Traceability of lime content in long-term lime-treated soils by EDTA titration based on nitric acid

The lime content of lime-treated soils measured by the traditional disodium ethylenediaminetetraacetate (EDTA) titration method based on NH4Cl (ammonium chloride) attenuates with curing time, which makes the original lime addition during onsite construction difficult to trace. In this study, the mechanism of this deviation was identified and a modified EDTA titration method based on HNO3 (nitric acid) is proposed to recover the original lime content. Exchangeable Ca2+ cations from lime are gradually transformed into acid-soluble forms (calcium carbonate, calcium (alumino)silicate hydrate constituents etc.) by carbonation and pozzolanic reactions. However, use of the traditional 10% ammonium chloride solution with weak acidity releases only the exchangeable Ca2+; consequently, the measured lime content drops to 52–70% of the original addition after 40 days of curing. In this work, leaching with 2 mol/l nitric acid solution (NAS) combined with heating in a water bath at 50°C for 1 h was identified as the optimum pretreatment, which dissolved the exchangeable and the acid-soluble Ca2+ simultaneously. The relative error between the preset lime content and the measured lime content obtained by NAS-based EDTA titration method for lime-treated soils after curing from 0 to 90 days was less than 2%. This confirms that the results of the proposed method are therefore independent of curing time. The NAS-based EDTA titration method provides an accurate and reliable technique to trace the original lime content in engineering scenarios requiring quality traceability and forensic evaluation.