Insights from historical ACR-affected concrete: freeze–thaw resistance under chloride exposure

This study investigates how mineralogical and microstructural alterations associated with alkali–carbonate reaction (ACR) may influence the salt–frost resistance of approximately 90-year-old concrete from the Rupnik Military Line. Historical limestone- and dolostone-aggregate concretes (HCL and HCD) were compared with laboratory-prepared counterparts made with CEM III/B cement and aged for six months in 1 M NaOH at 60 °C (LC3NH60 and RC3NH60). The specimens underwent freeze–thaw cycling while immersed in 3% NaCl. Mass loss and dynamic modulus were monitored, compressive strength was determined for the laboratory series, and post-exposure phases were characterised by XRD and SEM–EDS. The historical concretes deteriorated substantially more than the laboratory-prepared specimens and exhibited pronounced within-group variability. After 25 cycles, mean mass losses were 71.2% for HCL and 47.7% for HCD, whereas after 30 cycles they were approximately 1.33% for LC3NH60 and 0.62% for RC3NH60. Thus, the dolostone-containing concrete showed lower mean mass loss in both series, although the historical HCL–HCD difference should be interpreted as a descriptive trend. RC3NH60 retained its compressive strength, while neither laboratory mixture exhibited a progressive strength loss. Dynamic-modulus values for severely scaled historical specimens primarily described the ultrasonic response of the remaining cross-section and did not independently demonstrate the absence of internal deterioration. Post-exposure analyses identified a greater relative abundance of monocarboaluminate in the limestone-containing concretes and of hydrotalcite-like phases in the dolostone-containing concretes, together with chloride-bearing AFm phases. These associations are consistent with possible effects of ACR-related evolution on chloride interaction and freezing behaviour, but the mechanisms were not measured directly or isolated from aggregate mineralogy, binder chemistry and pore structure. ACR did not necessarily imply reduced durability under the investigated conditions. Accelerated ageing produced relevant dedolomitisation-related phases and provided a useful controlled reference, but it neither reproduced the complete evolution of the historical concrete nor established that ACR is generally beneficial.