CA1161071A - Method for lowering the viscosity of and liquefying portland cement mixes by means of lignosulfonate - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method for lowering the viscosity of and/or liquefying Portland cement mixes, especially Portland cement concrete, by means of lignosulfonate.
According to the method such a lignosulfonate is added to the mix whose purity is at least 85 per cent.
In addition to other conventional additives, an accelerator of cement known per se, whose quantity is 0,2 to 2.0 times as high as the quantity of lignosulfonate, is also added.

Description

Method for_lowering the viscosity of and liquefyin~
Portland cement mixes by means of lignosulfonate The subject of the present invention is a method for lowering the viscosity of and liquefying a Portland cement.
The strength of various cement products normally de-pends on the ratio of the quantity of water used for the mix to the quantity of cement, i.e. on the water-to-cement ratio of the product or mix. The reason for this is that, when being hydrated, the cement binds a certain maximum quantity of water, whereas the remainder - mostly the main part - of the water produces cavities and porosity, so-called capillary porosity in the hardened product.
When the working of the concrete mix is insufficient -especially in the case of viscous mixes - so-called macropores are additionally produced, which also contribute to a deterioration of the strength.
By means of additives that disperse cement particles to plasticize the concrete mix, even in little doses, it is possible to reach a considerably improved workability with unchanged water-to-cement ratio or, also, a reduced requirement of water with unchanged workability. In both cases a lower overall porosity is achieved in the cement product, and thereby the strength is increased.
Plasticizing additives additionally act so that, owing to their surface activity, they disperse such agglomera-tion of fine1y-divided particles as are always present in cement. This yields a better and more homogeneous distribution of cement in the product and thereby a more efficient utilization of the binder agent, which results in higher strength.
Plasticizing additives have been known and in use for a very long time. More recent agents in the field of additives are plasticizers whose dispersive effect on cement particles is even stronger than that of the normal plasticizers that were used previously.

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These additives that have a high surface acti-vity towards cement particles comprise mainly sulfonated and polymerized condensation products of formaldehyde and melamine and of formaldehyde and naphtalene, respectively.
Also, modified lignosulfonates, i.e. condensation products of lignosulfonate and other organic substances, are used, but their e~ficiency has not been of the same order as that of the substances mentioned above. The dif~erence between plasticizing and liquefying substances is not 10 ~ sharp. By means of plasticizing additives it is usually possible to achieve a reduction of about 15 per cent in the quantity of water, whereas by means of liquefiers one often achieves a reduction of up to ~0 per cent in the quantity of water and an increase of 40 to 50 per cent in strength.
It is characteristic of liquefiers that they include a great number o~ polar groups, a number of acid functional groups (COOH, SO3H or their corresponding salts), as well as that they have a high molecu~ar weight.
Their precise functional mechanism is not known, but it is known that they are adsorbed in a more or less non-reversible way onto the surface of the cement particles, whereby these particles cannot be reunited with each other. The various clinker components and their hydration products present in cement adsorb organic additives in different ways, for which reason different cement qualities and types behave in different ways, e.g., when liquefiers are dosed. This is why empiric experiments are always necessary in order that it should be possible to determine the effect of an additive on a certain cement mixture or concrete.
Even i~ liquefiers permit the solution of many concrete-technological problems, their use has not obtained the extension that had been expected. This is partly due to high cost of these synthetic additives, which means a considerable additional expense, and to certain work-hygienic drawbacks, resulting from formaldehyde.

Moreover, the efficiency of sulfonated melamine and of naphthalene resin seems to be reduced rapidly as pozzuo-lanas are added. AS slag, ~ly ash, and other pozzuo-lanas are to-day added to normal Portland cement in many countries, in order to reduce the energy expenses, this means that the possibilities of use of li~uefiers based on melamine and naphthalene are reduced.
The efficiency of liquefiers based on lignosulfonate normally seems to be lower than that of the substances mentioned above, and - despite the abundant availability of inexpensive lignosulfonate - their use has not been equally extensive.
Unmodified lignosulfonates as sodium ~NaLS) and calcium salts (CaLS) have bèen used as plasticizers for a rather long time. It has not been possible to use lignosulfonates in such high doses that a liquefying effect could have been reached. This comes, in many cases, from the quite serious secondary effects that are accompanied by the dosage of lignosulfonates, i.e. from the sugars and saccharic acids contained in lignin pro-ducts, the quantities of said sugars and saccharic acids varying significantly from one cellulose cooking batch to the other. Among such secondary effects could be mentioned the considerable retardation of the hardening even with dosages of 0.2~ out of the weight of cement.
This effect comes out clearly from the following Table 1 (V.5. Ramachandran, Thermochim. Acta 4 (1972)).

