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Evaluation

3 Evaluation

When FGS is introduced in an aqueous dispersion medium, hydrolysis and hydration processes begin to take place with the formation of calcium hydroxide and calcium hydrosilicates of different basicity [20]. In an aqueous dispersion medium, FGS particles are surrounded by water dipoles and adsorbed on the FGS surface (Fig. 3). The FGS particles acquire a negative charge of the surface. A double electric layer is formed on the interphase surfaces, which, together with the dispersed phase, forms FGS micelle (micelle 1) (Fig. 4).

Fig. 3. The mechanism of interaction of FGS with water micelle structure (micelle 1).

Fig. 4. Scheme of FGS: 1 – aggregate of FGS; 2 - layer of potential-determining ions; 3 - counter ions of a dense part of a double electric layer; 4 – counter ions of the diffuse layer; 5 – FGS micelle.

As the hydration reaction proceeds, reaction products are formed on the FGS surface and the solution becomes saturated with Са²⁺ and ОН¯ ions. The formed Са²⁺ ions with a larger charge and size than the Н⁺ ion replace it in the diffusion and adsorption layers, penetrate to the surface of colloidal particles with active sites, causing the chemisorption process (Fig. 5). The Са²⁺ ions, which are in excess, become potential-forming, the FGS surface is charged positively (FGS micelle - micelle 2) (Fig. 6).

Fig. 5. The mechanism of the chemisorption process on the FGS surface.         

Fig. 6. Scheme of FGS micelle structure (micelle 2).

Thus, in the aqueous suspension of FGS, two types of micelles (micelle 1, micelle 2) are formed in which the slag particles are charged negatively and positively, respectively. The presence of a double electric layer in FGS particles due to the electrostatic stability factor causes the aggregate and sedimentation stability of the “FGS-water” system.

To increase the amount of FGS particles with a positive charge of surface, slag suspensions were exposed to ultrasonic dispersion. Suspensions with concentrations of 10 g/l (1%); 30 g/l (3%); 50 g/l (5%) were used for the studies.

Table 3 shows the results of an experiment on the establishment of aggregative and sedimentation stability of FGS suspensions in an aqueous dispersion medium using ultrasonic dispersion at ν = 44 kHz, τ = 15 min and t_dm = 25 °C. Table 4 presents a comparative analysis of the sedimentation rates of FGS particles after UST and without UST.

Table 3. Time and rate of sedimentation of FGS particles in an aqueous dispersion medium.

Table 4. Comparative analysis of sedimentation rates of FGS particles in suspension after UST and without UST.

From the data given, it follows that when the slag suspension was under ultrasonication, the stability of FGS particles increased by an average of 2-3 times in comparison with slag suspensions which were not under ultrasonication.

Under the influence of ultrasonic vibrations, the slag particles are vigorously mixed in an aqueous dispersion medium and destroyed, forming FGS particles with a smaller size and a larger number of active surface centers than in the case of mechanical mixing. The slag particles after the UST also form micelles 1 and 2. The process of micelle formation with the participation of finely dispersed slags passes much faster after the UST than the same process in systems without the use of UST. Water-dispersed systems using UST behave more stable, the electrostatic factor of aggregative stability is enhanced.

To assess the effect of the prepared FGS suspensions after the UST under the recommended dispersion conditions and without the use of UST, a cement stone test was carried out on the basis of PC with the addition of a slag suspension. The results of the experiment are given in Table 5.

Table 5. Comparison of the strength characteristics of the samples with the slag suspension after the UST under the recommended dispersion conditions and without the use of UST.

From the results presented in Table 5, it follows that the strength characteristics of samples with FGS suspensions prepared using UST under the recommended dispersing conditions are higher than without the use of UST. For the first day of hardening, strength increased by 19 - 39%; for 28 days of hardening - by 19 - 36%. This confirms the effectiveness of the use of UST in the preparation of slag suspensions.

Also, studies were conducted to determine the influence of FGS suspensions prepared under dispersion conditions deviated from the recommended ones on the strength of samples. The results are summarized in Table 6.

Table 6. Strength characteristics of samples with a slag suspension using UST under dispersion conditions deviated from the recommended ones.

It follows from Table 6 that during the ultrasonic treatment of slag suspensions at t_dm> 40 °C, the strength characteristics of samples with the use of slag suspensions are significantly reduced. An increase in the dispersion temperature accelerates the thermal motion of the particles and deepens the coagulation of FGS, which is confirmed by the data given in Table 7.

Table 7. Time and rate of sedimentation of FGS particles in an aqueous medium after the UST under dispersion conditions deviated from the recommended ones.

It is established that the properties of the material modified by slag suspensions depend on the temperature at which dispersion is carried out. If the dispersing conditions are not observed and the slag suspension is heated, then a rapid coagulation process begins, which results in the aggregation of FGS particles. The mixing of cement with such a suspension results in an uneven distribution of FGS particles in the volume of the material and a deterioration in the strength characteristics of samples with FGS. Thus, to obtain a cement stone with increased strength characteristics, it is recommended to control the temperature of the dispersion process of FGS suspensions. The optimal dispersion temperature should vary between 25 ± 2 °C.