Durability of Supplementary Cementitious Material (SCM) Mortars Under In Situ Geothermal Water Exposure: Site-Specific Field Responses at Three Thermal Complexes in Turkey

Concrete in geothermal facilities meets waters whose composition varies between sites and over time, yet field evidence on supplementary cementitious materials (SCMs) in such waters is scarce. Ten mortars, a Portland cement reference and silica fume, blast furnace slag and Class F fly ash at 5, 10 and 15% replacement were exposed for nine months in perforated containers in the supply wells of three thermal complexes in north-eastern Turkey. Compressive strength and four transport indicators were measured at 3, 6 and 9 months; nine-month strength and capillary uptake were compared with laboratory controls of the same age, with bootstrap uncertainty and multiple comparison control. At Pasinler and Aziziye Ilıca, two sodium chloride–bicarbonate waters at 34.5–37 °C, the field specimens averaged 8.8 and 5.8 MPa above their controls, and slag gave the highest strengths (101.2 and 98.2 MPa). At Nene Hatun, a sulphate-bearing water at 46–49 °C, the mean field control difference was 0.4 MPa; the reference combined the highest strength with the highest water uptake of its shortlist. Relative to the controls, capillary uptake fell at Nene Hatun, changed little at Pasinler and rose at Aziziye Ilıca. SCM selection for geothermal service is therefore site-specific, and strength alone can misrank mortars at sulphate-bearing sites.