Chiang Mai J. Sci. 2020; 47(4) : 786-795http://epg.science.cmu.ac.th/ejournal/Contributed Paper

Physical and Mechanical Properties of Diatomite/Metakaolin - based Geopolymer for Construction MaterialsSuwanan Thammarong [a,b], Narumon Lertcumfu [c], Pharatree Jaita [a,d], Nuttaporn Pimpha [e], Tawee Tunkasiri [a,f], Gobwute Rujijanagul [a,d,f,g] and Pruchya Malasri*[a,f,g][a] Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai, 50200,

Thailand.[b] Graduate School, Chiang Mai University, Chiang Mai, 50200, Thailand.[c] Faculty of Gemological Sciences and Applied Arts, Rambhai Barni Rajabhat University, Chanthaburi, 22000,

Thailand.[d] Science and Technology Research Institute, Chiang Mai University, Chiang Mai 50200, Thailand.[e] National Nanotechnology Center, National Science and Technology Development Agency, Thailand Science

Park, Pathum Thani 12120, Thailand.[f] Materials Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai, 50200, Thailand.[g] Research Center in Physics and Astronomy, Faculty of Science, Chiang Mai University, Chiang Mai 50200,

Thailand.

*Author for correspondence; e-mail: malasri0425@gmail.comReceived: 3 July 2019Revised: 11 May 2020

Accepted: 18 May 2020

ABSTRACT In this study, the geopolymer materials were prepared by geopolymerization process, using

calcined diatomite (from Lampang province) and metakaolin (from Ranong province) with alkali activators (NaOH and Na2SiO3 solutions). The fresh slurry was casted in a plastic mold with cubic shape and cured at room temperature. Effects of calcination temperature of diatomite were investigated. Material characterizations, including XRD, XRF, and SEM, were used in this work. The maximum density of the geopolymer samples was 1.43 g/cm3, observed for the sample containing diatomite which calcined at 700 °C. To determine the mechanical property (compressive strength), the geopolymers were tested after the curing process (for 28 days). The result suggested that the compressive strength of the samples can be linked with the porosity of the samples, where the sample contained diatomite which calcined at 700 °C had the highest compressive strength of 18.90 MPa.

Keywords: calcined diatomite, metakaolin, geopolymer composite, compressive strength

1. INTRODUCTIONGeopolymer is one of promising alternative

materials for many applications, which was first discovered by Davidovits [1]. The geopolymer materials have attracted much attention due to their many excellent properties such as high compressive strength, low creep, good acid

resistance, low shrinkage, short setting, and curing time [2]. Furthermore, the geopolymer materials are green building materials as they can reduce CO2 emissions in the cement manufacturing process [3]. Therefore, this type of material is increasingly developed for many building projects [4]. The

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structure of geopolymer comprises the SiO4 and AlO4 tetrahedral linked network by oxygen sharing, and the positive ions of Al3+ can balance the negative charge in IV-fold coordination [4], with the empirical formula for geopolymer

Mn [−(SiO 2) z −AlO 2] n · wH2O (1)

, where M is the monovalent alkali metal or cation such as K+, Na+, Ca2+, Li+, n is the degree of polycondensation or polymerization, and z is 1, 2, 3, or higher [4]. The geopolymer forming process is called geopolymerization which often has the reaction between chemical agents (i.e. alkaline activator) and aluminosilicate at temperature less than 100 °C. The alkaline activator for using in the geopolymerization process, are commonly sodium hydroxide (NaOH), potassium hydroxide (KOH), sodium silicate (Na2SiO3) and potassium silicate (K2SiO3) [5], while the aluminosilicate minerals are minerals composed of aluminium, silicon, and oxygen such as, metakaolin [2], fly ash [2], dolomite [2], and diatomite [6].

Diatomite is a sedimentary rock, and mainly composes of the siliceous skeletal remains of diatoms, which are a single cell plant living in salt and fresh waters. The skeletons possibly consist of various components such as alumina amorphous hydrated, opaline silica, H2O, and crystallized silica [7]. Normally, the main resource of natural diatomite for the northern Thailand is in Lampang province, where it consists of mainly clay minerals and iron oxide [6, 8]. The diatomite is usually used as raw materials in conventional products (i.e. thermal insulating bricks, filtration devices, and refractories) and advanced applications (i.e. catalytic and biological supports, functional fillers, and adsorbents) because it contains a large amount of silica in diatomite [9-10].

Kaolin is a clay mineral, which has a main chemical composition of Al2Si2O5(OH)4 [11]. When the kaolinite is calcined at 500-900 °C, the dihydroxylation reaction can occur and then the crystal structure is collapsed to produce

amorphous metakaolin (Al2Si2O7) according to losing chemically-bound water and damaging the lattice structure [12-13]. Normally, the metakaolin is white ultrafine powder. It can be dehydroxylated form anhydrous alumino-silicate and mineral kaolinite. The kaolinite is one of the aluminosilicate materials, which is described as the main pozzolanic materials, including Al2O3 and SiO2 [14] and normally dissolved in an alkali-silicate solution [3]. Nevertheless, the calcined kaolinite has less a dissolving reaction than raw kaolinite, therefore, the metakaolin can be used for geopolymerization [12-13]. The metakaolin material has many significant properties such as highly specific area, less impurity durability, and high strength [3, 14]. Therefore, a geopolymer, containing metakaolin, can present many good mechanical properties [15].

Normally, there are many factors which affect to the properties of the geopolymerization reaction such as the ratio of Si/Al, Na2SiO3/NaOH, NaOH concentration, type of aluminosilicate, curing temperature, and curing time [16]. In this work, the effect of calcination temperature (of diatomite) and curing time on the physical and mechanical properties (i.e. compressive strength) were investigated. The geopolymer samples were cured at room temperature with different curing time.

2. MATERIALS AND METHODSThe geopolymer materials were synthesized

by the geopolymerization process. Diatomite (DM) and metakaolin (MK) were used as starting materials in this study. Kaolinite mineral obtained from Ranong province, Thailand. Metakaolin was synthesized from kaolinite minerals (kaolinite minerals dehydration at 750 °C for 3 h). Diatomite obtained from Lampang province, Thailand. The diatomite was dried at 70 °C for 24 h in incubators to remove moisture, then crushed and calcined at 500 - 1200 °C for 2 h using heating and cooling rates of 5 °C/min. The chemical composition of metakaolin and diatomite was determined by X-ray

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Fluorescence (XRF), as shown in Table 1 [10]. Sodium hydroxide (NaOH) pellets, from Praarthit brand (Thailand), was mixed with deionized water with a concentration of 15 M and cooled up to room temperature. Sodium silicate (Na2SiO3) was acquired from World Chemical Industrial (Thailand) with a mole ratio of SiO2/Na2O = 2.19 and a specific gravity at 20 °C of 1.54 g/cm3. The Na2SiO3 solution was diluted with water (61.5 vol % of Na2SiO3 and 38.5 vol % of H2O). The Na2SiO3 was mixed with NaOH solution (15 M) in the ratio of 1.5 by weight percent, and then leaved it cool down to room temperature. Diatomite and metakaolin (the ratio of DM/MK = 1.33) were mixed together until they were homogeneous, then were mixed with the alkaline solution (Na2SiO3 and NaOH solution) until a homogeneous slurry was obtained. The fresh slurry was casted into molds (1.5 ×1.5×1.5 cm3) and shaken for 5 min to remove air bubbles from samples. During the curing process, the samples were covered with plastic film to prevent moisture loss. Phase formation of the geopolymer and the starting materials were identified by an X-ray diffraction technique (XRD) with using Rigaku Smartlab model from 10° to 60° with a step of 0.01°. The microstructure of metakaolin, calcined diatomite powders, and geopolymer samples were determined by a scanning electron microscope (JSM-IT300). Besides, the density of geopolymer samples aged for 28 days was examined as described in ASTM C 138 and the porosity was measured using ASTM C 642. The compressive strength of the diatomite/metakaolin-based geopolymer

samples (cube specimens) was determined in accordance with ASTM C109, using a compressive test (Hounsfield universal testing machine), with using at least three specimens. To study the effect of curing time, the samples were cured for 7, 14, 21, and 28 days at room temperature.

3. RESULTS AND DISCUSSIONThe XRD patterns of metakaolin and diatomite

are shown in Figure 1(a). The diatomite contained many phases such as magnesium aluminum silicate, iron magnesium aluminum silicate, muscovite, mica, quartz, kaliophilite, rubidium oxide, hematite, rutile, silicon phosphide, and anatase [10]. The metakaolin composed of many phases such as kaolinite, muscovite, mica, quartz, and rutile [10]. However, both powders had quartz as the major phase. Normally, the calcination is necessary to remove the hydroxyl group from the structure of diatomite with the reaction of the water molecules (Dehydroxylation) and the elimination of other organic substances in diatomite [17]. Thus, effects of calcination temperature were studied in this work. The XRD patterns of calcined diatomite are shown in Figure 1(b). It was found that the XRD patterns of calcined diatomite showed many crystalline phases such as quartz, aluminum phosphate, phosphorus oxide, srilankite, and iron oxide. However, 500 - 1000 °C and 1100 - 1200 °C samples contained quartz (SiO2) and cristobalite (SiO2) as a major phase, respectively. Moreover, the phase of iron oxide (Fe2O3) was clearly seen at a calcined temperature greater than or equal to 700 °C.

Table 1. The chemical compositions of metakaolin (MK) and diatomite (DM).

Compound(wt%) Al2O3 Fe2O3 K2O MgO MnO Na2O P2O5 SiO2 TiO2 CaO Rb2O ZrO2

MK 38.60 0.73 0.948 0.015 0.002 0.214 0.051 55.64 0.87 0.06 − −

DM 6.008 17.38 1.947 0.107 − − 0.883 59.69 0.71 0.69 0.044 0.118

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Figures. 2(a) – 2(h) show the calcined diatomite microstructures, using the SEM technique. The shape of calcined diatomite particles was an irregular cylindrical shape of diatoms skeletons, which consist of regularly pores and round cylindrical. It had a length of 5-15 µm and a cylindrical radius of 5-8 µm. The calcined diatomite mostly contained micropores which was obtained from DM porous [17]. In addition, the structure of calcined diatomite with calcined at 1100 °C and 1200 °C showed the collapsed-skeleton structure [10].

XRD patterns of the diatomite/metakaolin-based geopolymers are shown in Figure 3. The phases of the diatomite/metakaolin-based geopolymers were similar to that of the starting materials. This indicates that the major phases of diatomite/metakaolin-based geopolymers are SiO2 as quartz (for G-DM 500 – G-DM 1000) and cristobalite (for G-DM 1100 – G-DM 1200). In addition, the XRD peaks of all samples showed a broad hump at 2θ around 22-30° which indicated a high degree of disorder of silicate glass phase in geopolymer, which is a characteristic owing

to geopolymerization. This suggests that the samples contained the amorphous phase [18]. The formation of amorphous phase may be due to the dissolution of metakaolin and calcined diatomite in an alkaline activator. The results also show that the materials had restructured and transformed. This is properly due to a changing of the local bonding environment of the material [19].

The SEM images in Figure 4 (a) – (h) illustrate microstructure of the samples. DM, MK, and matrix of geopolymer were clearly observed. However, a large amount of DM was occurred due to the non – dissolving of DM and MK (some amount of MK). Furthermore, the matrix of geopolymer consisted of less dense zones and defects such as cracks and pores. The samples also exhibited a heterogeneous in the microstructure.

The density and porosity of the studied geopolymers with different calcined diatomite powders (at the curing time of 28 days) are presented in Figure 5. The density of geopolymer samples increased as the calcination temperature of diatomite increased from ~1.05 g/cm3 for the 500 °C sample to ~1.43 g/cm3 for the 700 °C sample, and then

Figure 1. XRD patterns of some starting materials (metakaolin and diatomite) (a) diatomite calcined (b) at different temperatures.

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Figure 2. SEM micrographs of the calcined diatomite powders, calcined at 500 °C (a), 600 °C (b), 700 °C (c), 800 °C (d), 900 °C (e), 1000 °C (f), 1100 °C (g) and 1200 °C (h).

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decreased for further calcination temperatures. However, the porosity data showed an opposite trend with the density data. The increasing trend of the density for the 500-700°C samples is properly due to the hydroxyl group and organic substances in diatomite were eliminated at a higher calcination temperature [17]. This can help to improve the reaction between the calcined powder and alkaline activators [20]. However, the decreasing trend of the density (for the 800-1000 °C samples) may be due to the increase of the degree of crystalline of the quartz, which can result in a lower ability of the reaction between calcined powder and alkaline activators [21]. This can produce a mismatch interface and voids between the powders and alkaline activators. For the 1100- 1200°C samples, there was a transformation from the quartz to be the cristobalite phase. This can reduce the rate of reaction to form the geopolymer composites. [21] Thus, it can be seen that the heat treatment affected to the density of the geopolymer composites.

Compressive strength of the geopolymers (with different calcined powders) and curing times (7, 14, 21, and 28 days) is shown in Figure 6. The compressive strength tended to increase with the calcined temperature up to 700 °C and then decreased for further calcination temperatures. The trend of compressive strength matched well with the porosity trend, as it is well known that a higher porosity sample often has a lower compressive strength. Furthermore, overall trend of the compressive strength was similar for each curing time condition. An increase of compressive strength with a longer curing time is propoerly due to the fact that the longer period of the curing time can improve the ability of polymerisation, as a result in a higher compressive strength value [22]. In this work, the highest compressive strength (18.90 MPa) was observed for the 700 °C smaples (G-DM 700), cured for 28 days at room temperature.

Figure 3. XRD patterns of the geopolymer with different calcined diatomite (at the curing age of 28 days): 500 °C (a), 600 °C (b), 700 °C (c), 800 °C (d), 900 °C (e), 1000 °C (f), 1100 °C (g) and 1200 °C (h).

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Figure 4. SEM micrographs of geopolymer samples with different calcined diatomite(cured for 28 days at room temperature): 500 °C (a), 600 °C (b), 700 °C (c), 800 °C (d), 900 °C (e), 1000 °C (f), 1100 °C (g) and 1200 °C (h).

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Figure 5. Density and porosity of the samples cured for 28 days at room temperature.

Figure 6. The compressive strength of diatomite/metakaolin-based geopolymers at 7, 14, 21, and 28 days and various calcined temperature of diatomite.

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4. CONCLUSIONSThe effect of calcination temperature of

diatomite on the phase formation, microstructure, density, porosity, and compressive strength of a diatomite/metakaolin-based geopolymers were investigated. The geopolymer materials were successfully synthesized by geopolymerization. The morphology of geopolymer samples showed inhomogeneous microstructure, which consisted of geopolymer gel, metakaolin, and diatomite skeletons. The maximum bulk density of the geopolymer was 1.43 g/cm3 at the calcination temperature of 700 °C which corresponded to the lowest of the porosity. The highest compressive strength was found to be 18.90 MPa for the 700 ºC sample. The trend of compressive strength was related with the porosity of the samples.

ACKNOWLEDGEMENTSThis research work was supported by the

National Research Council of Thailand, partially supported by Chiang Mai University, Research Center in Physics and Astronomy, and Materials Science Research Center, Basic Research Fund, and Global Partnership. Science and Technology Research Institute, Chiang Mai University is also acknowledged.

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