|
|
Description of Tests Procedures
|
Sampling of the material to be tested:
The applicability and the informative value of ceramic laboratory tests depend largely on professional sampling. In this context sampling is the withdrawal of parts of a raw material or a blend where the material composition corresponds to the properties of the total quantity of the raw material to be tested. Samples taken unprofessionally are not representative for the total raw material deposit or the blend to be processed and may lead to incorrect statements and may result in false recommendations and measures. For our laboratory tests we generally assume that we receive representative raw material samples and samples of finished blends taken according to DIN 51061 (part 2). All samples should be marked and packed individually to avoid mix-ups and to preserve the original humidity. For every sample we need a designation of the pit and an indication as to the depth where the samples were taken to find out suitable mixing ratios. Depending on the scope of the tests we need a sufficient quantity of samples. Approximately 15 - 20 kg are required for an individual test. We need about 50 to 100 kg of each raw material to develop a factory blend.
Determination of moisture content
DIN 51 062 chapter 5.2
The moisture content is the share of water existing in a humid raw material or in a humid mixture, given in percent, whereby the quantity of water refers to the weight of the humid material (wet base). A sample is weighed into a vessel for its determination. This sample is dried in a dry box at 110°C until a constant weight is reached, then it is weighed again. The humidity on arrival of the sample and the preparation water contents are determined in the same way as the moisture content. However, in our reports we indicate the existing share of water in the material principally in percent with reference to the dry weight (dry base).
Preparation and test sample production:
Preparation and shaping of the raw materials and mixtures
We prepare individual raw materials as to their consistency and hardness and in conformity with the planned plant technology. For a conventional preparation raw materials will be pre-crushed with a jaw crusher as far as necessary. Then they are crushed in a laboratory roller mill with a gap width of a 0.8 mm. Additionally, a cross hammer mill with exchangeable mesh bottom is available. The raw materials are weighed according to their mixing ratio, considering their actual moisture, and are then homogenized in a kneading mixer where the necessary preparation water is added. The amount of water to be added depends on the raw material, the product to be made and the manufacturing method (e.g. stiff extrusion – standard extrusion). The homogenized body is then extruded with a vacuum extruder into various, different shaped test pieces. Depending on the designed plant and the product, we use penetrometer values between 0.8 and 4.0 kg/cm² for the extrusion of the test pieces. Bodies with a considerably softer consistency are processed to produce the so-called soft mud bricks.
Determination of the drying characteristics of raw materials, blends and green products:
Determination of the raw material-specific drying sensitivity
The determination of the drying sensitivity is carried out with test pieces with a size of 70 × 40 × 18 mm, which are produced as stress free as possible. This test method determines the raw material-specific drying sensitivity and it serves as a basic comparison between different kinds of raw materials. The test pieces freshly produced with a laboratory extruder are put into a drying box with an air circulation system having an airflow of 1 m/sec, but without humidity control system. They will be dried out completely at different constant test temperatures between 25 and 100°C. The maximum test temperature at which the test samples could be fully dried out without showing cracks is divided by ten and this figure is used as an index for the "drying sensitivity" (e.g. test temperature 70°C, drying sensitivity step 7). The drying sensitivity is divided into 8 steps from 3 to 10. The higher the step the less sensitive the material in the drying process.
Apart from the drying sensitivity specific for the raw material, the drying sensitivity to be expected for original green bricks is largely influenced by preparation, shaping, the weight and geometry of the green bricks and other factors that might occur during the process.
The drying sensitivity step alone is therefore not really a reliable statement on the drying sensitivity and the drying time of green bricks. In order to receive sufficiently reliable statements it is necessary to make tests with original green bricks, taking into consideration the dryer type to be used, the setting patterns, the ventilation system and last but not least a good deal of experience.
Determination of the data from freshly pressed test samples:
Four test pieces, sized 70 × 40 × 18 mm, out of the test series are weighed directly after shaping and dried in a dry box at a temperature of 110°C until their weight remains constant. The preparation water content is calculated from the wet and dry weight according to the formula:
PW = (Weight_wet-Weight_dry) / weight_dry × 100
The KELLER HCW ceramic laboratory always uses the dry weight as a reference to avoid any misunderstandings. A test piece with a diameter of 33 mm and a length of 40 mm is weighed directly after its production and its density is calculated to determine the density of the freshly extruded material. In addition to these analyses made from the freshly extruded material the so-called Bigot curve of the material is recorded. A freshly extruded test bar of approx. 120 × 20 × 15 mm is put into a dry box without air circulation at a constant temperature of 40°C. The weight loss and the shrinkage of the test sample are continuously recorded until the end of shrinkage. The result is a typical characteristic for the raw material.
The test pieces, which are not used for these analyses, are completely dried in a dry box, carefully avoiding any drying cracks.
