Hexavalent Chromium in Cement Manufacturing: Literature Review - PCA R&D Serial No. 2983
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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 reservedKEYWORDS
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.
iTABLE OF CONTENTS
Page
Keywords………………………………………………………………………………......... i
Abstract…...……………………………………………………………………..…………... i
Reference……………………………………………………………………………………. i
Table of Contents…………………………………………………………………………... ii
Introduction…………………………………………………………………………………. 1
Chromium in Cement……………………………………………………………………….. 1
Potential Sources of Chromium…………………………………………………………..... 2
Formation of Hexavalent Chromium ……………………………………………………… 4
Reducing Agents ………………………………………………………………………….. 7
Conclusion………………………………………………………………………….….….. 10
Acknowledgements……………………………………………………………………….. 11
References………………………………………………………………………….……... 11
iiHexavalent 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:
_______________________
*Affiliated Consultants, CTLGroup, 5400 Old Orchard Road, Skokie, IL 60077 USA (847) 965-7500,
www.ctlgroup.com
1• 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), Cr6+. 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).
2Table 1. Reported Chromium Content of Raw Materials
Total Chromium Concentration
Sprung and
Raw Material ATILH 2003 Bhatty 1993* Rechenberg 1994
Limestone 2-20 ppm 1.2-16 ppm 0.7-12 g/t
90-109 ppm
Clay 50-200 ppm (clay/shale) 20-90 g/t
Marl 50-200 ppm NA NA
Clay- Schiste 40-110 ppm NA NA
Iron oxide 20-450 ppm NA NA
Mill scale 5000-50000 ppm NA NA
Foundry sand 50-40000 ppm NA NA
Fly ash 200-250 ppm NA NA
Bauxite 200-1100 ppm NA NA
*Bhatty references 1992 Environmental Toxicology Institute report
NA: not available (no values provided)
Fuels
Many fuel types are used in the cement industry, and the chromium content will vary
accordingly. Overall, considering the fuel consumption is 10-15% of the kiln feed, the
contribution to the total chromium content of the clinker is minor. Table 2 provides some
typical values for fuels. Coal contributes minimal total chromium, while supplemental fuels
may contribute more. For example, with usage of tire-derived fuel, steel belts would
theoretically contribute more total chromium.
Table 2. Reported Chromium Content of Fuels
Total Chromium Concentration
Sprung and Rechenberg 1994
Fuel ATILH 2003 Bhatty 1993
Fossil fuels (coal, oil) 0-100 ppm 5-80 ppm (coal) 1-50 g/t (coal)
Lignite, pet coke etc 0-280 ppm NA 2.3-6.1 g/t (lignite)
By products 0-400 ppm NA 97 g/t (tires)
*Bhatty references 1992 Environmental Toxicology Institute report
NA: not available (no values provided)
Refractory Brick
While low-chromium brick is currently more common in use today, refractory brick containing
high levels of chrome have been used in cement kilns. Use of this refractory could contribute
to a surge in chromium levels to the clinker during the first use of kiln after re-bricking
(Klemm 1994). Klemm (2000) refers that these bricks could also contribute Cr (VI) during its
service life in the rotary kiln, chrome-bonded refractory brick was exposed to the clinker
coating and reactive alkalies circulating in the hot kiln gas stream. The chemical reactions that
take place can result in a significant amount of trivalent chromium being oxidized to
hexavalent chromium on the surface and within the bulk of the refractory, and the formation of
alkali chromates and calcium chromate. Potassium chromate and sodium chromate are highly
soluble in water, whereas calcium chromate has only a limited solubility.
