## 3 Results and Discussion
### Structure and Properties of Polymer-Sand Protective Coatings
At W = 0 and W = 0.036, rubbery virgin negatively charged BSMs and positively charged saturated BSMs-PDADMAC systems glue sand particles together. These microscopic events provide the elasticity of the polymer-sand crusts with no marked influence on their strength. At W not higher than 0.077, unbound and free polycation act like an additional glue, thus reinforcing the polymer-sand crust and contributing to strength growth. With an increase in the PDADMAC content within 0.077 < W < 1.0, the interval unbound PDADMAC chains prevail in the composition. Within this region, the sharp growth in the rigidity of polymer-sand crusts can be attributed to an ability of multi-charged PDADMAC macromolecules to penetrate deeper into the sand sample and to glue more particles in the sand aggregates. One of the most important operational characteristics of polymer-sand crusts is associated with their water resistance. To estimate this parameter, the crusts were sprayed with water. The removed sand was collected, dried and weighted. The loss of the weight was measured as S = W\({}_{\text{Sr}}\)/W\({}_{\text{S0}}\) \(\times\) 100%, where W\({}_{\text{Sr}}\) and W\({}_{\text{S0}}\) are the weights removed and initial sand, respectively (Table 2). The results obtained are evidence that water-resistant crusts are formed when compositions with 0 < W < 0.2 are used. An increase in PDADMAC content in the formulation is accompanied by the growth in the loss of sand under water spraying. Hence, the use of BSMs for sand treatment resulted in the formation of low-strength, elastic and water-resistance crust; whereas, the PDADMAC-based crust is characterized by the opposite properties--high strength, brittle and poor water resistance. Obviously, in order to realize the desired combination of operational characteristics of polymer-sand crusts, PDADMAC-BSMs mixtures should be used. Thus, water-resistant samples with no loss of sand can be prepared at 0 \(\leq\) W \(\leq\) 0.2. A further increase in W initiates a removal of sand with water. The correlation of water resistance and PDADMAC content is associated with the fact that an excess of unbound water-soluble PDADMAC is prone to be easily washed out from the inter-particle space of polymer-sand crust. Moreover, PDADMAC results in the hydrophilization of the surface of sand particles enhancing their slippage relative to each other. For these reasons, the accumulation of unbound PDADMAC at W > 0.2 decreases the water resistance of the polymer-sand crust and provides a \"flow\" of moistened polymer-sand composition (Figure 6a); whereas, polymer-sand samples treated by PDADMAC-BSM formulations with W \(\leq\) 0.2 are stable (Figure 6b). Thus, the variation of PDADMAC content in the formulations allows the control of operational characteristics of a polymer-sand crust. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Deposited** & **Water** & \multicolumn{5}{c}{**PDADMAC–BSM Formulation, W**} \\
**Formulation** & **(Control)** & **0** & **0.036–0.2** & **0.333** & **0.5** & **1** \\ \hline
**1** & **2** & **3** & **4** & **5** & **6** & **7** \\ \hline Loss of sand, S, \% & \(30\pm 3\) & 0 & 0 & \(11\pm 2\) & \(19\pm 3\) & \(39\pm 5\) \\ \hline \end{tabular}
\end{table} 
Table 2: Loss of sand from the polymer-sand crusts prepared with PDADMAC–BSM formulations. In order to reveal the \"structure-property\" correlation for the polymer-sand crusts, electron microscopic studies were carried out. Figure 7 shows electron micrographs of the outside surface (a), the edge (b) and transverse fault (c) of the polymer-sand crust. The treatment of sand with the polymer formulations had no significant influence on a sand porous structure (Figure 7a,b). The role of PDADMAC-BSM mixtures is associated with the formation of a polymer film on the sand particle surface, binding them with each other at the points of inter-particle contacts (Figure 7c). The thickness of film was estimated as 0.25-1 \(\upmu\)m.