Temperature-dependent multi-ion sulfate attack and gamma-ray shielding of carbon nanomaterial-modified mortars

Carbon nanomaterials can improve the initial mechanical and transport properties of cementitious materials, but it remains unclear whether these benefits are retained when sulfate attack occurs together with chloride or magnesium ions at elevated temperature while gamma-ray shielding must also be maintained. This study therefore evaluated the mechanical, transport, microstructural and gamma-ray attenuation responses of a reference mortar and mortars containing 0.05 wt% carbon nanotubes or 0.05 and 0.10 wt% graphene nanoplatelets. Following curing, specimens were continuously immersed for four months in Na 2 SO 4 (S), NaCl + Na 2 SO 4 (CS), or MgSO 4 + Na 2 SO 4 (MS) solutions at 25 °C and 50 °C. The experimental program included measurements of compressive and flexural strength, ultrasonic pulse velocity, electrical resistivity, bulk density and linear attenuation coefficients at 662, 1173 and 1333 keV, supported by factorial ANOVA, XRD and SEM-EDS. Significant mixture × solution × temperature interactions were obtained for all measured responses (p ≤ 0.003). Relative to the 28-day baseline, compressive strength changed by +1% to +56% after four months, with the highest absolute value of 84.60 MPa obtained for GNP-0.05 under CS at 25 °C. Electrical resistivity decreased by 3–60% across all conditions, indicating increased ionic transport, while CS reduced GNP flexural strength by 6–26% even where compression and gamma attenuation increased. GNP-0.10 under CS at 50 °C increased attenuation by 25% and 24% at 1173 and 1333 keV, respectively. XRD and SEM-EDS identified sulfate and chloride-bearing reaction products together with pores, microcracks and local Mg-rich alteration. The findings suggest that the investigated exposure period represents an intermediate stage in which continued hydration and reaction product formation occur alongside ionic penetration and developing deterioration, depending on the exposure conditions. Therefore, short-term improvements in mechanical strength or gamma-ray attenuation should not be regarded as indicators of long-term sulfate resistance, emphasizing the need to consider mechanical, transport and shielding performance collectively when assessing mortar mixtures.