High Temperature Hydration of Low Density Cement Slurries Weighted with Hollow Microspheres

Hollow microspheres are made of borosilicate glass and contain in their molecules abundant SiO₂. In laboratory experiment, several pieces of set cement of low density cement slurries containing hollow microspheres are tested at 120 ℃ and 150 ℃ respectively for their properties such as compressive strength, permeability, microstructure and adjustability of thickening process, confirming the effects of hollow microspheres on the high temperature hydration characteristics of cement slurries. Laboratory study shows that at both 120 ℃ and 150 ℃, the siliceous components of the hollow microspheres participate in the hydration process of the cement, thereby inhibiting or compensatory inhibiting the strength of the set cement. At 120 ℃ only 16% hollow microspheres can in 28 d inhibit the decay of the compressive strength. At 150 ℃, silica fume and hollow microspheres have to be used together, with both amounted in the cement to at least 40%, to inhibit the decay of the compressive strength of the set cement. At elevated temperatures, hollow microspheres’ reaction with other components in the cement slurry results in the formation of significant pore contours on the surfaces of the hollow microspheres. With the extension of aging time, the total porosity of the set cement increases. The pores increased in the set cement from the cement slurry without silica fume are mainly capillary pores with diameters in the range of 50-100 nm, the permeability of the set cement increases significantly with aging time from the initial 0.003 mD to 0.011 mD, while the net increment of the permeability is not relatively small. On the other hand, the pores increased in the set cement from the cement slurry treated with silica fume are mainly gel pores with diameters less than 50 nm, and the permeability of the set cement basically does not change greatly with aging time. In the set cement without the treatment of silica fume, the Ca(OH)₂ diminishes in 14 d, reacting with the glass microspheres to produce C–S–H gels. In the hydration products of the cement slurry treated with silica fume, no Ca(OH)₂ is found and the reaction proceeds swiftly to form tobermorite phase with good ability to resist strength decay. The thickening time of the hollow microsphere weighted low density cement slurries is adjustable, and the silica fume and the hollow microsphere do not negatively affect the early-stage hydration process of the cement slurries at high temperatures.