Rawmill - Control of Minor Elements

Control of Minor Elements

Apart from the major oxides (CaO, SiO₂, Al₂O₃ and Fe₂O₃) the minor oxides are, at best, diluents of the clinker, and may be deleterious. However, cement raw materials are for the most part dug from the Earth's crust and contain most of the elements in the periodic table in some amount. The manufacturer therefore selects materials so that the deleterious effects of minor elements are minimized or kept under control. Minor elements that are frequently encountered are as follows:

• Fluorine is beneficial to the kiln process in that it allows alite to form at lower temperature. However, at levels above 0.25% in the clinker, delayed and erratic cement setting time results.

• Alkali metals (primarily sodium and potassium) cause processing problems because they form volatile salts in the kiln system. These evaporate in the kiln burning zone and re-condense in the cooler regions of the preheater, causing blockages. Alkalis are also deleterious to concrete, potentially causing alkali silica reaction. For this reason, many standards limit alkalis (typically expressed as "total equivalent soda" which is Na₂O + 0.658 K₂O). Typical specification limits are in the range 0.5–0.8%.

• MgO causes problems at levels over 2.5%. Small amounts are accommodated in solid solution in the clinker minerals, but above 2.5%, "free" MgO exists in the clinker as periclase. This can slowly hydrate to Mg(OH)₂ with expansion in the hardened concrete, causing cracking. Careful processing of the clinker to keep the periclase in a microcrystalline form allows levels up to 5% to be managed without serious effect. All standards limit MgO, typical limits being in the range 4-6%.

• P₂O₅ at levels above 0.5% starts to cause slow setting and low clinker reactivity.

• Chlorine produces very volatile salts and consequent preheater blockages, and is usually limited to below 0.1% in rawmix.

• TiO₂ is ubiquitous, but is rarely present at levels (~1%) that might cause problems.

• Chromium can end up as chromates (Cr) in cement, particularly when the clinker is high in sulfate. Chromates cause allergic contact dermatitis in cement users, and for this reason cement Cr content is limited in many standards to 0.0002%. Typical natural rawmixes contain around 0.01% Cr₂O₃, and at this level, Cr formation can be controlled. Chromium present in the cement as Cr is of no consequence.

• Mn₂O₃ is not deleterious, acting as a substitute for iron. But it contributes more color to the cement than iron, and high-Mn₂O₃ cements (>1%) are almost black.

• ZnO is encountered in some rawmix additives (as well as tires used as kiln fuel). At levels above 0.2%, it causes slow setting and low clinker reactivity.

• Strontium and barium act as calcium replacements, and only start to reduce clinker reactivity at levels of 1.5% and 0.2% respectively.

• Toxic heavy metals: among these, low levels of arsenic, selenium, cadmium, antimony and tungsten are not a problem, because they are absorbed in the basic clinker structure as anions. On the other hand, mercury, thallium and lead must be carefully controlled because they can be emitted as volatile halides in the kiln exhaust.