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Table 1 Effect of lignosulfonate on properties of Portland cement concrete (Lignin purity 60 %) Additive of Water~ Reduction Setting time (h) Compression cement cement in quantity at resistance to strength as com-(% by weight) ratio of water penetration pin pared to refer-(%)enc~ sample (~) _ _ r 5N/mm~ 3,5N/mm~ 24h 3d 7d 28d 0 0,630 3-1/4 51~0 100 100 100 0,07 0,599 5 4 5 3/4101 104 103 102 0,13 0,599 5 4 6-1/495 108 111 101 0,18 0,580 8 4-3/4 7-1/2100 110 107 109 0,2~ 0,580 8 5-1/4 8-1/4107 115 112 115 By means of plasticizers based on lignosulfonates, it `15 is easily possible to obtain a reduction of 5 to 10 per cent in the requirement of water, but at the expense of a hardening time of 30 to 60 per cent higher length. The retardation increases when the dosage is increased, and in combination with the use of a low-heat cement the retard-ation may be very expensive and totally abnormal.
The retarding effect of lignosulfonate dosage onmaturing and hardening has been utilized, e.g., in the preparation of concrete used for oil drilling.
Lignosulfonates contain a number of functional groups which~ on one hand, help the adsorption of the product onto the surface of the cement particles and, on the other hand, owing to their hydrophilicity, bind water to the same product. E.g., owing to OH-groups, the adsorption may be even completely irreversibler in which case it is called chemisorption.
The lignosulfonates seem to be adsorbed in the first place onto aluminate phases (C~A and C4AF) and onto their hydration products. It looks like the ligno-sul~onates were adsorbed very strongly onto those metamorphic calcium aluminate hydrates that are formed as in-termediate products ~C2AH8 and C4AHlg, respectively) of hydration o aluminate minerals and prevent a conversion into a more stable cubic C3AH6 form, which is the final hydration product. The reaction mechanism is, however, not fully known, and contradictory study results are also encountered in many cases.
It has also been noticed that the morphological structure of these intermediates has been changed into a structure which is assumed to yield higher strength.
It is commonly known that a C3A component of cement also affects the hydration speed of a silicate phase in a decisive way.
Under these circumstances, the components of the lignosulfonate used that are adsorbed onto the a]uminate phase and its hydration products indirectly control the entire hydration process of cement.
The object of the present invention is to eliminate the practical difficulties that have been typically related to the varying quality of impure lignosulfonates and to provide a concrete product that matures and hardens in a desired and reproducible way and in which as binder is typically used Portland cement or its known ~ommercial variations.
Surprisingly it has come out that, if lignosulfonate is purified and treated in the way to be described below to a purity higher than 85 per cent, an efficient liquefier is obtained for Portland cement and for its variationsr in whose use no variability occurs and who does not cause un-controllable retardation of the maturing and hardening of concrete.
More specifically, according to the invention there is provided a method for lowering the viscosity of and liquefying a Portland cement mix, especially a Portland cement concrete, comprising adding the following to a Portland cement mix: a lignosulfonate having a purity of at least B5 percent, said lignosulfonate being added in a quantity of from 0.3 to 1.5 percent by weight of the Portland cement used; and an accelerator in a quantity of 6~7~

0.2 to 2.0 times the quantity of lignosulfonate selected from the group consisting of alkali carbonates consisting of soda, potash, sodium bicarbonate, potassium bicarbonate, and lithium bicarbonate; water soluble calcium nitrate, and calcium formiate; and the small-molecule organic amine, triethanolamine.
By means of the invention, remarkable advantages are obtained. Thus, by addition of a little quantity of an accelerator in itself known and in the first place affecting the hydration of the aluminate phase, it is possible to control the setting time of a cement mix or concrete within wide limits. If necessary, the method can also be used for purposes of increasing the porosity. On the other hand, if it is desirable to avoid an increase in porosity, the necessary compensation can be easily accom-plished by means of deaerating agents in themselves known~
In the present invention, an entirely new lignin quality has now started being used as a plasticizer of cement. By cement is meant the normal Portland cement and its typical commercial variations, such as sulfate-proof cement, low-heat cement, low-alkali cement or blast furnace Portland cement, various pozzuolana cements, etc.
It is commonly known that lignosulfonates can be purified to a very high purity, e.g., by using ultra-filtration/ whereby a purity of up to 97 per cent by weight is reached in the purity of a lignin product. This means that among the lignin product there are very little quantities of inorganic salts and of sugars or polysac-charides of low molecular weight. Further it i9 known, e.g. from U~S. Patent 3,251,820, that lignin can be purified by utilizing liquid extraction. When proceeding in accordance with the said U.S. patent, lignin sulfonate purities of 99 to 100 per cent can be achieved.
The retarding properties of commercial lignin products ', ~6~L~7~

