°C 800°C 600°C VV V

M 0

M 4

10

M 20

V

V

V Q

M 30

M

V

V QM M 40

Q M 50

V

Not treated

QM 60

70

80

2 theta (°) Fig. 5. X-ray diffraction patterns of V2–Mk (a), V4Mk (b) and V4–Al (c) panels before and after thermal treatments at 600 °C, 800 °C, 1000 °C and 1200 °C.

found for the metakaolin based matrix (2.2 · 10−5 °C−1) in agreement with the behavior (slopes) of the dilatometric curves (Fig. 6a). The CTE variation was within 1 · 10−5 °C−1 for each composite, about 8 times lower than that of Mk geopolymer matrix (Fig. 6b). The better hightemperature stability of the composites, demonstrated by the smoother curves of dimensional change and of CTE (Fig. 6), would reflect in lower risk of failure of a building element since, as a general rule, lower thermal strain results in lower thermal stress [41]. A second run up to 1200 °C (dotted lines in Fig. 6a) shows the disappearance of the phenomena linked to the evaporation of water entrapped in the geopolymer binder and starting from 150 °C, a continued expansion occurs with a CTE in the range 300–800 °C of ~ 10 · 10−6 °C− 1 for samples V2–Mk and V4–Mk, while ~ 15 · 10−6 °C−1 for samples V4–Al, which are linked to the different

compositions and phase transformation occurred during the thermal treatments. In V2–Mk and V4–Mk, the viscous flow starts at higher temperature than in the first run for the presence of crystalline phases formed during the first heating up. In sample V4–Al the expansion during the second run continues till the end of the measurement at 1200 °C (Fig. 6a): the higher thermal resistance of α-Al2O3 reduces the shrinkage and keeps the volume stable at high temperature [39]. 3.4.3. Thermal conductivity The thermal conductivity of metakaolin-based geopolymers lies in the range 0.4–0.8 W·m− 1·K−1 [42], while values of below 0.2 W·m−1·K−1 are observed increasing the total porosity [43]. The values of thermal conductivity and thermal resistance of the samples are reported in Table 4. A statistical analysis model [44] was used to

272


Table 4 Weight loss (ΔW%) and shrinkages of 10 cm × 10 cm × 3 cm composites after heating at 1200 °C; CTE values in the range 300–800 °C obtained after the 1st and 2nd runs in dilatometric analysis; average thermal resistance and thermal conductivity; average flexural resistance (σf ± Δσ) and compressive strength σc ± Δσ. Sample

ΔW%


−9 −9 −9

Shrinkage ΔL/Lo %

CTE (300–800 °C) 10−6·K−1

Side

Thickness

1st run

2nd run

−10 −12 −1

−12 −15 −3

17.8 12.3 17.2

10.6 9.7 15.6

Thermal resistance (m2K/W)

Thermal conductivity 10 °C (W/m·K)

Flexural strength σf ± Δσ (MPa)

Compressive strength σc ± Δσ (MPa) //

┴

2.643E−2 3.360E−2 1.975E−2

0.189 ± 0.003 0.178 ± 0.003 0.256 ± 0.004

2.4 ± 0.3 1.2 ± 0.3 2.3 ± 0.7

2.0 ± 0.2 1.0 ± 0.1 1.2 ± 0.6

3.1 ± 0.3 1.7 ± 0.4 2.6 ± 0.4

determine the uncertainty associated with the conductivity measurements. Fig. 7 shows the correlation between the density and the thermal conductivity of the samples. This experimental correlation allows to evaluate the performance of different samples as a whole, permits the comparison between different admixtures in terms of porosity dimension and distribution relapses on thermal conductivity and consents the potential confrontation to commercial solutions already available into the market. In terms of obtained results, the use of type 4 vermiculite (bigger particles) resulted in more dispersed density values compared to samples prepared with type 2 vermiculite. At equal density, V2–Mk has slightly higher thermal conductivity than V4–Mk. This is a typical phenomenon in building materials that show a wide spread of possible results at the same density level due to the porosity distribution and morphology, as previously mentioned [45]. Since the thermal

dL/Lo /%

a [4]

1.0 0 [2] [4]

-1.0 -2.0 [1] Geopol. Matrix [2] V2-MK [3] V4-MK [4] V4-Al

-3.0 -4.0 -5.0 0

500

T.Alpha*10-5 2

[2]

