The use of cement and matters needing attention

The raw materials available for Aether cement include limestone, bauxite, gypsum, iron raw materials and marl. Its introduction of calcium sulfoaluminate (C4A3S) mineral, production at a lower temperature (1 225 ~ 1 300℃), compared with Portland cement (1 400 ~ 1 500℃), can significantly reduce production energy consumption, tons of cement can reduce CO2 emissions by 25% ~ 30%. The mineral composition of Aether cement is: Belite (C2S) 40% ~ 75%, calcium sulfoaluminate (C4A3S) 15% ~ 35%, iron phase (C2(A, F)) 5% ~ 25%. Examples of Aether cement phase composition: C4A3S: 35.5%, β-C2S: 4.1%, α '-C2S: 44.0%, C12A7:0.7%, C2(A,F) : 12.4%, C2AS: 0.9%, CaSO4:2.4%.

Aether cement hydration process:

① C4A3S hydration:

C4A3S+2CS+38H→C3A·3CS·H32 +2AH3

② C2S starts to hydrate: C2S+AH3 +5H→C2ASH8

③ C2S hydration, C2(A,F) start hydration:

C2S+C2(A,F)+10H →C2(A,F)SH8+2CH

C2S + C2(A,F)+5H→C3(A,F)SH4+CH

④ Medium and long term hydration:

2C2S+C2(A,F)SH8 +(x-4)H→C3(A,F) SH4 +C3S2Hx

3 BCT cement 

BCT cement, short for Belite-Calciumsulfoaluminate-Ternesite cement, was invented by Heidelberg Company in Germany and has applied for invention patent. Semi-industrial scale experiments were carried out using a test kiln with a diameter of 0.3m and a length of 7.6m, and several tons of BCT clinker with different compositions were produced. The whole production process (including discharge) of BCT cement clinker is similar to that of ordinary cement clinker, and the performance of BCT cement mortar produced is proved to be good. The next step will be to scale up to industrial scale and optimize manufacturing technology.

The core technology of BCT cement is the introduction of Ternesite in the clinker mineral system. Ternesite (C5S2S), consisting of 2 C2S and 1 CS, has always been considered inert, and studies have found that Ternesite is an active clinker mineral with a hydration reaction between aluminate and Belite. Unlike the traditional OPC clinker calcination process, which requires rapid cooling to retain the high reactivity of the clinker, the formation temperature of Ternesite is between 950 and 1 200 ° C, so the BCT clinker requires a slower cooling process or a relatively long residence time after the burning zone.

The raw materials required for BCT cement production are similar to ordinary Portland cement, and industrial waste residues such as limestone, marl, fly ash and industrial by-product gypsum are the raw materials. The introduction of calcium sulphoaluminate (C4A3S) and calcium sulphosilicite (C5S2S) in BCT cement clinker, produced at a lower temperature (1250 ~ 1300℃), CO2 emissions are 30% lower than traditional OPC cement clinker, and it is expected to save fuel and power consumption by 10% ~ 15%. The mineral composition of BCT cement clinker is: C5S2S: 5% ~ 75%, C2S: 1% ~ 80%, C4(AxF1-x)S: 5% ~ 70%, and secondary phases: 0 ~ 30%. (Patent protection scope) The best mineral composition is: C5S2S: 20% ~

0%, C2S: 20% ~ 50%, C4(AxF1-x)S: 20% ~ 45%, secondary phase: 10% ~ 20%.

x ranges from 0.1 to 1, preferably from 0.8 to 0.95.

4 Conclusion

In the clinker system of the above three types with low calcium minerals C2S and C4A3S as the dominant minerals, in addition to the high Belite cement of the silicate system, more than one low-energy, low-calcium, high-early-strength mineral is introduced into the cement of other systems, such as C4A3S, C5S2S and so on. High Belite content and the introduction of low calcium early strength minerals have become the trend of international low-carbon cement research.

However, from the study of Aether cement and BCT cement systems, C4A3S and C5S2S will decompose at 1 300℃. Therefore, the sintering range of cement clinker system introduced with this mineral is narrow, which puts forward higher requirements for process control level. In addition, too much introduction of the above low calcium early strength minerals into the system is not conducive to achieving high performance of the cement system. In terms of cement performance, although the cement of these systems has high early strength characteristics, it has corresponding adverse effects on construction performance and cement durability. For example, the setting process of the cement containing C4A3S system is difficult to control, so in most cases, it is only used as a special engineering material, limiting its wide application.