Potential use of ceramic waste as precursor in the geopolymerization reaction for the production of ceramic roof tiles

Journal Pre-proof Potential use of ceramic waste as precursor in the geopolymerization reaction for the production of ceramic roof tiles A.R.G. Azevedo, C.M.F. Vieira, W.M. Ferreira, K.C.P. Faria, L.G. Pedroti, B.C. Mendes PII:

S2352-7102(19)32020-0

DOI:

https://doi.org/10.1016/j.jobe.2019.101156

Reference:

JOBE 101156

To appear in:

Journal of Building Engineering

Received Date: 26 September 2019 Revised Date:

27 November 2019

Accepted Date: 25 December 2019

Please cite this article as: A.R.G. Azevedo, C.M.F. Vieira, W.M. Ferreira, K.C.P. Faria, L.G. Pedroti, B.C. Mendes, Potential use of ceramic waste as precursor in the geopolymerization reaction for the production of ceramic roof tiles, Journal of Building Engineering (2020), doi: https://doi.org/10.1016/ j.jobe.2019.101156. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

CRediT author statement

Azevedo, A.R.G.: Conceptualization, Writing - Original Draft, Writing - Review & Editing and Visualization Vieira, C.M.F.: Supervision and Project administration Ferreira, W.M.: Methodology Faria, K.C.P.: Methodology Pedroti, L.G.: Supervision and Project administration Mendes, B.C.: Writing - Original Draft and Writing - Review & Editing

Potential use of ceramic waste as precursor in the geopolymerization reaction for the production of ceramic roof tiles Azevedo, A.R.G.1,2*; Vieira, C.M.F.2; Ferreira, W.M.2; Faria, K.C.P.2; Pedroti, L.G.3; Mendes, B.C.3 1

UFF - Federal Fluminense University, TER – Department of Agricultural and Environmental Engineering; Rua Passo da Pátria, 156, Niterói, Brazil 2

UENF - State University of the Northern Rio de Janeiro, LAMAV – Advanced Materials Laboratory; Av. Alberto Lamego, 2000, 28013-602, Campos dos Goytacazes, Brazil

3

UFV - Federal University of Viçosa, DEC – Department of Civil Engineering; Av. P.H. Rolfs, 36570-000, Viçosa, Brazil

*Corresponding author. E-mail addresses: [email protected] (Afonso R. G. de Azevedo)

Abstract

The red ceramic industry is responsible for generating high amounts of solid wastes around the world from manufacture process failures, such as ineffective firing and issues related to the products transportation. Besides the necessity of clean alternatives to discard the solid wastes, the civil construction industry has been demanding the development of better technological properties new materials. One an example of those new materials is the geopolymeric materials, characterized by the gain of mechanical strength at early ages, high fire resistance, low water absorption and refractoriness. All these characteristics imply that geopolymers are suitable for civil construction applications. The characterization of clay bricks waste, named chamotte, and its use as an alternative precursor to produce geopolymeric materials, such as roof tiles for buildings are the aims of this present work. The chemical characterization, particle size distribution, X-ray diffraction, specific mass, pozzolanic activity index (PAI) and scanning electron microscopy (SEM) were performed, in addition to the technological tests carried out on the geopolymer specimens, such as flexural

strength, water absorption, linear shrinkage and apparent porosity. The chemical and mineralogical analysis proved that the waste is rich in silica and alumina, which are fundamental compounds for the geopolymers synthesis. The chamotte also has fine particles and high pozzolanic reactivity. Thus, this waste has great potential to be used as a raw material for obtaining of ceramic roof tiles by means of geopolymeric reactions. Keywords: Geopolymers; Ceramic waste; Roof tiles.

1. Introduction

The red ceramic industry handles great amounts of financial resources around the world and generates large numbers of jobs, being an important segment of the overall production chain [1].

Some countries, such as Brazil, Italy and China, are major producing and

consuming markets for ceramic products. For example, Brazil earned about $ 4.5 billion last year from the red ceramic industry, showing the importance of the sector [2].

