Experimental

2 Experimental

In this work, a granulated blast-furnace slag subjected to grinding in a laboratory jet mill LHL-1 with a limitation of the upper grinding boundaries to a size of 1 micron was used as an object of study. The characteristics of the granulated blast-furnace slag, including chemical and phase compositions, the slag quality factor, and the basicity factor, are given in Table 1. The granulometric composition of finely ground slag (FGS) is presented in Table. 2.

Table 1. Characteristics of granulated blast-furnace slag.

Chemical composition of slag, %						Phase composition of slag, %		Slag quality factor Fq	Basicity factor Fb
CaO	SiO₂	Al₂O₃	MgO	Fe₂O₃+ FeO	Other	Glass	Crystalline phase	Slag quality factor Fq	Basicity factor Fb
45.40	38.20	8.10	3.20	0.80	4.30	93.20	6.80	1.45	1.1

Table 2. Granulometric composition of FGS.

Particle size, micron	0-0.5	0.5-1.0	1.0-1.5	1.5-2.0	2.0-3.0	3.0-4.0	4.0-5.0
Fraction content, mass.%	8.21	30.44	23.35	17.20	11.77	5.89	3.14

It follows from Table 1 that the slag basicity factor Fb = 1.1; Oxides of CaO and SiO₂ predominate in the FGS. For which reason the presence of basic calcium orthosilicates, mainly Ca₂SiO₄, is expected in FGS.

It follows from Table 2 that the FGS obtained in a laboratory jet mill has a predominant particle size of 0.5 - 1.0 micron. Such a size of FGS allows it to be attributed to submicron particles, which, like nanoparticles, tend to aggregate. The introduction of FGS into the cement composition as a dry powder does not allow obtaining stable results of physical and mechanical tests. For that reason slag suspensions based on FGS were obtained, and studies were conducted to establish the aggregative and sedimentation stability of suspensions in an aqueous dispersion medium [20]. It is established that the sedimentation process of FGS is divided into 3 periods, and the particles precipitate most rapidly in the first sedimentation period. Our earlier studies on the dispersion and homogenization of carbon nanotubes (CNTs) in a water-polymer dispersion medium by the method of acoustic cavitation (ultrasonic dispersion) made it possible to obtain CNT suspensions capable of evenly distribution in the volume of the cement matrix [21-24]. It is assumed that the ultrasonic treatment (UST) of the slag particles will lead to the destruction of larger particles and a decrease in the rate of their precipitation in the first sedimentation period.

Dispersing of FGS was carried out on the device UZDN-I. To effectively separate FGS in suspension, studies were carried out to optimize the dispersion parameters: ultrasonic vibration frequency (ν), dispersion time (τ), and dispersion temperature. The definition of each optimal parameter passed with the constancy of the others. In studies on optimization of the dispersion conditions, a suspension of FGS with a concentration of 50 g/l (5%) was used. The indicator of the dispersion quality of FGS was the compression strength of diurnal samples in the size of 20x20x20 mm, mixed with the same suspension with different dispersion parameters, i.e.

Rc _(day) = f (τ, t) (1)

Determination of the optimal time of ultrasonic treatment of FGS suspension was carried out by establishing the dependence Rc _(day) = f (τ), where τ = 10, 20, 30, 40, 50, 60 minutes. The parameters t = const, ν = const. The obtained dependence is shown in Fig. 1a. It was found that for τ ≥ 15 min, the compression strength is constant, and the optimal dispersion time of FGS suspension (concentration 50 g/l) is 15 minutes.

The effectiveness of the dispersion of FGS depends on the temperature control of the dispersion process. Determination of the dispersion temperature was carried out by establishing the dependence Rc _(day) = f (t), where t = 17, 20, 25, 30, 40, 50 ^оС. The parameters τ = const, ν = const. The obtained dependence is shown in Fig. 1b. It was found that a decrease in compression strength was observed in diurnal samples at t_dm ≥ 35 °C. At high temperatures, two competing processes occur simultaneously: diffusion transfer of particles (Brownian motion of particles) leading to an increase in the uniformity of the disperse system and coagulation, which causes the consolidation of particles’ aggregates. In the system, there is a violation of the balance of processes, which leads to a decrease in the efficiency of particle dispersion. Thus, it is recommended to carry out temperature control of the dispersion process of FGS at a temperature of 25 ± 2 °C.

a.
b.

Fig. 1. Selection of dispersion parameters for suspensions of FGS: a) Rc _(day) = f (τ); b) Rc _(day) = f(t).

The frequency of ultrasonic dispersion is constant and equal to 44 kHz.

Therefore, the dispersion and homogenization of FGS in the suspension should be carried out for 15 minutes with a frequency of ultrasonic vibrations of 44 kHz and a dispersion temperature of 25 ± 2°C.

The established dependences Rc _(day) = f (τ, t) are coherent with the curves of distribution of FGS (Fig.2) obtained on the Mastersizer 3000 (UK), which confirms the possibility of using the indirect method proposed by us in assessing the uniformity of FGS distribution both in suspension and in the volume of the cement matrix.

a.
b.

Fig. 2. The distribution curves of FGS particles in the suspension after UST: a) at t = 25^oC; τ = 15 min; b) at t = 50 °C; τ = 15 min.

From the graphs in Figure 2, it follows that carrying out ultrasonic dispersion in the absence of temperature control leads to a shift in the size of FGS particles towards the coarsening. This is explained by the fact that an increase in the dispersion temperature causes an accelerated flow of coagulation processes in FGS suspensions and leads to a regular increase in the kinetic energy that FGS acquires according to the following equation:

, (2)

where - particle diameter, m; – density, kg/m³; = 1,38·10^-23 J/K – Boltzmann's constant; Т – temperature of the dispersion medium, K; – coefficient of dynamic viscosity of the medium, Pa·s,

Therefore, the most significant parameter in assessing the quality of separation and homogenization of FGS particles in the suspension is the dispersion temperature. To maintain a constant dispersion temperature, it is necessary to carry out the dispersion of slag suspensions using a thermostat.

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