Set-accelerating admixture with improved stability
[Technological field]
The present invention relates to a set-accelerating admixture, to a method for its
5 preparation and to its use notably for accelerating the setting of a hydraulic composition
and for attaining high early strengths.
[State of the art]
It is customary to add admixtures to hydraulic compositions, in order to modulate
10 the properties thereof during application and after hardening.
It is thus known how to modify the hydraulic setting characteristics by adding setaccelerating
agents and set-retardants.
Accelerating the setting is particularly of interest economically since it allows an
increase in the manufacturing rate and also allows working under winter conditions.
15 Certain salts, notably alkaline salts like sodium chloride or earth alkaline salts like
calcium chloride are widely used as accelerators for setting and hardening Portland
cement.
The capability of these salts of improving mechanical strengths in compression
may however be limited in the case of cements with low clinker content, because these
20 salts accelerate more particularly the hydration of the phases of the clinker.
In order to limit corrosion caused by an excessive content of chlorides, patent
US 4,318,744 proposes admixtures including an alkanolamine.
However, the formulation of an admixture comprising several compounds
associated with the various admixtures may pose problems. Thus, formulations of
25 admixtures that associate sulfates and alkaline salts are not stable at low temperature,
since they tend to crystallize when stored under cold conditions, because of low solubility
of sodium sulfate. In order to avoid precipitation of the alkaline sulfate of the solution, it is
then necessary to store these admixtures in a heated environment.
[Technical problem]
Therefore the object of the present invention is to propose an admixture for
hydraulic compositions which avoids the mentioned drawbacks, and notably which
reduces the setting time, gives the possibility of ensuring good compressional strength at
an early stage, limits corrosion phenomena and which is stable at low temperature.
35
2
[Description of the invention]
According to the invention, this object is achieved according to the invention by a
set-accelerating admixture for hydraulic compositions comprising in an aqueous solution:
o the reaction product of an alkanolamine with a concentrated strong
acid;
o sulfate anions; and
o alkaline or earth alkaline cations;
having a pH comprised between 5 and 12.
The term of ((strong acid )) is meant to designate an acid for which the acidity
10 constant K, in an aqueous solution is such that pK, < 1.7. In these acids, all the acid
molecules are dissociated.
By the term of (( concentrated acid )) is meant an acid having a water content of
less than 30% and preferably less than 10% by weight.
The term of (( aminium salt )) designates a salt of a protonated amine.
15 By c( low temperature )) in this context is notably meant a temperature of less than
20°C, preferably less than 5°C and notably less than 0°C.
By the term of (( hydraulic composition )) is meant a composition comprising water
and a hydraulic binder.
By the term of (( hydraulic binder )) is meant any compound having the property of
20 being hydrated in the presence of water and the hydration of which allows a solid to be
obtained having mechanical characteristics notably a cement like Portland cement,
aluminous cement, pozzolanic cement or further anhydrous calcium sulfate or
semihydrate. Hydraulic binders based on Portland cement described in the NF EN 197-2
standard may further include pozzolanic materials such as slags from blast furnaces,
25 flying ashes, natural pozzolans, silica fumes. The hydraulic binder may in particular be a
cement according to the EN 197-1 standard and notably a Portland cement, and in
particular a cement of the CEM I, CEM II, CEM Ill, CEM IV or CEM V type according to
the NF EN 197-1 cement standard.
By the term of (( concrete n, is meant a mixture of hydraulic binders, of granulates,
30 water, optionally additives, and optionally mineral additions. The term of concrete also
comprises mortars.
Hydraulic binders based on Portland cement may further include mineral additions.
The expression of (( mineral additions s designates slags (as defined in the NF EN 197-1
cement standard in paragraph 5.2.2), steel-making slags, pozzolanic materials (as defined
35 in the NF EN 197-1 cement standard in paragraph 5.2.3), flying ashes (as defined in the
NF EN 197-1 cement standard in paragraph 5.2.4), calcined shales and clays (as defined
3
in the NF EN 197-1 cement standard in paragraph 5.2.5), limestones (as defined in the NF
EN 197-1 cement standard in paragraph 5.2.6) or further silica fumes (as defined in the
NF EN 197-1 cement standard in paragraph 5.2.7) or mixtures thereof.
