Indirect Method for Determining the Plasticity of Clays
Abstract Generated abstract
This paper proposes an indirect method for assessing clay plasticity by measuring the resistance of pressed dry-clay briquettes to washing in moving water. The procedure forms standardized cylindrical briquettes from air-dry, sieved clay, washes them under fixed reciprocating-motion conditions for one minute, and calculates an index from the mass retained relative to the original briquette mass. Tests on several clays indicate that the index differentiates plastic, moderately plastic, and slightly plastic materials, while low-plasticity clays require repeated determinations or evaluation through mixtures with a highly plastic reference clay. The study further argues that plasticity indices in clay mixtures and clay-sand mixtures vary regularly and can be approximated by a parabolic relation, allowing indices for mixtures or weakly plastic materials to be estimated graphically or by calculation.
Full Text
INDIRECT METHOD FOR DETERMINING THE PLASTICITY OF CLAYS
I. V. GEBLER
We propose an indirect method for determining the plasticity of clays on the basis of the washing away by water of briquettes prepared from dry clay.
The experiments are carried out as follows. Clay in the air-dry state is ground in a porcelain mortar and sieved through a sieve with openings of 1 mm; a rough sample of about 3 g is taken. In a hand press the clay is made into a briquette in the form of a cylinder with the following dimensions: diameter 10 mm, height 12 mm. The briquette is weighed to an accuracy of 0.01 g and placed in a brass cartridge \((d — 15\ \text{mm},\ H — 47\ \text{mm})\) with a mesh bottom (opening 2 mm); the cartridge is fastened in the head of the rod of an apparatus providing reciprocating motion of the cartridge in the vertical direction. The following regime is established. The stroke of the rod with the cartridge when moving up and down is 40 mm; the number of double strokes is 60 per minute. A beaker is placed under the cartridge. Distilled water is poured into the beaker while a stopwatch is started simultaneously. Washing continues for one minute. After the separation of drops from the cartridge has ceased, the clayey slurry is filtered on a weighed filter; the latter, with the sediment in the air-dry state, is weighed. The index \((I)\) is calculated by the following formula:
\[ I = \frac{a - b}{a} \times 100, \]
where \(a\) is the weight of the briquette, and \(b\) is the weight of the slurry.
The value of the index \(I\) is taken in whole numbers, as the mean of two determinations.
The work was carried out with various clays. The results are given in Table 1.
Table 1
| Clays | \(I\), avg. |
|---|---|
| No. 1. Chasov-Yar clay, plastic | 97 |
| No. 2. White clay from the environs of the city of Tomsk, plastic | 94 |
| No. 3. Red clay from the environs of the city of Tomsk, moderately plastic | 53 |
| No. 4. Red clay from the environs of the city of Tomsk, slightly plastic | 33 |
| No. 5. Black clay from the environs of the city of Tomsk, plastic | 72 |
For plastic clays, the discrepancy in parallel determinations is insignificant, fluctuating within the range from 0 to 2.8% of the mean value from two determinations.
For clays of low plasticity, the discrepancy in parallel experiments is far greater, and in order to obtain usable figures one should make 3–4 determinations and take the mean of the closer ones, or determine the plasticity for them by means of mixtures, as will be shown below.
The property of water disintegration of the pressed briquette, as can be seen, corresponds to the plasticity of the clay.
Fig. 1.
Fig. 2.
Experiments carried out by the method we propose, but with briquettes obtained by drying a clay paste, established that the indices obtained do not correspond to the actual plastic properties of the clays tested.
To characterize the variability of the plasticity indices in mixtures, experiments were made for mixtures of clays No. 2 and No. 4; for mixtures of clay No. 2 with quartz sand of a grain size of 1–0 mm. The results are given in the graph (Fig. 1).
The curve (Fig. 1) is close to a branch of a parabola having its vertex at point \(a\).
We construct a graph by a broken line rotated through \(90^\circ\), taking the axis \(OX\) as the abscissa axis and the line \(OY\) of the percentage composition of the mixture as the ordinate axis, taking as the origin point \(O\), corresponding to the value of the plasticity index of the more plastic clay \(I = 97\), plotted strictly vertically.
