Chromium is a naturally occurring element found in rocks, soil, animals, plants, and volcanic dust and gases. The most stable forms are Chromium 0, trivalent chromium (Chromium III), and hexavalent chromium (Chromium VI). Although white cement does not have a high level of Chromium VI because of the raw material mix, many Portland types of cement do. Water-soluble hexavalent chromium can cause an allergic reaction to workers exposed to cement with high Chromium VI concentrations, often in the form of what is commonly known as cement dermatitis or eczema. Compounds containing chromium have different solubility and toxicity. The most stable and common oxidation states of chromium are Chromium VI and Chromium III. Chromium VI, in contrast to Chromium III, is relatively well absorbed in human skin. When producing clinker in kilns at high temperatures, Cr³⁺ is oxidized. Among oxidized species, Cr⁶⁺ is the most toxic.
The European cement industry Directive 2003/5C/EC prohibits the sale of cement containing more than 2 ppm of soluble Chromium VI when hydrated. The reduction of Chromium VI to meet the requirements of the European directive has become an integral part of cement production. Producers must provide information on shelf life, as the reducing agent is effective for a limited period.
The most widespread means for achieving chromium reduction is the addition of small quantities of a chemical called a "Reducing Agent" to the cement. The amount of soluble hexavalent chromium depends on the fineness of Portland cement, on type and amount of gypsum, and on the presence of other additives. When water is added to the cement, the reducing agent converts most of the soluble Chromium VI to insoluble Chromium III. A reducing agent in sufficient amounts must be added to ensure that any residual soluble chromium is below the 2-ppm limit. Typical reducing agents include ferrous sulfate, tin sulfate, and tin chloride; with Ferrous sulfate the most widely used reduction substance.
When cement is ground together with ferrous sulfate and mixed with water, chromates and ferrous salts are released into the solution, while the pH quickly increases due to cement hydration. Fe²⁺ ions form insoluble hydroxides, their redox potential drops. In particular as the pH increases, their redox potential drops faster than the redox potential of Cr⁶⁺, and Fe (OH)₂ becomes a strong reducing agent. Soluble chromates are reduced to Cr (OH)₃. The reaction of reduction is as follows
CrO₄²⁻ + 3Fe(OH)₂ + 4H₂O → Cr(OH)₃ + 3Fe(OH)₃ + 2(OH)⁻
Unfortunately, there are some drawbacks associated with the use of chromium reducers. In addition to being hazardous, stannous chloride suffers from bad solubility and high prices, not to mention the chloride content. In general, the Ferrous, Magnesium, and Stannous Sulfates present in amounts higher than 1% by mass of cement in process water may retard generally the hydration at the early age of cement with the retardation effect increasing with increased concentration. On the other hand, the drawback of ferrous sulfate heptahydrate's use as a Chromium VI reducer is that when added during grinding, it undergoes partial dehydration to monohydrate salt, with the consequent loss of reducing activity due to its high water solubility and the high temperatures in the mill. It is also a corrosive agent and in the long term, it can damage the components of both the grinding system and storage silos. In addition, if excessive ferrous sulfate heptahydrate is added to cement, the color of the cement becomes green, which is an unacceptable result. The use of dried ferrous sulfate heptahydrate can be problematic as it is dusting and it has a low melting point. The ferrous sulfate hygroscopicity may cause a reduction in the fluidity of the powder at all stages of handling, which compounds the workers’ health problems. Formulated products can be designed to mitigate these drawbacks.
With air and or moisture permeating through packaging or storage silos, the effect of reducing agents decreases from the time of manufacture until the cement is used. Producers are required to provide in the form of shelf life for their cement, a guarantee that the cement will release less than 2-ppm Chromium VI by weight of cement, when water is added to it, assuming it has been stored correctly. The shelf life will depend on a number of factors including cement Chromium VI content, type and addition rate of reducing agent, type of packaging, storage, etc. Information on the appropriate storage conditions for cement and its shelf life must be marked on the packaging, for packed cement and/or on the delivery ticket for bulk cement. The cement that has exceeded its shelf life should not be used.
When users are using any cement to produce other products they should ensure the blended product is within the declared shelf life and also the final product does not contain other sources of Chromium VI that could increase the overall level of Chromium VI above the limit. The same applies to formulated cement products containing a blend of ingredients. Manufacturers or formulators are responsible for ensuring adherence to standards and for declaring shelf life.