in cement mos~ly result from free sugars or saccharic acid, which are always present in normal lignin~ A
typical purity of the lignin contained in the spent liquor from cellulose cooking is of the order of 40 to 60 per cent by weight, and with a corresponding fermented spent liquor, from which alcohol, yeast, pe~ilo, or any other typical fermentation product has been prepared, a lignin content of 65 to ~0 per cent in the sulfite spent liquor is obtained as calculated from the dry substance~
All of the known lignin plasticizers mentioned above had been prepared by using such impure lignin products as the starting material.
In the tests performed by us we have noticed thatr when only such a lignin product, purified either by ultrafiltration or by liquid extraction, was used as a plasticizer of the concrete mix, i.e. for dispersing the cement particles, only slight retardation in the hardening of the concrete was noticed. In such a case the retarda-tion does not result from sugars present but from the OH-and OCH3-groups involved in the structure of lignin, which groups bind Ca- and A1-containing ions efficiently in the hardening cement paste, and thereby reduce the mobility of these ions. Such a retarding effect is hardly encountered with the known plasticizers of cement, which are, among other things, sulfonated naphtalene condensates and sulfonated melamine-formaldehyde condensates. When one starts improving such a substantially pure lignin produck in respect of its dispersing properties, whereby typical procedures applied are, e.g., desulfonating oxy-genation, alkali treatment, slight ozonization, etc.treatment which reduces the numbers of sulfone yroups, no detrimental oxydation products are produced ~rom the impurities either. In the known methods (e.g. U.S. Patent 4,088,640)~ in which very strong ozone treatment is applied, these detrimental impurities are oxydated into a harmless form. If this treatment is~ however, allowed to .~,. .:,, remain at a slight form, which would be enough for desulfonation of lignin, e.g., the sugars would remain saccharic acids, which would still be efficient retarders of the hardening of the concrete.
When substantially pure lignosulfonate products are used for dispersing cement, as a simultaneous effect a very slight retardation in ~he hardening o~ concrete is obtained. This property of slight retardation in itself can be compensated easily in known ways.
Since lignosulfonates are primarily adsorbed on the aluminate phase of cement and prevent any other reaction, it is to be expected that accelerators affecting the hy-dration of the aluminate phase are here efficient.
Soda (Na2CO3) as well as bicarbonate (NaHCO3) are well-known accelerators, which havet also when used together with the lignin preparation used, proved capable of compensating the retardation in the setting time. This comes out from Table 2.
Also useful as accelera~ors are potash, potassium bicarbonate and lithium bicarbonate.

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- 8a -Table 2 Plasticizing of rapid cement by means of lignosulfonate and sodium carbonate.
Immediate flow =
Test LS 95% LS 99%~Melam. Na2CO3 Water/ Increase T~80-0) Density (%) (~) form. cement in cone (h.min) (kg/m3) sulfon. (%) ratio diameter ~ (mm) 1. ~ 0,50 33 4.13 2065

2. 0,5 - - 0,5 0,50 1~3 0.37 2224

3. 0,5 - - 0,3 0,50 9S 3.12 2220

4. 1,0 - - 1,0 0,50 150 0.35 2216

5. - 0,5 - 0,3 0,50 100 0.~0 2200

6. - 1,0 - 0,6 0,50 135 0.46 2156

7. - - 0,5 ~ 0,50 53 3.53 2143

8. - - 1,0 - 0,50 78 4.30 2113

9. 1,5 - - 2,0 0,50 81 3.56 2102

10. 0r5 0r50 105 6.32 2220 (NaHCO3)

11. 1,0 - - 2,5 0,40 73 0.31 221 (TEA)

12. 0,5 - - 0,5 0,50 130 0.07 2277 (TEA)

13. 0,5 - - 0,1 0,45 75 0.28 2195 (TEA)