The new graph has the form shown in Fig. 2. Here \(I = OX - 97\), and each value \(I_n\) will be equal to \(97 - X_n\).
The equation of the parabola in this case will be \(Y^2 = 2pX\).
For mixture 2 (Fig. 1) we find the value of the parameter \(2p\) at
\[ X_{\mathrm{II}} = 97 - 82 = 15 \quad \text{and} \quad Y_{\mathrm{II}} = 50; \quad 2p = \frac{Y_{\mathrm{II}}^2}{X_{\mathrm{II}}} = \frac{50^2}{15} = 167. \]
For the first point of the parabola,
\[ Y_{\mathrm{I}} = 25, \quad X_{\mathrm{I}} = \frac{625}{167} = 3.8. \]
For the third point,
\[ Y_{\mathrm{III}} = 75 \quad \text{and} \quad X_{\mathrm{III}} = \frac{5625}{167} = 33.8. \]
From these coordinates we construct a branch of the parabola, find the values \(I_{\mathrm{I}}\) and \(I_{\mathrm{III}}\), and compare them with the experimental values in Fig. 1.
Computed:
\[ I'_{\mathrm{I}} = 97 - 3.8 = 93.2, \]
\[ I'_{\mathrm{III}} = 97 - 33.8 = 63.2. \]
Determined:
\[ I_{\mathrm{I}} = 92, \]
\[ I_{\mathrm{III}} = 61. \]
The agreement of the numbers, as is evident, is quite good.
The plasticity index for clay No. 4 \((I_{\mathrm{IV}})\) can now be found without experimental determination by an analogous calculation or directly from the graph, if the branch of the parabola is continued to its intersection with the horizontal at \(Y_{\mathrm{IV}}=100\); in this case \(I_{\mathrm{IV}}\) is equal to 37.
Thus, for low-plasticity clays (brick clays, loams, loess, etc.), the plasticity indices can be found by means of mixtures of them with some highly plastic clay, which is constant and should serve for this purpose.
For a mixture of both clays in the ratio 1:1, the value \(I_{\mathrm{II}}\) is carefully determined experimentally, and then the value of the parameter \(2p\) is calculated for \(Y_{\mathrm{II}}=50\), and then, for \(Y_{\mathrm{IV}}=100\), the value of the plasticity index of the low-plasticity clay is found.
The branch of the parabola, constructed from several points at the same parameter, will serve to find the plasticity index of mixtures at any ratios.
Fig. 3.
For a mixture of clay No. 2 \((I=94)\) with sand, the parameter \(2p\) (at \(Y_{\mathrm{II}}=50\)) \(\simeq 109\); the calculated values for \(X_{\mathrm{I}}\) and \(X_{\mathrm{III}}\) are respectively 5.7 and 51.7, which gives \(I_{\mathrm{I}}=88.3\) and \(I_{\mathrm{III}}=42.3\), while the experimental mean values for them (Fig. 1) are \(I=90\), \(I=39\).
For sand the plasticity index should be equal to zero. The calculation gives (at \(Y_{\mathrm{IV}}=100\)):
\[ X_{\mathrm{IV}}=\frac{10000}{109}\simeq 92 \]
and \(I_{\mathrm{IV}}=94-92=2\), i.e., a number close to zero.
The branch of the parabola drawn through the calculated points, as a correcting curve, characterizes the depletion of clay by sand (Fig. 3).
Conclusions
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The water resistance of briquettes made of clay paste, neither in still water nor in moving water, in no way corresponds to the plastic properties of clays.
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The described method for determining the water resistance of pressed briquettes made from dry clays gives, for them, sufficiently distinct characteristics that can be used for the classification of clays.
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It has been established that in mixtures of clays with different plasticity, and also of clays with sand, the plasticity indices change quite regularly along a curve that may, with sufficient approximation, be taken as a branch of a parabola. This makes it possible to find the plasticity indices of various clay mixtures by calculation or graphically.
Literature
- Sokolov A. Izvestiya Petrogradskogo tekhnologicheskogo instituta, XIX, 1908.
- Budnikov P. P. Ceramic Technology, part 1, ONTI, 1937.
- Rodin S. Collection of Experimental Works on Clays, 1927.
- Keram-Rundschau. 8, 816, 528, 540, 1909.