Determination of the data of dried test samples:
During production a 50 mm wide so-called shrinkage marking is applied to all samples sized 70 × 40 × 18 mm lateral to column direction. A further test piece of 70 × 60 × 10 mm for each firing series receives the same shrinkage markings in column direction and lateral to column direction. When the test pieces are completely dried out these markings are used to determine the drying shrinkage. A total of 10 test pieces sized 120 × 20 × 15 mm are produced to determine the dry bending strength. Likewise, they are completely dried out with the utmost care. When the test pieces are cooled down five of them are broken with a three-point bending test device with a distance of 100 mm between the bearings. The dry bending strength is calculated from the force necessary to break the test piece and the real dimensions of the individual test piece. The remaining five test pieces are weighed in dry state and are stored for 48 hours under standard ambient room conditions. The clay minerals reabsorb water from the ambient air, which generally results in a measurable loss of strength. After their storage the test pieces are weighed again and are then treated with the above mentioned bending test device. The bending strength after reabsorbtion, the reabsorbed humidity and the loss of strength are then calculated.
Tests in the test dryer:
During the description of the determination of the drying sensitivity it was mentioned that it is very difficult to determine the drying sensitivity of a brick produced in a plant from the raw material data only. KELLER HCW owns two test dryers for a more precise determination of the drying sensitivity of the original green bricks and for a more accurate determination of the drying times. These dryers are fully equipped chamber dryers. They consist of a drying chamber with approximately 5 m³ usable volume where different air distribution systems can be installed. The drying curves (time, air temperature and relative air humidity) are programmed and run automatically. All relevant data (air temperatures and humidity, green brick temperatures, shrinkages and, if applicable, weight loss) are recorded during the test and are available for evaluation. The drying chamber, the control system and all necessary units (fans, heating burners, air humidifiers, etc.) are housed in a container. Thus the system is self-supporting except for the supply of air, energy and water. One of the two dryers is made up as a swap body with telescopic landing legs. It can easily be transported by truck to a brick plant to make tests with the customer’s different brick sizes directly on site.
Determination of the characteristics of the fired raw materials, blends and products:
Tests in an electric kiln, firing tests
All test pieces are fired in electrically heated laboratory kilns to test the firing behavior. The heating rate for each firing series is 180 K/h up to top temperature. Generally, five firing series with different types of test pieces are run at different maturing temperatures. The height of the maturing temperature is adjusted between 800 and 1300°C depending on the requirements of the future product. A soaking time of 3 h is maintained for the maturing temperature. Cooling down takes about 15 h in accordance with the natural heat loss of the kiln.
Determination of ceramic data of fired test pieces
The shrinkage markings applied to the test pieces in clay column direction and lateral to the clay column are again evaluated after firing to determine the total shrinkage (wet => fired). The firing shrinkage (dry => fired) can be calculated from the dry shrinkage and from the total shrinkage. The firing shrinkages and the total shrinkages are determined with the help of small uncored test pieces sized 70 × 40 × 18 mm lateral to the column direction (TB) and on small test plates sized 70 × 60 × 10 mm in column direction and lateral to the column direction. In addition, all test pieces are weighed before and after firing to determine the loss on ignition. One test piece of each firing series is cut after firing to find out whether there are any textures and residual black hearts of organic material. The results are noted down. A test piece of each firing series is put into a vessel with distilled water to determine the water absorption and the bulk density. This vessel will be heated until it boils. The test pieces are boiled for half an hour and afterwards cool down in the water for four hours. When they are cold the water sticking to the test pieces is carefully removed and the test pieces are weighed. Additionally, a determination of the test piece's weight is made under water to determine the body volume. The water absorption and the bulk density for each maturing temperature are calculated from these data. The determination of the firing stability is made with rods sized 210 × 20 × 15 mm. They are set on supports at a distance of 200 mm and are put into the kiln during the firing tests. One of these rods is charged in the middle with a weight of 100 gr. to determine the deflection with load. The deflection of the rods is determined before and after firing and is an important measure for the firing stability. With a further test piece of each firing series we determine whether the material still contains soluble salts, which could affect the look of a fired product by causing efflorescences. A test piece is immersed into distilled water with one fourth of its height. It is left there until the water has completely evaporated. The result is then evaluated visually. When testing bodies for the production of common bricks or facing bricks, two cylindrical test pieces with a diameter of approx. 30 mm are also fired with each firing series. The end faces of this test pieces are cut with parallel faces before firing. The compressive strength reached by the blend after firing is determined with this test pieces. However, it gives only a clue as to the values, which will be reached in reality, as, due to other geometries, a perforation and other initial stress, these real values can only be determined with original fired bricks. When testing bodies for the production of roof tiles, pavers and floor tiles the breaking strength of the fired material will be determined. Two test rods sized 120 × 20 × 15 mm are also fired with each series. After firing they are treated with a bending test device with a distance of 100 mm between the bearings until they break. The breaking strength is calculated from the breaking load and the rupture cross-section.