Despite numerous attractions, the red ceramics industry segment still has major environmental barriers, which hinder the industry's progress and its certification in countries with more rigorous legislation [1]. The main environmental issues related to this industry are the exploration of natural raw materials (i.e. clays), energy resources used for sintering step and the emission of gases in the atmosphere, contributing for its pollution. As the environmental problems related to this industry it’s important to mention the extraction of natural raw materials such as clay, energy resources used for the sintering stage of the pieces, and the release of gases in the atmosphere, contributing to their use. Thus, the use of modern and more sustainable practices has been the focus of several studies around the world [3].

In addition to the environmental problems directly related to that industry previously mentioned, there is also the generation of solid waste in the production process of ceramic pieces. These wastes are generated by failures in the sintering stage, which forms products with inappropriate visual appearance, technological problems or deviations of geometric values, making their use unfeasible [4]. Natural losses also occur in the transportation, drying and cutting steps. Some research has suggested losses in this segment of about 5% [5].

Brazil has developed over the last decade a modern legislation related to the management of both industrial and municipal solid wastes, which brought significant advances to the civil construction sector. On the other hand, the industries were forced to adapt to this legislation, especially those that generates high amounts of waste, such as the red ceramic industry. Thus, one topic of the industrial waste management is the adoption of proper disposal and reuse of these materials [6]. Several studies have already addressed the reuse of waste from the red ceramic industry, called chamotte, in traditional ceramic or cementitious materials [7, 8 and 9].

The application of ceramic materials for civil construction, traditionally manufactured, causes many environmental damages, as well as several technological issues such as the need to be in accordance to the technical standards related to mechanical strength and water absorption, which are fundamental parameters for the viability of using ceramic products [5]. An alternative that has been studied in the last decades is the use of geopolymerization reactions for the production of ceramic pieces. This option results in advantages from both environmental and technological points of view, such as the no need for sintering, the possibility of reusing solid wastes in the production of these materials and the manufacture of pieces with higher mechanical strength and lower porosity, which is beneficial for some applications [10 and 11].

Geopolymers can be defined as a class of materials derived from inorganic polymers. The SiO4 and AlO4 tetrahedral form a three-dimensional network by sharing all their oxygen atoms, and this network must be stabilized by the presence of monovalent cations [12, 13]. The geopolymerization reaction occurs between silicoaluminates sources in an aqueous and alkaline medium, which are called precursor and activator solution, respectively.

Most research related to geopolymers uses metakaolin, a silicoaluminate obtained from kaolinite calcination, as the precursor agent and uses solutions composed of sodium such as sodium hydroxide (NaOH), sodium carbonate (Na2CO3), sodium silicate (Na2O.nSiO2) and sodium sulfate (Na2SO4). However, the ones with the greatest reaction potential are potassium-based activators because potassium have a larger atom size and a greater tendency for association with water molecules [13].

The use of these traditional materials, such as metakaolin (precursor) and sodium-based solutions (activator), for example, is a problem regarding the dissemination and use of this technique, given the existence of environmental impacts from the extraction of natural resources and the associated high costs, especially in alkaline solutions. Thus, the search for new materials that promote technological and environmental advantages to the products obtained through the geopolymerization is a topic of great scientific importance.

Potassium hydroxide or sodium hydroxide has the role of dissolving the raw materials and sodium silicate acts as a binder, but this fact is not well explained or understood in the literature [14]. An important fact is that for the activation of aluminosilicates, in the synthesis process of geopolymers, activators with pH values greater than 13 and near 14 are required [15].

The selection of activator depends on the precursor and desired characteristics of the geopolymer products. The choice of the ideal activator should considerer the balancing of the Al3+ and Si4+ charges in the tetrahedral obtained from geopolymerization and the negative charge of the AlO4- group is responsible for the balancing of positive Na+ and / or K+ charges [16]. The concentration to be used is also an important factor, because the combination of solutions determines the product type and the final strength of the geopolymer. NaOH concentrations should be in the range of 5-16 moles and the KOH concentration must be in the range of 4-8 to enable its use [17, 18, 19 and 20].

