The subject matter of the article comprises cement-bentonite-water hardening slurries with an addition of fluidized-bed fly-ash resulting from the combustion of hard and brown coal. The main objective of the study was to determine the filtration resistance of hardening slurries in the context of long-term exposure to the filtration of an aggressive substance in relation to a cement binder. A nitric acid aqueous solution with a concentration of 0.5 M was used, which modelled acid aggressiveness. The authors studied the hydraulic conductivity of the slurries as a function of time (18 months) for the exposure to a 0.5 M solution of nitric acid. Changes in the phase composition and hardening slurry surface structures were analysed in terms of their filtration resistance to the action of acid aggressiveness. The comparative base were samples subjected to filtration in tap water (neutral environment). The article reviews a methodology for studying hydraulic conductivity (k10) of hardening slurries. It also presents a study involving the phase composition using the X-ray diffraction analysis (XRD) method, infrared (IR) spectroscopy and showing an image of the hardening slurry surface structure. The findings of other researchers that the application of fluidized-bed fly-ash additives positively impacted improving the resistance of the cement matrix to acid aggressiveness were confirmed. The hydrated colloidal compounds, identified within the subsurface zone of the slurry, which has been in contact with an acidic environment, provide extra sealing of the material structure, thus significantly limiting the access of aggressive substance to the interior of the slurry.