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PCA R&D Serial No. 2983

Hexavalent Chromium in Cement

Manufacturing: Literature Review

by Linda Hills and Vagn C. Johansen

©Portland Cement Association 2007 All rights reserved

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KEYWORDS

ACD, alkali, allergic contact dermatitis, cement, chromate eczema, chrome, chromium, clinker, Cr (VI), Cr (III), ferrous sulfate, finish mill, health, hexavalent, kiln atmosphere, manganese sulfate, Occupational Safety & Health Administration, OSHA, oxidation, portland cement, raw materials, reducing agent, refractory brick, solubility, stability, stannous chloride, stannous sulfate, storage period, trivalent.

ABSTRACT

With regard to chromium and health and safety aspects, the water-soluble compounds of chromium in cement are most relevant, specifically compounds of the form Cr (VI). Chromium in the cement can originate from: 1) raw materials or fuel, 2) magnesia-chrome kiln refractory brick, if used, 3) wear metal from raw mill grinding process, if chromium alloys are used, and 4) additions such as gypsum, pozzolans, ground granulated blast furnace slag, mineral components, and cement kiln dust. The cement process, specifically kiln conditions, can influence how much Cr (VI) will form. In the kiln, oxidizing atmosphere will play the largest role, with more oxygen in the burning zone leading to increased Cr (VI) formation. Alkali concentration is also of importance, since Cr (VI) in clinker is primarily in the form of chromates. In the finish mill, thermodynamically favorable conditions for oxidation to Cr (VI) exists, including high air sweep, moisture from gypsum dehydration, cooling water injection, and grinding aids, along with the high pH of the cement. Several materials have been used to reduce the level of soluble Cr (VI) formation. The most widely used material is ferrous sulfate; other materials include stannous sulfate, manganese sulfate, and stannous chloride. Some of these materia2ls have limitations such as limited stability, limited supply, and possible influence on cement performance. In all cases, some form of dosing and mixing equipment is required.

REFERENCE

Hills, Linda M and Johansen, Vagn C., Hexavalent Chromium in Cement Manufacturing: Literature Review , SN2983, Portland Cement Association, Skokie, Illinois, USA, 2007, 16 pages.

Hexavalent Chromium in Cement

Manufacturing: Literature Review

By Linda M. Hills and Vagn C. Johansen*

INTRODUCTION

The purpose of this review is to summarize information related to hexavalent chromium, Cr (VI), in the portland cement industry. The catalyst for initiating this project was the content restrictions and labeling/marketing of cements based on the level of Cr (VI) in cement in Europe, and the potential for a similar situation in North America in the near future. European Directive 2003/53/EC was implemented January 2005 and is binding in the UK and other EU member states (European Parliament 2003). As outlined in reference BCA (2006), the directive: 1) prohibits the placing on the market or use of cement or cement preparations which contain, when hydrated, more than 2 ppm (0.0002%) of soluble Cr (VI); 2) requires that where cement or cement preparations have a soluble Cr (VI) content of 2 ppm or less, when hydrated, due to the presence of a reducing agent, their packaging should be marked with information on the period of time for which the reducing agent remains effective (i.e. packing date, suggested storage conditions, and suggested storage period); and 3) permits the placing on the market and use of cement or cement preparation not meeting the two requirements above only when it is for use in totally automated and fully enclosed processes where there is no possibility of contact with the skin. In regards to possible similar restrictions in North America, at the time of this report, the Occupational Safety and Health Administration (OSHA) has exempted portland cement from its standard for occupational exposure to hexavalent chromium (OSHA 2006). This report includes information on: 1) potential sources of chromium to the manufacturing process, 2) the process of chromium oxidation in the cement kiln, including the dependant variables, and 3) the use of additives to reduce the level of hexavalent chromium formation in hydrated cement.

