Effect of Autoclave Treatment Regime on the Quality Characteristics of Dense Silicate Tiles
Abstract Generated abstract
This study examines how autoclave treatment conditions affect structure formation and quality in dense silicate facing tiles made from lime sand mixes. Specimens of differing mix activity and compaction pressure were aged, steam cured under varied maximum pressures and holding times, and evaluated for chemically bound water, lime assimilation, bulk density, compressive strength, frost resistance, and microstructure. The results indicate that higher steam pressure, longer holding at maximum pressure, and greater pressing pressure promote hydration and silicate hardening, increasing calcium hydrosilicate formation, strength, and frost durability. For the investigated compositions, the authors identify 10 atm steam pressure with a six hour hold as an optimal general regime, while noting that adequate strength can also be obtained at 8 atm when pressing pressure is sufficiently high.
Full Text
EFFECT OF AUTOCLAVE TREATMENT CONDITIONS ON THE QUALITY CHARACTERISTICS OF DENSE SILICATE TILES
N. S. DUBOVSKAYA, L. F. MENYAILO, V. V. SAVELIEV
(Presented by the scientific seminar of the Department of Technology of Silicates, Inorganic Substances, and Electrochemical Production)
The strong structure of dense silicate materials begins to form during the aging of the green body before autoclaving. During this period, the hydration properties of the ground quicklime appear; according to studies by Osin [1], Rabinovich [2], Butt et al. [3], and Solinskii et al. [4], this lime hardens, unlike slaked lime, as a hydration binder.
Intensive structure formation in a pressed specimen proceeds under steam conditions.
The heat-treatment regime plays an essential role in the process of silicate hardening. This article presents the results of studies on the influence of the autoclaving regime on the process of formation of a strong structure in dense silicate tiles. The study was carried out with mixes of various activity, formed into products at different pressing pressures. The holding time of the mix was 7 hours; that of the green body before autoclaving, 2 hours. In the steamed specimens, the amount of chemically bound water, the mechanical compressive strength, and frost resistance were determined. In addition, bulk density was measured. In the course of the chemical processes and the formation of the strength of the product and its frost resistance, a number of regularities were observed.
The physico-mechanical properties of the steamed products are directly dependent on the autoclaving time and the magnitude of the maximum steam pressure. The experimental results are given in Table 1.
As can be seen from the table, with an increase in steam pressure in the autoclave
Table 1
Dependence of product characteristics on the duration of holding at maximum pressure (mix activity in %)
| Hydrothermal treatment regime | Holding duration, h | Bulk density, kg/dm³ | Content of chemically bound water, % | Strength, kg/cm² | Frost resistance |
|---|---|---|---|---|---|
| 8 ata | 4 | 1.72 | 7.1 | 75 | after 50 cycles, have surface cracks |
| » | 5 | 1.80 | 7.3 | 150 | after 50 cycles, have surface cracks |
| » | 6 | 1.82 | 7.8 | 180 | after 50 cycles, have surface cracks |
| 10 ata | 3 | 1.91 | 7.9 | 225 | after 50 cycles, do not have cracks |
| » | 4 | 1.93 | 7.9 | 280 | after 50 cycles, do not have cracks |
| » | 5 | 1.93 | 8.1 | 300 | after 50 cycles, do not have cracks |
and with an increase in the duration of holding at this pressure, the quality characteristics of the products improve, owing to the creation of favorable conditions for silicate hardening.
In this work, the dependence of the assimilation of lime into hydrate neoformations on the degree of compaction of the green body and the duration of its holding at maximum pressure was investigated.
The results of the investigations are presented in Table 2.
