Liberty 200

The elemental composition of the cement precursors as well as the release of calcium and strontium ions in cell culture medium were analysed by Optical Emission Spectrometry (ICP-OES, Liberty 200, Varian, Clayton South, Australia). In particular, 20 mg of precursor powders were dissolved into 2 ml of nitric acid (HNO₃, Sigma Aldrich), followed by dilution to 100 ml with bi-distilled water. Then, the release of calcium and strontium ions was evaluated by submerging disk-shaped samples (diameter = 10 mm; height = 2 mm) in Dulbecco's modified Eagle's medium (DMEM/F12 GlutaMAX (Gibco, Carlsbad, CA) supplemented with 10% Fetal Bovine Serum and 1% Penicillin-Streptomycin (PAA, Pasching, Austria) and aged at 37°C and 5% CO₂. At each time point (after 1, 2, 3, 7 and 14 days) of incubation the supernatants were completely removed from the sample and collected for ion quantification (in mM): the supernatants were diluted in 0.5 ml HNO₃ and 8.5 ml bidistilled water. Three specimens for each precursor powder and cement formulation were tested. The amount of Sr²⁺ ions (in weight) released at each time point was also referred to the correspondent amount in the precursor powder (in weight) (Fig 1C).

Drinking waters analyzed in this study came directly from the water resources that supply residents in the Krupina district. The source of drinking water was exclusively groundwater, which was pumped from wells and captured springs. Water samples were taken at regular intervals in 2014−2015 and water hardness and total dissolved solids were determined in a certified reference laboratory of the State Geological Institute of Dionýz Štúr. The analyses were carried out in accordance with the Slovak drinking water standard (Government Regulation No. 496/2010 Coll., which lays down requirements for water intended for human consumption and quality control of water intended for human consumption). The total dissolved solids (TDS) of drinking water were determined using the gravimetric method on water samples dried at 180 °C [35 ]. The water hardness was expressed as the sum of the molar concentrations of dissolved Ca and Mg. Calcium and Mg were measured using inductively coupled plasma-optical emission spectrometry (ICP-OES; Liberty 200, Varian, Palo Alto, CA, USA).