CHROMIUM IN CEMENT

The “chromium” content of cement generally refers to compounds containing chromium. An important consideration is the oxidation state of chromium in these compounds. The most often discussed forms in the cement industry are Cr (III) and Cr (VI). Cr (III) because it is the major form of chromium in cement, and Cr (VI) because it has received the most attention regarding health issues. Chromium has also been detected in the form of Cr (IV) and Cr (V) (Mishulovich 1995) although during cement hydration, these forms disproportionate to Cr (III) and Cr (VI) (Johansen 1972). A description of the trivalent and hexavalent states of chromium in clinker and examples of the compounds in each are provided below:

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*Affiliated Consultants, CTLGroup, 5400 Old Orchard Road, Skokie, IL 60077 USA (847) 965-7500, www.ctlgroup.com

• Trivalent chromium, also Cr (III), Cr3+. Compounds include chromic oxide, chromic sulfate, chromic chloride, and chromic potassium sulfate (APCA 1998). Compounds with Cr (III) are stable, and therefore the form found in quarried materials, and most prevalent in clinker and cement. Since these compounds are the most stable, having low solubility and low reactivity, their impact on the environment and living systems is low.
• Hexavalent chromium, also Cr (IV), Cr 6+. Compounds include chromium trioxide, chromic acid, sodium chromate, sodium dichromate, potassium dichromate, ammonium dichromate, zinc chromate, calcium chromate, lead chromate, barium chromate, and strontium chromate (APCA 1998). Compounds of hexavalent chromium are strong oxidizers and unstable (Mishulovich 1995). It’s solubility in water is related to reported health risks, as described further below.

Chromium in some form is present in portland cement in generally trace amounts. The form of this chromium is important to reported health risks. The form of particular interest is Cr (VI) due to it’s solubility in water. For example, when dissolved, Cr (VI) can penetrate unprotected skin and is transformed into Cr (III), which combines with epidermal proteins to form the allergen that causes sensitivity to certain people (Chandelle 2003). This allergic problem only occurs in certain individuals who are particularly sensitive; once sensitization is induced, this condition, allergic contact dermatitis (ACD), may be triggered by very small amounts of subsequent exposure to chromate ions. This sensitivity can exacerbate the severity of chemical burns brought on by the high pH of hydrating cement. The amount of Cr (VI) in clinker and cement can originate from: 1) oxidation of total chromium from the raw materials or fuel entering the system based on conditions of the clinker burning process, 2) magnesia-chrome kiln refractory brick, if used, 3) wear metal from crushers and raw mill grinding process, if chromium alloys are used, and 4) additions of gypsum, pozzolans, ground granulated blast furnace slag, mmineral components, cement kiln dust, and set regulators. The cement manufacturing process, specifically the kiln and possibly finish mill conditions, can influence how much Cr (VI) will form.

POTENTIAL SOURCES OF CHROMIUM

The amounts of total chromium and soluble hexavalent chromium found in clinker and in hydraulic cements may originate from a variety of sources, as exemplified in this section.

Raw Materials

All quarried raw materials for cement manufacture contain very small or trace quantities of total chromium, which is a common element in the earth's crust. The increasing use of many by-product raw materials such as metallurgical slag, spent catalyst fines, flue gas desulfurization gypsum, lime sludge, etc., may contribute additional amounts, however little published data was found on many of these by-product materials. Total chromium from the primary raw materials varies with the type and origin; typical values are given in Table 1. Most quarried raw materials contain no water soluble chromium as Cr (VI), and chromium is usually in oxidation state Cr (III) (ATILH 2003). Cr (VI) levels in fly ashes and electro filter dust are reported in the range of about 0.5 ppm and 0.3 ppm respectively (ATILH 2003).

Grinding Media

If chromium alloys are used in grinding media and crushers, they may contribute metallic chromium. Klemm (1994) reports that in clinker ground with chrome alloy balls containing 17-28% chromium, the hexavalent chromium content of the cement may increase to over twice that in the original clinker. However, the reduction in use of such materials over recent years makes this a less likely source of chromium. Regarding conversion to Cr (VI) during finish grinding, possible favorable conditions are discussed in the following section of this report.

Additions

Additions of gypsum, pozzolans, ground granulated blast furnace slag, mineral components, cement kiln dust, and set regulators may be potential sources of chromium. ATILH (2003) reports total chromium content in gypsum to be 3.3-33 ppm.

FORMATION OF HEXAVALENT CHROMIUM

Conversion to Cr (VI) in cement manufacturing takes place in the kiln and possibly to some small degree in the final grinding stages.