Table 2
Assimilation of lime and binding of water during autoclaving
| Specific pressure, kg | Content of assimilated lime (numerator) and water (denominator) at holding time, h: 4 | Content of assimilated lime (numerator) and water (denominator) at holding time, h: 5 | Content of assimilated lime (numerator) and water (denominator) at holding time, h: 6 | Compressive strength limit, kg/cm², at holding time, h: 4 | Compressive strength limit, kg/cm², at holding time, h: 5 | Compressive strength limit, kg/cm², at holding time, h: 6 |
|---|---|---|---|---|---|---|
| Steam pressure 8 ata | Steam pressure 8 ata | Steam pressure 8 ata | Steam pressure 8 ata | Steam pressure 8 ata | Steam pressure 8 ata | Steam pressure 8 ata |
| 160 | $\dfrac{5.75}{6.75}$ | $\dfrac{6.0}{6.79}$ | $\dfrac{6.25}{7.1}$ | 140 | 180 | 210 |
| 180 | $\dfrac{5.90}{6.9}$ | $\dfrac{6.3}{6.9}$ | $\dfrac{6.5}{7.3}$ | 160 | 210 | 230 |
| 200 | $\dfrac{5.95}{7.1}$ | $\dfrac{6.38}{7.4}$ | $\dfrac{6.7}{7.5}$ | 220 | 240 | 260 |
| Steam pressure 10 ata | Steam pressure 10 ata | Steam pressure 10 ata | Steam pressure 10 ata | Steam pressure 10 ata | Steam pressure 10 ata | Steam pressure 10 ata |
| 160 | $\dfrac{6.1}{6.9}$ | $\dfrac{6.4}{7.1}$ | $\dfrac{6.7}{7.4}$ | 260 | 280 | 310 |
| 180 | $\dfrac{6.4}{7.1}$ | $\dfrac{6.7}{7.4}$ | $\dfrac{6.9}{7.5}$ | 280 | 290 | 330 |
| 200 | $\dfrac{6.9}{7.3}$ | $\dfrac{7.1}{7.5}$ | $\dfrac{7.1}{7.7}$ | 300 | 320 | 350 |
As can be seen from Table 2, with an increase in the specific pressing pressure and in the duration of holding at maximum pressure, the hydration and silicate-hardening processes proceed to a greater extent, as evidenced by the increased amount of chemically bound water and calcium hydroxide.
The density of the molded green body at the maximum pressing pressure provides a larger contact surface between the interacting sand and lime under steam conditions. The amount of cementing substance formed in this process—calcium hydrosilicates—determines the high mechanical strength. The latter is directly dependent on the amount of chemically bound water and the amount of calcium oxide that has entered into reaction [5].
For the compositions and autoclaving regimes investigated, this dependence has the following character (Table 2): at a mass activity of 8% and steam pressure in the autoclave of 8 and 10 ata, with increasing density of the green body and increasing holding time, the amount of chemically bound calcium hydroxide and water increases. This is accompanied by a uniform increase in strength, and, as can be seen under the microscope in transmitted light, the cementing substance proves to be a complex fine-crystalline intergrowth with a higher degree of crystallization.
The results obtained make it possible to draw the following conclusions:
- In the production of dense facing tiles, at a mass activity of 8%, the optimum autoclaving regime should be taken as: 1.5—6—1.5 hours at a maximum steam pressure of 10 ata.
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If the pressing pressure is maintained at \(200\ \text{kg}/\text{cm}^2\), the autoclaving regime may be the same, but with a steam pressure of 8 atm, since the strength of the products is quite sufficient.
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In order to shorten the production cycle, it is advisable to mold the products at \(160\ \text{kg}/\text{cm}^2\) and steam-cure them at a pressure of 10 atm with a holding time of 4 hours.
REFERENCES
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B. V. Osin. Ground quicklime and its use in construction by the method of I. V. Smirnov. Gosstroiizdat, 1940.
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I. V. Rabinovich, B. V. Osin. Electrical conductivity of lime during slaking and setting. ZhPKh, l. 1946.
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Yu. M. Butt, L. N. Rashkovich. Hardening of binders at elevated temperatures. Stroiizdat, 1965.
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D. S. Saminskii et al. On the mechanism of hardening of finely ground lime-sand binder without hydrothermal treatment. VNIITISM, scientific communication, 28, Promstroiizdat, 1957.
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Yu. M. Butt, N. A. Maier et al. Chemical properties and microstructure of calcium hydrosilicates. Collection of Works of ROSNIIMS, 17, 1960.