Indeed, it is seen that this formulation is stable under cold conditions, so that the
5 admixture may be stored at low temperature in a prolonged way without observing any
precipitation.
Moreover, the admixture allows the preparation of hydraulic compositions having a
shortened setting time but nevertheless having good compressional strength at an early
stage, notably at 1, 2, 7 and 28 days.
10 The admixture according to the invention first of all contains the product of an
alkanolamine with a concentrated strong acid. The alkanolamine has an accelerating, in
particular hardening effect. In the case when the admixture is added to the hydraulic
binder prior to grinding, it may also act as a grinding agent, in order to facilitate grinding.
Preferably, the alkanolamine is an amine having a molar mass comprised between
15 50 and 400 glmol. it may notably be selected from triethanolamine (TEA),
triisopropanolamine (TIPA), diethanolamine (DEA), diethanolisopropanolamine (DEIPA)
and tetrahydroxylethylethylenediamine (THEED) and mixtures thereof. Preferably the
alkanolamine is triethanolamine.
As this will be explained in more detail later on, the admixture according to the
20 invention contains the alkanolamine as a reaction product with a concentrated strong acid.
It may be assumed that the alkanolamine forms a salt with a strong acid similar to
the reaction of other amines. Although the exact composition of the obtained product has
not yet been specifically elucidated, it is assumed that the alkanolamine forms by reaction
with a strong acid, an adduct, for example as an aminium salt (also called ammonium
25 salt). However the occurrence of a different reaction notably at the alkanol groups is not
excluded.
The compound form may be present in the aqueous solution in a dissociated form
or not, depending on the pH of the solution.
The admixture preferably includes 0.05 to 0.6 mollL of reaction product of the
30 alkanolamine.
The sulfates present in the admixture according to the invention have the function
of ensuring acceleration of the setting of the hydraulic composition while limiting the
presence of chloride ions, the excessive presence of which may generate corrosion
phenomena. Moreover they are particularly efficient as set-accelerating agents for
35 cements with low clinker content.
4
The admixture according to the invention preferably comprises a content of sulfate
anions comprised, in the limit of their solubility, between 0.1 and 1.5 mollL.
These anions may be introduced into the admixture notably by means of alkaline
or earth alkaline metal sulfates, notably sodium, potassium or calcium sulfates.
5 Alternatively, it is also possible to introduce the sulfates via the addition of sulfuric
acid, as this will be explained later on.
Quite unexpectedly, it was discovered that the adjustment of the pH of the
admixture to a suitable value gives the possibility of ensuring the stability of the
formulation.
10 Within this framework, it was observed that a pH comprised between 5 and 12
allows storage at a temperature of 5°C for 15 days without the appearance of
precipitations visible to the naked eye.
The admixture according to the invention therefore preferably has a pH comprised
between 5 and 9, and at most preferentially between 5.5 and 7.
15 The admixture according to the invention may moreover contain an additional
set-accelerating agent. By the term of (( set-accelerating agent)) is meant a compound, the
presence of which in the hydraulic composition increases the hydraulic setting rate of the
composition. Their performances are notably indicated in the US standard ASTM C494.
This is most often a salt, which may notably be an inorganic compound, and it may
20 notably be selected from sodium chloride, calcium chloride, sodium thiocyanate, calcium
thiocyanate, sodium nitrate and calcium nitrate and mixtures thereof.
The admixture according to the invention preferably comprises a chloride anion
content comprised between 1.5 mol/L and 5 mollL.
However, it is preferable to limit the chloride content in the binder to a value not
25 exceeding 0.1% by weight of binder so that the concrete may be used in the making of
reinforced concrete in order to reduce the risk of corrosion. This value may even be lower
for applications of the pre-stressed concrete type. The supply of chlorides by the means of
admixture should therefore be limited as much as possible.