The SiO2/Al2O3 ratio is another important factor to consider, as the three-dimensional arrangements are organized according to the disposal of silicon and aluminum atoms that alternate in tetrahedral coordination dependent on SiO2/Al2O3 ratio [18]. In 1976 Davidovits adopted another terminology for polysialates, calling them geopolymers. These polysialates are named according to their SiO2/Al2O3 ratio [17].

Precursors are the primary sources of silicon and aluminum oxides responsible for the geopolymer structure that, once in contact with activating materials, react and form the structural chains of the material polymeric network. Metakaolin is a thermally activated material composed of aluminosilicates with high pozzolanicity. This material can be generated by calcining kaolinitic clays at temperatures between 650°C and 800°C. The temperature of calcination depends on the purity and crystallinity of the clays [12].

The word kaolin is also used to denote a group of hydrated aluminum silicates, which includes some minerals such as kaolinite and haloisite. The prefix meta is used to highlight the last hydration of the series, which in the case of metakaolin is dehydration due to firing [10].

The metakaolin is composed by 52% SiO2, approximately 40% of Al2O3 and about 8% of other elements which are considered impurities, such as feldspar, mica, gypsum and organic matter [21]. According to Davidovits (1994), due to the transformation of kaolin into metakaolin, the material becomes much more reactive, acquiring pozzolanic properties [22]. One possibility to consider is the replacement, in whole or in part, of the traditional metakaolin by the chamotte, which is a burnt clay that has gone through a grinding process in various grain sizes. This material is considered a waste from the ceramic industry and has an inert behavior. Thus, it can be used to provide stability to ceramic masses as it facilitates the drying process, reducing shrinkage and warping [20].

Due to the manufacture process of low quality pieces and production failures, many of these pieces are discarded after the firing process, generating a large amount of ceramic wastes also called chamotte or grog. This waste is rich in SiO2 powder or granular SiO2, which may suggest a potential use as a precursor in geopolymers [23]. Some research using chamotte as a metakaolin substitute has observed large amounts of SiO2 and Al2O3, which are necessary elements for the geopolymerization reaction. The presence of silica in quartz phase was also verified in the waste by both chemical and mineralogical analysis [19, 20 and 24].

According to Vieira (2004), in Campos dos Goytacazes, a city located in the state of Rio de Janeiro - Brazil, which is one of the major producer of ceramic products in Brazil, the loss is relatively low, between 0.5% and 1% at firing stage by adopting quality control criteria. However, ceramic industries that do not adopt the same criteria can have losses that reach 10% only at the firing stage, which entails a huge environmental liability of the sector [23].

Ceramic tiles are artifacts generally produced by the pressing process of a ceramic mass that has characteristics necessary for the correct use in roofs and their subsequent firing [25]. The tiles have their own characteristics for their use, meeting the normative criteria of each region, giving users comfort and reliability [26]. The mechanical strength and porosity properties are determinant in the quality classification of the roof tiles, they must be able to support the weight of their structure and also the human weight during the roof assembly and resist the sun and rain exposition maintaining a low water absorption [27].

1.1 Research Objectives:

The objective of this research is the chemical characterization, particle size analysis, Xray diffraction, real specific grain mass, pozzolanic activity index (PAI) and Scanning Electron Microscopy (SEM) of the ceramic waste (chamotte or grog) in order to evaluate its potential to replace metakaolin as a precursor agent in the production of geopolymers. This study also aims the assessment of technological performance of the geopolymeric products and the viability of their application in the production of ceramic roof tiles comparing two manufacturing processes molding and pressing.

2. Methods and materials 2.1 Materials and Characterization The waste applied in this work, called grog or chamotte (Figure 1a), is the waste of clay bricks unused in the manufacturation step. The material was collected in a ceramic industry of the ceramic district of Campos dos Goytacazes, RJ, Brazil. This waste was ground and homogenized in a medium sieve, reaching particle size compatible with the application in ceramic materials.

