ARID ZONE JOURNAL OF ENGINEERING, TECHNOLOGY & ENVIRONMENT AZOJETE September 2021. Vol. 17(3):389-402 Published by the Faculty of Engineering, University of Maiduguri, Maiduguri, Nigeria. Print ISSN: 1596-2490, Electronic ISSN: 2545-5818 www.azojete.com.ng Corresponding author’s e-mail address: changliab@gmail.com 389 ORIGINAL RESEARCH ARTICLE STRENGTH PROPERTIES AND MICROSTRUCTURAL CHARACTERIZATION OF METAKAOLIN GEOPOLYMER CONCRETE SYNTHSIZED AT AMBIENT TEMPERATURE C. H. Salihu1*, A. E. Abalaka2, R. A. Lafiya-Araga3, B. J. Olawuyi2, I. Kariim4 1Department of Building, School of Environmental Sciences, Modibbo Adama University of Technology Yola, Adamawa State, Nigeria. 2Department of Building, School of Environmental Technology, Federal University of Technology Minna, Niger State, Nigeria. 3Department of Chemistry, School of Physical Sciences, Federal University of Technology Minna, Niger State, Nigeria. 4Department of Chemical Engineering, School of Engineering and Engineering Technology, Federal University of Technology Minna, Niger State Nigeria *Corresponding author’s email address: changliab@gmail.com ARTICLE INFORMATION ABSTRACT Geopolymer concrete has been gaining extensive attention in recent years due to its numerous advantages over Ordinary Portland cement concrete in terms of reduced carbon footprint, improved mechanical strength, durability as well as chemical resistance. However, production of geopolymer concrete is usually affected by several factors such as the synthesis temperature, nature of the source material and the type of alkaline activator used. For these materials to have wider application within the Nigerian construction industry, there is a need to synthesize the concrete at ambient temperature and to examine the suitability of native Alkaleri kaolin to produce geopolymer concrete. The study presents the strength and microstructural properties of Metakaolin Geopolymer Concrete synthesized at ambient temperature. Alkaleri calcined kaolin from Bauchi state Nigeria was used as the main geopolymer precursor with sodium hydroxide and sodium silicate as the alkaline activating agent. The Geopolymer concrete was prepared with Silicon/Aluminium ratio of 2.0 and 2.5 (Geopolymer concrete M1 and M2 respectively) and cured for 3, 7, 14, 28, and 90 days at ambient condition (average temperature of 26°C and average relative humidity of 61± 15%). Fourier Transform Infra-Red, X-ray Diffraction, Thermo-Gravimetric Analysis, Scanning Electron Microscopy as well as compressive strength and split tensile strength test were conducted on the Geopolymer concrete at appropriate curing age to examine their microstructure and strength properties. The Fourier Transform Infra-Red revealed that there was an immediate geopolymeric reaction between the metakaolin and the alkali activator. The X-ray Diffraction showed that the raw metakaolin sample and both Geopolymer concrete M1 and M2 were amorphous in nature; while the Geopolymer concrete M2 (Silicon/Aluminium ratio of 2.5) exhibited good dissolution of the kaolinite which resulted in a more compact and stable structure in comparison with Geopolymer concrete M1 (Silicon/Aluminium ratio of 2.0). This was also confirmed by the Scanning Electron Microscopic images of the Geopolymer concretes. The Thermo- Gravimetric Analysis revealed that both Geopolymer concrete M1 and M2 were thermally stable at 300°C and at 800°C, only the organic phase of the geopolymer decomposed. The strength properties (compressive and tensile strength) of Geopolymer concrete M1 and M2 increased with increase in the curing age, and Geopolymer concrete M2 displayed slightly higher strength in all the curing ages as compared with the Geopolymer concrete M1. This implies that the higher the Silicon/Aluminium ratio, the higher the mechanical strength of the geopolymer. Both the Geopolymer concrete samples attained a compressive strength that can be acceptable for structural use as normal strength concrete grade C20/25 as specified in the requirement of BS EN 206- 1 2000 at 28 days curing. © 2021 Faculty of Engineering, University of Maiduguri, Nigeria. All rights reserved. Submitted 27 March, 2021 Revised 21 May, 2021 Accepted 28 May, 2021 Keywords: Kaolin Geopolymerization ambient temperature Silicon/Aluminium ratio strength properties Arid Zone Journal of Engineering, Technology and Environment, June, 2021; Vol. 17(2):389-402. ISSN 1596-2490; e-ISSN 2545- 5818; www.azojete.com.ng Corresponding author’s e-mail address: changliab@gmail.com 390 1.0 Introduction Concrete has gained universal recognition as the most utilized man-made construction material. Consequently, the global consumption of concrete which is estimated to be about 10 billion tons yearly, is believed to rise exponentially primarily been driven by urbanization, industrialization as well as economic growth and expansion taking place around the world (Thapa and Waldmann, 2018). Making cement for concrete involves heating pulverized limestone and clay to a temperature reaching 1450°C. They chemically interact to form the cementitious compounds in Portland cement. The limestone or calcium carbonate (CaCO3) is broken down to carbon dioxide gas and calcium oxide when heated to extreme temperatures (Naqi and Jang, 2019). The calcination of the limestone as well as the use of fossil fuels for the heating process releases carbon dioxide into the atmosphere, contributing to global warming. The emissions by cement manufacturing process contributes about 5-7% to the global carbon dioxide emission, estimating it to one tone of carbon dioxide been released to the atmosphere when one tone of Ordinary Portland cement is manufactured (Das et al., 2018; Andrew, 2018). Also, about 1.5 to 1.8 tons of limestone and 0.4 tons of clay are needed to produce every ton of Portland cement (British Geological Survey, 2005). Therefore, the production of Portland cement is an extremely resource and energy intensive process. The increased use of cement in concrete causing environmental concerns in terms of emission of carbon dioxide during cement manufacture has brought pressure on the Scientists and Engineers to research on other alternative supplementary materials in order to reduce cement consumption in the construction industry. In that regard, geopolymer cements have been proposed in recent years by researchers (Lateef et al., 2016; Shabarish et al., 2018 and Liang et al., 2016) as a greener and more sustainable alternative to OPC and also to address the environmental problems related to the disposal of industrial waste and by-products while reducing costs. Consequently, geopolymerization became popular and geopolymer binders such as Fly Ash (FA), Granulated Blast Furnace Slag (GBFS), Metakaolin (MK) and Silica Fumes (SF) which are termed alumina-silicate materials were adopted and considered more ecologically friendly alternative to Ordinary Portland Cement as their production does not involve limestone calcinations. Geopolymer binders or cements are used together with aggregates and an alkaline activating solution through a process called geopolymerization to produce geopolymer concretes. Geopolymer concrete (GPC) have been proven by research (Al-Shathr, 2016; Neupane et al. 2018) to have superior advantages than Ordinary Portland Cement concrete. Such advantages include but not limited to high early strength, exceptionally high thermal and chemical stability and improved durability that makes this type of concrete acceptable for construction work. However, geo-polymerization of alumina-silicate materials is affected by several factors which include the synthesis temperature, actual chemical composition of the source material, its particle size distribution, the type of alkaline activator used and the curing temperature (Lateef et al., 2016). In recent times, investigations have centred on the mechanical, microstructural and durability properties of geopolymer concrete (Diaz-Loya et al. 2011; Bashir et al., 2017; Fernandez- Salihu et al.: Strength properties and microstructural characterization of metakaolin geopolymer concrete synthsized at ambient temperature. AZOJETE, 17(3):389-402. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng Corresponding author’s e-mail address: changliab@gmail.com 382 Jimenez et al., 2006). On factors that affect the geopolymerization, Barbosa et al. (2000) specified on the nature of source material, that geopolymers made from calcined source materials, such as MK, FA, and GBFS yield higher compressive strength when compared to those synthesized from non-calcined materials, such as kaolin clay. Xu and Van Deventer (2000), carried out a study on 15 different natural alumina-silicate materials and reported that alumina-silicates with increased rate of dissolution after polymerisation attained higher compressive strength. The authors concluded that the chemical composition of the alumina- silicate materials such as the percentage of Calcium Oxide (CaO), Potassium Oxide (K2O) as well as the molar ratio of Silicon/Aluminium (Si/Al) in the source material, the type of alkaline activator and the molar ratio of (Si/Al) in the alkaline solution during dissolution had significant effect on the compressive strength of the GPC. According to Khale and Chaudhary (2007) the Si/Al ratio of the source material substantially affected the