Thus, it is recommended in certain standards, notably the EN 196 and 197
30 standards for cement, and the EN-934 for concrete, to limit the chloride content of cement
to 1,000 ppm by weight of chlorides. The admixture according to the invention preferably
provides less than 500 ppm, and most particularly less than 300 ppm of chlorides to the
cement.
According to a second aspect, the invention aims at a method for preparing an
35 accelerating admixture for hydraulic compositions comprising the steps:
5
(1) adding a concentrated strong acid to an aqueous solution of
alkanolamine;
(2) adding to the product obtained in step (1) an aqueous solution of an
alkaline or earth alkaline chloride and/or sulfate; and
5 (3) adjusting the pH to a value from 5 to 12, before or after step (2).
The collected experimental elements seem to indicate that a transformation occurs
in step (1). Indeed, the mixture of a solution of sulfate salts with the alkanolamine does not
have the same infrared spectrum as the mixture of the alkanolamine with one of a
concentrated sulfuric acid. This teaching may be generalized to the mixtures of
10 alkanolamines with other strong acids or anions.
Preferably the acid is a strong acid, and most particularly a concentrated acid.
Among the suitable strong acids, are of particular interest the acids which do not
introduce new ions into the hydraulic composition. Therefore inorganic strong acids will
therefore be preferred and among the latter sulfuric acid, hydrochloric acid and nitric acid.
15 Preferably, the anion of the selected acid is different from the anions introduced
into the aqueous solution in step (2).
Thus, according to an embodiment of the invention, step (1) is carried out by
adding concentrated sulfuric acid and step (2) is carried out by adding a solution of an
alkaline metal chloride.
20 According to another embodiment, step (1) is carried out by adding concentrated
hydrochloric acid and step (2) is carried out by adding a solution of an alkaline metal
sulfate.
An additional set-accelerating agent may if necessary also be added in step (2).
The molar ratio between the amine and the acid in step (1) is advantageously
25 comprised between 0.2 to 0.6.
Step (3) is advantageously carried out by adding a base, notably a strong base
and most particularly a base selected from sodium hydroxide and potassium hydroxide.
The composition of the admixture has not yet been determined accurately.
However, as mentioned, it was shown that the addition of a concentrated strong acid to
30 the alkanolamine solution modifies the composition of the product.
Also, according to a third aspect, the invention aims at a set-accelerating
admixture for hydraulic compositions obtainable by the thereby described method.
According to a fourth aspect, the invention aims at a method for preparing a
hydraulic composition, comprising the step of adding an admixture according to the
35 invention to the hydraulic binder, before, during or after grinding. The admixture may thus
6
be used in cement plant grinders or be directly incorporated in plants which proceed with
the mixing of Portland cement and of pre-ground additions.
The admixture may however also be used at the moment of the preparation of the
hydraulic composition, for example by being added into the mixing water.
5 Preferably, this method is applied in that the admixture is added with a dosage
from 500 to 10,000 ppm by weight based on the weight of the hydraulic binder.
The method for preparing a hydraulic composition according to the invention is
particularly useful for hydraulic binders described in the NF EN 197-2 standard including a
lower clinker content and a higher content of pozzolanic materials such as slags from
10 blast furnaces, flying ashes, natural pozzolana, silica fumes. The cements designated as
CEM II, CEM Ill, CEM IV and CEM V, are thus preferred for this invention.
Also, the preparation method is more preferred when the hydraulic binder is a
cement containing less than 75%, and preferably less than 65% by weight of clinker.