14. 0,5 - - 0,05 0,38 85 0.~4 2252 ~` 9 To the samples 2 to 9, 0.2 % of Na-gluconate and 0.5 % of tributylphosphate were added~ as calculated from the quantity of cement.
As comes out from the tests, even a ligno-sulfonate addition of 0.3 to 0.5 %, as calculated from the quantity o~ cement, produces good plasticization.
A ~ertain ~etardation in the ~etting time can b~ rloti¢ed when lignosulfonate alone is added, but this retardation can be compensated by means o~ a little addition of soda 10 ~ or bicarbonate.
By increasing the lignosulfonate addition from 0.5 ~ to, e.g., 1.1 % no improved workability is obtained, but instead the setting time becomes longer.
The reduction in the quantity of water, as com-pared with a mortar with no additives, is about 30 %.
In the preceding table T(80-o) indicates the hardening time of cement mortar within which the flow of the cement mortar on the table is changed from 80 mm to 0 mm. In the table there are three columns for different liquefiers; the first two involve pure lignosulfonate products prepared in differenk ways, and khe third column contains, as a reference, a common commercial melamine-~ormaldehyde condensate sulfonate.
From Table 2 it is noticed that~ when pure lignosulfonate is used, a very little addition of alkali carbonate is sufficient to compensate the retardation of the hydration of cement produced by the lignin itself.
~urther, the examination of the effect of lignosulfonates of different degrees of purity on the hardening of cement mortar was continued. Cement and sand were used at the ratio of 1:3, and the temperature continue~ to be 30C.

:1~6ill?~1 Table 3 Immediate flow =
Increase Test LSA 100~ LSA 95% LSA 91~ LSA 80% Na2CO3 in cone T(80-0) (%) (%) (~) (%) diameter (h.min) (~) (mm)

15. 0,5 - - - 0,5 101 0.43

16. - 0,5 - - 0,5 103 0.37

17. - - ~,5 - 0,5 95 1.37

18. - - - 0,5 0,5 82 3.11

19, - - - 0,75 1,1 102 5.04 In the preceding Table 3 the effect of the purity of LSA on the plasticity and retardation of cement mortar was examined. The product had been purified by ultra-15 filtration, by using membranes GR 6 produced by DeDanske Sukkerfabrik, and the products had been taken from the same batch or spent liquor from cellulose cooking at different stages of purification~ The 100%
pure ligno sulfonate product had been prepared by extraction by means of an amine into an organic phase and removed ~rom the organic phase by means of NaOH-treatment.
All of the lignin products discussed above had been prepared from the spent liquor of Na-bisulfite cook, wherein the sulfonation degree is 0.3~ This relatively low sulEonation degree has been noticed to be favourable as compared, e.g., with a Ca-lignosulfonate prepared by means of ultrafiltration, wherein a typical sulfonation degree is, owing to the acidity of the cook, of the order of 0.45 to 0.5. Further, it has been noticed that from such cellulose cooks in which the sulfonation degree is very low, such as catalysed alkali-sulfite cooks, an even more advantageous lignin product is obtained to be used fcr dispersing cement (see our Canadian Patent 1,141,620).
In such , V~

a spent liquor ~rom alkali sul~ite cellulo~e cooking the sulfonation degree of the lignin can be even as low as 0.15. The sulfonation degree means here the number of sulfone groups in the lignin structure per one aromatic ring.
Now it has been noticed further that, when li~nosulfonates that have been purified either by extrac-tion or by ultrafiltration are treated under slightly alkaline conditions with heat and/or when the sample has 10 ~ been at the same time oxygenated either with air or, e.g. 3 with hydrogen peroxide, their sulfonation degree can be easily lowered to about 0.2 and even below this without lignin in itself still beginning to decompose and, in a known way, to form vanillin and other known decomposition products. When substantially sugar-free lignins are treated under slightly oxydating conditions~ there is no risk of saccharic acids being formed under certain conditions nor of their further oxydation under other conditions. This means tha~ when oxygenation is continued ~urther, pure lignosulfonates constantly obtain proper-ties continuously changing in the same direction.

