INFLUENCE OF COLLOIDS

ON THE TECHNOLOGICAL PROPERTIES OF LOAMS

P. G. Usov, N. F. Voronova, E. A. Guber

(Presented by the scientific seminar of the Department of Silicate Technology)

In the study of loams of the Tomsk Region it was established [1] that the clay fraction, along with the clay minerals proper—montmorillonite and hydromicas—which are crystalline formations with a stoichiometric ratio of elements, also contains amorphized material: colloids of various degrees of maturity without stoichiometric ratios of elements.

The purpose of our investigations was to establish the influence of colloids on the technological properties of loams. The composition of the colloids was established earlier [2] and is represented by magnesial silicates corresponding to the formula MgO·$n$SiO$_2$, in which $n$ varies from 1 to 2.5. Such minerals may be kerolite or minerals of the sepiolite group—attapulgite, palygorskite—and hydrates of aluminum and iron. All of them dissolve readily in Tamm’s reagent and in acids.

In order to determine the influence of colloids on technological properties, it is necessary to remove them from the natural loam. The removal of colloids was carried out by chemical and physical methods. Loams with a high content of colloidal matter were subjected to investigation.

In the chemical method of removing colloids, 0.5 N HCl was used. Acid of this concentration does not destroy the clay substance, but only dissolves the colloidal particles [2].

Physical removal of colloids was carried out by fivefold decantation of a turbid clay suspension, a layer 10 cm thick, consisting of fractions smaller than 0.005 mm, 1 h 20 min after shaking. Colloids adsorbed on the clay particle are held tightly by it, and their removal occurs together with the clay particle.

When the rock was treated with 0.5 N HCl, removal of the colloids was almost complete; with the physical method, only partial removal of them occurred.

For the samples treated by the physical and chemical methods, the following technological indices were determined: molding moisture content, plasticity, air and total shrinkage, and mechanical strength. For the sample treated by the physical method, the grain-size composition was determined by the pipette method.

From Table 1 it is seen that the grain-size composition of the loam changes somewhat after partial removal of the fraction smaller than 0.005 mm.

For the loam under study, molding moisture content and plasticity were determined. The data are given in Table 2.

From the data in the table it may be concluded that the sample treated with 0.5 N HCl sharply changes its molding moisture content and plasticity. In practice, such a loam becomes of little use for manufac-

Table 1

Table 2

for producing red construction brick without special additives. The loam, however, after partial removal of the finely dispersed fractions, differs little in its properties from its natural composition.

In determining shrinkage during drying and firing, it was established that, for natural loam treated by the physical method, the shrinkage values differ little. Loam treated by the chemical method gives low shrinkage during drying and firing; i.e., after treatment with 0.5 N HCl the rock becomes low-plasticity and lean. These data are presented in Table 3.

Data on the change in the mechanical strength of the specimens are given in Table 4.

From the data presented it is evident that it is precisely in the finely dispersed mass of the clay that, during firing, those physicochemical processes occur which transform the loam into an artificial stone of definite strength, porosity, and frost resistance.

Table 3

Table 4

From the studies presented earlier [3], it has been established that the magnitude of mechanical strength is affected by the temperature of the water used for tempering the clay. Since at low temperatures the clay particles swell less in water and form solvation shells of lesser thickness, the structural-mechanical strength of the green body and of the finished brick decreases sharply.

Under production conditions, the clay must be moistened with hot water or steam in order to create a thicker solvation shell on the clay particle; this is directly related to the mechanical properties of the finished products.

Conclusions

1. The mechanical strength of products is formed mainly as a result of reactions in the solid state, the development of which is directly dependent on the plastic preparation of the masses.

2. When freshly mined loams are processed, aggregates of coagulated colloids are not completely destroyed; therefore, to make better use of their properties, it is necessary to practice preliminary weathering of the loams followed by soaking with hot water during processing.

3. Colloids are a cementing, adhesive substance between clay particles, ensuring the strength of dry and fired products.

Literature

1. P. G. Usov. Pyrochemical activity of red brick clays, Izv. SO AN SSSR, No. 8, Novosibirsk, 1958.

2. E. A. Guber. Material compositions of cover loams of Tomsk Oblast, Candidate dissertation, Tomsk, 1965.

3. P. G. Usov, E. A. Guber, N. F. Voronova. Low-melting clays of Tomsk Oblast and the physicochemical bases of their processing, Materials of the scientific-practical conference on problems of the development and distribution of productive forces of Tomsk Oblast, Tomsk, 1969.