3Grinding 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,FeII)(Cr,Al,FeIII)2O4] is mixed with sodium carbonate and free lime and roasted in a
rotary kiln with excess air at around 1200oC. 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
40 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 C3S with Cr2O3
heated in air, Johansen (from Bhatty, 1993) concluded that Cr (III) oxidizes to Cr (V) above
700oC and then is reduced to Cr (IV) above 1400oC, 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, C3S Belite, C2S Aluminate, C3A Aluminoferrite, C4AF
0.39 0.87 0.04 0.55
Table 4. Distribution of Chromium Between Clinker Phases (ATILH 2003)
Burning Chromium content in clinker phases, %
Burning
Temperature Alite Belite Interstitial Phase Metallic
atmosphere
(°C) (C3S) (C2S) (C3A and C4AF) Cr
1450 50 20 30 --
Oxidizing 1500 40 25 35 --
1550 40 25 35 --
1600 40 20 40 --
1450 25 5 20 50
Slightly 1500 30 5 20 45
Reducing 1550 45 5 20 25
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
5between 8 and 20% of the total chromium content (Lizarraga 2003). The content of total
chromium and Cr (VI) of cements are shown in Tables 6 and 7. Table 7 is a compilation of
data from different countries and shows large variation from location to location.
Table 5. Chromium Content of Clinker (ATILH 2003)
Chromium content of Clinker, ppm
Average Minimum Maximum
67.2 16.5 97.0
Table 6. Chromium Content of 94 Cement Samples (PCA 1992)
Total Chromium TCLP Extractable Cr(VI)
(ppm) (ppm)
Range 25-422 0.07-1.54
Average 76 0.54
Table 7. Chromium Content of Cement (ATILH 2003)
Total Chromium Water soluble Cr(VI)
Cement
(ppm) (ppm)
147 cements tested in 1963 NA 6-11.7
Portland Cements NA 1-30
From various European NA
0.003-20
countries
USA/CanadaREDUCING 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 (FeSO4*7H20) 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 (FeSO4H20) 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).
7• 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.”
8Others Agents
• A method described by Schremmer, Oelschlaeger and Boege (from Klemm 1994) to
achieve a low-chromate cement involved calcining the clinker under oxidizing
atmosphere followed by granulating or selecting sizes less than 10 mm, heating to
550ºC with waste coal dust to produce reducing atmosphere and cooling to 300ºC.
• A Japanese patent said to be “effective in diminishing hexavalent chromium” includes a
cement admixture comprised of an air-cooled blast furnace slag powder which consists
mainly of a melilite and has a CO2 absorption of 2% or higher and an ignition loss of
5% or lower. In the powder, the content of sulfur not in the form of sulfuric acid is
0.5% or higher and/or the concentration of non-sulfuric-acid-form sulfur which
dissolves away is 100 mg/L or higher (Morioka, Nakashima, Higuchi, Takahashi,
Yamamoto, Sakai, Daimon 2003).
• A European patent involves the addition of ammonium, alkaline metal or earth alkaline
metal disulphides and/or polysulphides (Cabria 2004).
• Sodium thiosulfate, sodium metabisulfite, and ascorbic acid was found unsatisfactory
due to incomplete chemical reduction of Cr (VI); zinc and aluminum powder required
large amounts to be effective and handling difficulties were encountered; and sodium
dithionate was effective at low concentrations but deteriorated rapidly with storage
(Fregert and Gruvberger from Klemm 1994).
Possible Feed Points
Figure 1 show the possible feed points for addition of ferrous sulfate at the cement plant.
Clinker Cement
Silo Storage
FeSO4
Mixer
FeSO4
Packing Plant
Cement Mill
Bulk
Discharge
Figure 1. Locations of possible feed points for FeSO4 at the cement plant.
Storage and Shelf Life
With at least some of the materials described above, there is reduced effectiveness with
exposure (time/temperature/humidity). The European directive requires that delivery
documents and cement bags be marked with information on the period of time for which the
reducing agents remain effective; BCA member companies have initially declared shelf life as
61 days (BCA 2006). Brookbanks (2005) outlines the recommended storage for packed
9cement 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 FeSO4 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 FeSO4 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 FeSO4 5/1000 FeSO4 10/1000 FeSO4
0 5.1 5.1 5.1
1 0.00 0.00 0.00
10 0.10 0.00 0.00
15 0.60 0.00 0.00
20 0.98 0.23 0.00
27 2.35 0.62 0.49
39 2.70 1.50 1.20
46 2.71 1.62 1.26
60 3.05 2.17 1.46
74 3.05 2.17 1.46
88 3.05 2.17 1.46
Table 9. Amount of Cr (VI) as Function of FeSO4 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 FeSO4 5/1000 FeSO4 10/1000 FeSO4
180 2.30 1.40 1.10
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
10play 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.
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