Formation in Kiln

The formation of Cr (VI) in the kiln system is dependant primarily on the oxygen level and alkali content. These relationships are discussed further below. The source of chromium input in the kiln feed is primarily as Cr (III). The conditions in the kiln include high amount of CaO, free lime, and alkalis due to the internal circulation of volatiles. Such conditions are favorable for oxidation of chromium to Cr (VI), the amount of which will depend on the oxygen pressure in the kiln atmosphere. The process is similar to the production of sodium chromate by which chromite ore [(Mg,Fe II)(Cr,Al,Fe III) 2 O 4 ] is mixed with sodium carbonate and free lime and roasted in a rotary kiln with excess air at around 1200o^ C. The sodium chromate is water soluble and washed out of the product. In the cement burning process, depending on the partial pressure of oxygen and availability of potassium and sodium chromates of these will form, and due to some chemical similarity between sulfate and chromate, the chromate will follow alkali sulfates in clinker (Fregert 1974). Mishulovich (1995) also indicates the importance of alkali concentration, since Cr (VI) in clinker is principally in the form of chromates. Numerous studies emphasize the importance of oxidizing conditions for conversion to Cr (VI), including Reifenstein and Paetzold (from Bhatty 1993), Boikova (from Bhatty 1993), and Fregert (1974). Mishulovich (1995) showed in laboratory studies the relationship between degrees of oxidation to oxygen content and concluded that limiting oxygen in the burning zone would decrease formation of Cr (VI) in the clinker. Lizarraga (2003) observed that insufficiently calcined clinker had low amounts of Cr (VI), supporting the view that the oxidation of chromium and formation of Cr (VI) takes place in the burning zone. In this study, plant tests were performed to investigate the possibility to decrease the amount of Cr (VI) in clinker by having reducing conditions in the kiln. This was obtained by adding fuel oil to the cooler, pet coke to various positions in the preheater, and to kiln feed at various rates. Determination of Cr (VI) in the clinker indicated some effect in terms of decreasing Cr (VI) to

0 mg/kg clinker from around 5 mg/kg. However it was concluded that the operational complications involved were much greater than addition of ferrous sulfate to the cement. Other than as alkali chromate, the chromium is distributed in solid solution in the clinker minerals as a function of burning conditions. In a laboratory study combing C 3 S with Cr 2 O 3 heated in air, Johansen (from Bhatty, 1993) concluded that Cr (III) oxidizes to Cr (V) above 700 o^ C and then is reduced to Cr (IV) above 1400o^ C, resulting in the presence of Cr (IV) and Cr (V) in solid solution. Table 3 shows the distribution of chromium in the individual clinker minerals according to Hornain (1971). Table 4 shows the relative distribution of chromium in laboratory clinker burned at different temperatures and oxidation conditions (the fact that belite holds a smaller fraction of the total amount of chromium reflects the low amount of belite in this clinker).

Table 3. Distribution of Chromium in Clinker Minerals (Hornain 1971)

Chromium content in clinker minerals %

Alite, C 3 S Belite, C 2 S Aluminate, C 3 A Aluminoferrite, C 4 AF 0.39 0.87 0.04 0.

Table 4. Distribution of Chromium Between Clinker Phases (ATILH 2003)

Burning^ Chromium content in clinker phases, % atmosphere

Burning Temperature (°C)

Alite (C 3 S)

Belite (C 2 S)

Interstitial Phase (C 3 A and C 4 AF)

Metallic Cr 1450 50 20 30 -- 1500 40 25 35 -- 1550 40 25 35 --

Oxidizing

1600 40 20 40 -- 1450 25 5 20 50 1500 30 5 20 45 1550 45 5 20 25

Slightly Reducing 1600 40 5 45 10

Formation in Finish Mill

Conversion to Cr (VI) in the cement may also take place in the finish mill. Wear metal from chrome alloy grinding media may provide metallic chromium. The finish mill provides thermodynamically favorable conditions for oxidation of metallic Cr to Cr (VI), including high air sweep, moisture from gypsum dehydration, cooling water injection, and grinding aids, along with the high pH conditions characteristic of portland cement (Klemm 2000) However, report APCA (1998) states that chromium from finish grinding will not oxidize to Cr (VI). “Chromium from finish grinding remains either as metal or gradually oxidizes to divalent chromium, trivalent chromium, or perhaps Cr (IV), but not hexavalent chromium.”

Chromium Levels in Clinker and Cement

The chromium content in clinker samples from a Belgian study are shown in Table 5. In regards to Cr (VI) levels in clinker, a Spanish investigation reports Cr (VI) content of clinker

REDUCING AGENTS

The use of materials to reduce the level of Cr (VI) formation is especially prevalent in the European cement industry due to the 2003 European Directive which prohibits sale of cement containing more than 2 ppm of soluble Cr (VI) when hydrated. Cement companies under this directive are adding reducing agents to comply with this directive. The description of materials used for this purpose, reported effectiveness, limitations, and other items of note are provided below. Examples of capital investment for storage and metering systems for reducing agents, and estimated cost of these agents for the European industry are provided in Cement International (2004).

