Simple Model for Alkali Leaching from Geopolymers: Effects of Raw Materials and Acetic Acid Concentration on Apparent Diffusion Coefficient

Abstract

This paper investigates alkali leaching from geopolymers under various concentrations of acetic acid solutions. The effects of the raw metakaolin purity as well as fly ash-based geopolymer mortars and pastes are considered. A new methodology for (acetic) acid attack is proposed, adapting standard approaches, where the concentration of the leached alkali in the exposure solution is measured over time. The applicability of a simple diffusion-based mathematical model to determine the apparent diffusion coefficient (Dapp) for geopolymer pastes and mortars was validated. At the end of the paste tests, microstructural alterations of the specimens’ cross-sections were analyzed microscopically, revealing occurrence of degradation across the outermost surface parts and, especially under acid attack, the formation of long cracks that connect the surface with the intact inner zone. Drastically different Dapp are discussed in terms of the differences in the mix designs, principally resulting in different alkali-binding capacities of the geopolymers, while the acid promoted dissolution and increased porosity. As a result of this interpretation, it was concluded that Dapp is governed mainly by the chemistry of the alkali release from the gel, as it overruled the effects of porosity and cracks.

Effect of Silica Fume on Metakaolin Geopolymers’ Sulfuric Acid Resistance

“…When geopolymers come into contact with an acidic solution, the charge-balancing alkali metals and the free alkalis in the pore solution are leached out of the system in the first step [ 40 ]. This may not necessarily lead to the destruction of the gel network, as the charge-balancing K + and Na + can be replaced by other cations [ 41 , 42 ]. The actual deterioration of the network results from the dealumination of the polymeric structure, where the aluminum is extracted due to the hydrolysis of the Si–O–Al bonds and the subsequent hydrolysis of the Si–O–Si bonds [ 40 ].…”

“…To evaluate the durability of geopolymers, in most studies, sulfuric acid [ 8 , 11 , 17 , 18 , 19 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 38 , 39 , 40 , 43 , 44 ], nitric acid [ 24 , 27 ], hydrochloric acid [ 18 , 20 , 21 , 22 , 39 , 45 , 46 , 47 ] and acetic acid [ 6 , 7 , 42 , 48 , 49 , 50 ] were applied. Tests with sulfuric acid may be seen as an accelerated laboratory-testing procedure compared to microbially induced sulfuric acid attack in sewers [ 51 ], as it can take several years to build up a pH of 1 on a biofilm at a sewer surface [ 52 ].…”

Thermally Treated Waste Silt as Filler in Geopolymer Cement

“…The same pattern was observed with the K-based geopolymers. The increasing of the pH values was due to the leaching of the alkaline elements into the leachant [ 34 , 35 ]. With increasing calcination temperatures for both the K- and Na-based geopolymers, the number of alkaline elements participating in the geopolymerization process and creating bonds in the potassium and sodium alumina–silicate gels increased [ 36 ].…”

Acid resistance of alkali-activated materials: recent advances and research needs

“…The depth to which this neutralisation proceeds is often considerable, and samples with dimension of a few centimetres can be completely neutralised (pH < 10 as determined by spraying a phenolphthalein solution) within the period of a typical laboratory experiment (several weeks to few months) or field exposure (several months to few years), particularly if the material is a low-Ca AAM [15,17]. For the comparatively simple case of deterioration of metakaolinbased AAMs in acetic acid, the leaching of alkali ions can be modelled using a single apparent diffusion coefficient for each combination of a specific material and leaching conditions [18,19]. However, the inclusion of even low to moderate amounts of GGBFS in the precursors can significantly reduce the depth of the neutralised layer [15].…”