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<p>Concrete sewer surfaces made with Portland Cement (PC) consistently exhibit deterioration products such as gypsum and amorphous silica, despite relatively high measured in-situ concrete&#xa0;surface pH values (typically 4–2) in relation to the lower expected pH of the attacking acid. This is because the alkalinity of concrete neutralises the sulphuric acid produced by Sulphur-Oxidising Bacteria (SOB), masking the true acidity at the surface. Consequently, the actual pH driving corrosion is problematic to measure directly, yet it is hypothesised to be as low as pH 1. This study investigates the corrosion mechanisms of PC binders under controlled sulphuric acid attack at pH 1, 2, and 4 using a laboratory titration method coupled with reactive transport modelling, and microstructural analyses, i.e., X-ray diffraction&#xa0;(XRD), Scanning Electron Microscopy (SEM), and Quantitative Evaluation of Minerals by SEM (QEMSCAN). Results&#xa0;show that at pH 1, the most aggressive condition, gypsum and amorphous silica concentrations were highest, with their content decreasing as pH increased. Reactive transport modelling confirmed that reduced availability of SO₄<sup>2</sup>⁻ and H⁺ ions at pH &gt; 1 limits gypsum formation, implying that SOB must generate acid close to or below pH 1 to account for the observed gypsum formation (and associated surface zonation) on the exposed surface. The significance of this study lies in bridging laboratory, modelling, and field observations to advance understanding of biogenic sulphuric acid corrosion. These insights improve the accuracy of durability predictions and support the development of effective mitigation strategies for concrete sewer infrastructure.</p>

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Biogenic acid attack of portland cement: reactive transport modelling, laboratory studies, and field observations of effect of different pH

  • 期刊论文

  • 发表日期：

    2025年11月29日

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  • Alice Titus Bakera,
  • Tom Damion,
  • Mark Alexander,
  • Piyush Chaunsali

摘要

Concrete sewer surfaces made with Portland Cement (PC) consistently exhibit deterioration products such as gypsum and amorphous silica, despite relatively high measured in-situ concrete surface pH values (typically 4–2) in relation to the lower expected pH of the attacking acid. This is because the alkalinity of concrete neutralises the sulphuric acid produced by Sulphur-Oxidising Bacteria (SOB), masking the true acidity at the surface. Consequently, the actual pH driving corrosion is problematic to measure directly, yet it is hypothesised to be as low as pH 1. This study investigates the corrosion mechanisms of PC binders under controlled sulphuric acid attack at pH 1, 2, and 4 using a laboratory titration method coupled with reactive transport modelling, and microstructural analyses, i.e., X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Quantitative Evaluation of Minerals by SEM (QEMSCAN). Results show that at pH 1, the most aggressive condition, gypsum and amorphous silica concentrations were highest, with their content decreasing as pH increased. Reactive transport modelling confirmed that reduced availability of SO₄2⁻ and H⁺ ions at pH > 1 limits gypsum formation, implying that SOB must generate acid close to or below pH 1 to account for the observed gypsum formation (and associated surface zonation) on the exposed surface. The significance of this study lies in bridging laboratory, modelling, and field observations to advance understanding of biogenic sulphuric acid corrosion. These insights improve the accuracy of durability predictions and support the development of effective mitigation strategies for concrete sewer infrastructure.