In this study, natural rock asphalt with a high softening point was selected as an oil-well cement admixture. The effects of the asphalt, which was subjected to a low-temperature plasma treatment, on the mechanical properties and microstructure of the cement paste were investigated. The results show that the hydrophilicity of the surface of the rock asphalt, and the interfacial bonding between the asphalt and cement matrix were improved with the use of the low-temperature plasma technology. The addition of the treated rock asphalt reduced the compressive strength of the cement paste but improved its tensile strength. Triaxial stress-strain curves, obtained under static and multicycle loads, showed that the rock asphalt improved the toughness of the oil-well cement paste. The addition of the 3% rock asphalt obviously enhanced the mechanical deformability of the cement paste. The results of the scanning electron microscopy analysis showed that the rock asphalt can deflect cracks and prevent crack propagation. The ability to restrain crack propagation improved as the adhesion between the treated hydrophilic asphalt and matrix cement improved. The results of the nitrogen-adsorption tests and mercury-intrusion porosimetry tests showed that the incorporation of the rock asphalt reduced the number of harmful pores present within the cement composite. The results of the X-ray diffraction (XRD) and thermogravimetric (TG) analyses performed on the asphalt-cement composites showed that the asphalt slightly retards cement hydration.
Natural rock asphalt
Low-temperature plasma technology
Cementing is the key to oil and gas exploitation. It is the process of injecting cement slurry into the annular space that exists between the casing and formation within a wellbore, during drilling operations. The hardened cement slurry within the annular space is known as the cement sheath, and it provides structural support and zonal isolation [l]. The cement sheath is a brittle material, with low tensile strength and poor fracture toughness [2,3]. The effects of the cementing conditions, as well as the down-hole temperature and pressure conditions, on the cement sheath can cause destabilization, and expansion of the cracks within the cement stone. These effects can also impair the integrity of the cement sheath, and result in poor adhesion between the casing and formation [4,5]. The failure of the cement sheath induces annular channelling and can even trigger huge economic losses and endanger life. Therefore, it is important to improve the toughness and tensile strength of cement paste by tailoring its microstructure to prevent crack propagation.
Studies have shown that asphalt can improve the toughness of cement-based materials. Liu et al.  studied the mechanical properties of cement-emulsified asphalt mortar (CEAM) with varying asphalt-to-cement (A/C) ratios. The results showed that the toughness and 28-day elastic modulus of the CEAM increased and decreased, respectively, as the A/C ratio increased. Tian et al.  reported that the elastic modulus of a hardened cement asphalt binder decreased as the A/C ratio increased. Huang et al.  studied the effect of reclaimed pavement material containing asphalt and aggregate, also known as reclaimed asphalt...