Effect of relative humidity on the fracture and creep behaviors of cement paste: Insights from micro-indentation and 1H NMR testing

The durability of cement-based materials in extreme environments, such as high-altitude areas and deep underground, is closely linked to relative humidity (RH), while durability is mainly affected by the fracture and creep behaviors. Herein, the fracture and creep properties of cement paste (0.3 w/c and an 85:15 cement-to-fly ash ratio) under various RH conditions are examined via micro-indentation tests, and moisture distribution and pore structure are also studied by 1 H NMR and nitrogen adsorption tests. Results show that as RH decreases from 98% to 11%, the micro elastic modulus, indentation hardness, and fracture toughness increase by 48.7%, 113.3%, and 39.5%, respectively, which is attributed to the uneven water distribution and its lubricating effect. Both creep displacement and rate decline, while creep modulus increases from 23.9 GPa to 122.1 GPa, due to the hindered sliding of the C-S-H gel layer resulting from reduced moisture. Meanwhile, a decrease in saturation of gel pore (<10 nm) and transition pore (10–100 nm) can be observed, dropping by 38.3% and 49.1%. Pore saturation—particularly in gel pores—exhibits a positive correlation with both creep modulus and elastic modulus. Additionally, the micro-creep properties align with the Isotache creep model, indicating that the creep behavior of cement paste involves particle sliding and rearrangement.