Effect of Sulfur Containing Cement Raw Material on the Quality of Portland Cement Clinker

Among the raw materials used to produce Portland cement, there are two sources of sulfur. One is that clay or shale often contains a small amount of sulfur, or pyrite , sulfate or other organic sulfur compounds. The second source of sulfur is fuel. After the raw meal enters the rotary kiln, the sulfur-containing compound is eventually oxidized to sulfur trioxide in a typical oxidizing atmosphere. Since the cement raw meal is alkaline, when it enters the rotary kiln, it will absorb the gaseous sulfide produced by the coal combustion, so the sulfur compounds in the raw material gradually increase. When the raw material enters the high temperature zone, all the calcium sulfate present will be Partially decomposed, so the sulphur content in the raw meal is first increased to the maximum and then decreased. A considerable amount of sulfur eventually leaves the kiln with the fly ash.

The reaction situation when the ordinary silicate is fired by using the tailings containing FeS ₂ or ZnS, PbS or the like as a compound is the same as described above.

The tailings have a sulfur content of 3% to 5%. If the amount of tailings is 30% when blended with limestone , the prepared high-sulfur cement raw material content is 1.5%. When the cement is fired with high sulfur raw materials, The residual SO ₃ content in the clinker is related to the calcination operation control. For example, when a cement raw material having a sulfur content of 1.34% to 1.61% is used and calcined with coal containing 3.4% sulfur, since the operation control is good, the residual SO ₃ content in the clinker is only 0.56% to 0.68%. Sulfur 0.23～0.35%; however, the sulfur content of the cement raw material is unchanged. When the fuel is changed to the coal with a sulfur content of 1.1%, the SO ₃ content in the clinker is increased by 0.78% due to poor control of the calcination operation. ~1.2%, sulfur 0.92% to 0.98%. It can be seen that the sulfur content in the clinker can be controlled by the calcination operation.

When high-sulfur raw materials are fired, the content of SO ₃ in the clinker should be strictly controlled, which is unfavorable for the strength of the cement. According to the test, when the raw material (chemical composition is shown in Table 1) has a sulfur content of 1.34% to 1.58%, after calcination in the rotary kiln, most of the sulfur in the raw material escapes in the form of SO ₂ and remains in the clinker. The SO ₃ is 0.6% to 0.78%, and the individual is 1.2%. This is in line with the requirement that the SO ₃ content in ordinary Portland cement clinker should be less than 1.5%. For the physical properties test results (Table 3) of cement clinker with SO3 content of 0.56% to 1.2% (see Table 2 for chemical composition) (Table 3), although all cement clinker has good coagulation and stability, it is used as cement clinker. When the sulfur content is high (S>0.92%), the strength of the cement clinker is low, and the compressive strength of 28 days is about 50-100 kg/cm ^{2 lower} , and especially the early tensile strength is greatly reduced. Therefore, when burning cement with high sulfur raw materials, it is very important to properly control the content of SO ₃ in the clinker.

Table 1 Composition of raw materials into the kiln

Table 2 Chemical composition of Portland cement clinker (%) (1)

Table 2 Chemical composition of Portland cement clinker (%) (2)

Table 3 Physical properties of cement