Table 4 Immediate flow =
Test LSA 95% LSA 95% LSA 95% Na2C03 Increase T(80-o) SA 0,30 SA 0,24 SA 0~21 (%) (mm)

20. o,5 - - o,5 102 o,47

21. - 0,5 ~ 0,5 125 0.42

22. - - 0,5 0,5 134 0.l~3 The LSA-products used in the tests 20, 21 and 22, which are here characterized by different sulfonation degrees (SA), were prepared by mixing appropriate quan-i5 tities of hydrogen peroxide into an alkaline water solution of lignosulfonate and by heat-treating the product for certain periods of time. The conditions of the said procedure were the following:
A 16~ Na-lignosulfonate-water solution was prepared, to which 4% of NaOH and 0 to l.S% of hydrogen peroxide, as calculated from the weight of lignin, and about 2% of lime milk, as calculated from the weight of lignin, were added. The reaction times used were 30 min., 40 min., and 55 min., whereby the given sulfonation degrees were reached at 100C. The NaOH used was later converted into Na2CO3 by means of CO2. A corresponding desulfona-tion can be produced in a known way even without oxygena-tion by incubating the lignin under stirring and in the presence of Ca(OH~. In such a case, however, reaction times of the order of 2 to 4 hours are obtained.
Example on desulfonation of LSA
A solution was prepared that contained 16% of 100-percent lignosulfonate, to the solution was added 4% of NaOH and 2% of Ca(OH~2, and the solution was boiled for 2.5 hours. The boiling was continued for periods of 2.5 hours and samples were always taken between these periods.
In the starting situation the sulfonation degree of the LSA was 0.30, and after 2.5 hours it had been lowered to 0.20, and the sulfonation degree was not lowered from this further even by subsequent treatments.
The same solution as above was in addition to mere incubation also treated with a quantity of 2% of hydrogen peroxide, and the same degree of sulfonation, 0.195, which was by cooking alone reached in 2.5 hours, was now reached after a treatment of 5 minutes.
The first test was repeated so that the incubation was continued for 1 hour only, whereby the degree of sulfonation was lowered to 0.~4.
The determinations of sulfonation degree were performed from well filtered liquors by determining the total sulfur in the liquor.

Previously, in this connection, compensation of the retarding effect caused by pure lignosulfonate by means of soda has been discussed. ~he same compen-sation of retardation can be obtained by using soluble calcium salts (e.g., Ca-formiate, CaC12, Ca(N02)2 and Ca(N03)2), among which the most advantageous one is calcium nitri~e because o~ its other advantageou~ e~eots.
It is well known that with lignosulfonate plasticizers usually, e.g., CaC12 is used in order to compensate the retarded hydration caused by the said plasticizers. Then it is necessary to use large quantities of the calcium chloride product, which causes corrosion, because the lignin product used normally contains sugars and/or saccharic acids, whos~ retarding effect is remarkably high.
Table 5 Test LSA 95% N~2C03 N~HC03 C~C12 C~(COOH)2 Ca(N02)2 Strength(MPa) ~ ) (%) (%) (~) ~%) 1 d 2 d 3 d _ . ,_ .... _ _ , . .. __.... __

23 0,5 o,5 _ _ _ _ 22,2 43,~ 45,8

24. 0,5 ~ 1,0 - _ 20,3 38,5 42,8

25. 0,5 - - 1,0 - - 18,5 24,0 36,0

26. 0,5 - - - 1,0 - 21,2 26,0 2B,o

27. 0,5 _ _ _ _ 1,0 24,2 28,2 29,0

28, 0,5 - - - _ _ 19,5 24,9 25,4 The lignosulfonate used, whose purity was 95 %
and sul~onation degree 0.30, when compared with commercial liquefiers based on melamine and naphtalene, was noticed to give both better workability and higher strength, which comes out from the ~ollowing Table 6.

-` 13a6~7~l Table 6 Concrete 8 mm, 300 kg RHC/m3 Test ¦ Liquefier Watel~r~ Volu- Air , cement S :rength (MN/m ) metric con-1 ratio 1 d 3 d 7 d 28 d weight tent - ¦ -- - - ( k~l ) ( 96 ) 2g. _ 0,60 16,230,5 36,1 2,309 3,9 30. M 0,57 15,832,1 36,8 2,309 3,9 31. N 0,57 17,332,4 38,1 2,350 3,0 32. P 0,56 14,233,4 40,4 2,312 4,0 33. LS 0,5 % 0,5022,2 43,4 45,8 2,369 3,0 34. LS 1,0 % 0,50 _ 18,4 30,6 _ 2,351 2,4 The cement used was RHC of a fineness grading of 450 m2/kg. Workability (i.e. flow on a vibrator table as per DIN Standard) was in all cases 12 cm. The quantity of additive was in all cases 0.5% of the cement quantity.
Out of the liquefiers indicated in the table, M = based on melamine N = based on naphthalene P = based on lignosulfonate, and LS = based on a lignosulfonate in accordance with the invention (purity 95~).
As is seen, the water/cement ratio can be reduced by about 17% without a change in workability, whereby an increase of 37% in strength is obtained after 24 hours.
An increase in the LSA quantity to 1~ causes a consider able retardation in the development o strength.

