The plasticity expressing in China’s brick and tile industry is now proposed by the Swedish A. Aterberg in 1911.Plasticity index is expressed as the range of water content when the clay is in a plastic state,whose value is equal to the difference between flow limit and plastic limit.Flow limit, also known as liquid limit, is water amount that clay enters a flowing state.Plastic limit is water amount that clay can be rolled into 3mm diameter.

According to this method, the plasticity index is used to classify clay as: high plasticity clay >15%; medium plasticity clay 7%-15%; low plasticity clay<7%.

This method has been widely used for many years in the fields of soil science and engineering geology.Although China's brick and tile industry has been using it for many years, it is only for soft and dispersed clay raw materials.For coal gangue, shale, etc., which rely on reducing particle size to obtain plasticity, there is a great deal of deviation in using this method.For example, the plasticity of raw materials such as coal gangue and shale depends on crushing to reduce the particle size to a certain degree, adding water to the particles after decomposing (aging), and it is changeable during processing.If the hard clayey shale or coal gangue rich in illite is ground to a sufficiently fine degree (such as cement fineness),the plasticity index may reach a pretty high level.And formed moisture content and drying shrinkage may be larger than common clay materials, which totally differ from common brick clay.

The plasticity index of common brick clay is relatively stable.In terms of this type of material, which depends on crushing process reducing particle size to obtain plasticity, its plasticity index is variable during processing.For example, when a certain coal gangue is crushed below 0.9 mm in the laboratory, according to geotechnical test method, the plasticity index is measured as only 7.2%.However, after adding 40% of excessively fired vermiculite (essentially no plasticity),then adding water, aging, and crushing on the roller, vacuum extrusion,plasticity index of the small sample actually reached 10.5%.For example, when other shales were crushed to less 0.9 mm, the plasticity index is measured as 8.4% according to geotechnical test method.However, after adding 40% (by weight) of fly ash,then mixing with water, aging, and crushing,plasticity index of the small sample extruded by the vacuum extruder reached 9.5%.This is still the result after revalidation according to the geotechnical test method,which fully demonstrates that the current geotechnical test method used in the brick and tile industry can’t correctly reflect the characteristics of raw materials such as coal gangue and shale during processing and forming.In order to further prove the fact that raw materials such as shale or coal gangue relys on reducing the particles size to obtain plasticity, the hard shale in a place near Shijiazhuang was crushed into two groups in the laboratory.One group all can pass 0.9 mm sieve; the other can all pass 0.5 mm sieve.However, the shale of the same mineral composition differs greatly in its plasticity index only due to its different grain sizes, one being 4.8% (0.9 mm) and the other being 8.9% (0.5 mm).In order to further verify this phenomenon, this two groups were blended together in different proportions to determine their plasticity index, drying line shrinkage rate and drying sensitivity index. The results are shown in the following table.

From the above table, it can be clearly seen that the plasticity index, dryness sensitivity index, and dry linear shrinkage of the mixture tend to increase with the increase of the particle composition below 0.5 mm in the mixture.This fully shows that the use of geotechnical test method can not fully evaluate the properties of plastic materials such as shale and coal gangue that rely on reducing particle size.From the above analysis, the plasticity degree has a great relationship with the particle size of clay minerals.

For example, assuming that the type and total amount of clay minerals contained in a kind of clay are exactly the same as that in a certain type of shale, plasticity index measured by the geotechnical test method is much higher than shale,because  dispersed clay mineral particles in the clay are uniform and small.If the shale is fully crushed, so that the clay minerals in the shale reach a finely dispersed state like the clay mineral particles contained in the clay, it is possible that the plasticity index measured by the geotechnical test method will be the same as that of the clay.However, it can’t be achieved in actual production,causing a great plasticity fluctuation during processing raw materials such as coal gangue and shale.Which method should be used to describe and compare the plasticity of these raw materials?

First of all, it is necessary to limit the particle size composition of this kind raw materials after crushing.According to the practice and research of bricks and tiles produced by shale in USA for many years, shale crushing can provide 0.053 mm (270 mesh) or more particles.And it is thought that shale crushed should have three levels of particle grading:(1)The decorative coarse grain should occupy 0-3% (the grain size is generally 1.2-2.4 mm, sometimes it can reach 9.5 mm. It can be used without coarse grain decoration);(2)Filled granules should occupy: 20-65% (1.2-0.3 mm, this granules part is to limit the excessive shrinkage, cracks and deformation of the granules);(3)Plastic particles should account for 35-50% (below 0.053)

This indicates the minimum of plastic particles less than 0.053 mm after comminution,whether it is shale or coal gangue. The amount of particles smaller than 1 mm, or less than 0.05 mm, which was previously proposed in the design, is a very inaccurate method.If the gangue is completely crushed into 1 mm diameter pellets, it is possible that this material will not have the plasticity for forming.Before determining materials plasticity such as coal gangue and shale, the particles content less than 0.053 mm should be measured after the raw materials are crushed, and the content of this component should be controlled over 40%.This limit value can also be used as a basis for equipment selection during plant design and basic requirements for product quality control.In addition,this type of material relying on reducing particle size to obtain plasticity, whose particle size are falling in the processing of crushing, stirring, adding water, aging, milling, vacuuming, etc., or due to particles loosening caused by water effect.Therefore,determining this plasticity should be done at the outlet of the extruder or use other expression methods,in addition to the case where the limit after the comminution is less than 0.053 mm, the particle composition is greater than 40%.Another noteworthy situation is that when crushing hard or semi-rigid raw materials such as shale and coal gangue,the particle size distribution of the crushing material is very narrow,due to improper crushing equipment or processes.The direct results are poor strength of green body, or the difficulty in molding, or the poor frost resistance of the sintered product. Because unreasonable particle distribution will cause that the particles in the green body can’t reach the closest aggregation state.