On the Role of Rock Lithotype, Porosity, and Permeability in Shear Bond Strength of Rock-Class G Cement Paste Interfaces

Summary In this study, we examine two rock lithotypes and the influence of their physical and mechanical properties on the shear bond strength and failure modes of rock-cement paste interfaces. Specifically, we investigate Class G cement paste bonded with two rock types—Botucatu sandstone and Roman travertine. Comprehensive physical and mechanical characterizations of these materials were conducted, and shear bond strength tests on the interfaces were performed under confining pressures ranging from 0 MPa to 20 MPa (2,900 psi) using cylindrical samples with inclined interfaces. The results indicate that the porosity and permeability of Roman travertine did not impact the shear bond strength of its interface with cement paste. In contrast, these factors significantly influenced both the strength and failure mode of the interface with Botucatu sandstone. Notably, the high porosity and permeability of Botucatu sandstone, along with its grain and matrix structure, resulted in a robust interface where the shear bond strength surpassed that of the cement paste. Conversely, for interfaces with less porous Botucatu sandstone and both porous and less porous Roman travertine, the shear bond strength was lower than the individual strengths of the rock and cement paste. In addition, we explore the effects of rock surface roughness and the impact of high pressure and high temperature (HPHT) on the cement paste curing process. Our findings indicate that the Mohr-Coulomb strength envelopes of the rough interface exhibit behavior very similar to that of the cement paste, and the strength envelopes of interfaces cured under HPHT closely resemble those cured under ambient conditions.