The quartz sand used to complete the solid mass (Figure 1b) is from river, purchased from the commercial market, being widely used in civil construction. This sand went through the same sieving process on the 42 mesh sieve. The metakaolin used is commercial, called Metakaolin HP Ultra, from Metacaulim do Brasil, purchased in 20 kg bags (Figure 1d). Potassium hydroxide (85%) (Figure 1c) and sodium silicate powder (Figure 1e) (Na2SiO3 - 49% SiO2 and 23% NaOH) were used to prepare the activating solution. These alkaline materials are very used to produce geopolymeric matrices, being constantly reported in the literature [18].

Figure 1. (a) Processed ceramic waste (grog). (b) processed sand. (c) Potassium hidroxide (85%). (d) Metakaolin HP Ultra. (e) Sodium silicate poder Na2SiO3 (49% SiO2 and 23% NaOH).

Initially the waste was analyzed through a characterization process, aiming at a better knowledge of its properties and allowing a thorough study in relation to the literature data, several works have already evaluated the importance of the characterization process of wastes for applications in ceramic, cementitious and geopolymeric materials, demonstrating its correlation with technological properties [28, 29 and 30]. The chemical analysis was performed, by mean of the X-ray fluorescence (XRD), model AXIOS, belonging to the Mineral Technology Center (CETEM). Another characterization technique was the determination of pozzolanic activity index, according to ABNT NBR 5751 (2015), through this technique, it is possible to determine how much waste and consistency (water / binder) will be used in each test from the material density [31]. Materials that exhibit pozzolanic

activity are reactive silica and alumina sources and have the potential to be activated with alkaline solutions, resulting in a product with adequate mechanical strength (Firdous et al., 2018) [32]. The X-ray diffraction (XRD) was performed in a conventional diffractometer (Shimadzu,

XRD

7000) used for qualitative mineralogical analysis, with Co-Kα

monochrome radiation at a speed of 1.5° (2θ) per minute. The crystalline phases were identified by means of comparison of intensity and position of Bregg peaks with those of JCPDS-ICDD files. The physical properties were also determined, through the combined process of sieving and sedimentation according to the NBR 7181 (ABNT, 1984) [33]. The main objective of this test is to obtain the particle size distribution in order to enable a combined analysis of the morphology. The real specific mass of the grains were determined according to the NBR 6508 (ABNT, 1984) [34]. The morphology and chemical analysis of specific points in the material were obtained by means of Scanning Electronic Microscopy (SEM) and Energy-Dispersive Spectroscopy (EDS). Particle size analysis of metakaolin was performed with MSS Mastersizer equipment using isopropyl alcohol as a dispersing medium. The test provides the distribution and diameter frequency profile of the spray material. 2.2 Formulation and preparation of geopolymerized specimens The specimens were prepared using various compositions with grog and metakaolin as precursors. Two conformation techniques of the geopolymerized specimens were evaluated in this work: pressing and molding, being possible to verify the influence of each one on the final properties. The compositions are described in Table 1.

Table 1. Composition of geopolymerized specimens

Composition

Precursor

Activator

Conformation

Water/solids

process

ratio

Sand

Molding

0.60

Sand

Pressing

0.42

Sand

Molding

0.41

Sand

Pressing

0.26

Aggregate

Sodium silicate and MM

Metakaolin potassium hidroxide Sodium silicate and

MP

Metakaolin potassium hidroxide Sodium silicate and

GM

Grog potassium hidroxide Sodium silicate and

GP

Grog potassium hidroxide

According to the literature, the SiO2/Al2O3 ratio must be in a specific range which depends on the intended product. Thus, a value of approximately 4 was adopted for all formulations in this work [22]. For the preparation of the alkaline solutions, a magnetic mixer was used in which the solution was around 10 minutes for complete homogenization. The solutions were produced 24 hours prior to specimen preparation for proper cooling. Pressed specimens were made using chamotte and metakaolin as precursors. The granulated geopolymeric mass was prepared using solution / (chamotte + sand) and solution / (metakaolin + sand) ratios of 0.26 and 0.42 respectively. The samples were formed by uniaxial pressing in a hydraulic press, using a rectangular steel mold with dimensions 114.5 mm x 2.25 mm x 10 mm, as shown in Figure 2a. The molded specimens were made using two other types of formulations, with solution/ (chamotte + sand) and solution/ (metakaolin +

sand) ratios equal to 0.41 and 0.60 respectively. The mixture was poured into 115 x 30 x 20 mm acrylic molds as shown in Figure 2b.

