NaCl gradient controls Ca leaching of C-S-H in oil well cement at 260℃: From strength enhancement to degradation

With development of deep offshore and salt-layer resources, the advancement of high-temperature salt-resistant cement becomes a globally urgent priority for cementing technology reserves. However, the coupled effects of NaCl and high temperature on the microstructure of cement pastes remain unclear. This paper investigated the microstructure change of cement pastes with different concentrations of NaCl cured at 260°C and 21 MPa using multi-scale characterization. The results showed that NaCl led to the decalcification of C-S-H, while Na + and Al 3+ enhanced the polymerization degree of C-S-H through charge compensation. This process increased the packing density of C-S-H and promoted the formation of reyerite, thereby improving compressive strength of cement pastes. However, higher NaCl concentration led to leaching of interchain calcium, causing structural degradation of xonotlite. The reduced free energy of Ca dissolution enhanced the thermodynamic spontaneity of transformation from xonotlite to wollastonite. This transformation caused the strength retrogression of cement pastes at high temperature.