Use of cement

According to the China Cement Association, in the first three years, the country invested in the construction of 463 new dry process cement production lines, and the new clinker capacity reached 547 million tons, which means that at least about 200 million tons of backward technology capacity was squeezed out and became excess capacity. But "it is the overcapacity in the cement industry that has increased the motivation of cement companies to pursue technological progress and improve management, and sufficient market competition has optimised the allocation of resources." Kong Xiangzhong said.

Stability:

In the hardening process, if the cement does not produce uneven volume deformation, no cracks, bending and other phenomena, it is called qualified volume stability; If there is an uneven change in the volume of the cement after hardening, causing harmful expansion that will crack the building or even collapse, it is called unqualified stability. This cement cannot be used in engineering.

If the cement contains too much free calcium oxide or magnesium oxide, especially if the particles are coarse, and the storage time in the factory is short, it will produce a phenomenon of unqualified stability. Because the calcium oxide and magnesium oxide are not fully matured (above 1000 ° C), the hydration is very slow, and the hydration begins after the cement is set under the condition that the cement stone is constrained, and the cracking phenomenon will be formed after the volume expansion. In addition, if the content of sulfur trioxide in the cement is too much, calcium aluminate sulfate will be generated, and the volume will expand, which will also cause poor stability.

Test the stability of cement according to GB/T 750-1992. The test process is to use the standard consistency of cement paste, make it into a certain shape (diameter 70 ~ 80mm, the center thickness is about 10mm, the edge is thinner) test cake, put into the boiling box boiling for 4h, if the cooked test cake by naked eye observation no cracks, no bending phenomenon with a ruler inspection, it is called stable; Otherwise, it is unqualified.

In addition to the above cake test method, there are also Redwood clip method and length measurement method. Although the last two methods have quantitative numerical limits, the methods are complex and the reproducibility is poor. The cake testing method has a series of advantages such as simple equipment, easy operation, sensitive reaction, intuitive observation and good reproducibility, so it is still listed as the national standard method.

Boiling only provides insight into the damage caused by free calcium oxide. Because the overheated magnesium oxide hydration rate is slower than the overheated calcium oxide, so the boiling method can not find the uncertainty caused by magnesium oxide, only through high temperature, high pressure steam test, can judge this phenomenon. The uneasiness caused by sulfur trioxide can only be tested by cold cake method and water immersion method, that is, the test cake is placed in water at 20 ° C ±3 ° C for 28 days to check whether there is instability. Because when the temperature exceeds 60 ~ 70 ° C, it will not be able to form calcium sulfoaluminate that produces volume expansion.