Figure 2. (a) Manufacturing of specimens by pressing. (b) Samples produced in acrylic mold (molding process).

To produce the molded specimens, the entire volume of the alkaline solution was added to the sand and mixed manually for about 5 minutes. The precursor was added in an equal amount of sand and mixed for a further 5 minutes. Then the mixture was poured into the molds. For the pressing process, the mixture was prepared in the same way; however, as it is a semi-dry mass, it was necessary to pass it in a 10 mesh sieve to obtain a more homogeneous mixture. The specimens were pressed (Figure 3a and 3b) applying a load of 5 tons.

Figure 3. (a) Molded and (b) pressed specimens, with the grog as precursor.

The molded and pressed specimens (Figure 4a e 4b) were cured at room temperature for 24 hours, with the upper surfaces covered by plastic to prevent the excessive moisture loss to the environment. After this period, the specimens were removed from the molds and cured at room temperature and in a drying oven at 60 ºC for a period of 7 and 14 days. Some studies have already evaluated the effect of the curing condition and its period on the properties of geopolymers, as well as their durability, avoiding the appearance of efflorescences that may affect the physical integrity and the aesthetics of the specimens [35 and 36]. The curing temperature at 60 °C was adopted based on the study, that observed that metakaolin-based geopolymers presented higher mechanical strength when cured at this temperature for 168 hours (7 days) [32]. The specimens were identified according to the composition and curing conditions (temperature and time) as showed in Table 2. Table 2. Composition and curing conditions.

Identification

Curing temperature

Curing period

MM1/GM1/MP1/GP1

Room temperature (25°C)

7 days

MM2/GM2/MP2/GP2

Oven at 60°C

7 days

MM3/GM3/MP3/GP3

Room temperature (25°C)

14 days

MM4/GM4/MP4/GP4

Oven at 60°C

14 days

Figure 4. (a) Molded specimens (b) and pressed, both using metakaolin as a precursor.

2.3 Technical evaluation of geopolymerized specimens After the curing period, the samples were subjected to tests for the evaluation of technological characteristics, comparing them with normative values and showing their potential application as roof tiles for civil construction. The flexure strength (three-point test) of the geopolymer samples was determined according to the standard C674-77(ASTM) [37] standard, in order to verify the strength of the geopolymer pieces using an INSTRON universal testing machine, model 5582, with a loading rate of 0.5mm/min. For the determination of water absorption, the mass of the dried samples and after 24 hours of immersion in water was measured, according to procedures described by the standard ABNT NBR 15310 (2005) [26]. The apparent porosity is a property that indicates the content of open porosity, and it was performed using a hydrostatic balance for the measurement of the specimens immersed mass. The apparent specific mass (ASM) of the specimens was determined according to the procedures described by the standard C 373-88 (ASTM, 1994) [38]. The linear shrinkage was calculated by measuring the difference in sample length

before and after curing. The measurements were made using a digital caliper with a resolution of 0.01 mm.

3. Results and Discussion 3.1 Characterization of the precursors Table 3 shows the chemical composition of the grog and metakaolin used in this work. Table 3. Chemical composition (%) of the grog and metakaolin.