compressive strength of the resulting geo-polymeric products, which increased almost linearly with the Si/Al ratio of geopolymer precursors. A similar conclusion was also pointed out in the study conducted by He et al. (2011) on MK and Red Mud – FA GPC to evaluate the effect of the source materials on the strength and microstructural properties. Correspondingly, in another study, Najet et al. (2013) found that the Si/Al ratio of geopolymer precursor has a significant effect on the mechanical properties and the microstructure of the resulting geo-polymeric products. The authors suggested that more effort should be made to investigate the effects of compositions of alumina-silicate materials from different origin and geopolymers induced under some reaction conditions such as low temperature synthesis (Rovnanik, (2010). Some studies (Duxson et al., 2007; Kong et al., 2007) carried out on the geopolymeric synthesis of MK to produce GPC revealed that the chemical composition of MK, synthesis and curing temperature significantly affect the final properties of a geo-polymeric product as well as recommend the need to research on locally available MK to understand the extent of reactivity when influenced by different experimental parameters such as Si/Al and ambient temperature synthesis. Based on these, Nigeria as a developing country has a competitive advantage in the development of this sustainable construction material, because of the large deposit of kaolin scattered in different part of the country. Therefore, the study focuses on a developing field that utilizes cheap and abundant alumina-silicate materials such as kaolin from Alkaleri in Bauchi State, Nigeria to make GPC at ambient temperature aimed at examining the strength and microstructural properties with respect to the curing ages of the GPC. This is to identify what variable works best for the indigenous kaolin at the Nigerian climatic condition. 2.0 Materials and Methods 2.1 Kaolin The kaolin clay which is off-white in color and in powdered form was purchased from local miners at Pali district of Alkaleri Local Government Area, Bauchi State, Nigeria. The kaolin sample was calcined in a laboratory furnace at a temperature of 700°C for 3hours to form MK. The metakaolin obtained was sieved in a 75µm sieve and its chemical composition was examined by way of X-ray fluorescence, which was done at Nigerian Geological Survey Agency Kaduna, following the method outlined by Usman et al. (2020). Arid Zone Journal of Engineering, Technology and Environment, September, 2021; Vol. 17(3):389-402. ISSN 1596-2490; e-ISSN 2545- 5818; www.azojete.com.ng Corresponding author’s e-mail address: gukopn@unijos.edu.ng 383 2.2 Alkaline Solution 12M of Sodium hydroxide solution and sodium silicate solution containing 30.2% SiO2, 22.8% Na2O and 47% water of crystallization were used as the alkali activator to induce the geopolymerization process. The alkaline activating solution was prepared 24 hours prior to the mixing of concrete (Kamarudin et al., 2011). 2.3 Aggregate Aggregate mostly provide volume stability to concrete and improves its durability. Aggregate which forms 75-80% of the volume of concrete is classified as fine and coarse aggregate (Neville 2000). Sharp river sand was used as fine aggregate because of its lower silt and clay content which can adversely affect the strength of the concrete. It was prepared at saturated surface dry condition following the procedure as outlined in ASTM C128. While crushed granite was used as the coarse aggregate. The coarse aggregates were kept at saturated surface dry condition by soaking in water for 24 hours and surface drying it with a dry towel until all surfaces were dry before experimentation. 2.4 Experimental Procedures For metakaolin (MK) geopolymer concrete (GPC), a total number of fifty (50) 100 x 100 x 100mm concrete cubes as one of the specified size by ASTM C39/C39M-03 (for compressive strength tests) and thirty (30) 150mmꝊ x 300mm concrete cylinders as specified by ASTM C496/C496M-17 (for split tensile tests) were casted. The experiment was designed for twenty- five (25) cubes, that is (5 cubes for each curing age) and fifteen (15) cylinders (3 cylinders for each curing age) for each Si/Al ratio GPCM1 (Si/Al: 2.0), GPC M2(Si/Al: 2.5). The ASTM standard stipulated a minimum of three specimens for an average strength value at a particular curing age, hence the selection of five (5) number of specimens for compressive strength test and three number of specimens for tensile strength test at each curing age. The curing ages considered for the study were 3, 7, 14, 28- and 90-days. Tables 1, 2, and 3 gives details of number