In the hardening process, if the cement does not produce uneven volume deformation, cracks, bending, etc., it is called volume stability qualified; if the volume of the cement changes unevenly after hardening, causing harmful expansion, it will cause the building If the object cracks or even collapses, it is called unqualified stability. This kind of cement cannot be used in engineering.
If the cement contains too much free calcium oxide or magnesium oxide, especially if the particles are coarser, and the storage time in the factory is short, the stability will be unqualified. Because this kind of overheated (above 1000°C) calcium oxide and magnesium oxide have not been fully matured, their hydration is very slow. After the cement is set, it will begin to hydrate under the condition of cement stone. After volume expansion occurs, It will form a cracking phenomenon. In addition, if the content of sulfur trioxide in the cement is too much, calcium sulfoaluminate will be formed, which will expand in volume, which will also cause poor stability.
Check the cement stability according to GB/T 750-1992. The inspection process is to use standard consistency cement paste, make it into a certain shape (diameter 70-80mm, center thickness about 10mm, edge gradually thinning) test cake, put it into the boiling box and boil for 4 hours, such as after boiling When no cracks are found in the test cake by naked eye, and there is no bending phenomenon when inspected with a ruler, it is called stable stability; otherwise, it is unqualified.
In addition to the test cake method mentioned above, the stability test methods include the Lei's clip method and the length measurement method. Although the latter two methods have quantitative numerical limits, they are complex and have poor reproducibility; while the test cake method has a series of advantages such as simple equipment, convenient operation, sensitive response, intuitive observation, and good reproducibility. Still listed as the national standard method.
The boiling method can only detect the destructive effect of free calcium oxide. Because the hydration rate of overfired magnesium oxide is slower than that of overfired calcium oxide, the instability caused by magnesium oxide cannot be found by boiling method. This phenomenon can only be judged by high temperature and high pressure steaming test. The instability caused by sulfur trioxide can only be tested by cold cake method and water immersion method, that is, the test cake is cured in water at 20℃±3℃ for 28 days, and then check whether there is any instability. This is because when the temperature exceeds 60-70°C, calcium sulfoaluminate that generates volume expansion cannot be formed .
Since the national standards have set limits on the content of magnesium oxide and sulfur trioxide, the two inspections of the autoclave method and the water immersion method are generally not required.
The first type: "look" mud to know the pros and cons
First of all, look at the packaging quality from the appearance. Check whether a laminated woven bag with good moisture-proof performance and not easy to break is used, and check whether the label is clear and complete. Generally, the cement produced by a regular manufacturer should be marked with the following content: registered trademark, place of origin, production license number, implementation standard, packaging date, bag net weight, factory number, cement variety, etc. Inferior cement is often unclear. Second, carefully observe the color of the cement. Generally speaking, the normal color of cement should be grayish white, and if the color is too dark or changed, there may be too much other impurities.
The second type: "smell" mud analysis quality
The “smell” here is not smelling. After all, smelling cement is not safe for consumers. The “smell” here is “listening”. Listening to merchants introduces the ingredients of cement to infer the quality of cement. In order to sell at low prices, some domestic small cement plants violated cement standards and used cement mixtures too much . They did not strictly follow the national standards for the ratio of raw materials. The performance of their products can be imagined. The regular manufacturers are very strict in the selection of cement raw materials, and the cement produced has the characteristics of moderate setting time, high bonding strength and good durability .
The third type: "asking" the source of the mud
It is mainly to inquire about cement manufacturers and production techniques to see if their "origin" is regular and whether the production techniques are advanced. At present, most of the cement products in the illegal building materials decoration market are produced by small vertical kiln technology, not only the product quality is very unstable, but also the enemy of environmental protection; and some professional manufacturers adopt new dry rotary kiln production and adopt advanced computer technology. Control and management can ensure the stable quality of cement products.
The fourth type: "cut" the mud to know the lifespan
In this step, fingers are mainly used to "sign pulse" to the cement to distinguish the length of time it left the factory. Cement also has a shelf life. Generally speaking, the strength of cement after 30 days from the factory date will decrease. The strength of cement after three months of storage will decrease by 10%-20%, and after one year, it will decrease by 25%-40%. Cement that can be used normally should not be damp and agglomerate. For high-quality cement, use your fingers to twist the cement powder to feel fine particles. Packing low-quality cement will get damp and agglomerate during opening inspection; for low-quality cement, use your fingers to twist the cement powder and feel rough, indicating that the cement is coarse and abnormal in fineness, and has low strength and poor viscosity when used.
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