Sample

SiO2

A2O3

Fe2O3

MgO

K2O

Na2O

TiO2

P2O5

CaO

Grog

57.30

32.70

3.50

-

1.40

0.78

1.00

0.13

0.83

Metakaolin

52.20

39.20

3.20

0.10

1.20

-

1.00

-

0.20

Both grog and metakaolin have great amounts of SiO2 and Al2O3, besides the presence of other minor oxides such as Fe2O3, K2O, TiO2 and CaO. One of the most relevant factors related to the suitability of these materials for geopolymer production is the SiO2/Al2O3 molar ratio. Davidovits (2002) [22] recommends a molar ratio between 3.5 and 4.5. However, what really determines the ratio of aluminum to silicon is the threedimensional structure formed by aluminosilicates and their characteristics and applications [19]. For the production of ceramic materials from geopolymerization reactions, some studies indicate that a SiO2/Al2O3 molar ratio of at least 1 is necessary, and the structure formed is named polissialato-OS [22]. The SiO2/Al2O3 molar ratios of the grog and metakaolin are 1.48 and 1.13, respectively, both being able to be applied in geopolymerization process.

The real specific mass of the grains is a useful information, since it is necessary to determinate the material amount for pozzolanic activity index (P.A.I.) and consistency analysis. The real specific mass of the grog is very close to the metakaolin supplied by Metakaolin Brazil. The amount of water required for the P.A.I. was defined according to the NBR 7215 [39] standard and the consistency index. The value was expressed in terms of water/binder ratio, equal to 0.66. The result of pozzolanic activity index is shown in Table 4. It was satisfactory, since the NBR 5751 (2015) [28] standard specifies a minimum value of 6.0 MPa. Table 4. Real specific mass and pozzolanic activity index of the grog and metakaolin.

Material

P.A.I with lime (MPa)

Specific mass (g/cm3)

Grog

7.7

2.60

Metakaolin

8.3

2.54

The XRD pattern of the grog is shown in Figure 5. The detected peaks reveal the presence of mica (M), feldspar (F), quartz (Q) and hematite (H). One can notice the absence of kaolinite, which can be explained by the fact that the material was fired at 600°C, occurring the transformation of kaolinite into metakaolinite (amorphous phase).

Figure 5. XRD pattern of grog sample.

Figure 6 shows the energy-dispersive spectroscopy (EDS) performed on the grog sample. The grog presents particles with different morphologic aspects. Many angular particles smaller than 30µm were found, confirming the particle size analysis. The identification of the present elements Si, Al, Fe, Na and K corroborates the chemical analysis of the waste shown in Table 3. In Table 5 and Figure 7 it is shown the physical characterization of the grog.

Figure 6. (a) Scanning Electronic Microscopy (b) energy-dispersive spectroscopy (EDS) of

the grog. Table 5. Particle size distribution of the grog.

Fraction

Percentage (%)

Particle size (µm)

Silt

66.8

2 < x ≤ 63

Clay

20.3

<2

Sand

12.9

63 ≤ x < 200

Figure 7. Particle size distribution of the grog.

The grog is composed by 66.8% of silt fraction, 20.3% of clay fraction and a small amount of sand (12.9%). Studies have indicated that the smaller the particles, the higher the reactivity of the material, because of the greater dissolution of aluminossilicates. Therefore, the fine grain size of the chamotte is a positive aspect for the production of geopolymers [40]. In Table 6 shows the result of laser granulometric analysis of the metakaolin used.

Table 6. Percentage granulometric values of metakaolin.

Material

D10%

D50%

D90%

Metakaolin

1.65 µm

9.01 µm

40.39 µm

The result shown in Table 6 is the diameters that correspond to the cumulative distribution in diameters of D10%, D50% and D90%. It is a material much finer than the Portland cement mill clinkers used in Brazil. This high fineness influences the demand for kneading water in cementitious materials and contributes to the manufacture of geopolymeric materials [32]. 3.2 Results of the technological tests 3.2.1 Flexural strength Figure 8 shows the results of flexural strength of the specimens produced from metakaolin and grog as precursors, respectively, cured at 7 and 14 days under two conditions: room temperature (25°C) and drying oven at 60°C.

(a)

(b)

Figure 8. Flexural strength graph of molded and pressed specimens, using: (a) metakaolin and (b) grog as precursors.

