Suitability of metakaolin-based geopolymers for underwater applications
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ORIGINAL ARTICLE
Suitability of metakaolin-based geopolymers for underwater applications Joud Hwalla . Marianne Saba . Joseph J. Assaad
Received: 5 April 2020 / Accepted: 6 August 2020 Ó RILEM 2020
Abstract The development and use of geopolymers (GPs) considerably increased in the construction industry. This paper assesses the suitability of metakaolin-based GP mortar and concrete mixtures for underwater applications, including the performance comparison to cementitious (CEM)-based materials. Results showed that the flow times and rheological properties of GP mixtures are significantly higher (i.e., about 2 to 5.5-times) than CEM-based ones. This was mostly related to the high viscosity of the alkaline activator that promotes hydrogen bonding and entanglements between interstitial solution and solid particles. The GP mixtures showed no signs of bleeding and remarkably lower levels of washout loss, reflecting excellent stability as well as superior resistance to dilution, erosion, and washout phenomena upon underwater placement. Their residual compressive, flexural, and pull-off bond strengths varied from 65 to 95% (as opposed to 25–70% for CEMbased materials), revealing the suitability of metakaolin-based GPs for underwater placement. Keywords Geopolymers Metakaolin Underwater concrete Washout loss Rheology Strength
J. Hwalla M. Saba J. J. Assaad (&) Department of Civil and Environmental Engineering, University of Balamand, PO Box 100, Al Kurah, Lebanon e-mail: [email protected]
1 Introduction The understanding of underwater concrete (UWC) constituents and placement conditions is essential for successful repair or new construction of marine, hydraulic, and wastewater structures. Generally, it has been shown that the risks of washout loss increase for mixtures possessing higher workability (i.e., slump flow larger than around 600 mm), given the reduced cohesiveness and proneness of matrix separation upon underwater casting [1–3]. Compared to concrete cast in dry conditions, the residual compressive strength could improve from 72 to 90% when UWC washout decreased from 13 to 4%, respectively [4]. The drop of in situ strength is normally attributed to washing out of the cement matrix as well as alteration of concrete homogeneity due to coarse aggregate segregation during casting process. The use of high-range water reducers (HRWRs) is common to facilitate concrete spreading from the point of discharge without increasing the water-to-binder ratio (w/b) [5–7]. Antiwashout admixtures (AWAs) and finely ground pozzolanic additives are often incorporated to reduce the dilution of cementitious matrix, which enhances the resistance against erosion of exposed surfaces and maintains acceptable residual bond strengths to existing substrates and embedded steel reinforcement [4, 8, 9]. In rheological terms, the plastic UWC should possess relatively low yield stress, yet high plastic viscosity that combats washout and segregation
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