Ferrous Sulfate

• Natural heptahydrate (FeSO 4 *7H 2 0) is found as an alteration product of iron sulfides as the mineral melanterite, or can be an industrial by-product. It is soluble in water, its aqueous solutions are oxidized slowly by air when cold and rapidly when hot, and the oxidation rate is increased under alkaline conditions (Klemm 1994). Should not be overheated to avoid partial oxidization and dehydration, leading to reduced solubility.
• Various ferrous sulfate types differ in active-substance content, particle size, and chemical and physical properties; the product selected for use is determined on metering location and temperature, and storage conditions (Kuehl 2006).
• Monohydrate form (FeSO 4 H 2 0) has been demonstrated to be a successful reducing agent by Valverde, Lobato, Fernandez, Marijuan, Perez-Mohedano, and Talero (2005)
• Addition rate is usually 0.5% by mass and is generally added as powder which may require addition and blending equipment.
• The dosing process can influence the effectiveness of the reducer. The addition of the reducer in the cement mill involves thermal, mechanical and chemical stress, which can accelerate the chemical reaction of the reducers and decrease its effectiveness (Kehrmann and Bremers 2006). These authors conclude heptahydrates are particularly effective if added to cement in format of granulates versus ground in the finish mill, whereas monohydrate, which contain less crystallized water are less susceptible to high temperature and can be ground with clinker.
• May effect cement quality. Excess sulfate may result in lower concrete strength, expansion, and possible internal sulfate attack. At high dosages, there can be concerns of increased water demand, long setting time, and possible concrete discoloration or mottling (Sumner, Porteneuve, Jardine, and Macklin 2006).
• Regarding reduction results, 0.35% ferrous sulfate reduced 20 ppm of Cr (VI) in cement to less than 0.01 ppm in aqueous slurry, however, high temperature and humidity in simulated grinding minimized this reduction capacity (Fregert, Gruvberger and Sandahl from Klemm 1994).

Modifications:

• Bhatty (1993) reports several studies in which the ferrous sulfate was dissolved as 20% solution and added as admixture in concrete/mortar.
• A free-flowing powder was reported by Rasmussen (from Klemm 1994) by mixing with fly ash, gypsum, or other absorbing powder and drying at 20-60ºC.
• The use of “ferrogypsum” is described by mixing the ferrous sulfate with “green salt” (waste product from titanium dioxide manufacture) and gypsum (Norelius from Klemm 1994).

• A Belgium patent application includes a method for fluidization of moist ferrosulfate heptahydrate by adding flyash or fumed silica as a desiccant (Degre, Duron, and Vecoven, 2005).
• A patent by Kehrmann and Paulus (2004) involves cement, iron (II) sulfate, and an acidifying agent for reducing the pH. The acidifying agent provides an acidic environment in the cement, thereby influencing the reactivity of iron (II)-sulfate and increasing the storage life.

Stannous Sulfate

• Manufactured product which can withstand relatively high temperatures without degradation, enabling addition to finish mill.
• Storage stability is longer than ferrous sulfate (Bonder 2005).
• More effective at low dosages compared to ferrous sulfate (Sumner, Porteneuve, Jardine, and Macklin 2006). These authors also discuss a patent pending liquid additive to increase storage stability.
• Can be available in form of suspension, which may not require expensive metering/addition equipment.

Manganese Sulfate

Larsen (from Klemm 1994) discusses that a cement composition containing manganous sulfate is effective in reducing the content of water-soluble chromate. Mangenese compounds are much more oxidation stable than iron compounds in dry cement at high temperatures and have a high chromate-reduction efficiency. Klemm (1994) reports a study in which clinker with 19.7 ppm Cr (VI) interground in laboratory ball mill with 5% gypsum and 0.75% manganous sulfate resulted in water-soluble chromate after leaching of 0.0 ppm.

