Evaluation of the Effect of Silica Fume on Amorphous Fly Ash Geopolymers Exposed to Elevated Temperature

magnetochemistry Article Evaluation of the Effect of Silica Fume on Amorphous Fly Ash Geopolymers Exposed to Elevated Temperature Ong Huey Li 1,2, Liew Yun-Ming 1,2,* , Heah Cheng-Yong 1,2, Ridho Bayuaji 3, Mohd Mustafa Al Bakri Abdullah 1,2, Foo Kai Loong 4, Tan Soo Jin 1,2, Ng Hui Teng 1,2 , Marcin Nabiałek 5, Bartlomiej Jez˙ 5 and Ng Yong Sing 1,2 1 Geopolymer and Green Technology, Centre of Excellence (CEGeoGTech), Universiti Malaysia Perlis (UniMAP), Perlis 01000, Malaysia; [email protected] (O.H.L.); [email protected] (H.C.-Y.); [email protected] (M.M.A.B.A.); [email protected] (T.S.J.); [email protected] (N.H.T.); [email protected] (N.Y.S.) 2 Faculty of Chemical Engineering Technology, Universiti Malaysia Perlis (UniMAP), Perlis 01000, Malaysia 3 Department of Civil Infrastructure Engineering, Faculty of Vocations, Institut Teknologi Sepuluh Nopember, Surabaya 60111, Indonesia; [email protected] 4 Institute of Nano Electronic Engineering (INEE), Universiti Malaysia Perlis (UniMAP), Perlis 01000, Malaysia; [email protected] 5 Department of Physics, Faculty of Production Engineering and Materials Technology, Cz˛estochowaUniversity of Technology, Al. Armii Krajowej 19, 42-200 Cz˛estochowa,Poland; [email protected] (M.N.); [email protected] (B.J.) * Correspondence: [email protected] Abstract: The properties of amorphous geopolymer with silica fume addition after heat treatment was rarely reported in the geopolymer field. Geopolymer was prepared by mixing fly ash and alkali activator. The silica fume was added in 2% and 4% by weight. The geopolymer samples were ◦ Citation: Li, O.H.; Yun-Ming, L.; cured at room temperature for 28 days before exposed to an elevated temperature up to 1000 C. Cheng-Yong, H.; Bayuaji, R.; The incorporation of 2% silica fume did not cause significant improvement in the compressive Abdullah, M.M.A.B.; Loong, F.K.; Jin, strength of unexposed geopolymer. Higher silica fume content of 4% reduced the compressive T.S.; Teng, N.H.; Nabiałek, M.; Jez,˙ B.; strength of the unexposed geopolymer. When subjected to elevated temperature, geopolymer with et al. Evaluation of the Effect of Silica 2% silica fume retained higher compressive strength at 1000 ◦C. The addition of silica fume in fly Fume on Amorphous Fly Ash ash geopolymer caused a lower degree of shrinkage and expansion, as compared to geopolymer Geopolymers Exposed to Elevated without the addition of silica fume. Crystalline phases of albite and magnetite were formed in the Temperature. Magnetochemistry 2021, geopolymer at 1000 ◦C. 7, 9. https://doi.org/10.3390/ magnetochemistry7010009 Keywords: geopolymer; fly ash; amorphous; silica fume; thermal performance Received: 28 November 2020 Accepted: 4 January 2021 Published: 6 January 2021 1. Introduction Publisher’s Note: MDPI stays neu- Geopolymer is one of the alternatives to ordinary Portland cement (OPC), as a con- tral with regard to jurisdictional clai- struction material, because it is more environmentally friendly [1,2]. The OPC production ms in published maps and institutio- involves the burning of large amounts of fuel, thus resulting in almost 5% of total global nal affiliations. CO2 emissions [3]. Geopolymers are inorganic materials, introduced by Davidovits [4], and are made by mixing the solid aluminosilicate source with liquid alkaline activator, which is then cured at ambient temperature or a slightly higher temperature (<100 ◦C). The aluminosilicate sources, which consist of high silica and alumina, could be obtained Copyright: © 2021 by the authors. Li- from natural resources (kaolin and metakaolin) or industrial by-products (fly ash, bottom censee MDPI, Basel, Switzerland. ash and rice husk ash). In addition, geopolymer could be used as an alternative to OPC This article is an open access article because to its properties such as higher early strength, being superior against thermal distributed under the terms and con- ditions of the Creative Commons At- resistance and lower CO2 emission [5]. tribution (CC BY) license (https:// Geopolymers are generally recognized as superior systems with respect to thermal creativecommons.org/licenses/by/ resistance. The amorphous aluminosilicate structure with three-dimensional arrangement 4.0/). Magnetochemistry 2021, 7, 9. https://doi.org/10.3390/magnetochemistry7010009 https://www.mdpi.com/journal/magnetochemistry Magnetochemistry 2021, 7, 9 2 of 14 enables geopolymers to possess ceramic-like properties and exhibit high thermal resis- tant [6]. Studies have found that geopolymers are incombustible and do not spall with extended heating as geopolymer contains lower Ca(OH)2 content in comparison to OPC [7]. Besides, geopolymers also exhibit superior thermal stability as no smokes or toxic gases are released when they are heated [8]. The excellent thermal stability of geopolymers not only increases its endurance at high temperature, but also alters its other properties and geopolymer structure. Deteriora- tion of strength of geopolymer at elevated temperatures was commonly reported [9,10]. However, an increase in mechanical strength of geopolymer when exposed to elevated temperature has also been reported [11]. Park et al. [12] observed the occurrence of fur- ther geopolymerisation reaction in class F fly ash geopolymer at 400 ◦C, which leads to increasing strength. The compressive strength dropped at a temperature beyond 400 ◦C, as result of the initiation and propagation of thermal cracks. In contrast, Wongsa et al. [13] found that higher residual strength of fly ash geopolymers incorporating natural mullite and zeolite was achieved at 900 ◦C, which is associated with the increase in crystallinity of the matrix. Heat treatment would initiate phase transformation in a geopolymer matrix. Crystalline nepheline (NaAlSiO4) is most reported in the heated geopolymer [14,15]. Based on Rovnaník and Safránková [15], albite (NaAlSi3O8) was also formed, except from nepheline, boosting the flexural strength of fly ash geopolymers. Payakaniti et al. [16] studied the relationship between thermal stability and the magnetic properties of high calcium fly ash geopolymer, and observed the formation of new phases of nepheline and zeolite and an increased hematite peak above 800 ◦C. The antiferromagnetic behaviour of hematite decreased the magnetic properties of geopolymers and eventually switched from ferromagnetic to paramagnetic properties when the temperature reached 1200 ◦C. The incorporation of nanoparticles leads to the enhancement of physical structure and mechanical strength of geopolymer matrices. Silica fume is one of the potential nanoparticles that could be introduced into geopolymer as it is a highly effective pozzolanic and possesses extreme fineness and high silica content [17]. Incorporation of silica fume in- duces Si content, which improves the yield of geopolymer gels and results in a strength increment [18–21] with lower porosity and water absorption [22,23]. Liu et al. [21] reported superior compressive strength of 151 MPa when 30% of silica fume was incorporated into the fly ash/slag geopolymer concretes. It has been proven that the addition of silica fume results in improvement of mechanical strength, durability, and structure properties of geopolymers. Despite understanding the thermal resistance of pure geopolymers without additives, and the mechanical strength development of geopolymer with silica fume addition, the influence of silica fume on the properties of geopolymers at elevated temperatures is not known. The past literature mostly covers the evolution of microstructure and mechanical strength under the influence of a silica additive. Therefore, this paper evaluates and compares the effect of the addition of silica fume on physical, mechanical properties and thermal resistance of fly ash geopolymers. A quan- titative research study was performed on fly ash geopolymer with varying dosages of silica fume (0%, 2%, and 4%). 2. Methodology 2.1. Materials The Class F fly ash used in this work was sourced from Manjung Coal-Fired Power Plant, Perak. The chemical composition of the fly ash is displayed in Table1. The fly ash is made up mainly of SiO2 (56.3%) and Al2O3 (28.0%). Figure1a demonstrates that the fly ash is highly amorphous with some crystalline inclusions of quartz, mullite and hematite. The fly ash particles revealed in Figure1b have spherical shapes with smooth surfaces and wide size distributions. Magnetochemistry 2021, 7, 9 3 of 14 Magnetochemistry 2021, 7, x FOR PEER REVIEW 3 of 15 Table 1. Chemical composition of Class F fly ash determined using X-ray Fluorescence (XRF). Table 1. Chemical composition of Class F fly ash determined using X-ray Fluorescence (XRF). Compound SiO2 Al2O3 Fe2O3 CaO Others Compound SiO2 Al2O3 Fe2O3 CaO Others Mass (%) 56.3 28.0 6.86 3.89 4.95 Mass (%) 56.3 28.0 6.86 3.89 4.95 FigureFigure 1. 1. ((aa)) XRD XRD diffractogram diffractogram and and ( (bb)) Scanning Scanning electron electron microscope microscope (SEM) (SEM) micrograph micrograph of of Class Class F F fly fly ash. ash. SodiumSodium hydroxide (NaOH)(NaOH) pellets pellets with with an assayan assay (total (total alkalinity alkalinity calculated calculated as NaOH) as NaOH)of 98.0–100.5% of 98.0–100.5% were supplied were supplied by Progressive by Progressive Scientific Scientific Sdn. Sdn. Bhd., Bhd., Selangor. Selangor. Technical Tech- nicalgrade grade liquid liquid sodium sodium silicate silicate (Na2 SiO(Na32)SiO with3) with 60.5% 60.5% H2O, H 30.1%2O, 30.1% SiO2 SiOand2 9.4%and 9.4% Na2O Na with2O with3.2 SiO 3.22 /NaSiO22/NaO ratios2O ratios was was supplied supplied by South by South Pacific Pacific Chemicals Chemicals Industries Industries (SPCI) (SPCI) Sdn. Sdn. Bhd., Bhd.,Malaysia. Malaysia. Nano-silica Nano-silica (silica (silica fume) fume) with with an average an average particle particle size ofsize 7 nm,of 7 surfacenm, surface area areaof 395 of ±39525 ± m252 /gm2/g and and density density of of 36.8 36.8 kg/m kg/m33,, suppliedsupplied by Sigma-Aldrich Sigma-Aldrich Corporation, Corporation, waswas added added into into fly fly ash ash geopolymers geopolymers as as the the additive.

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