REOLOGICKÉ VLASTNOSTI CEMENTOVÝCH ZMESÍ OBSAHUJÚCICH POPOLČEK RÔZNOU VEĽKOSŤOU ZŔN THE RHEOLOGICAL PROPERTIES OF CEMENT PASTES CONTAINING FLY ASH WITH DIFFERENT PARTICLE SIZE DISTRIBUTION REOLOGICKÉ VLASTNOSTI CEMENTOVÝCH ZMESÍ OBSAHUJÚCICH POPOLČEK S RÔZNOU VEĽKOSŤOU ZŔN THE RHEOLOGICAL PROPERTIES OF CEMENT PASTES CONTAINING FLY ASH WITH DIFFERENT PARTICLE SIZE DISTRIBUTION

The paper presents the analysis of fly ash influence on rheological properties of cement pastes with content different type of fly ash concerning particle size distribution. It was pointed that rheological properties of cement pastes containing fly ash from bituminous coal (hard coal) depend mostly on content fly ash in cement and on the quantity of fine fraction in fly ash as well as on the shape of fly ash particle.


Introduction
Fly ash presence in cement result in modification of the rheological properties of cement pastes, which means the influence on the rheological properties of the fresh concrete mixes and mortars [1][2][3][4].
Analysis of studies of many authors concerning the influence of the fly ash on the rheological properties of cement paste shows, that the addition of fly ashes to cement can have the result in improvement or deterioration of rhelogical properties [5][6][7][8][9]. Different rheological reactions have been observed of cement pastes from cement with additions of fly ash and are the summary of influence of many factors superposition [10][11]. They are mainly such factors as type and quantity of fly ash in cement, particle size, shape and porosity of grains of ash and also contents of unburned coal in ash.
In this study the researches were carried out aiming at the defining the role of factors which have influence on rheological properties of pastes from cement including fly ash.

Materials
Low-calcium fly ashes (A, B) from the bituminous coal combustion were used. The chemical composition of the materials used is given in table 1.
The phase composition of fly ashes was characterized by XRD. The following crystalline phases have been detected: quartz, mullite, hematite.
Chemical composition of fly ashes A and B Table 1 Specific surface area of fly ashes (Blaine) Table 2 Fly ash -cement blends designed for rheological studies were obtained by homogenizing the components. The ashes A and B were applied in the raw state (A, B) and the B fly ash was ground in a laboratory mill to obtain a larger Blain's specific surface area (B1, B2). The fly ash content in cement was 20, 40, The paper presents the analysis of fly ash influence on rheological properties of cement pastes with content different type of fly ash concerning particle size distribution. It was pointed that rheological properties of cement pastes containing fly ash from bituminous coal (hard coal) depend mostly on content fly ash in cement and on the quantity of fine fraction in fly ash as well as on the shape of fly ash particle.
and 80 percent wt. The Blain's specific area of the fly ash used are given in table 2.

Methods
The rheological measurements were carried out using the rotative viscosimeter type Rheotest RV -2.1, with the modified surfaces of both cylinders. All the cement -fly ash samples were prepared and measured following the same procedure and in the same conditions. The tests were performed at a constant temperature 21 °C and at a constant water to solid ratio 0.4. Measurements started 10 minutes after mixing with water. The rheological properties of pastes with fly ashes were determined from the flow curves at growing and reduced rates of shearing in the range from 0 to 146 s Ϫ1 . The yield value and plastic viscosity were determined from the descending part of flow curve, according to the Bingham's model.
The particle size analysis of fly ash was made by the laser analiser type LAU -10.32 fractions were determined in range 0,5 -200 m. On the base of size analysis of fly ash the following parameters characterised the file of particles of fly ash were determined [12]: average diameter of particle d , conventional diameters (D25, D50, D75), spherical shape coefficient ⌿ and contents of grains less then 24 m in % wt.
They were calculated by equations:

Results of analysies
In table 3 Table 4 presents calculated parameters characterising particles of the fly ashes. Analysis of the results shows, that content of fly ash from bituminous coal in cement causes the decreasing of the yield value and plastic viscosity of pastes. Moreover, it was stated that the cement pastes including low-calcium fly ash A an B with the similar chemical contents and specific surface display significant differences of rheological properties (tab. 3). In aim to explain those differences the size analysis was made of fly ash by the laser diffraction method.    Calculated parameters of particles of fly ashes Table 4 Fig The spherical shape coefficient calculation relating to the grains of fly ash A and B (tab. 4) moreover showed, that the shape of fly ash grains A is closer to the spherical shape comparing with B grains. That was confirmed by the microscope analysis SEM.
The above explains, why the pastes containing fly ash with the similar chemical composition and specific surface show differences in rheological properties of cement pastes. Figure 7 presents the influence of spherical shape coefficient ⌿ of fly ash grains and their quantities in cement on yield value of paste. Grinding of ashes results in the growth of participation of fine fractions and increase of spherical shape coefficient of fly ash grains (tab. 4), this explains the improvement of rheological properties of pastes with these ashes (tab. 3).

Conclusions
G Content of fly ash from bituminous coal in cement has the result in decrease of yield value and plastic viscosity of pastesthe more significant, the greater is contents of fly ashes in cement. G Rheological properties of pastes including fly ash from bituminous coal depend mainly on the participation of fine fractions in ashes and on shape of particles of fly ashes. The level of fluidity of cement-ash pastes is more visible at the greater number of fine fractons Ͻ 24 m in ashes and the more spherical shape of fly ash particles.