Acid attack on hydrated cement: effect of organic acids on the degradation process

Abstract Owing to their ability to form buffer solutions, the attack of organic acids on concrete structural components can be highly aggressive. This work considers the changes in microstructure, chemical and phase composition in hardened Portland cement paste (hcp) exposed to acetic acid/sodium acetate or citric acid/sodium citrate buffer solutions. The degradation products were investigated using 29 Si and 27 Al NMR spectroscopy with XRD and ICP-OES. Exposure to acetic acid/sodium acetate at pH 3.9 ≤ pH ≤ 5.5 decalcifies hcp to produce aluminosilica gels (0.1 ≤ Al/Si ≤ 0.3) with Si predominately in Q 3 /Q 4 sites and NBO values (non-bridging oxygen per Si atom) 0.6 ≤ NBO ≤ 0.9. Cross-linking processes causing the formation of the gel from C–A–S–H dreierketten incorporate Al, originally in crystalline phases and C–A–S–H phases. Degradation by citric acid/sodium citrate is governed by the precipitation of expansive calcium citrate which continuously removes degraded surface material. Pore-blocking at the degradation front inhibits acid transport deeper into the material. A new mathematical expression is presented which enables the calculation of NBO for aluminosilica gels of known Al/Si ratio from 29 Si NMR spectra despite overlapping signals. The expression was verified by a stochastic computer model based on a Si quartz lattice with substituted Al and vacancies. The model simulated the measured 29 Si NMR spectra of aluminosilica gels.