Abstract
This investigative study aims to study the mechanical and morphological properties of fly ash (FA)-based geopolymer paste as a repair material when applied on ordinary Portland cement (OPC) overlay concrete. The first part of this study investigates the optimal mix design of FA-based geopolymer paste with various NaOH concentrations of 8, 10, 12, and 14 M, which were used later as a repair material. The second part studies the bonding strength using a slant shear test between the geopolymer repair material and OPC substrate concrete. The results showed that a shorter setting time corresponds to the higher NaOH molarity, within the range of 53 and 30 min at 8 and 14 M, respectively. The compressive strength of FA-based geopolymer paste was found to reach 92.5 MPa at 60 days. Also, from the slant shear test results, prism specimens with 125 mm length and 50 mm wide have a large bond strength of 11 MPa at 12 M. The scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) analysis showed that the OPC substrate has a significant effect on slant shear bond strength, where the presence of free cations of Ca2+ on the OPC substrate surface contributed to the formation of calcium alumina-silicate hydrate gel (C-A-S-H) by building various cross-links of Ca-O-Si.
Highlights
Massive efforts and attempts are made to reduce the share of using ordinary Portland cement (OPC)as the main source of concrete production
Tanakorn et al [11] studied bond strengths of a geopolymer mix made from fly ash and of slag, and the results revealed that a high bonding strength (>10 MPa) can be obtained when sodium hydroxide with sodium silicate solution is used as an alkali activator, which was in contrast to those mixes where sodium hydroxide solution or sodium silicate solution were independently used
The resulting calcium hydroxide from the hydration process of OPC, in particular, the Ca2+ ions, latter contributes with the aluminasilicate from the fresh fly ash (FA)-based geopolymer paste in the formation of calcium alumina-silicate hydrate gel (C-A-S-H) gel is expressed by Equation (6): SiO2 (OH)4 +2Al(OH)4 −1 +Ca2+ → [C − A − S − H] gel
Summary
Massive efforts and attempts are made to reduce the share of using ordinary Portland cement (OPC). Pacheco-Torgal et al [5] investigated the production of a geopolymer using tungsten mine waste, where calcium hydroxide was added as an additive to the geopolymeric mixture Their results showed a high bonding strength of the geopolymeric binders formed when compared to that of commercial repair products. Hani et al [12] investigated the bond strength of a high calcium-based geopolymer repair material (GRM) using both slant shear and split tensile tests. Based on the literature described above, there is a lack of studies that clearly describe the main bonding mechanism between geopolymer repair material and OPC concrete substrate. The authors attempted to investigate fresh and hardened properties of high calcium fly ash-based geopolymer as a repairing material, including setting time, morphological analysis, and a bonding strength mechanism against OPC substrate. The effect of variation of alkali activator concentration has been investigated
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