One can notice the increase of flexural strength with age for most compositions. The MM1 and MM3 samples presented strength values of 6.47 MPa and 8.27 MPa, respectively. The same trend was observed for samples MM2 and MM4. For the pressed samples MP1 and MP3, the flexural strength results were 1.34 MPa and 6.55 MPa; the strength of the specimens cured at 14 days was about five times higher. The GM1 and GM3 samples presented strength values of 0.44 MPa and 1.78 MPa. For the GM2 and GM4 samples, the results were about 10 MPa and 17.5 MPa. The strength improvement was also observed in samples GP1 and GP3 [41]. For both grog and metakaolin-based geopolymers, samples cured at 60°C (GP2, GP4, MP2 and MP4) did not presented increase of strength with the age. This fact can be explained by the occurrence of cracks caused in the pressing process. The high standard deviation observed for the MP4 and GP4 samples suggests that this hypothesis is valid. Some research has already shown that this increase in mechanical strength, due to the curing condition, is influenced by the appearance of cracks in the specimens, which indicates greater brittleness and integrity due to internal micropores [42]. Therefore, for pressed specimens, prolonged curing period at controlled temperature was not determinant [40]. Assessing the effect of curing temperature, the samples cured at 60°C showed higher values of flexural strength proving the rise of strength with the increase of curing temperature at the same period. The flexural strength reached 30.44 MPa for MP2 sample and 17.51 for GP4 sample. According to Chen et al. [41], the increase of curing temperature accelerates aluminosilicate dissolution reactions; with the higher availability

of Si dissolved in the system, more Si – O bonds are formed, resulting in mechanical strength improvement. Considering as reference the values presented in the literature, the minimum flexural strength required for the manufacture of roof tiles is 6.5 MPa [43]. The MM1 and MM2 samples did not reach acceptable values for roof tile manufacturing, as well as GM1, GM3 and GP1 ones. Some reasons for the low flexural strength are the short curing time and the degree of compaction, which are insufficient for the geopolymerization reactions to occur satisfactorily. On the other hand, all other samples presented values above the minimums suggested in some studies [23, 43]. It is also possible to observe that the degree of compaction influences the flexural strength. For metakaolin-based gepolymers, it was observed a significant increase of resistance in the pressed specimens in relation to the molded ones for curing at 60°C. For grog-based samples, this increase was verified for all curing conditions, except at 60ºC for 14 days. Indeed, the higher degree of compaction is related to a smaller number of voids in the geopolymer structure [44, 45]. 3.2.2 Linear shrinkage The results of linear shrinkage of the specimens using metakaolin and grog as precursors, according to the curing period and temperature is shown in Figure 9. Linear shrinkage increased with increasing curing time and temperature. Molded samples have higher values of linear shrinkage, since in their formulation they contain more water in relation to the amount of solids [46].

(a)

(b)

Figure 9. Linear shrinkage graph of molded and pressed specimens, using: (a) metakaolin and (b) grog as precursors.

Pressed samples have lower linear shrinkage under all tested conditions. This lower shrinkage is related to the higher degree of compaction and lower water content. The dimensional variation values should not exceed ± 2.0% for roof tiles, according to NBR 15310 (2009) [26]For metakaolin-based geopolymers only the pressed specimens and MM1 samples met the recommendations. For grog-based geopolymers all pressed compositions also met the specifications, besides the GM1 and GM2 samples. The pressing method, once again, was more efficient in obtaining adequate linear shrinkage geopolymers. It was observed that GP2 and GP4 present very close linear shrinkage values, considering the standard deviation. These results corroborate the flexural strength values that were approximately constant for these samples [20]. This fact is related to the water content in the GP4 sample, which was insufficient to complete geopolymerization and obtain an expected flexural strength for the applied curing time [21].

