The most widely used group of sealing materials are bentonite–cement mixtures. This material is most often a mixture of bentonite, cement, water and fillers—most often fly ash and ground blast furnace slag, sometimes other binding additives, such as lime meal, are also used. The composition of the material is variable, and it is designed to reach a filtration rate of 1∙10⁻⁸ m/s and a compressive strength of 0.3 MPa after 28 days of maturation. Since the composition of bentonite–cement slurries varies over a wide range, the resulting filtration rate and strength results also vary widely. The proportions of the individual components are 3–7% bentonite, 2–32% cement and 18–43% fillers. Such mixtures obtain filtration coefficient values in the range of 1∙10⁻⁹ to 3∙10⁻⁸ m/s and uniaxial compressive strengths at the level of 0.5–3.5 MPa [19,20,21,22]. Slag–alkaline systems are also used as sealing and reinforcing slurries. This binder produced from ground blast-furnace slag has the ability to set and harden, especially in aqueous environments, and its hydration products are hydrated calcium silicates and aluminosilicates with high durability. It is characterized by high sulfate resistance and good strength parameters. Previous studies have shown that sealing slurries based on a slag–alkali binder have compressive strengths of 10–38 MPa and very good resistance to sulfate corrosion (shrinkage within 0.1–0.3) [23,24,25,26]. The basic component of clay–cement slurries is clays, which give specific properties to viscoplastic bodies. Since there is a whole range of clays with different degrees of crystallization, their properties are variable. This is particularly important because in technological practice, clays found in the area of the hydrotechnical work performed are usually used. For this reason, the necessary information on the characteristics of the clay minerals are important. It is generally known that clays do not dissolve in water, and when they are stirred, they change to a colloidal state. Clay grains in suspension have a size from 2 to 100 µm. Due to the packet structure, three groups of clay minerals are distinguished. Minerals with a packet structure (octahedral layer to tetrahedral layer) of 1:1, 1:2 and mixed are shown in Figure 1.

Representatives of these groups are kaolinites, smectites and chlorites. Not all clay minerals have equal hydration and dispersion properties in aqueous environments [27,28,29]. Kaolinite and kaolinite-like minerals have a high degree of dispersibility and can achieve good plasticity in aqueous environments. However, kaolinite lacks ion-exchange and hydration properties [30]. Chlorites in some range can be characterized by the above properties. In contrast, the minerals of the smectite group are characterized by all of the above properties. The features of clay rocks depend not only on the size of the clay particles and their quantitative composition but also on their crystallographic structure. The different crystallographic structures of clay minerals affect their different behavior in water in the presence of cations, in particular the rheological properties [31,32,33]. The spontaneous dispersion of clay particles is most common in smectites. Weak bonding between packets of this mineral occurs between exchangeable cations. This bond depends mainly on the type of exchangeable cation and does not interfere with the water penetration and their swelling, as well as the formation of complex packets. The spontaneous swelling and disintegration of the agglomerates into fine particles of minerals, which are included in sealing slurries, cause the structure formulated in this way to strongly restrict groundwater flow. However, sealing slurries based on clay with strong exchangeable properties are not long lasting. Their properties change depending on the moisture content of the system [34]. The clays included in clay–cement grouts are inert with respect to the various types of aggressive groundwater. The most resistant structures in clay–cement slurries are obtained on the basis of polymineral clays of the kaolinitic and chloritic types. Thanks to the specific characteristics of these minerals, a suitable environment is created for crosslinking processes in the slurries. When using clays for the preparation of sealing slurries, it is important to know the granulometric, mineralogical and chemical compositions. The chemical composition of clays determines their ability to form a homogeneous and dispersed suspension under the action of water. The mineralogical composition can determine the nature of clays: calcareous—with a small percent of limestone and calcium carbonate; sandy—with a high quartz content (80–90%). For sealing slurries, clay is recommended to contain the following amounts of oxides: 63–69% SiO₂; 19–29% (Al₂O₃ + Fe₂O₃); 1.6–2.6% MgO; and 0.28–3.29% CaO [2]. The granulometric composition determines the suitability of the clay for sealing slurries in terms of the rheological properties. The content of the sand and colloidal fraction determines such important characteristics of the clay slurry as the static lateral deformation, viscosity and stability. The literature shows that a sand fraction with a content of more than 10% by weight negatively affects the process of internal structure formation. Large and heavy sand particles, having a certain surface electric charge due to the fact of their structure, can entrain charged clay particles behind them and sediment. It is generally recommended that the clay fraction, with a particle size of 0.01–0.05 mm or smaller, should be approximately 89%, and the sand content should not exceed 3–5% [35,36].