The invention will be better understood by means of the following examples and
15 with reference to the three figures which show:
Fig. 1: the infrared spectrum of residues of the solution at the end of step (1) of
Example 1 and of a solution of triethanolamine and sodium sulfate of
equivalent proportions;
Fig. 2 the instantaneous heat flow measured by isothermal calorimetry on a TAM
20 Air device from TA Instruments of a cement slurry with and without the
admixture according to the invention; and
Fig. 3 and the accumulated heat measured by isothermal calorimetry on a TAM
Air device from TA lnstruments of cement slurry with and without the
admixture according to the invention
[Examples]
EXAMPLE 1
Step (1). In a suitable 3L container provided with a magnetic stirrer, 192 g of
30 triethanolamine with 85% purity by weight provided by BASF were dissolved in 1,466mL
of water at room temperature. 342 g of concentrated sulfuric acid solution (96% by weight
content) were then introduced. It is observed that the temperature of the reaction mixture
rises to 35°C.
Step (2). To 240 mL of the solution obtained in the previous step, are added
35 53.81mL of water and 14.5g of NaCI. Stirring is maintained until a limpid solution is
formed.
7
Step (3). The pH of the obtained solution was adjusted to a value of 7 by adding
7.69 g of a 50% by weight sodium hydroxide solution.
a. Studv of the compound formed in step (11
5 The identity of the compound formed in the solution at the end of step (I), after
adding the acid, has not yet been elucidated with certainty.
In order to characterize the product formed in the triethanolamine acid solution, the
solution obtained after adding sulfuric acid is dried for 24 hours in the oven heated to a
temperature of 105°C. The solid residue is analyzed by Fourier transform infrared
10 spectroscopy, as compared with the residue of a solution with equivalent proportions of
triethanolamine and sodium sulfate. The obtained spectrum (see Fig.1) shows a
composition difference of the compounds present in these solutions,
Without intending to be bound by any theory, it is presently assumed that the
amine forms an aminium sulfate in the presence of the acid.
15
b. Cold stability accordina to the pH
In order to study the impact of the pH on the stability of the admixture according to
the invention, admixtures were made according to Example 1 having a different pH.
For this purpose, the amount of KOH solution added in step (3) of the procedure
20 indicated in Example 1 was varied in order to obtain solutions with a pH comprised
between 4 and 12.
The aspect of the prepared samples, having a pH ranging from 4.5 to 9.0, was
determined immediately after preparation before placing them at a temperature of 5°C.
The aspect of the solutions is then checked at regular intervals for a period of 15 days.
25 The results of this study are recorded in Table 1 below.
Table 1: Cold stability of the admixtures according to the pH
Stability
NO
YES
YES
YES
YES
Aspect
Visible crystallization after 24h
Visible crystallization after 5 days
No visible crystallization after 15 days
No visible crystallization after 15 days
No visible crystallization after 15 days
Formulation
EX. 1
EX. 1
EX. 1
EX. 1
EX. 1
pH
4.5
5.1
5.8
7.0
9.0
8
c. Action reinforcin~th e compressional strength at an early stage
In order to validate the performance of the admixture according to the invention in
different applications, the compressional strength was measured at 1, 2, 7 and 28 days
according to the EN-196-1 standard, of a mortar prepared with a cement admixed with the
5 admixture according to the invention comprising 58% by weight of Portland cement, which
consists of about 95% by weight of clinker and of 5% by weight of gypsum, and 42% by
weight of slag.
The mortar specified by the aforementioned standard is prepared by mixing:
- 450 g of hydraulic binder,
- 1,250 g of standardized sand;
- 225 g of water containing the admixture of the invention.
In order to measure the mechanical compressional strength, prisms are made with
dimensions of 4 x 4 x 16 cm. For each term, three prisms are prepared and two
measurements are made per prism, which makes a total of six measurements for each
15 term. Table 2 groups the average values for each term,
Table 2: Application performance of the admixture
It emerges that the strengths obtained with the admixture according the invention
20 are at least equivalent, or even better than for the control without any admixture.
It is noted as regards these results that the dosage of 3,000 ppm is more favorable
for the strength at 28 days and the dosage of 5,000 ppm is favorable for the strength at
1 day.
d. Studv by isothermal calorimetry
Isothermal calorimetry measurements were conducted in order to study the effect
of the admixture according to the invention on the hydraulic setting process. With
9
isothermal calorimetry, it is possible to measure the heat emitted over time during the first
hours of the setting of a hydraulic binder.
