TECHNOLOGICAL PARAMETERS OF LIME–SAND MIXES FOR DENSE FACING TILES

N. S. DUBOVSKAYA, L. I. BERCHENKO, V. V. SAVEL'EV

(Presented by the scientific seminar of the Department of Technology of Silicates, Inorganic Substances, and Electrochemical Production)

To obtain silicate products of high density, a number of methods are used—[1, 2]. One of the most effective is the method of utilizing the capacity of lime for hydration hardening [3, 4].

The aim of the studies carried out by us for the Kopylovo silicate-brick plant was to establish the optimum activity and amount of water for tempering the mixes, the optimum periods of holding the moistened mix before molding and the molded green body before autoclaving. For the experimental studies, factory-burned quicklime was used, as well as a joint “grinding” with an activity of 40–45%, obtained in a ball mill and having a fineness of grinding of 5300 cm²/g (according to Tovarov), and sand from the plant quarry.

The procedure for preparing the mix was as follows: the sand and ground material, dosed by weight, were mixed in the dry state; water and the remaining lime (up to the required activity of the mix) were added; the mix was additionally mixed; at the end of mixing the temperature of the mix was measured, and the mix was left undisturbed for the entire holding period before molding. Every hour a sample was taken to determine the amount of chemically bound water; at the same time the temperature of the mix was measured. At the end of the holding period the mix was removed from the container, loosened, additionally moistened, and passed three times through a sieve with openings of 2.5 mm. After this, specimens were molded at the adopted pressing pressure, their bulk density was measured, and the specimens were left for holding before autoclave treatment.

The sand used in the work was sufficiently clean and met the requirements for granulometric composition; the lime, according to GOST 5179—59, was of the first grade.

The studies were carried out for mixes with activities of 8, 10, and 12%, tempered with 15% water. The experimental mixes were selected after a series of preliminary tests. As the experimental results showed, the quality of the products is greatly influenced by the composition of the mix. During the process of slaking, the holding time of the mix before molding, as well as during the period of curing of the mix and holding of the molded green body, physicochemical processes associated with lime hydration take place in the mix. During the holding period of the mix an increase in chemically bound water is observed in it (Table 1), which indicates a continuously proceeding process of lime hydration. From the held mixes were molded on

Table 1

Change in the amount of chemically bound water during curing of the mix

In a hydraulic press, experimental specimens—cylinders with a diameter and height of 3 cm—were molded. The pressing pressure was taken as 160, 180, and 200 kg/cm². After the bulk density was measured, the specimens were stored in air for 2, 4, 6, 8, 10, and 12 hours before hydrothermal treatment, and were steamed in an autoclave according to the following regime: pressure increase to 10 ata—1.5 h, holding at 10 ata—5 h, pressure reduction to 4 ata—1.5 h. The hydrothermal treatment was carried out in a laboratory vertical-type autoclave with electric heating. After steaming, the specimens were subjected to external inspection and physicomechanical tests (bulk density, compressive strength, and frost resistance were determined). The test results are given in Tables 2, 3, and 4.

Analyzing the experimental data, the following conclusions may be drawn:

1. As a result of fine grinding of lime (to a specific surface of 5200 cm²/g), the lime exhibits the properties of a hydration binder.

Table 2

Physicochemical characteristics of the green body and of the steamed product made from mixes with different activity
(curing time of the mix 7 h)

Note. Specific pressing pressure 180 kg.

Table 3

Effect of the water–lime factor on the physicochemical characteristics of the green body and the steamed product
(mix activity 8%, curing time before molding 7 h, green body—1 h)

Table 4

Effect of pressing pressure on the physicomechanical characteristics of the green body and of the steamed product (activity of the mix—8%; holding time of the mix—7 hours; green body—1 hour)

1. For these properties to be manifested to a greater extent, the water–lime factor of the lime–sand mixture should be within the range 1.3–1.5. In this case, no swelling of the lime is observed during slaking; the green body and the products have maximum volumetric weight.

2. The activity of the mix, in order to ensure optimum density and strength of the products, must be not less than 8%.

3. The holding time of the tempered mixture before molding is 7 hours, and that of the molded green body before steaming is 1 hour; this ensures the production of articles free from cracking.

4. When the recommended parameters of the mix and the regime of its preparation are observed, the hydration properties of ground lime are utilized to the maximum extent; at the same time, it is possible to obtain dense facing silicate materials with improved physicomechanical characteristics.

5. The optimum pressing pressure in molding tiles should be taken as 200 kg/cm². It provides maximum density of the green body and of the products and promotes the processes of silicate hardening of the mixes in the autoclave.

REFERENCES

1. M. S. Shvarzaid. Assortment of structural and facing components made of autoclave-hardened concretes. NIIStrоitekhniki, Academy of Sciences of the USSR. Scientific and technical report, 1947.

2. M. S. Shvarzaid, V. E. Pesselnik. Silicate slabs in exterior cladding of buildings. In: Autoclaved Materials and Products. Publishing House of the Academy of Sciences of the USSR, Moscow, 1956.

3. Academy of Construction and Architecture of the USSR. Facing silicate brick. Moscow, 1959.

4. V. V. Osin. Quicklime as a new binder. State Publishing House for Building Materials, 1954.