Stannous Chloride

The use of tin salts is described by Magistri and Padovani (2005), who describe higher reduction efficiency over iron salts, superior stability and duration of reduction with time, and absence of surface discoloration. The disadvantage of use in the cement industry, as stated by these authors, is the high cost and reduced availability. A patent using liquid additive with stannous chloride is provided by Jardine, Cornman, Chun, and Gupta (2005). Included in the patent description is the use of an antioxidant and/or oxygen scavenger, which is believed to extend the shelf-life and effectiveness of the stannous chloride reducing agent. A similar methodology is described by Magistri and Padovani (2005), in which a triple emulsion is employed involving the aqueous reducing agent solution in an emulsion surrounded by a layer of organic solvent, which is dispersed in a second aqueous solution. The organic solvent functions as a barrier which impedes contact between the reducing agent and atmospheric oxygen, preventing loss in performance with time.

Zinc (II) Salts

A patent by Alter and Rudert (2004) involves zinc (II) or stannous salts mixed with a fine hydraulic binder or finely ground (10000-18000 cm2/g) blast furnace slag to provide a “physiologically-effective industrial protective means for preventing the harmful effects of tetravalent chromium compounds in cements.”

cement as stored in unopened bags clear of the ground in cool dry conditions and protected from excessive draught, whereas recommended storage for bulk cement is to be stored in silos that are waterproof, clean, and protected from contamination, dry with stock rotated in chronological order with dispatch dates marked on delivery documents. One documentation of storage conditions was reported by Lizarraga (2003). In this study, different levels of FeSO 4 was added to cement stored in sacks. The amount of Cr (VI) was followed up to 88 days in individual sacks stored at ambient and up to 180 days for sacks stored in plastic cover. Tables 8 and 9 show the results demonstrating the dependence of storage conditions. Important differences in analytical results were observed between samples from the outer and the inner part of a sack, so homogenization of samples before analysis was needed. A later study by this author investigated an accelerated test to determine period of effectiveness for reducing agents (Lizarraga 2006). A study reported by Valverde, Lobato, Fernandez, Marijuan, Perez-Mohedano, and Talero (2005) did not show a decline in the reducing power of either ferrous sulfate heptahydrate or monohydrate after storage in a dry environment for three months.

Table 8. Amount of Cr (VI) as Function of FeSO 4 Dosage and Time for Cement Stored at Ambient Conditions in Individual Sacks (Lizarraga 2003)

mg/kg Cr(VI) mg/kg Cr(VI) mg/kg Cr(VI) Days 1/1000 FeSO 4 5/1000 FeSO 4 10/1000 FeSO 4 0 5.1 5.1 5. 1 0.00 0.00 0. 10 0.10 0.00 0. 15 0.60 0.00 0. 20 0.98 0.23 0. 27 2.35 0.62 0. 39 2.70 1.50 1. 46 2.71 1.62 1. 60 3.05 2.17 1. 74 3.05 2.17 1. 88 3.05 2.17 1.

Table 9. Amount of Cr (VI) as Function of FeSO 4 Dosage and Time for Cement Stored Under Plastic Cover in Individual Sacks (Lizarraga 2003).

mg/kg Cr(VI) mg/kg Cr(VI) mg/kg Cr(VI) Days 1/1000 FeSO 4 5/1000 FeSO 4 10/1000 FeSO 4 180 2.30 1.40 1.

CONCLUSION

Chromium in the cement can originate from a variety of sources, including raw materials, fuel, refractory, grinding media, and additions. With regard to health and safety aspects, the water- soluble compounds of chromium in cement are most relevant, specifically compounds of the form Cr (VI). The manufacturing process, specifically kiln conditions, can influence how much Cr (VI) will form. Oxidizing atmosphere will play the largest role, with more oxygen in the burning zone leading to increased Cr (VI) formation. Alkali concentration is also of importance, since Cr (VI) in clinker is primarily in the form of chromates. The finish mill may

play a role, as the thermodynamically favorable conditions for oxidation to Cr (VI) exists, including high air sweep, moisture from gypsum dehydration, cooling water injection, and grinding aids, along with the high pH of the cement. The most widely used material used to reduce the level of soluble Cr (VI) formation in hydrated cement is ferrous sulfate; other materials include stannous sulfate, manganese sulfate, and stannous chloride. Some of these materials have limitations such as limited stability, limited supply, and possible influence on cement performance. In all cases, some form of dosing and mixing equipment is required.

ACKNOWLEDGEMENTS

The information reported in this paper (SN2983) was conducted by CTLGroup with the sponsorship of the Portland Cement Association (PCA Project Index No. M05-04). The contents of this report reflect the views of the authors, who are responsible for the facts and accuracy of the data presented. The contents do not necessarily reflect the views of the Portland Cement Association.