of cubes cast, material composition, specimen number and ratios adopted for the GPC samples. Table 1: Number of cubes for curing age/Si/Al ratio Curing Age No. of cubes for compressive strength test GPCM1(Si/Al: 2.0) GPCM2(Si/Al: 2.5) No of cylinders for tensile strength test GPC M1 GPC M2 3 3 3 3 3 3 3 3 3 3 15 15 3 7 14 28 90 Total 5 5 5 5 5 5 5 5 5 5 25 25 Table 2: Volume of materials for Geopolymer concrete Specimen Si/Al Ratio MK (kg) Fine aggregate (kg) Coarse aggregate (kg) NaOH (kg) Na2SiO3 (kg) Water (kg) GPC M1 2.0 1.87 2.52 5.90 0.41 0.53 0.65 GPC M2 2.5 1.87 2.52 5.90 0.26 0.68 0.65 Salihu et al.: Strength properties and microstructural characterization of metakaolin geopolymer concrete synthsized at ambient temperature. AZOJETE, 17(3):389-402. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng Corresponding author’s e-mail address: changliab@gmail.com 384 Table 3: Ratios of materials for Geopolymer concrete Specimen Si/Al Na2SiO3/NaOH SiO2/Na2O Concentration of NaOH (Mol/L) H2O/MK GPC M1 2.0 1.3 2.4 12 0.35 GPC M2 2.5 2.6 2.4 12 0.35 2.4.1 Geopolymer Concrete Production. For the purpose of this study/research, MK GPC was prepared at Si/Al ratios of 2.0 and 2.5, cured at ambient temperature for 3, 7, 14, 28 and 90 days. The experiment adopted the separate mixing method as reported by Rattanasak and Chindaprasirt (2009). The Alkaline activating solution was prepared 24 hours prior to the time of mixing the concrete. This was done by dissolving 480g of Sodium Hydroxide pellets in one liter of water and allowed to cool down to room temperature. The Sodium silicate solution was weighed based on the Si/Al ratio and added to the sodium hydroxide solution (Usman et al., 2020). Predetermined quantity of dry MK powder for each Si/Al ratio and the alkaline solution (table 2) were poured into the concrete mixer and the precise quantity of water according to the water/cement ratio was also added. These materials were mixed for 20 minutes and the fresh geopolymer matrix was poured into 50mm cubic steel molds which were used to examine the microstructure after geopolymerization. The required measure of fine and coarse aggregates (table 2) was poured into the geopolymer paste and the mixing continued for another 20 minutes. The fresh GPC obtained was then transferred into 100mm cubic steel molds for compressive strength test and 150mm Ɵ x 300mm cylindrical steel molds for tensile strength test. Curing of the geopolymer specimens was done in a laboratory ambient environment for 3, 7, 14, 28 and 90 days. The average temperature of the laboratory as at the time of curing was 26.6°C. The relative humidity was 62% at 3- and 7-days curing, 31% at 14 and 28 days curing and 22% at 90 days curing age. During curing, all specimens were covered with a plastic film to prevent quick drying of the concrete specimens. 2.4.2 Microstructural Characterization of Geopolymer Fourier Transform Infra-red (FTIR), X- Ray Diffraction (XRD), Thermo-Gravimetric Analysis (TGA) and Scanning Electron Microscopy (SEM), were the key tools used in this study for investigating the composition, microstructure, chemical elements and the thermal properties of the constituent materials used in the experiment and the geopolymer produced. 2.4.2.1 Attenuated Total Reflectance – Fourier Transform Infra-Red (ATR-FTIR) Fourier Transform-Infrared (FTIR) spectroscopy was used to characterize and identify the geopolymerization by means of transmitting the infra-red radiation directly through the sample. This was done at National Research Institute for Chemical Technology (NARICT) Zaria Kaduna State, Nigeria. The specimens were ground to powder and small amount of potassium bromide (KBr) was added to each sample and they were placed in a mould. Cold press machine was used to press the mould which contains the geopolymer powder and potassium bromide (KBr) at 6 ton for 3 minutes to form pellets for examination. Shimadzu FTIR 8400s Fourier Infra-Red Spectrometer (U.S.A) was used to evaluate the functional group of the sample. FTIR absorption spectra were Arid Zone Journal of Engineering, Technology and Environment, September, 2021; Vol. 17(3):389-402. ISSN 1596-2490; e-ISSN 2545- 5818; www.azojete.com.ng Corresponding author’s e-mail address: gukopn@unijos.edu.ng 385 recorded in the range of 400 – 4000cm-1 using a spectral resolution of 2 cm-1. The spectrum of each sample represents an average of 32 scans (Mousa 2013). 