Potential application of geopolymers as protection coatings for marine concrete: II. Microstructure and anticorrosion mechanism
Introduction
Over the last decades, geopolymers have been studied aiming at toxic metal immobilization (van Jaarsveld et al., 1999, Zhang et al., 2008), waste management (Duxson et al., 2007a, van Deventer et al., 2007), and fire resistance (Cheng and Chiu, 2003) and construction repair (Hu et al., 2008). Prior research (Zhang et al., 2010) has shown that geopolymers can potentially serve as coatings for the protection of concrete structures exposed in marine environments. According to different anticorrosion mechanisms, three types of concrete protection materials have already been used: coating, pore blocker and pore liner (Medeiros and Helene, 2009). However, the anticorrosion mechanism of geopolymers, particularly the effective bonding to existing concrete, is not clear yet.
Current studies of geopolymer microstructure were focused on the morphology as well as its relationship between compositions and mechanical properties. Microstructure largely determines its final properties and performances, such as Young's modulus (Duxson et al., 2005) and mechanical strength (Yip et al., 2005). Numerous studies have investigated the interface between geopolymers and aggregates (Lee and van Deventer, 2004, Goretta et al., 2007, Subaer and Van Riessen, 2007) and the bonding strength between geopolymer and cement mortar (Hu et al., 2008). To date, little is known about the microstructure of geopolymers bound to hardened cement under marine conditions. Here, we further investigate the correlations between the microstructure of the geopolymers and their low water permeability and high anticorrosion property in sea water.
Section snippets
Materials
Metakaolin (MK) and granulated blast furnace slag (GBFS), ordinary Portland cement (OPC), standard sand, polypropylene (PP) fiber and alkaline activator solution were described previously (Zhang et al., 2010). MK was in fact the mixture of the calcined products of the kaolinite and some other impurity minerals. To compare the effects of GBFS content and liquid/solid ratio on the microstructure of the geopolymer, including pore structure and morphology, 4 reaction systems were designed as shown
Microstructure
There were many tubular crystal particles besides the bulk lamellar metakaolinite particles (Fig. 1a). These tubular particles belong to the calcination product of halloysite. The product from alkali silicate activation of MK was compact and relatively flat with 10% GBFS (Fig. 1b). An interesting feature was the special lamellar crack trace on the whole amorphous fracture surface. Corresponding mechanism accounting for the formation of this feature is not clear yet, but probably is caused by
Conclusion
The microstructure and pore structure of geopolymer were investigated by using SEM, MIP and BET. The results revealed the mechanism accounting for the low water permeability, good anticorrosion in sea water and high bonding strength to paste and mortar. Geopolymers synthesized with 90% MK and 10% GBFS at a liquid/solid ratio of 0.60 ml/g had a porosity of 22.3% and 94% open pores < 20 nm. In contrast, the well prepared OPC pastes had a porosity of 29.5% and 73.7% open pores > 50 nm. The physical
Acknowledgements
The authors thank the support by the Graduates Research & Innovation Program (CX098_126Z) of Jiangsu province as well as the suggestion and help on experiments from Professor Yang Nanru and Engineer Chen Yue.
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