Materials Ground
Rawmixes are formulated to contain a correctly balanced chemistry for the production of calcium silicates (alite and belite) and fluxes (aluminate and ferrite) in the kiln. Chemical analysis data in cement manufacture are expressed in terms of oxides, and the most important of these in rawmix design are SiO2, Al2O3, Fe2O3 and CaO. In principle, any material that can contribute any of these oxides can be used as a rawmix component. Because the major oxide required is CaO, the most prevalent rawmix component is limestone, while the others are mostly contributed by clay or shale. Minor adjustments to the chemistry are made by smaller additions of materials such as those shown below.
Typical rawmix component chemical analyses:
| Oxide | Gray Limestone | White Limestone | Marl | Clay | Sand | Millscale | Kaolin | Bauxite | Fly ash |
|---|---|---|---|---|---|---|---|---|---|
| SiO2 | 6.6 | 2.1 | 14.1 | 61.6 | 98.0 | 1.3 | 46.1 | 11.1 | 48.1 |
| Al2O3 | 1.5 | 0.3 | 3.3 | 17.5 | 0.9 | 1.2 | 38.5 | 54.4 | 26.5 |
| Fe2O3 | 0.7 | 0.13 | 1.3 | 7.5 | 0.13 | 96.8 | 0.35 | 9.7 | 6.3 |
| CaO | 48.4 | 53.7 | 43.8 | 1.1 | 0.1 | 0.3 | 0.3 | 0.6 | 4.7 |
| MgO | 2.0 | 0.8 | 0.7 | 1.1 | 0.0 | 0.6 | 0.1 | 0.1 | 1.2 |
| Na2O | 0.07 | 0.02 | 0.07 | 0.5 | 0.02 | 0.11 | 0.01 | 0.05 | 0.3 |
| K2O | 0.27 | 0.08 | 0.43 | 1.9 | 0.37 | 0.05 | 0.09 | 0.05 | 1.3 |
| TiO2 | 0.06 | 0.02 | 0.15 | 0.8 | 0.06 | 0.30 | 0.9 | 2.1 | 1.5 |
| Mn2O3 | 0.03 | 0.01 | 0.02 | 0.12 | 0 | 0.63 | 0 | 0.09 | 0.07 |
| LoI950 | 40.0 | 42.7 | 35.8 | 6.8 | 0.3 | 0 | 13.7 | 20.8 | 9.1 |
Note: LoI950 is the Loss on ignition at 950 °C, and represents (approximately) the components lost during kiln processing. It consists mainly of CO2 from carbonates, H2O from clay hydrates, and organic carbon.
Using these materials, typical rawmixes could be composed:
- Mix 1: General-purpose cement: 88.0% gray limestone, 8.9% clay, 2.2% sand and 0.9% millscale.
- Mix 2: Sulfate-resisting cement: 87.6% gray limestone, 5.2% clay, 5.0% sand and 2.2% millscale.
- Mix 3: White cement: 82.3% white limestone, 6.8% kaolin and 10.9% sand.
The chemical analyses of these rawmixes would be:
| Oxide | Mix 1 | Mix 2 | Mix 3 |
|---|---|---|---|
| SiO2 | 13.46 | 13.91 | 15.55 |
| Al2O3 | 2.91 | 2.30 | 2.96 |
| Fe2O3 | 2.16 | 3.14 | 0.14 |
| CaO | 42.69 | 42.47 | 44.23 |
| MgO | 1.86 | 1.82 | 0.67 |
| Na2O | 0.11 | 0.09 | 0.02 |
| K2O | 0.41 | 0.35 | 0.11 |
| TiO2 | 0.13 | 0.10 | 0.09 |
| Mn2O3 | 0.04 | 0.05 | 0.01 |
| LoI950 | 35.8 | 35.4 | 36.1 |
The raw materials and mixes shown are only "typical": considerable variations are possible depending on the raw materials available.
Read more about this topic: Rawmill
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