Modeling the degradation of Portland cement pastes by biogenic organic acids
Introduction
Concrete and cement-based materials operate in chemically aggressive conditions that may damage cement microstructures and limit the material service life. The activities of microorganisms can generate such aggressive aqueous solutions in several specific environments. The main mechanisms of biodegradation usually relate to hydrate alteration and mineralogical transformation by ion exchange between acidic effluents and the cement-based material. Bacteria catalyze the generation of sulfuric acid by hydrogen sulfide oxidation in sewage disposals, which strongly impacts the durability of concrete sewer pipes (e.g. [1], [2]). Bacteria and fungi metabolisms produce biogenic carboxylic acids in agro-industrial environments (silage effluents, liquid manure), which can severely damage concrete floors and storage structures (e.g. [3], [4]).
A case study of the detrimental impact of microorganisms on ordinary Portland cement matrices has been previously investigated by means of a bioleaching test with the fungus Aspergillus niger[5]. Fungi are liable to colonize cement matrices and lead to either a direct (physical) attack by the biofilm coating the material, which results in hyphae penetrating through the accessible pores, and/or to an indirect (chemical) attack by their metabolites [6]: chemical attack is predominant [7], [8]. The major organic acids secreted by the fungal culture during the studied bioleaching test consisted of acetic, butyric, lactic and oxalic acids. Cement pastes experienced a substantial leaching of calcium and a significant mechanical degradation highlighted by the drastic decline in Young's modulus.
The complexities arising from having many dissolved and mineral species as well as coupling with advection and diffusion mechanisms are now readily handled by reactive transport codes. These numerical tools can be used to investigate degradation processes at the laboratory scale (leaching tests) as well as to assess of the long-term behavior of similar materials at field scales in view of performance and service life modeling. Many studies have already dealt with the modeling of decalcification of cement-based materials in leaching tests by pure water [9], [10], [11], [12], [13], [14] or ammonium nitrate solutions [15]. Complex relationships between microstructural changes and effective diffusivity have been developed in some models [11], [12], [14]. However, to our knowledge, reactive transport modeling has never been applied for examining the generic case of biogenic acid degradation of cement matrices. Small organic acids have been reported to be important in the weathering of soil due to their ability to complex aluminum [16]. An important question is, therefore, to determine whether microorganism attack is a special case of acid attack on concrete [17], or whether complexation by the conjugated bases is effective too. Another question is the protective effect of organic salt (mainly calcium oxalate) precipitation at the cement surface against biodegradation [4], [18].
Thermodynamic data to permit the calculation of interaction between commonly-found organic acids (acetic, butyric, oxalic) and hydrated phases as well as ions in the pore solution of cement at 25 °C are readily available. This paper aims at modeling the fungal bioleaching test [5] with the HYTEC reactive transport model [19] by taking into account: i) the production of organic acids by microorganisms, ii) the chemical mechanisms occurring both in the cement pore water and the bioleaching reactor, iii) the mineralogical alteration of the cement matrix, and iv) the coupled evolution of porosity and diffusivity. The chemical effects (acidity, complexation, salt precipitation) of the biogenic organic acids are analyzed in details, and the model capacity for simulating the extent of cement degradation is estimated with a view to further applications of material performance with respect to biodegradation.
Section snippets
Microorganism and culture medium
The acidophilic fungus A.niger is ubiquitous and commonly found in soil environments. It is known to colonize mortar [20], and is consequently a suitable candidate for assessing the durability of cement materials with respect to bioleaching phenomena. The composition of the fungal growth medium used in this study is given in Table 1. The fungal culture medium provided the essential nutritional elements needed by the microorganisms for growth and, due to the phosphates, buffered the lixiviating
Reactive transport modeling
To describe the evolution of the mineral phases in time and in space requires the coupling of chemistry with hydrodynamic migration processes. HYTEC has been developed for the purpose of solving migration and chemical processes and can be used for calculating profiles as a function of time, type of attack solution and cement composition [19]. HYTEC is based on a finite volume scheme with representative (homogenized) elementary volumes (REV) for mass transport and a sequential iterative
Organic acid chemistry
The major organic acids secreted by the fungal culture during the bioleaching test consisted of 4 carboxylic acids: acetic, butyric, lactic and oxalic acids. Acetic (pKa = 4.75), butyric (pKa = 4.80), lactic (pKa = 3.9) and oxalic (pKa1 = 1.2, pKa2 = 4.2) acids are all weak acids though the first oxalic acid function is relatively strong.
In aqueous solution, Al3+ and Ca2+ cations are complexed by the conjugated bases of the organic acids (i.e. acetate, butyrate and oxalate anions). Fig. 1 reports the pH
Conclusion
Ordinary Portland cement pastes were severely attacked during bioleaching with an A.niger fungal culture, especially by biogenic organic acids (acetic, butyric, lactic and oxalic), and to a lesser extent respiration-induced carbonic acid. Biodegradation was not affected by any physical disruption such as hyphae penetration. The availability of thermodynamic data on organic acids (pKa, aluminum, calcium, magnesium complexes) has allowed for a coupled chemical and hydrodynamic modeling of this
Acknowledgments
The constructive and detailed comments of the anonymous reviewers are gratefully acknowledged as well as our colleagues, Jan van der Lee and Vincent Lagneau, for their tremendous contributions to the development of HYTEC.
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