REFERENCES

Alter, Gabriela and Rudert, Volkhart, Physiologically-Effective Industrial Protective Means , World Intellectual Property Organization, publication number WO/2004/052806, 2004.

APCA Hexavalent Chromium Task Force, SN2219, Portland Cement Association, Skokie, Illinois, USA, 1998, 13 pages.

ATILH 2003 Chromium in Cement, origin and possible treatments (in French), ATILH Center for information and documentation, September 2003.

Bhatty, Javed, Chromium in Portland Cement: Literature Review , SN1971, Portland Cement Association, Skokie, Illinois, USA, 1993, 10 pages.

BCA (British Cement Association), Cement Industry Update: Special Issue on Chromium (VI) Directive , http://www.cementindustry.co.uk/PDF/Special%20Cement%20Industry %20Update%20on%20Chromium%20(VI).pdf, accessed from website 2006.

Bonder, Wolfgang, “Chromate VI Reduction”, World Cement , November 2005, pages 19-110.

Brookbanks, Peter, The New Chromium (VI) Directive , Institute of Concrete Technology, Briefing Note BN 04/05, 2005, 3 pages.

Cabria, Flavio, Process for Preparing Cement with a Low Hexavalent Chromium Content , European Patent Office, Patent Application No. 04425201.3, 2005.

Cement International, “Chromate Reduction – A New Challenge for the Cement and Concrete Industry”, Vol. 2, No. 5, 2004, pages 51-53.

Chandelle, Jean-Marie, Chromium VI in Cement, Global Cement and Lime Magazine , Surry, U.K, July-August 2003, pages 12-13.

Lizarraga, Serafin, “Study of the Behaviour of Ferrous Sulphate Heptahydrate as a Hexavalent Chromium Reducing Agent”, Cemento Hormingon , No.887, April 2006, pages 4-8.

Magistri, Matteo and Padovani, Davide, “Chromate Reducing Agents” , International Cement Review , October, 2005, pages 49-56.

Mansfield, S., BCA Safety Information Sheet: Cements, Chromium, Ferrous Sulfate and Other Reducing Agents , BCA Document No. ST/IS/14b, British Cement Association, December

Mishulovich, Alex, Oxidation States of Chromium in Clinker , SN2025, Portland Cement Association, Skokie, Illinois, USA, 1995, 6 pages.

Morioka, Minoru; Nakashima, Yasuhiro; Higuchi, Takayuki; Takahashi, Mitsuo; Yamamoto, Kenji; Sakai, Etuo; and Daimon, Masaki, Cement Admixture, Cement Composition, and Cement Concrete Made Therefrom , World Intellectual Property Organization, publication number WO/2003/035570, 2003.

OSHA (Occupational Safety & Health Administration), Occupational Exposure to Hexavalent Chromium , http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table= FEDERAL_REGISTER&p_id=18599, accessed from website 2006.

PCA (Portland Cement Association), An Analysis of Selected Trace Metals in Cement and Kiln Dust , SP109T, Portland Cement Association, Skokie, Illinois, USA, 1992, 56 pages.

Sprung, S. and Rechenberg, W., “Levels of Heavy Metals in Clinker and Cement”, Zement- Kalk-Gips , Vol. 47, No. 7, 1994, pages 183-188.

Sumner, M.; Porteneuve, C.; Jardine, L.; and Macklin, M., “Advances in Chromium Reduction”, World Cement , March 2006, pages 33-36.

Taylor, M.G., BCA Information Sheet: The New Chromium VI Directive for Cement: Timetable for Implementation , BCA Document No. ST/IS/14a, British Cement Association, September

Taylor, M.G., BCA Information Sheet: The Chromium (VI) Directive for Cement: Sheet 7, Compliance Protocol for Chromium (VI) Content of Cement, BCA Document No. ST/IS/14g, British Cement Association, November 2005.

Taylor, M.G., BCA Information Sheet: Cements and Water-Soluble Chromium (VI ), BCA Document No. ST/IS/12, British Cement Association, October 2003.

Valverde, J.L.; Lobato, J.; Fernandez, I.; Marijuan, L.; Periz-Mohedano, S.; and Talero, R., “Ferrous Sulphate Mono and Heptahydrate Reduction of Hexavalent Chromium in Cement: Effectiveness and Storability”, Materiales de Construccion , No. 279, Vol. 55, July/Aug/Sep 2005, pages 39-52.