2.4.2.2 X-ray Diffraction (XRD) X-ray diffraction on MK and resulting geopolymers was done to give more details about the microstructure and chemical composition of the GPC. Following the procedure reported by Soleimani et al. (2012), the GPC specimens were grounded into powder and scanned with CuKα radiation having generator voltage of 45 kV and tube current of 40mA. The diffractive patterns of the MK and GPC were gotten at 1° per 3.5 minutes over an interval of 2θ = 5° - 80° and steps of 2θ = 0.02°. The procedure was performed at the Nigerian Geological Survey Agency in Kaduna, Nigeria using Empyrean XRD diffractometer by P-Analytical B.V (Netherlands). 2.4.2.3 Thermal Gravimetric Analysis (TGA) Thermal Gravimetric Analysis is a method of thermal analysis in which changes in weight of a sample is measured as a function of temperature or time in a controlled atmosphere. TGA was carried out at Step-B Laboratory of Federal University of Technology, Minna using TGA 4000 manufactured by PerkinElmer (Netherlands). 13.836 mg of GPC M1 and 16.367 mg of GPC M2 in powdered form were scanned (heated) from 50oC to 900oC at 10oC /min in a nitrogen environment at a flow rate of 20 ml/min (Rosas et al., 2014) 2.4.2.4 Scanning Electron Microscopy (SEM) Scanning Electron Microscopy (SEM) was carried out at Chemical Engineering Department of Ahmadu Bello University, Zaria on the impact fractured surfaces of the different compositions of the geopolymer. The concrete specimens were cut into thin sections and coated with gold before transferring to the sample holder for imaging using JEOL-6400 Model Scanning Electron Microscope (Kamarudin et al., 2011). The images were captured at 5mm working distance with the accelerating voltage of 15kv. The Emission current was 10µA which was adjusted from time to time. 2.4.2.5.1 Strength Properties of Geopolymer Concrete Compressive strength and split tensile strength tests were performed on the GPC to examine their mechanical properties at different Si/Al ratio (2.0 and 2.5) and at different curing ages considered (3, 7, 14, 28 and 90 days). Compressive strength test of the geopolymer specimens was performed according to the specifications of ASTM C39/C39M-03. Specimens for the compressive strength test were crushed using the Compression Testing machine (10KN) MODE_JYS2000A at the Building Department laboratory, FUT Minna. An average crushing load of 5 specimens was obtained for each of the curing age and the compressive strength (N/mm2) was calculated by dividing the maximum crushing load (N) by the area of the concrete cubes (mm2). Salihu et al.: Strength properties and microstructural characterization of metakaolin geopolymer concrete synthsized at ambient temperature. AZOJETE, 17(3):389-402. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng Corresponding author’s e-mail address: changliab@gmail.com 386 Split tensile strength test was carried out on the GPC sample at the appropriate curing age using the modified form of ASTM C496/C496M-17. The concrete cylinders were crushed using the Universal testing machine UTM3000 (30KN) at the Building department Laboratory, FUT Minna. For better comparison of result and to obtain an average tensile load, 3 samples were tested for each curing age. The tensile strength was found by dividing the maximum crushing load by the cross-sectional area of the cylinder. (1) 3.0 Results and Discussion 3.1 Chemical Composition of Metakaolin Table 4 present the oxides found in metakaolin, having Al2O3 and SiO2 as the major oxides and traces of other oxides. As seen on the percentages of the major oxides, the metakaolin has Si/Al ratio of 1.65 making it adequate for geopolymerization (Kim, 2012). Table 4: Chemical composition of Metakaolin Oxides Al2O3 SiO2 SO3 K2O CaO TiO V2O5 Cr2O3 MnO Fe2O3 Mass (%) 34.30 54.70 - 0.771 0.385 3.87 0.17 0.054 0.01 2.02 Oxides Ga2O3 ZnO IrO2 Re2O7 Eu2O3 L.O.I NiO Yb2O3 CuO Mass (%) 0.041 0.01 0.07 0.13 0.084 3.30 0.006 0.040 0.016 Source: XRF Result from Nigerian Geological Survey Agency Kaduna 3.2 Sieve Analysis of Fine and Coarse aggregate The results of the sieve analysis for the fine and coarse aggregate are shown on the grain size distribution curve (Figure 1). Figure 1: Grain size distribution curve of fine and coarse aggregate Coefficient of Uniformity (Cu) and coefficient of curvature (Cc) of fine aggregate Coefficient of uniformity (Cu) = 2.96 Coefficient of curvature (Cc) = 0.95 0 10 20 30 40 50 60 70 80 90 100 0.01 0.1 1 10 100 C u m u la ti ve p as si n g (% ) Sieve size (mm) Fine Aggregate Coarse Aggregate Arid Zone Journal of Engineering, Technology and Environment, September, 2021; Vol. 17(3):389-402. ISSN 1596-2490; e-ISSN 2545- 5818; www.azojete.com.ng Corresponding author’s e-mail address: gukopn@unijos.edu.ng 387 Classification of sand = Uniformly graded or poorly graded since it did not meet the criteria Cu ≥ 6 for well graded soil and 1