Strength and Microstructure of Class-C Fly Ash and GGBS Blend Geopolymer Activated in NaOH & NaOH + Na2SiO3

Abstract

In this paper, class-C fly ash (FA) and ground granulated blast-furnace slag (GGBS)-based geopolymer activated in NaOH and NaOH + Na 2 SiO 3 was studied regarding setting time, compressive strength, porosity, microstructure, and formation of crystalline phases. When comparing the effects of alkali type on the FA and GGBS geopolymer composites, results revealed that NaOH has a lesser effect in developing strength and denser microstructure than does NaOH + Na 2 SiO 3, since the addition of Na 2 SiO 3 provides the silica source to develop more compact structure. Incorporation of Na 2 SiO 3 reduced the crystallinity and the paste was more amorphous compared to NaOH activated pastes. The class-C FA and GGBS blends resulted in prolonged setting time, reduced strength, and loose matrix with the increase in fly ash content. The un-reactivity of calcium in blends was observed with increasing fly ash content, leading to strength loss. It is evident from XRD patterns that calcium in fly ash did not contribute in forming C-S-H bond, but formation of crystalline calcite was observed. Furthermore, XRD analyses revealed that the reduction in fly ash leads to the reduction in crystallinity, and SEM micrographs showed the unreactive fly ash particles, which hinder the formation of a denser matrix.Materials 2020, 13, 59 2 of 15 is also formed in Portland cement. Its formation leads to short setting time, reduced porosity, and increased compressive strength [9,10].Fly ash has been investigated as a precursor material in geopolymer mortar. The mechanical properties of fly ash-based geopolymer depends on the chemical composition of fly ash. As explored by Davidovits, Izquierdo [11] in their study, the fly ash containing low calcium (class-F) produced a workable geopolymer; however, the high calcium in fly ash did not contribute in producing the suitable geopolymer due to flash setting time. The flash setting time in the high-calcium fly ash was confirmed in studies [12,13] where they used borax in the mixture to delay the setting time. However, regardless of this problem, the class-C fly ash has been found to help improve the strength properties due the presence of adequate calcium content, which may forms C-S-H gel in the matrix [14].Ground granulated blast-furnace slag (GGBS) is another by-product material rich in amorphous calcium, silica, and alumina, which makes it suitable to be used as a binder in the construction industry [15]. Formerly, GGBS as a precursor material in geopolymer has been investigated. It is worth mentioning that the GGBS activated in alkali solution possess a high strength and good resistance to the chemicals compared to Portland cement. However, the reduced setting time makes it susceptible to high shrinkage and develops micro cracks, which increases the porosity leading to the strength loss [16]. Since the GGBS is rich in calcium, the main reaction product is C-S-H gel, which may coexist with the geopolymer gel depending on the chemical composition of the GGBS, alkali type, and alkali amount [4,17,18].The ...

Strength, Shrinkage and Early Age Characteristics of One-Part Alkali-Activated Binders with High-Calcium Industrial Wastes, Solid Reagents and Fibers

“…The binary binder CS2 showed the formation of mainly C-A-S-H and C-S-H binding phases (with C 4 AlH 13 compound having a lower aluminum content of 3.6%) due to the higher calcium content in the system, owing to the composition of reagent 2 (Figure 15b). Similar binding phases/gels were characterized in recent studies on fly ash and slag-based pastes/mortars [57,58,71]. This additional C-S-H gel formation further densified the microstructure and improved compressive strength compared to binder CS1.…”

Incorporation of Waste Glass as an Activator in Class-C Fly Ash/GGBS Based Alkali Activated Material

“…Sodium silicate “Na 2 SiO 3 ” and sodium hydroxide “NaOH” are two common activators, either combined (in a certain ratio) or used individually to manufacture geopolymer-type binders [ 4 , 5 ]. It is reported that, Na 2 SiO 3 , when incorporated in NaOH solution, shows promising results in improving the mechanical properties of activated precursors compared to only NaOH-activated precursors [ 3 , 6 ]. Generally, NaOH works as a mineralizer, which breaks the covalent bonds present in the aluminosilicate precursor material(s), therefore, releasingthe Si and Al into the system [ 7 ].…”

“…Given that the research on a waste glass-based alkali activator is limited and no research has been conducted on the influence of this activator on the properties of high calcium (Ca) alkali-activated material (AAM), it suggests that the research reported here is warranted. Class-C FA and GGBS composed of high Ca was selected for our previous study [ 6 ]. It was determined that the Class-C FA compared to GGBS is less reactive when activated by NaOH-4M solution, but when these materials were activated by NaOH+Na 2 SiO 3 solution, the dissolution of FA and GGBS was enhanced.…”

Properties of a New Insulation Material Glass Bubble in Geopolymer Concrete

“…Fly-ash based geopolymer concretes have been synthesised from different power stations, resulting in substantially different particle size, morphology, and composition due to the different coal powders and combustion conditions in those plants [ 2 , 30 , 33 , 34 , 35 , 36 , 37 , 38 ]. Fly-ash is excellent for improving early strength and durability, rendering it suitable as the primary source materials in the synthesis of geopolymer concrete [ 2 , 33 , 36 , 37 , 38 , 39 , 40 ].…”