Abstract

Objective Fiber reinforcement proves to be an effective approach in enhancing the mechanical properties of oil well cement.Sepiolite fibers， known for their remarkable dispersibility in cement slurry and strong bonding to the cement matrix， are promising candidates. However， the presence of high impurity levels and significant water absorption in natural sepiolite fibers not only hinder the fluidity of cement slurry but also compromise the compressive and tensile strengths of cement paste， thereby constraining the broader utilization of sepiolite fibers in cement-based materials. To overcome these challenges，this study employed a hydrochloric acid-hydrothermal purification method to refine natural sepiolite fibers， followed by surface coating with SiO₂ using tetraethyl orthosilicate （TEOS）.The impact of modified sepiolite fibers on the properties of cement slurry，mechanical strength，and microstructure of oil well cement was systematically investigated. The findings and methodologies presented in this study offer valuable insights into harnessing sepiolite fibers in various applications involving cement-based materials.

Methods This study focused on the purification of natural sepiolite fibers using the HCl-hydrothermal method， followed by the coating of SiO₂ on the purified sepiolite surface using TEOS to produce modified sepiolite fibers. Scanning electron microscopy （SEM） was employed to characterize both unmodified and modified sepiolite fibers. Subsequently， these fibers were separately incorporated into cement slurry to evaluate their impact on flowability and rheological properties. Additionally， the mechanical properties of the resulting cement paste， including compressive strength， tensile strength， and stress-strain curves， were examined to evaluate the influence of sepiolite fibers. The reinforcement mechanism of both unmodified and modified sepiolite fibers in oil well cement was analyzed using X-ray diffraction （XRD）， thermal analysis （TGA）， and SEM.

Results and Discussion Unmodified sepiolite fibers exhibited a needle-like， columnar morphology with a smooth surface， while modified sepiolite fibers displayed a thin film coating. Energy-dispersive X-ray spectroscopy （EDS） analysis revealed an increased Si content on the surface of the modified sepiolite fibers， indicating SiO₂ attachment from the TEOS hydrolysis. The specific surface area of unmodified sepiolite fibers decreased from 4.518 m² /g to 2.598 m² /g after SiO₂ coating， attributed to SiO₂ forming in and filling the internal pores of the fibers. Cement slurry containing modified sepiolite fibers showed superior flowability with improved rheological properties compared to unmodified fibers. This improvement was attributed to the SiO₂ coating reducing water absorption of the fiber， and weakening binding and adsorption of solvents by the bundled fiber structure of sepiolite， thus enhancing slurry flowability. At the same mass fraction of sepiolite fibers， cement-based material with modified fibers exhibited higher compressive and tensile strengths compared to those with unmodified fibers under identical curing times. Optimal mechanical properties were observed at 5% mass fraction of modified sepiolite fibers. After 7 days of curing， the compressive and tensile strengths of the modified sepiolite fiber cement-based material were 10.98% and 10.58% higher， respectively，than those of the unmodified sepiolite fiber cement slurry. The stress-strain curve from uniaxial compression demonstrated a significant increase in peak stress and strain of cement paste with sepiolite fiber addition， alongside a reduction in its elastic modulus. Specifically， at a 5% mass fraction of modified sepiolite fibers， the cement paste exhibited a peak stress of 35.76 MPa and a strain of 3.97×10^-2 ， indicating significant reinforcing and toughening properties. The toughening and strengthening effects of modified sepiolite fibers on the cement paste exceeded those of unmodified fibers. Sepiolite fiber addition did not induce the formation of new phases in the cement paste. Unmodified fibers inhibited cement hydration， while a 5% mass fraction of modified fibers facilitated it. Microscopic morphology results showed improved adherence of modified fibers to hydration products， leading to enhanced bonding with the cement matrix. Modified sepiolite fibers improved cement paste toughness through mechanisms involving fiber fracture and pull-out.

Conclusion This study investigated the impact of SiO₂-coated sepiolite fibers on the properties and mechanical characteristics of oil well cement slurry. The introduction of SiO₂ coating onto the fiber surface effectively reduced water absorption and enhanced the flowability of cement slurry containing sepiolite fibers. Optimal performance was achieved with a 5% mass fraction of SiO₂-coated sepiolite fibers， significantly enhancing the mechanical properties and toughness of oil well cement slurry. Furthermore,the interface bonding between SiO₂-coated sepiolite fibers and the cement matrix was notably improved. The preparation process for SiO₂-coated sepiolite fibers was found to be simple and compatible with cement slurry， offering valuable insights for their application in cement-based materials.

Keywords：sepiolite fiber； oil well cement； mechanical property； silica

张春梅（1977—），女，副教授，博士，硕士生导师，研究方向为固井新材料研发。E-mail：200531010045@swpu. edu. cn；

程小伟（1977—），男，教授，博士，四川省学术和技术带头人，博士生导师，研究方向为先进胶凝材料及其在固井中应用。E-mail：chengxw@swpu. edu. cn。

DOI：10.13732/j.issn.1008-5548.2024.04.005