[3] [1] XX X[3] 1000 T/°C

/K-1

b

Ref T = 30°C

0 [4] -2 -4

[2] [3] XX

-6

[1] Geopol. Matrix [2] V2-MK [3] V4-MK [4] V4-Al

-8 0

500

[1] 1000

T/°C

Fig. 6. Dilatometric analyses of the 1st and 2nd runs (dotted lines) on samples V2–Mk, V4–M and V4–Al and geopolymer MK matrix (a) and curves of the CTE values in the range (30-T) related to the 1st dilatometric run (b). The symbol X on the curves indicates that the shrinkage exceeded the detection range of the dilatometer.

conductivity of pure alumina (corundum, α-Al2O3) is about 33 W/mK [46], the use of alumina instead of metakaolin in the starting mixture resulted in samples with higher conductivity values: 0.18–0.19 W/mK for metakaolin-based samples (V2–Mk, V4–Mk) and 0.26 W/mK for alumina-based samples (V4–Al). 3.5. Mechanical properties 3.5.1. Flexural strength The average values of flexural strength are reported in Table 4 and the trends of the curves are shown in Fig. 8a. Samples V2–Mk and V4– Al present a similar average flexural strength of ≈2.4 MPa. Conversely sample V4–Mk possesses half of the flexural strength (1.2 MPa). It is well known that porosity amount and morphology affect the mechanical properties of brittle materials [47,48]. In particular the presence of expanded vermiculite aggregates in light weight concrete materials increases the thermal insulation properties but decreases the mechanical properties [10]. The higher flexural strength of V2–Mk sample in comparison with V4–Mk might be attributed to the lower porosity as well as more homogenous structure in V2–Mk samples. The expanded vermiculite aggregates can be considered critical defects, since they are not strong enough to carry the load [10] in respect with the geopolymer binder: the smaller dimension of type 2 aggregates in comparison to type 4 minimize the detrimental stress localization at the interface between matrix and aggregates. The trends of the curves (Fig. 8a) reveal that the presence of vermiculite helped dissipating the fracture energy, thus reducing the brittle behavior under fracture. These preliminary tests show that the values and the curves are analogous to those of other lightweight building materials with comparable densities, such as cellular concrete (b 1 MPa) or plaster board (~5 MPa). 3.5.2. Compressive strength The average values of compressive strength are reported in Table 4 and the trends of the curves are shown in Fig. 8b and c, respectively for the test parallel and perpendicular to the casting direction. The higher values of compressive strengths in both directions were registered in V2–Mk, thanks to the lower porosity, size and aspect ratio of expanded vermiculite type 2. Again the anisotropic structure of the expanded vermiculite–geopolymer composites (Fig. 2) affects the properties, being lower the compressive strength measured parallel to the casting direction. In details, the decrease in parallel compressive strength is 35.5%, 41.2% and 53.8% in V2–Mk, V4–Mk and V4–Al, respectively. In the parallel direction the rupture of the sample does not occur in catastrophic way (Fig. 8b), but it progressively involves the bridges among the thin plates and voids of the expanded vermiculite, that are manly aligned perpendicularly to the casting (and load) direction (Fig. 2). A clear toughening effect can be observed in the case of V4– Mk and V4–Al, because the load remains constant after failure, due to the dissipation of the deformation energy. When the aggregates (and plates and voids) are instead parallel to the applied load, the compressive strength is higher because of a lower area of voids under the loading surface. This is a consequence of the size effect on strength in brittle materials: the smaller is the volume under stress the higher is its fracture stress [47,48].


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Fig. 7. Correlation between thermal conductivity and density of V2–Mk, V4–Mk and V4–Al.

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Vermiculite and geopolymer-based composites result appropriate for the production of precast panels where the chemically active binder, suitable for chemical consolidation at low temperature, allows to avoid the use of high temperatures during the production process. These materials can be compared to lightweight or cellular concretes. The inclusion of lightweight aggregates improves the heat-insulating properties of the starting matrix, besides lowering the specific weight. The selected composite mixtures result suitable for a scale-up process, but a detailed feasibility analysis should be addressed in order to define the most suitable industrial process, time and cost-effective, in respect of the final product and final intended use. The composite mixtures have a good workability, they can be easily cast in molds and the final density of the material can be tuned by changing the quantity of light aggregates. The balance between thermal and mechanical properties suggests to use these materials for the production of pre-cast high-temperature insulating panels that could be mechanically anchored to a load-bearing structure.

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Acknowledgments

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The research activity was carried out in the frame of the Project “MATEC — New materials and new technologies for internal combustion co-generator prototype”, funded by the Ministry of Economic Development of Italy. The authors wish to thank Dr. Eng. Annalisa Natali Murri, Dr. Guicciardi and Mr. Cesare Melandri for the useful discussion about mechanical properties.

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References

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