The tests were conducted by preparing in a suitable flask, a cement slurry of the
CEM Ill type, containing 60% by weight of blast furnace slag and 40% by weight of
5 Portland cement, with a water-over-cement (W/C) mass ratio of 0.5, by adding to the
mixing water, 3,000 ppm of admixture according to Example 1 based on the weight of the
cement. As a comparison, the same cement slurry was prepared without any admixture
according to the invention.
Immediately after preparation, the flask is introduced with the slurry into an
10 isothermal calorimetry device set to a temperature of 20°C and the emitted heat is then
recorded for a period of 65h.
The results of the measurement (see Fig. 2) show that the presence of the
admixture according to the invention notably increases the heat flow from about 10 hours
of hydration.
15 This second peak is only observed for cements with the slag contents. It therefore
seems that the admixture according to the invention favorably interacts during hydration of
the slags. The admixture according to the invention is therefore particularly of interest for
set-accelerating cements with strong slag contents or other pozzolanic additions rich in
silicates and aluminates.
20 It is also seen that the total heat versus time increases for the slurry including the
admixture according to the invention.
These results show that the addition of the admixture according to the invention
induces an increase in the emitted heat over time, synonymous with accelerating the
setting reaction.
25
10

CLAIMS
1.- A set-accelerating admixture for hydraulic compositions comprising, in an
aqueous solution:
o the reaction product of an alkanolamine with a concentrated strong
acid;
o sulfate anions; and
o alkaline or earth alkaline cations;
having a pH comprised between 5 and 12.
2.- The admixture according to claim 1, further comprising an additional
set-accelerating agent.
3.- The admixture according to claim 2, wherein the set-accelerating agent is
selected from sodium chloride, calcium chloride, sodium thiocyanate, calcium
thiocyanate, sodium nitrate, calcium nitrate and mixtures thereof.
4.- The admixture according to one of claims 1 to 3, wherein the alkanolamine is
selected from triethanolamine (TEA), triisopropanolamine (TIPA), diethanolamine
(DEA), diethanolisopropanolamine (DEIPA) and tetrahydroxylethylethylenediamine
(THEED) and mixtures thereof.
5.- The admixture according to one of claims 1 to 4, having a pH comprised
between 5.5 and 7.
6.- A method for preparing a set-accelerating admixture for hydraulic compositions
comprising the steps: ,
(1) adding a concentrated strong acid to an aqueous solution of
alkanolamine;
(2) adding to the product obtained in step (1) an aqueous solution of an
alkaline or earth alkaline chloride and/or sulfate; and
(3) adjusting the pH to a value from 5 to 12, before or after step (2).
7.- The preparation method according to claim 6, wherein step (1) is carried out by
adding concentrated sulfuric acid and step (2) is carried out by adding a solution of
an alkaline metal chloride.
8.- The preparation method according to claim 6, wherein step (1) is carried out by
' adding concentrated hydrochloric acid and step (2) is carried out by adding a
solution of an alkaline metal sulfate.
9.- The method according to claims 6 or 7, wherein the molar ratio between the
amine and the acid in step (1) is comprised between 0.2 to 0.6.
10.- The preparation method according to one of claims 6 to 9, wherein step (3) is
carried out by adding. a base selected from sodium hydroxide and potassium
hydroxide.
11 .- A set-accelerating admixture for hydraulic compositions which may be /'.!
obtainable by the method according to one of claims 6 to 10.
12.- A method for preparing a hydraulic composition, comprising the step for
adding an admixture according to one of claims 1 to 5 to the hydraulic binder,
before, during or after the grinding.
13.- The preparation method according to claim 12, wherein the admixture is
added with a dosage from 500 to 10,000 ppm by weight based on the weight of the
hydraulic binder.
14.- The preparation method according to claims 12 or 13 wherein the hydraulic
binder is a cement containing less than 75% by weight of clinker.
15.- A hydraulic composition, containing a cement with a clinker content of less
than 75% by weight and an admixture according to one of claims 1 to 5.