In general, the metakaolin-based specimens presented higher values of linear shrinkage. This may be associated with the predominance of finer particle size particles requiring higher water / solids ratios to achieve the desired workability. Thus, more water was lost during the geopolymerization and curing process. 3.2.3. Water absorption and apparent porosity The water absorption and apparent porosity of pressed and molded specimens using metakaolin and grog as precursors is respectively shown in Figures 10 and 11. It is observed that the MM1, GM1, MP1, GP1 and MP3 specimens were not able to resist the test, occurring dissolution and considerable loss of mass during 24 hours of submersion in water, demonstrating that the geopolymerization process was not sufficient for the formation of a durable matrix. This fact made it impossible to determine the water absorption and apparent porosity.

(a)

(b)

Figure 10. Water absorption graph of molded and pressed specimens, using: (a) metakaolin and (b) grog as precursors.

(a)

(b)

Figure 11. Apparent porosity graph of molded and pressed specimens, using: (a) metakaolin and (b) grog as precursors.

The Metakaolin-based specimens showed low water absorption and apparent porosity values, below 8% and 16%, respectively, for all categories. In relation to grog specimens most mixtures reached low water absorption (less than 4%) and apparent porosity values. Only GM3 and GP3 samples presented higher water absorption (between 9 and 20%), indicating a low strength and high porosity structure [18]. Water absorption is directly associated to material porosity since more porous materials - with open and interconnected pores have greater water absorption [47]. Samples GP2 and GP4 obtained slightly higher values compared to GM2 and GM4, indicating that the compaction method influences the formation of open pores. The GM4 composition showed the lowest absorption and porosity values among all evaluated grogbased mixtures, corroborating the high flexural strength and low shrinkage [25]. The degradation condition of the samples due to no formation of a sufficient geopolymer chain to guarantee the material durability is demonstrated in Figure 12 [48]. The NBR 15310 (2009) [26] defines that water absorption should not exceed 22.0% for

roof tiles. In this case, all specimens met the normative requirement for both precursors and manufacturing process.

Figure 12. Pressed samples degradation: specimen before 24 hours immersed in water (a),

specimen after 24 hours immersed in water (b) e (c).

Conclusions

According to the results it can be concluded that:

• The ceramic waste, named grog, presented as main mineralogical characteristics the absence of kaolinite and possible presence of amorphous metakaolinite, besides the desirable density and consistency. The SiO2 and Al2O3 contents were similar to those found in metakaolin, as expected. The value of the pozzolanic activity index was in accordance with the standard (above 6 MPa), thus being compatible with the ceramics obtained by the conventional firing process.

• Regarding the technological analysis, it was verified that water content and curing temperature are decisive to improve the characteristics of grog and metakaolinbased geopolymers manufactured by pressing. In the case of specimens subjected to the curing process at room temperature, a longer period should be applied to

produce geopolymeric ceramic pieces with better technological characteristics and within the technical standard for use in civil construction.

• Considering the conditions and methods used, as well as the normative references, it was concluded that the geopolymerized ceramic artifacts produced in this work can be used in the civil construction field. Geopolymerization of ceramic waste as the metakaolin substitute and pressing method are feasible for the production of ceramic roof tiles, thus obtaining satisfactory mechanical and physical properties. Thus, the usual conformation of conventional ceramic tiles (pressing) can be maintained.

• Thus, the ceramic waste proved to be suitable for use in the production of geopolymer ceramics. This generates lower CO2 emission compared to the firing step in the production of conventional roof tiles, besides promoting the reuse of the waste, being an alternative that contributes to the sustainability and environmental preservation.

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Highlights

•

The water content and curing temperature are decisive to improve the characteristics of grog and metakaolin-based geopolymers manufactured by pressing;

•

The curing process at room temperature, a longer period should be applied to produce geopolymeric ceramic pieces with better technological characteristics;

•

Geopolymerization of bricks using the ceramic waste as the metakaolin substitute and the pressing method for their production is feasible;

•

The low water absorption presented by the method is also very efficient for roof tiles manufacturing;

•

The CO2 emission is low compared to the firing step in the production of conventional bricks and roof tiles, besides promoting the reuse of the waste.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: