INVESTIGATION OF KAOLIN BASED ONE PART GEOPOLYMERS

1Dumlupinar University, Hisarcık Vocational School, Land Registry and Cadastrate Pr., Kütahya ORCID No : http://orcid.org/0000-0002-8976-3594 2Dumlupınar University, Faculty of Engineering, Department of Civil Engineering, Kütahya ORCID No :http://orcid.org/0000-0001-5483-2871 3Dumlupınar University, Faculty of Engineering, Department of Metallurgy and Materials Engineering, Kütahya ORCID No :http://orcid.org/0000-0003-4926-8035


Introduction
Victor Glukhovsky, in the 1960s and 1970s made major contribution in identifying both calcium silicate hydrates, and calcium and sodium alumino-silicate hydrates as solidification products in geopolymerization (Davidovits, 2020).
Davidovits began to work on new heat-resistant "plastic materials" after the catastrophic fires in France in 1970/73. Davidovits stated that these requirements can only be provided by inorganic polymermaterials and reported that a mineral such as kaolin may form a binder by alkali activation at a temperature as low as 100-150 °C, instead of heat treatment at 1000-1300°C.He introduced the term of geopolymer in 1978 (Davidovits, 1988). Davidovits has defined Si-O-Al bond as sialate bond and Si-O-Si bond as siloxo bond for aluminosilicate structures and this aluminosilicate structures known as "inorganic polymers", "mineral polymers" (Davidovits, 1982). If Si/Al molar ratiosare 1, 2 and 3, this structures respectively defined as poly (sialate), poly(sialate-siloxo) and poly (sialate-disiloxo) (Davidovits, 1976).
The geopolymer is an amorphous semi-crystalline structure with a three-dimensional Si-O-Al polymeric network. Molecular sequences of geopolymer systems with different Si/Al ratios and share of all oxygen atoms of SiO4 and AlO4 tetrahedrals shown in Figure 1 (Palomo, Grutzeck and Blanco-Varela, 1999;Provis, Lukey and van Deventer, 2005). In quadruplex coordinated Al, negative chargesare balanced with cations such as Na + , K + , Li + , Ca +2 , Ba +2 and H3O + (Allahverdi, Mehrpour and Kani, 2008) Geopolymerization is an exothermic reaction and forms three-dimensional macro molecular structures through oligomers. The structure proposed by Davidovits is given in Figure 2 (Davidovits, 1991) (Davidovits, 1994). The presence of hydroxyl ions in the alkaline medium initiates the dissolution process of the aluminosilicates and liberates the silicate and aluminate species to promote further-polymerization reaction (Komnitsas and Zaharaki, 2007) (Davidovits, 1988). The degree of dissolution depends on fineness, ion-exchange ability, concentration of alkali solutions and the structure of the precursors. It has been suggested that the geopolymerization reactions occur simultaneously and in multiple steps (van Jaarsveld, van Deventer and Lukey, 2002), (Alonso and Palomo, 2001), (Dimas, Giannopoulou and Panias , 2009).
Second one is one-part geopolymers that demonstrate significant point on geopolymer technology having been described by the first time in 2007 (Koloušek, Urbanova, Andertova, Hulinsky and Vorel, 2007) (Shi, Fernández-Jiménez and Palomo, 2011). In one-part geopolymers, solid activators such as calcium hydroxide, sodium hydroxide, calcium carbonate, or sodium metasilicate are blended with geopolymer precursors like rice husk ash, silica fume, or fly ash, then just water is added to initiate the geopolymerization. The dissolution process of the solid phase starts rapidly after adding water to the dry mixture (Abdollahnejad, Pacheco-Torgal and de Aguiar, 2015) (Ma, Long, Shi, & Xie, 2018).
Researchers have studied on one-part geopolymer system because of its economic value, whereby geopolymer compound can be prepared by just adding waterto aluminosilicate-alkali mixes (Sturm, Greiser, Gluth, Jäger andBrouwers, 2015) (Ke, Bernal, Ye, Provis andYang, 2014) (Peng, Wang, Shen and Xiao, 2015). With this approach, it is not necessary to use highly alkaline compound or sodium hydroxide solutions which prevent problems with carbonation and ageing of these solutions as well as safety issues. (Hajimohammadi, Provis and van Deventer, 2008 ). But recent investigations (Peng, Wang, Shen and Xiao, 2015)support that one-part geopolymers show low mechanical strength.
The aims of the present study arei) to evaluate usability of kaolin as raw material in geopolymersii) to determine the effect of silica fume (SF) on properties of geopolymers.
For this purpose, mechanical, mineralogical, and microstructural analyses wereconducted on geopolymer samples.

Materials and Methods
In this study, research and publication ethics principles were followed.

Materials
The Blaine fineness of kaolin (K) used in this studywas 5000 cm 2 /g. XRD patterns reveals that SF was completely amorphous anditsspecific surface area (measured with the BET method) typically ranges from 150000 to 300000 cm 2 /g. (Yeğinobalı, 2009) (Tonak, Sipahi and Atay, 1997) (Aldred, et al., 2000). Ye et al. (2016) stated that the addition of silica fumes up to 20-25% clearly has a positive effect on the compressive strength of the one-part geopolymers.This is because silica fume, which has small particle sizes and consists of a high amount of amorphous silicon dioxide, is a highly reactive pozzolanic material (Jithendra and Elavenil, 2020).
In this study, 8.48 wt% of SF with respect to the mass of total solid were used to produce different geopolymer precursors.Sodium hydroxide pellets (98% purity) was obtained from a local market in Eskişehir/TURKEY.
Chemical composition ofKand SF obtained by using an X-ray Fluorescence Spectrometer (Panalytical Axios Spectrometer)at Dumlupinar University İLTEM department were given in Table 1.

Methods
Geopolymer samples were produced by one-part production method described below. Composition and production details of geopolymer series were given in Table 2.
One-part geopolymer(OPG): aluminosilicates (K or K+SF) and the alkali (NaOH pellets) were ground together with ring mill (Figure 3) for 30 minutes. After pre-cure in plastic bags for 1 or 2 day in ambient conditions, ready mix was only mixed with distilled water and pressed in 25 mm cylindrical mold ( Figure  4.a) under 12 tons with hydraulic press (Figure 4.b).
Studies have indicated that pre-cured samples show some minor cracks compared to open cured samples without pre-curing. During pre-curing, the binder gains some strength. At a low temperature such as room temperature, the water evaporation and hence the capillary forces are not too high. Therefore, crack formation is prevented. After pre-curing, the binder acquires enough strength to withstand capillary forces and gains its ultimate strength without too many cracks (Peys, Rahier and Pontikes, 2016).For this reason, the effect of the increase in pre-curing time on compressive strength was investigated.

Figure 3. Ring Mill
Steam Cure (SC) was applied to one-part geopolymer samples in the digital water bath shown in Figure 5 (a) and the samples (25x50mm cylinder) obtained were given in Figure 5 (b).  MPa/sec. All the reported results are the means of three samples. Phase analysis of raw kaolin and onepart geopolymer samples (K-2 andK+SF-2) were obtained by using Panalytical Empyrean XRD device at Dumlupinar University İLTEM department with a scanning speed of 1 ° in a minute between 5-70 °. FT-IR spectroscopy was performed between 400-4000 cm -1 wave number using Bruker Alpha brand FT-IR device for K-2 andK+SF-2. Zeiss Supra 50VP scanning electron microscope (SEM) was used for SEM microstructure analysis of K+SF-2 (the highest compressive strength).

Compressive Strength Analysis
SF addition has positivelyaffected the compressive strengthof geopolymers as expected( Figure  6).Extending the pre-cure time up to 2 days almost doubled the compressive strength of the 1-dayprecured geopolymer samples.
A study conducted in the literature, the 3-day compressive strength of calcined kaolin with amorphous structure and high reactivity found 63 MPa at most. In this study, calcined kaolin and NaOH or Na2CO3 were used (Peng et al., 2015). However, in our study, 7-day compressive strength was obtained with a maximum of 66.73 MPa with a mixture of uncalcined kaolin and NaOH, and this value was increased up to 88.57 MPa with silica fume additive.
As the Si/Al ratio in K+SF-2increased, intensity of peaks at the 2θ range from 20° to 25° decreased as well as the amorphous pattern considerably disappeared.Increasing Si/Al ratio increases the angles of sharp peaks (Lizcano, Kim, Basu and Radovic, 2012). Sample Code absorption bands and the Si:Al ratio in the aluminosilicate frame. According to this, as the amount of Al increases, the wavenumber decreases (Milkey, 1960). In the FT-IR analysis shown in Figure8, the bands in the 3600-3800 cm -1 wave number region are caused by hydration of water connected to the structure.
Peaks   Figure 9-10 showed the microstructure of the geopolymers at different magnifications. EDX (Energy dispersive X-ray) analysis of the study were given in Figure 11-15. The rod structures seen in Figure 9-10 are insoluble sodium hydroxide crystals. The weight ratio (%) of sodium in the structure at the selected areawas 29.86% which is the average of the spectrum 1-5.The reason of the high sodium ratio was insoluble sodium hydroxide crystals given in Figure 9-10. Sodium ratio in geopolymer structure in the spectrum 2 region shown in Figure 12was only 6.53 %.

SEM Microstructures and EDX Spectrum Analysis
SEM analysis shows that the geopolymer region is much denser than the insoluble alkali region. Joints at the grain boundaries and dense structure are manifested by the increase in compressive strength values in mechanical test results. It was determined by EDX analysis that sodium aluminosilicate structures were formed as a result of geopolymerization and the best results were observed in the samples produced by SF addition.
The EDX analysis of one-part geopolymer sample showed major element of Si, Al and Na. The physical and mechanical properties of geopolymer were a function of SiO2/Al2O3 and Na2O/Al2O3 ratios (Sathonsaowaphak, Chindaprasirt and Pimraksa, 2009).
The geopolymer sample with the respectively SiO2/Al2O3 ratios of 1.53,1.96 and the Na2O/Al2O3 ratios of 0.37, 1.37 (as showed in Fig. 12 and 14) verified with reported of SEM images which the presence of strong geopolymer.

Conclusion
Results of this experimental study can be summarized as follows:  The interatomic bonds examined in the FT-IR analysis revealsgeopolymer formation.
 The SEM observations supports the presence of geopolymer bonding at the grain boundaries.
 High strength one-part geopolymers were obtained using kaolin as aluminosilicate source.
 Kaolin can be preferred for geopolymer production considering economic and ecological reasons instead of metakaolin generally used for geopolymer production.
 37.56 MPa compressive strength was obtained in the sample K-1 containing kaolin + NaOH + water, which was pre-curing for 1 day, but 66.73 MPa compressive strength was obtained in the sample K-2 containing kaolin + NaOH + water for 2 days.
 48.64 MPa compressive strength was obtained in the sample K+SF-1 containing kaolin + Silica Fume + NaOH + water, which was pre-curing for 1 day, but 88.57 MPa compressive strength was obtained in the sample K+SF-2 containing kaolin + Silica Fume + NaOH + water for 2 days.
 In a kaolin based one-part geopolymer a decrease in the reaction rate was observed as the ratio of SiO2/Al2O3 increased. In addition, the increase in the ratio of SiO2/Al2O3 due to the addition of silica fume caused the formation of silica-rich phases and increased the amount of unreacted silica.
 Silica fume addition enhanced geopolymerization and the compressive strength of geopolymers.
The study was produced from the first author's thesis. Therefore, the article does not require legal / special permission (Çetin, 2017).

Author Contributions
In this article; Sezer ÇETİN, contributed to the concept, designed the research, experimental design, experimental work, analysis of results, discussed the results and reviewed the manuscript. Mehmet Uğur TOPRAK, contributed to the concept, designed the research, analysis of results, discussed the results and reviewed the manuscript. Seher GÜZ, contributed to experimental work, analysis of results, discussed the results and reviewed the manuscript.