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- 14a -Table 7 -Effect of pure (95~) lignosulfonate when fly ash (PFA) and ground blast furnace slag (MK) are used as an admixture for Portland cement tOPC) TextMix LSA 95% Water/ Na2CO3 Na-glu- TFB Flow ratio (%) cement conate ~%) (mm) ratio (%) (%) 35.100~ OPC 0,5 0,37 0,3 0,08 0,05 80 36.70~ OPC 0,5 0,37 0,3 0,08 0,05 140 +30~ PFA
37.70% OPC 0~5 0,37 0,3 0,08 0,05 135 +30% MK

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(TBF = tributylphosphate, in Table 7)~
Table 8 Effect of an addition of lignosulfonate on the air content o~ mortar Test Cement NaLS Water/ Flow T(80-o) Volu- Incr.
95% cemenk (mm) (h.min.) metric in (%) ratio weight quant.
(kg~l) of air ~ . ~ .. ....... ... .. . . .
10~ 3~.100~ RHC - 0,50 85 3.30 2,16 39.100% RHC0,15 o,48 70 2.26 2,05 5,2 40.70% RHC - 0,50 93 4.23 2,17 +30% M~
41.70% RHC 0,15 0,50 95 4.35 2,03 6,5 +30% MK
42.70~ RHC - ,47 80 4.20 2918 ~30% PFA
43.70~ RHC 0,15 0,42 83 3.07 2,10 3,5 +30% PFA
._ . . . . . . . . . . .. . .

The tests 38 to 43 prove that even with a little increase in the quantity of lignosulfonate an extra content Or air is achieved, whereby at the same time the setting becomes faster. The test was per~ormed with a mortar whose mix ratio was 1:3. The specific surface area o~ the cement was 430 m2/kg.

Claims (15)

Claims:

1. A method for lowering the viscosity of and liquefying a Portland cement mix, especially a Portland cement concrete, comprising adding the following to a Portland cement mix:
a lignosulfonate having a purity of at least 85 percent, said lignosulfonate being added in a quantity of from 0.3 to 1.5 percent by weight of the Portland cement used; and an accelerator in a quantity of 0.2 to 2.0 times the quantity of lignosulfonate selected from the group consisting of alkali carbonates consisting of soda, potash, sodium bicarbonate, potassium bicarbonate, and lithium bicarbonate; water soluble calcium nitrate, and calcium formiate; and the small-molecule organic amine, triethanolamine.

2. A method as claimed in claim 1, wherein such a ligno-sulfonate is used whose purity is 90 to 99 percent.

3. A method as claimed in claim 1, wherein such a lignosulfonate is used as has been purified by means of extraction to a purity of 90 to 99 per cent.

4. A method as claimed in claim 1, wherein about 0.5 per cent by weight of lignosulfonate and about 0.5 per cent by weight of accelerator are used.

5. A method as claimed in claim 1, wherein such a lignosulfonate is used as has been purified by means of ultrafiltration to a purity of 90 to 97 percent.

6. A method as claimed in claim 5, wherein the lignosulfonate has a purity of 92 to 96 percent.

7. A method as claimed in claim 2, wherein the lignosulfonate used is purified by means of ultra-filtration to a purity of 90 to 97 percent.

8. A method as claimed in claim 7, wherein the lignosulfonate has a purity of 92 to 96 percent.

9. The method of claim 3, wherein lignosulfonate is used in a quantity of 0.3 to 0.7 percent by weight and the accelerator in a quantity of 0.3 to 0.7 percent by weight as calculated from the quantity of cement used.

10. The method of claim 1, wherein lignosulfonate is used in a quantity of 0.3 to 0.7 percent by weight and the accelerator in a quantity of 0.3 to 0.7 percent by weight as calculated from the quantity of cement used.

11. A method as claimed in claim 1, wherein the sulfonation degree of lignosulfonate has been lowered to a level between 0.4 to 0.15.

12. A method as claimed in claim 5, wherein the sulfonation degree of lignosulfonate has been lowered to a level between 0.4 to 0.150

13. A method as claimed in claim 7, wherein the sulfonation degree of lignosulfonate has been lowered to a level between 0.4 to 0.15.

14. A method as claimed in claim 3, wherein the lignosulfon-ate has a purity of 96 to 98 percent.

15. A method according to claim 1 which further comprises the addition of a pozzuolana.