Generally, the determination of all ceramic data is made in accordance with the corresponding DIN or EN prescriptions.
Tests in the laboratory gas-fired kiln:
As already mentioned in the drying test chapter, it is also extremely important to carry out firing tests with dried original products. Keller owns two gas-fired kilns with a size of 50 × 85 × 82 cm and 90 × 90 × 150 cm (W × D × H). The kilns are computer-controlled and work with a preset firing program (time, temperature, oxygen and CO content). The composition of the atmosphere, the kiln temperatures and the temperatures of the product are measured and recorded during the firing tests. The setting patterns, the firing time and the atmosphere of the individual program steps are here again adapted to the requirements of the plant, to the blend and to the products. As with the firing tests in the electric kilns, the shrinkages and the water absorptions are determined after firing and the firing results are evaluated. The water absorption values are, whenever possible, determined from original bricks fired in the plant to compare them with the firing results of our tests. In this case the firing colors obtained are included in the comparison.
Determination of the thermal behavior and of the chemical analysis
STA (Simultaneous thermal analysis – Differential thermal analysis with integrated determination of the weight loss)
The simultaneous thermal analysis is made with two devices combined with each other.
a) Differential thermal analysis
A small quantity (approx. 20 mg) of the substance to be tested is put into an Al2O3 crucible and placed into a tube furnace, which is scavenged with air. A second crucible is situated in the furnace, containing Al2O3 powder, which is inert to temperature variations. Both crucibles are connected to one thermocouple each. These thermocouples are connected in a way to indicate only differences between them. The comparative substance being inert, all variations originate inevitably from the test substance. Endothermic (energy-consuming) reactions in the test substance, e.g. caused by emerging water or by the splitting of limestone into CaO and CO2 can here be determined. The exothermic (energy-emitting) reactions show a similar behavior, e.g. when organic substances burn out. A suitable calibration of the device with standard test samples allows a relatively exact determination of the energy contents. A special case is the determination of the free quartz content in the body. A change in volume occurs at approximately 575°C (dependant on grain size and heating-up speed), which is endothermic during heating up. As there are many other reactions during heating up in the same temperature range, the quartz content can only be detected in rare cases. The reaction, however, is reversible, i.e. the quartz which is still in the sample after firing runs through the same change in volume backwards and emits the energy absorbed during heating up. As this is the only reaction taking place in the cooling phase it can be used to determine the free quartz content.
b) Thermogravimetric analysis (loss-in-weight test)
The two crucibles described in the DTA chapter are additionally weighed during the tests and all variations of the weight are recorded, e.g. water losses from intermediate layer water or constitution water or emitted CO2. Together with the DTA curve and the dilatometer curve this test provides important data to elaborate the firing curve and to calculate the expected energy consumption.
TMA (Thermomechanical analysis, Dilatometer curve)
A test rod of approx. 20 mm length and 5 mm diameter of the material to be tested is made to determine the variation in length (dilatation). It is then put into a measuring device situated in a tubular furnace where it is fired. All variations in length are recorded during the test.
Together with the STA this analysis gives a good picture of the predominant mineral content of the tested material.
Chemical analysis:
A chemical analysis is not carried out in the KELLER HCW ceramic laboratory. Such a work is forwarded to a certified laboratory. In general, the sample is molten to a glass sample before making a radioscopic determination of the elementary oxides. The determination of fluorine and chlorine has to be carried out with the help of a pulverized sample, as these elements cannot be determined after the fusion. This laboratory also disposes of a scanning electron microscope, which provides an energy-disperse analysis when this is required.
Screen and sediment analyses:
Sediment analyses are also not carried out by the KELLER HCW ceramic laboratory but by specialized laboratories. However, KELLER HCW can make simple screen analyses.
General:
All data enclosed in a report are determined from the material given to us by the customer. We have to assume that these samples were expertly taken and that they correspond in their composition to the cross section of the pit or the blend, respectively. Any variations in the composition of the blend usually entail distinct variations of the ceramic values. Naturally, we do not take any responsibility for other ceramic values, which result from variations in the pit or from other body compositions. It is usually necessary to make large-scale geological tests to determine layers in the pit, which we cannot carry out as a routine.
It is not at all advisable to manufacture cutters or dies on the basis of the shrinkage data obtained from laboratory test pieces. The shrinkages of products obtained in the plant differ distinctly from those in the tests owing to different operating conditions (e.g. other preparation water contents, pressing powers, maturing temperatures, maturing holding times, modified body mixtures, material variations in the pit, different sizes, etc.).
The maturing temperature relates exclusively to the test pieces fired in the laboratory. The commissioning engineer has to adapt the maturing temperature of the tunnel kiln successively in accordance with operating conditions, body compositions and the product quality. The product quality is decisive for the final maturing temperature, i.e. the max. firing temperature set for the tunnel kiln can differ from the firing temperatures mentioned in the laboratory report.
top »
|
Contact:
