WO1997027153A1

WO1997027153A1 - Sulfate elevation in anionic polysaccharide-containing cementitious formulations for enhanced cement performance

Info

Publication number: WO1997027153A1
Application number: PCT/US1997/001111
Authority: WO
Inventors: Bryan Skaggs; Walter Rakitsky; John Swazey; Harold Dial
Original assignee: The Nutrasweet Company
Priority date: 1996-01-26
Filing date: 1997-01-24
Publication date: 1997-07-31

Abstract

The rheological properties of a variety of anionic polysaccharide- containing cementitious systems can be improved by increasing the level of soluble sulfate in the cementitious formulation. This can be achieved by introducing a sulfur-containing compound and/or by increasing the amount of alkali metal cation(s) in the formulation.

Description

_Sulfate Elevation in Anionic Polysaccharide-Containing cementitious Formulations for Enhanced Cement Performance

FIELD OF THE INVENTION

The present invention relates to novel anionic polysaccharide-containing cementitious systems having improved rheological properties, as well as methods for improving the rheological properties of prior art anionic polysaccharide-containing cementitious systems.

BACKGROUND OF THE INVENTION

The majority of buildings are constructed with cementitious materials or systems that vary widely in composition, design, and end use. As used herein, the term "cementitious system" refers to materials which, when mixed with an aqueous medium, bind, or impart an adhesive or cohesive behavior. Some examples include portland cements that are produced by burning limestone and clay, natural and artificial pozzolanic cements (pozzolanic materials react with lime to form calcium silicate hydrates) , slag cements, combinations of portland cement and granulated blast furnace slag, refractory cements (e.g., rapid set cements containing primarily calcium alu inate compounds, such as, for example, Ciment Fondu produced by Lafarge, and Luminite, produced by Lehigh Cement Company) , gypsum and desulfurized gypsum cements, expanding cements, fly ash, and the like. See, for example, Bye, G.C, "Portland Cement, Composition, Production and Properties", Perga on Press, New York, London, Ontario, Paris, Oxford (1983) ; Smith, Dwight K., "Cementing", Monograph Volume 4, Published by The Society of Petroleum Engineers, New York and Richardson, TX (1987) .

Fresh cement or concrete paste is comprised of a wide range of materials such as portland cement, fly ash, silica fume, sand, aggregate, i.e., small rocks and water. Mixing, transporting, and placing the fresh concrete presents a number of challenges as the paste must remain highly fluid and ideally should provide homogenous transport of all particles. This problem is compounded because excessive water is frequently added to the mixture in efforts to enhance flow. The hydration of portland cement, for example, typically requires some 25-28 percent water basis weight of cement (BWOW) . Unfortunately, excessive water can lead to a number of problems, such as, for example, bleed, sedimentation, reduced strength and durability, and poor bonding to structural reinforcement members.

Two types of materials can be used in admixture with cementitious formulations to enhance fresh paste flow, without the need to employ additional water, i.e., water reducers and superplasticizers. However, admixtures containing either material can induce excessive bleed and sedimentation. As used herein "bleed" refers to free water collection on the surface, while "sedimentation" refers to the segregation of particle size whether during placement, or static. Excessive bleed reduces durability and strength of the desired bond. In εome cases, bleed water channels form on horizontal structural components, thereby reducing bond strength and creating corrosion sites. Aggregate segregation reduces the surface wear properties causing increased maintenance costs.

Recent technologies have provided a new class of cement additives, the so called rheological modifiers, or viscosity modifying agents (VMA) . This class of additives comprises generally water-soluble polymers which function by increasing the apparent viscosity of the mix water. This enhanced viscosity facilitates uniform flow of the particles and reduces bleed, or free water formation, on the fresh paste surface. Underwater concrete placement designs frequently require a polymer admixture to reduce fines loss during placement (Khayat, Ka al Henri, "Effects of Antiwashout Admixtures on Fresh Concrete Properties", Published in the ACI Structural Journal, Title No. 92-M18, March-April, (1995)) . Unfortunately, this also increases the resistance of the fresh cement paste to flow and may induce excessive frictional pressure during conveyment.

Accordingly, there is still a need in the art for methods to treat the above-described problems of bleed, sedimentation, flow resistance, etc, encountered with existing cementitious formulations.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the present invention, it has been discovered that the rheological properties of a variety of anionic polysaccharide-containing cementitious systems can be improved by increasing the level of soluble sulfate in the cementitious formulation.

For example, the rheological performance of cementitious grouts containing welan gum is enhanced by manipulating soluble sulfate concentration. This performance increase is directly related to the soluble sulfate content of the cement formulation. In accordance with the present invention, it has been found that the rheological performance may increase by up to eighty percent or more. A number of methods are effective for increasing the soluble sulfate content of cementitious grouts, including direct addition of alkali sulfate.

The present invention provides an economical means to improve the performance properties of anionic polysaccharides in certain cements. Thus, it has been discovered that sulfate addition to anionic polysaccharide- containing cementitious systems (and/or enhancing the level of soluble sulfate in such systems by a variety of means) provides enhanced rheological control in a variety of cementitious systems. In addition, increasing the levels of soluble sulfate in anionic polysaccharide-containing cementitious systems provides a number of advantages relative to prior art systems, including enhanced rheological performance, improved free water and sedimentation control, improved water retention, as well as reduced fines loss during underwater placement.

Furthermore, enhancing the levels of soluble sulfate in anionic polysaccharide-containing cementitious systems enhances the stability of highly diluted microfine cementitious systems, and enhances the flow and workability of superworkable and self-leveling pastes. The homogenous set cement resulting from the invention treatment of cementitious systems promotes bond strength and eliminates the need for vibrated concrete. Efficient free or bleed water control eliminates unsightly vugs or voids adjacent to the form work and thus enhances the appearance of the finished concrete. The invention methods provide additional benefits as well, such as, for example, enhanced color delivery in pigmented concrete and stabilized bubble entrapment in so called foamed or cellular cement systems. When used for treatment of sprayable cementitious systems, the invention method reduces rebound and sag.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, there are provided methods to improve the solubility and performance properties of anionic polysaccharides in cementitious formulations. The invention method comprises increasing the amount of soluble sulfate in the cementitious formulation.

As readily recognized by those of skill in the art, the amount of soluble sulfate in a cementitious formulation can be increased in a variety of ways, such as, for example, by introducing into said formulation at least one sulfur-containing compound selected from soluble salts of sulfides, sulfites, sulfates, bisulfates, persulfates, thiosulfates, sulfanes, thionates, dithionates, thioantimonates, and the like, as well as mixtures of any two or more thereof. Alternatively, the amount of soluble sulfate in a cementitious formulation can be elevated by increasing the amount of alkali metal cation(s) in the formulation. As yet another alternative, the pH of the cementitious formulation can be increased (for example, by adding bicarbonate, soda ash, or the like to the formulation) , thereby releasing sulfate ions therefrom.

Specific compounds which can be employed to increase the amount of soluble sulfate in cementitious formulations include εyngenite, potassium zinc sulfate, potassium zirconium sulfate, ammonium iron sulfate, and the like, as well as lithium, sodium, potassium, ammonium, magnesium, calcium, rubidium, strontium, cesium, barium, thallium, aluminum, cobalt or copper salts of sulfides, disulfides, persulfides, sulfites, sulfates, bisulfates, persulfates, thiosulfates, sulfanes, thionates, dithionates, thioantimonates, and the like, as well as mixtures of any two or more thereof.

In accordance with the present invention, it has been found that desirable levels of soluble sulfate which impart the improvements described herein typically fall in the range of about 0.1 up to 10 wt %, based on the dry weight of the cementitious formulation. Those of skill in the art recognize that preferred levels of soluble sulfate which impart the improvements described herein will vary depending on the particular cementitious formulation employed. Preferred values typically fall in the range of about 0.5 up to 5 wt %, based on the dry weight of the cementitious formulation. When the amount of soluble sulfate in cementitious formulations is indirectly increased by increasing the amount of alkali metal cation(s) in a cementitious formulation, a variety of soluble alkali metal sources can be employed, such as, for example, one or more soluble salts of lithium, sodium, potassium, rubidium or cesium.

When one or more alkali metal-containing compounds are employed to increase the amount of alkali metal cation(s) in cementitious formulations, the amount of alkali metal cation-containing compounds employed typically falls in the range of about 0.1 up to 10 wt %, based on the dry weight of the cementitious formulation. Those of skill in the art recognize that preferred levels of alkali metal cation-containing compounds which impart the improvements described herein will vary depending on the particular cementitious formulation employed, and the particular application contemplated for such formulation. Preferred values typically fall in the range of about 0.5 up to 5 wt %, based on the dry weight of the cementitious formulation.

Cementitious formulations contemplated for use in the practice of the present invention include portland cement, pozzolanic cement, blast furnace slag cement, slag cement, masonry cement, oil well cement, aluminous cement (e.g., calcium alu inate cement), expansive cements, air entrained cement, gypsum cement, desulfurized gypsum cement, microfine cement, colloidal cement, as well as so called "mud to cement" systems, whereby a drilling mud is converted into a cementitious material during the completion process of certain subterranean wellbores, and the like.

Anionic polysaccharides contemplated for use in the practice of the present invention include xanthan gum, welan gum, gellan gum, rhamsan gum, S-657 or carboxylated cellulose ethers (CMC) . Presently preferred anionic polysaccharides contemplated for use in the practice of the present invention are welan gum and S-657.

Welan gum (also referred to as S-130) is described in detail in U.S. Patent No. 4,342,866, the entire contents of which are hereby incorporated by reference herein. Welan gum is a microbial polysaccharide produced under carefully controlled aerobic fermentation conditions by the organism Alcalige eε ATCC 31555. The primary structure comprises a linear tetrasaccharide repeat unit of D-glucose, D-glucuronic acid, D-glucose and L-rhamnose. Welan gum has a backbone repeat unit with a single side substituent. The side sugar can be either L-rhamnose or L-mannose. The molecular weight is estimated to be around 2 million. In solution, welan appears to exist as a double helix that builds viscosity through direct helical interaction via hydrogen bonding or through ion mediated association of helices. Intermolecular hydrogen bonding between the main chain and side chains contribute to the stiff conformation of the molecule. This εtructure produces a molecule with excellent heat stability, extremely high viscosity at low shear rates, and salt tolerance in high pH calcium environments. Because of its lack of hydrophobic substituents, welan gum has little activity at the air-water interface and generally does not cause foaming problems.

S-657 is described in detail in U.S. Patent No. 5,175,278, the entire contents of which are hereby incorporated by reference herein. S-657 is a microbial polysaccharide produced under carefully controlled aerobic fermentation conditions by the organism Xanthomonas ATCC 53159. The primary structure comprises a linear tetrasaccharide repeat unit of D-glucose, D-glucuronic acid, D-glucose and L-rhamnose. Thiε repeat unit has a side chain composed of two rhamnose substituents linked through 0-3 on the backbone 4-linked glucoεe reεidue (Moorhouse, Ralph, Structure/Property relationshipε of a Family of Microbial Polysaccharides, Industrial Polysaccharides: Genetic Engineering. Structure/Property Relations and Applications. Elseivier Science Publishers B.V., Amsterdam (1987)) . In contrast, welan gum (S-130), has the same backbone repeat unit with a single side substituent. The side sugar can be either L-rhamnose or L-mannose.

Gellan gums and rhamsan gums have been well characterized in the art and are well known to those of skill in the art. See, for example, Campana et al., in ■Carbohydrate Research 231:31-38 (1992) , and Moorhouse, supra.

Carboxylated cellulose ethers have also been well characterized in the art and are well known to those of skill in the art. See, for example, Industrial Gums. Polysaccharideε and Their Derivatives, Third Edition, Whistler and BeMiller, eds. (Academic Press, 1993) .

In accordance with another embodiment of the present invention, there are provided alternate methods to improve the solubility and performance properties of anionic polysaccharides in cementitious formulations. This alternate embodiment of the invention comprises increasing the amount of alkali metal cation(s) in the formulation.

Alkali metal-containing compoundε contemplated for use in this aspect of the invention are as described above. Performance enhancing amounts of alkali metal- containing compounds typically fall in the range of 0.1 up to 10 wt %, based on the dry weight of the cement formulation. Presently preferred amounts of alkali metal- containing compounds contemplated for use in the practice of the preεent invention fall in the range of about 1 up to

4 wt I, based on the dry weight of the cement formulation.

In accordance with yet another embodiment of the present invention, there are provided improved anionic polysaccharide-containing cementitious formulations, the improvement comprising increasing the level of soluble sulfate and/or soluble alkali metal-containing compounds in said formulation.

A variety of methods can be employed to increase the level of soluble sulfate and/or alkali metal cation- containing compounds in cementitious syεte ε, as described above. Desirable levels of soluble sulfate and/or alkali metal cation-containing compounds fall in the same general range as set forth hereinabove.

In accordance with εtill another embodiment of the present invention, there are provided modified anionic polysaccharides containing in the range of about 25 up to 99 wt % of a soluble sulfate, based on the weight of the anionic polysaccharide. Optionally, the above-described modified anionic polysaccharides may further contain in the range of about 25 up to 99 wt % of at least one soluble alkali metal cation-containing compound, based on the dry weight of the anionic polysaccharide.

In accordance with a still further embodiment of the present invention, there are provided modified anionic polysaccharides containing in the range of about 25 up to 99 wt % of at least one soluble alkali metal cation- containing compound, based on the dry weight of the anionic polysaccharide.

As readily recognized by those of skill in the art, constituentε of invention formulations can be combined in a variety of ways. Thuε, for example, conεtituentε of invention formulations can be pre- ixed during the final polysaccharide milling stage, or dry blended at any convenient time and place (e.g., at the job site). Moreover, the sulfate-containing compound (or sulfate- generating compound) can be added directly to the cementitious formulation, or the sulfate-containing compound can be pre-mixed with any one of the components thereof, e.g., the dry cement powder, the sand or aggregate, or with water, and then combined with the remainder of the components of the formulation.

In accordance with yet another embodiment of the present invention, there are provided cement compositions comprising dry cement and 0.2-15 wt % of any one or more of the above-described modified anionic polysaccharides, wherein the quantity of anionic polysaccharide(s) in the resulting composition is based on the weight of the dry cement. Presently preferred compositions comprise dry cement and 0.3-5 wt % of any one or more of the above- described modified anionic polysaccharideε.

As can be readily recognized by those of εkill in the art, modified polyεaccharide and cement can be mixed employing a variety of methods, such as, for example, any blending method commonly uεed in the art.

The invention will now be described in greater detail with reference to the following non-limiting examples.

Example 1

It has surprisingly been observed that rheological test resultε for different welan-containing cementε can vary widely for different portland cementε, as demonstrated by the data presented in Table 1. Thus, type I/II cements are mixed using 400 grams of cement, 0.4 grams of welan gum (0.1 wt %) , 4.0 grams of polynaphthalene sulfonate superplasticizer (1 wt %) . _Cement and water (400 grams each) are premixed for three minutes; superplasticizer and gum are dry blended together and added to the cement/water blend. Once superplasticizer and gum are added, the slurry is mixed an additional 10 minutes.

The cement test data are presented as Fann dial reading (FDR), and obtained using a Fann 35 (Rl, Bl, F0.2) rotating at 100 rpm.

Table 1

CEMENT¹ A B C D E F G

Cement test (FDR) 81 77 63 59 55 48 4 3

1 Seven different Portland cement samples as obtained from several different finished grinding plants were employed

The Fann dial reading (FDR) ranged from 43 to 81. The better performing cement is seen to be almost 90 percent more viscous than the poorest performing cement.

Example 2

Based on the results presented in Table 1, two types of testε were carried out in efforts to determine the cause for rheological variance. The first type of test analyzed the dry cement powder; these data are summarized in Tables 2, 3 and 4, relating to differential thermal analysis (DTA) , X-ray fluorescence (XRF) analysis and X-ray diffraction (XRD) analysis, respectively. Ta bl e 2

DTA of Cement Powders Illustrating the Presence of

Different Types of Sulfateε in Different Cements

The cement samples used were the same as described in the footnote to Table 1

From the above Table, Portland cement is seen to contain in the range of about 3-5 % gypsum (CaSO^•2H₂0) , which is added to the clinker prior to grinding. Although gypεum iε preεent, aε indicated in Table 2, there iε no apparent relationship between the various types of gypsum present and the performance of welan gum (K1C 376) added to the fresh cement paste.

XRF analysis was employed to determine the oxide content for the various cement powders described with

10 reference to Table 1 (i.e., Samples A-G) . The resulting data are presented in Table 3. Note particularly the influence of alkali (total) on the results of the cement test (FDR) .

Table 3 XRF analysis of Cement Powder

The cement samples used were the same as described in the footnote to Table 1

Note the relationship of total alkali metal content on the Fann dial reading. The total alkali metal content (expresεed as Na₂0) is seen to be directly related to the Fann dial reading. The phase composition for the various cements used in the above analyses was determined. The results are presented in Table 4.

Table 4

Phase Composition of Cement Samples Showing

Concentration twt % of the Four Phases

The cement samples used were the same as described in the footnote to Table 1. The four major phases of Portland cement are C₃S (tricalcium silicate) , C-S (dicalcium silicate) , C₃A (tricalcium aluminate) and C₄AF (tetracalcium aluminoferrite) .

These data show that there is essentially no relationship between the cement test FDR and the difference in the phaseε present in the different cement powders.

A second type of test was carried out to analyze the metals content in the fresh filtrate of freshly mixed cement paste. Data for thiε metalε survey in the fresh filtrate are εummarized in Table 5. The welan samples tested include a production welan gum (KIC 376) and an experimental welan gum, 93-212. The cement samples (containing an equal mass of water and cement) were mixed for 5 minutes and filtered to remove all solids. The resultant filtrate was then ana_lyzed to obtain the concentration of various elements in solution. The resulting data are presented in Table 5. _Note the influence of sulfur (S) , potassium (K) , and potassium + sodium on the performance (FDR) of the three gums tested in different cementitious systems.

Table 5 0 ICP Analysis of Cement Filtrate

ICP analysis refers to "inductively coupled plasma emission spectrometry" wherein a plasma (i.e., a partially ionized gas) is utilized to excite target atoms. Upon relaxation, the excited atoms or ions emit photons of characteristic frequency and number. The cement samples used were the same as described in the footnote to Table 1

Note the sulfur (expressed as ppm) concentration increases with increased Fann dial reading. Stated another way, the welan rheological performance is seen to be directly related to the sulfur concentration. Example 3

The influence of directly added sulfates on the performance of anionic polysaccharides, such as welan, was tested by dry blending sodium and potassium sulfate with cement and measuring the reεulting rheological performance.

Mix procedure - Type I/II cement, with no added sulfate, is mixed using 400 grams of water, 0.4 grams of gum, 4.0 grams of polynaphthalene sulfonate superplasticizer. Cement and water are premixed for three minutes. Superplasticizer and gum are dry blended together and then added to the cement/water premix. Once superplaεticizer and gum are added, the εlurry is mixed an additional 10 minutes.

Added εulfate samples contain 388 grams of cement, 9.72 grams of Na₂S0₄ and 2.17 grams of K₂S0₄. The samples are mixed with 400 grams of tap water for 3 minutes followed by addition of the dry blended superplasticizer (4.0 grams) and gum (0.4 grams). Once superplasticizer and gums are added, the slurry is mixed an additional 10 minutes. The resulting data are presented in Table 6. The data in Table 6 are obtained employing cement sample E, aε described in the footnote to Table 1.

Table 6 Enhanced Cementitious Grouts Containing Welan Gum

CEMENT E WELAN GUM¹ FDR² @ 100 RPM

Type I/II Experimental gum- 26 93-212

Type I/II + S0_A Experimental gum- 80 93-212

Type I/II Production gum- 55 K1C 376

Type I/II + SO_A Production gum- 73 K1C 376

KIC 376 welan gum is a production gum manufactured specifically for cementitious applications (available from Kelco, San Diego, CA as KELCO-CRETE; KELCO-CRETE is a registered trademark of Kelco, a Unit of Monsanto Company), while welan sample 93-212 is an experimental welan gum. FDR = Fann Dial Reading

These data demonεtrate a novel, economical method of enhancing the rheological performance of anionic polyεaccharideε, such as welan, in cement.

While the invention has been described in detail with reference to certain preferred embodiments thereof, it will be understood that modifications and variations are within the spirit and scope of that which is described and claimed.

Claims

CLAIMSThat which is claimed is:

1. A method to improve the solubility and performance properties of anionic polysaccharides in cementitious formulations, said method comprising increasing the amount of soluble sulfate in said formulation.

2. A method according to claim 1 wherein the amount of soluble sulfate is increased by introducing into said formulation at least one sulfur-containing compound selected from soluble saltε of sulfides, sulfiteε, sulfates, bisulfates, persulfates, thiosulfates, sulfaneε, thionates, dithionates, thioantimonates, or mixtures of any two or more thereof.

3. A method according to claim 1 wherein the amount of soluble sulfate is increased by introducing into said formulation at least one compound selected from: syngenite, potassium zinc sulfate, potassium zirconium sulfate, ammonium iron sulfate, or lithium, sodium, potasεium, ammonium, magneεium, calcium, rubidium, strontium, cesium, barium, thallium, aluminum, cobalt or copper salts of sulfides, disulfides, persulfides, εulfites, sulfates, bisulfateε, persulfates, thiosulfateε, sulfanes, thionates, dithionates, thioantimonates, or mixtures of any two or more thereof.

4. A method according to claim 1 wherein the amount of soluble sulfate falls in the range of about 0.1 up to 10 wt %, based on the dry weight of the cementitious formulation.

5. A method according to claim 1 further comprising increasing the amount of alkali metal cation(s) in said formulation.

6. A method according to claim 5 wherein the amount of alkali metal cation(s) is increased by introducing into said formulation one or more soluble salts of lithium, sodium, potassium, rubidium or cesium.

7. A method according to claim 5 wherein the amount of alkali metal-containing compound falls in the range of about 0.1 up to 10 wt %, based on the dry weight of the cementitious formulation.

8. A method according to claim 1 wherein said cementitious formulation is selected from portland cement, pozzolanic cement, blast furnace slag cement, slag cement, masonry cement, oil well cement, aluminous cement (e.g., calcium aluminate cement), expansive cementε, air entrained cement, gypsum cement, desulfurized gypsum cement, microfine cement or colloidal cement, a mud to cement εystem, or mixtures of any two or more thereof.

9. A method according to claim 1 wherein said anionic polysaccharide is εelected from welan gum, gellan gum, rhamεan gum, S-657 or carboxylated celluloεe ethers.

10. A method according to claim 1 wherein said anionic polysaccharide is welan gum.

11. A method to improve the solubility and performance properties of anionic polysaccharides in cementitious formulations, said method comprising increasing the amount of alkali metal cation(ε) in said formulation.

12. A method according to claim 11 wherein the amount of alkali metal cation(ε) is increased by introducing into said formulation one or more soluble salts of lithium, sodium, potassium, rubidium or cesium.

13. A method according to claim 11 wherein the amount of alkali metal-containing compound falls in the range of about 0.1 up to 10 wt %, based on the dry weight of the cement formulation.

