Thermodynamic Modeling and Experimental Validation of Acetic Acid Attack on Hardened Cement Paste: Effect of Silica Fume

Abstract

Concrete structures are increasingly becoming exposed to organic acid attack conditions, such as those found in agriculture and food-related industries. This paper aims to experimentally verify the thermodynamic modeling of cement pastes under acetic acid attack. For this, a modeling approach implemented in IPHREEQC via Matlab is described, and results are compared with measured pH and compositions of equilibrated solutions (MP-AES) as well as unreacted/precipitated solids (XRF, XRD and STA) for a wide range of acid concentrations. The 11% replacement of cement by silica fume (SF) led to a 60 or 70% reduction (measured or modeled, respectively) of Portlandite content in the hardened cement paste due to the pozzolanic reaction resulting in higher content of CSH phases, which has effects on the progression of dissolution processes and a resulting pH with increased acid concentrations. Considering that no fitting parameter was used, the model predictions showed good agreement with measured values of pH, dissolved ion concentrations and composition of the remaining (degraded) solids overall. The discrepancies here were more pronounced at very high acid concentrations (equilibrium pH < ~4), i.e., after the full dissolution of hydrate phases due to limitations in the model used to describe Al-, Si- and Fe-gel phases and/or identified experimental challenges in precipitation of calcium and aluminum acetate hydrates.

Thermodynamic Modeling of Sulfuric and Acetic Acid Attack on Hardened Cement Paste: Effect of Silica Fume

“…This is contrary to the model predictions, which show strong increase after a threshold acid dosage of 4 mM/ghcp. In contrast to H2SO4, for acetic acid (AcH), the applied modelling approach showed an overall good agreement with experiments over a very broad range of acid concentrations [8], both for PC and PC+SF (Figure 4b), resulting in a relative root square mean error of below 10% for pH and calcium (in solution and remaining solids), without applying any fitting parameter. This agreement is remarkable considering a particular challenge proved to be the development of a filtration and washing process.…”

“…Already in the case of acetic acid [8] several experimental challenges were identified. The most significant concerned the washing of the solid fraction during filtration.…”

“…Notably, the mixture without silica fume exhibited a higher fraction of Portlandite. The addition of SF (11% replacement of cement) resulted in a reduction of Portlandite content from 22% to 6.7% [8], assuming a nearly complete pozzolanic reaction in the model. Consequently, higher concentrations of C-S-H phases were observed (see Figure 6).…”

“… A transition zone, being a mechanically sound region, where the pH is continuously increasing until reaching the value of the undamaged core material. With a pH drop of the pore solution, portlandite dissolves first followed by calcium silicate hydrate (C-S-H), aluminate ferrite monosulfate (AFm) and aluminate ferrite trisulfate (AFt) [4][5][6][7][8].…”

Acid Neutralization Capacity of Metakaolin‐based Geopolymers

“…The method is therefore suitable for obtaining an initial overview of the neutralization behaviour of various cement types. In other experimental methods, such as the method used by Berger et al [9], the powdered samples are stored in the acid for more than 100 days until results are available for evaluation. These may be more accurate for this purpose, as the pH has significantly more time to approach a final value.…”