14. A method according to claim 11 wherein said cement formulation is selected from portland cement, pozzolanic cement, blast furnace slag cement, slag cement, masonry cement, oil well cement, aluminous cement (e.g., calcium aluminate cement) , expansive cementε, air entrained cement, gypsum cement, desulfurized gypsum cement, microfine cement or colloidal cement, a mud to cement system, or mixtures of any two or more thereof.

15. A method according to claim 11 wherein said anionic polysaccharide is selected from welan gum, gellan gum, rhamsan gum, S-657 or carboxylated celluloεe ethers.

16. A method according to claim 11 wherein said anionic polysaccharide is welan gum.

17. In an anionic polysaccharide-containing cementitious formulation, the improvement comprising increasing the level of soluble sulfate and/or alkali metal cation(s) in said formulation.

18. A modified anionic polysaccharide containing in the range of about 25 up to 99 wt % of a soluble sulfate, based on the weight of said anionic polysaccharide.

19. A modified anionic polysaccharide according to claim 18 wherein said soluble sulfate is provided as lithium sulfate, sodium sulfate, potassium sulfate, ammonium sulfate, magnesium sulfate, calcium sulfate, rubidium sulfate, εtrontium εulfate, ceεium εulfate, barium sulfate, thallium sulfate, aluminum sulfate, cobalt sulfate or copper sulfate.

20. A modified anionic polysaccharide according to claim 18 further comprising in the range of about 25 up to 99 wt % of at least one alkali metal cation-containing compound, based on the weight of said anionic polysaccharide.

21. A modified anionic polysaccharide according to claim 20 wherein said alkali metal cation is provided as a soluble salt of lithium, sodium, potassium, rubidium or cesium.

22. A modified anionic polysaccharide according to claim 18 wherein said anionic polyεaccharide iε εelected from welan gum, gellan gum, rhamsan gum, S-657 or carboxylated cellulose ethers.

23. A modified anionic polysaccharide according to claim 18 wherein said anionic polysaccharide iε selected from welan gum, gelan gum, rhamsan gum or S-657.

24. A modified anionic polysaccharide according to claim 18 wherein said anionic polysaccharide is selected from welan gum or S-657.

25. A modified anionic polysaccharide containing in the range of about 25 up to 99 wt % of at least one alkali metal cation-containing compound, based on the weight of said anionic polysaccharide.

26. A modified anionic polysaccharide according to claim 25 wherein said alkali metal cation iε provided as a soluble salt of lithium, sodium, potassium, rubidium or cesium.

27. A cement composition comprising dry cement and 0.2-15 wt % of modified anionic polysaccharide according to claim 18, wherein the quantity of anionic polysaccharide in said composition is based on the weight of the dry cement.

28. A cement composition according to claim 27 wherein said anionic polysaccharide is selected from welan gum, gellan gum, rhamsan gum, S-657 or carboxylated cellulose ethers.

29. A cement compoεition according to claim 27 wherein εaid anionic polysaccharide is welan gum or S-657.

30. A cement composition comprising dry cement and 0.2-15 wt % of modified anionic polysaccharide according to claim 20, wherein the quantity of anionic polyεaccharide in said composition is based on the weight of the dry cement.

31. A cement composition according to claim 30 wherein said anionic polysaccharide is selected from welan gum, gellan gum, rhamsan gum, S-657 or carboxylated cellulose ethers.

32. A cement composition according to claim 30 wherein said anionic polysaccharide iε welan gum or S-657.

33. A cement composition comprising dry cement and 0.2-15 wt % of modified anionic polysaccharide according to claim 25, wherein the quantity of anionic polysaccharide in said composition is based on the weight of the dry cement.

34. A cement composition according to claim 33 wherein said anionic polysaccharide is selected from welan gum, gellan gum, rhamsan gum, S-657 or carboxylated cellulose ethers.

35. A cement composition according to claim 33 wherein said anionic polysaccharide is welan gum or S-657.