FR2717466A1 - Use, as additive for controlling the flow threshold and / or viscosity, of a precipitated silica in cementitious pastes. - Google Patents

Abstract

The use of 0.5-5 wt %, based on the weight of cement, of precipitated silica as the viscosity and/or flow point controlling additive in mixed compositions consisting of cement and water, is disclosed. Cement pastes containing 0.5-5 wt %, based on the weight of cement, of such an additive, and the corresponding compositions after setting, are also disclosed.

Description

USE. AS A FLOW THRESHOLD AND / OR ADDITIVE

  VISCOSITY. OF PRECIPITATED SILICA IN CEMENT PASTA

  The present invention relates to the use, as additive for controlling rheological properties, of a precipitated silica in cementitious pastes based on

cement and water.

  It also relates to these cementitious pastes containing said silica

  precipitate and to the corresponding compositions after setting.

  The development and use of cementitious paste compositions are known

  for a very long time and are still the subject of numerous publications.

  It has already been proposed, in order to improve certain properties of these compositions, to add to it various adjuvants, in particular fluidizers, retarders

  and also silica smoke.

  The purpose of fluidizers is essentially to reduce the amount of water in

  mixing to use and deflocculate the grains of cement.

  The setting retarders have the sole function of delaying the setting of the composition. Silica smoke can act as a water retentant and help improve

  the resistance to chemical aggressions of the final composition.

  However, cementitious pastes do not necessarily have certain

  desired rheological properties.

  In addition, the control of these rheological properties is not always possible, whereas it can prove to be essential in particular during the processing of the dough.

cement on site.

  The term “rheological properties” is understood here to mean cementitious pastes, in particular their flow threshold, that is to say their resistance to flow (for low shear (less than 50 s-1 in particular, or even 10 s -1) and their viscosity (for a

  high shear (100 to 1000 s-1 in particular)).

  The object of the present invention is in particular to propose a control additive

  and / or improving these rheological properties relating to cementitious pastes.

  The invention consists in the use of precipitated silica, in an amount by weight of between 0.5 and 5% relative to the weight of cement, in such compositions with

cement and water base.

  By cementitious pastes is meant tempered compositions formed from

  cement and water (and possibly common additives).

  Other characteristics, details and advantages of the invention will become more apparent

  clearly on reading the description which follows.

  The Applicant has thus surprisingly discovered that the use of a certain amount of precipitated silica in a composition formed from cement and water (called mixing water) makes it possible in particular to control and / or improve the flow threshold and / or the viscosity of this composition, the addition of the precipitated silica generally taking place before or during the mixing operation. This cementitious paste is generally formed by kneading the products entering into its constitution (mixing operation); the order of introduction of said products in

the mixer can be any.

  According to the invention, the precipitated silica can be used as an additive for controlling and / or improving the flow threshold of the aforementioned cementitious pastes. The flow threshold of a cement paste corresponds to its resistance to flow (for a

  low shear, especially less than 50 s-1, or even 10 s-1).

  Thus, the addition of a certain quantity of precipitated silica in such compositions makes it possible in particular to be able to adjust the value of the flow threshold,

in particular under low shear.

  This use of precipitated silica allows an adjustment of the flow threshold (under low shear) to a low value; cementitious (tempered) pastes obtained

  can then penetrate very satisfactorily into porous media.

  The tempered compositions obtained are then usually self-leveling (putting them horizontally only requires a strip), preferably self-leveling (their

  horizontal positioning does not require any device or instrument).

  An adjustment of the flow threshold (under low shear) to a high value due to the use of a certain amount of precipitated silica is also possible; the cementitious pastes (tempered) obtained are then perfectly suitable for prefabrication and have a thixotropic consistency (and therefore optimal homogeneity) which thus give them a very satisfactory pumping ability, even after a period of rest, hence safety and flexibility for the user. According to the invention, the precipitated silica can be used as an additive for controlling and / or improving the viscosity of said cementitious pastes (under high shear (100 to 1000 s-1 in particular)), the quantity by weight of silica used being between

0.5 and 5% relative to cement.

  This use of precipitated silica makes it possible in particular to obtain a relatively low viscosity for said compositions (in particular under high shear),

  thus giving them good pumping ability.

  Finally, it makes it possible, preferably, to improve their ability to fit into the formwork without vibration, ensuring a surface appearance which generally does not require re-setting; it also reduces the pressure exerted

  on these formwork, hence a reduction in the cost of the latter.

  The term “precipitated silica” is understood here to mean a silica obtained by precipitation from the reaction of an alkaline silicate with a generally inorganic acid at an adequate pH of the precipitation medium, in particular a basic pH, neutral or slightly acid; the mode of preparation of the silica can be arbitrary (addition of acid on a silicate base stock, simultaneous total or partial addition of acid and silicate on a base stock of water or silicate solution, etc.) and is chosen according to the type of silica that

wish to get.

  The precipitated silica is advantageously used in the form of a powder (dry)

  or, possibly, in aqueous suspension (slurry or porridge).

  The content by weight of precipitated silica used is between 0.5 and 5% relative to the weight of cement; it is for example between 1 and 3% or between 2.5 and 4.5% relative to the weight of cement. Indeed, the use of precipitated silica at these relatively low contents gives excellent results, which represents an advantage.

undeniable of the invention.

  The precipitated silicas used according to the present invention preferably have a BET specific surface of at least 40 m2 / g, in particular included

  between 40 and 400 m2 / g, for example between 50 and 350 m2 / g.

  According to a particular embodiment, this surface is between 210 and

300 m2 / g.

  The BET specific surface is determined according to the BRUNAUER method-

  EMMET-TELLER described in 'The Journal of the American Chemical Society', vol. 60,

  page 309, February 1938 and corresponding to standard ISO 5764/1 (annex 1).

  Similarly, the precipitated silicas used according to the present invention preferably have an average particle size of at least 0.5 μm, in particular between 1 and 300 μm, in particular between 1 and 40 μm, or even between 1 and 20 μm; she

  may for example be between 1.5 and 16 pm.

  According to particular embodiments, this average particle size is

  between 1.5 and 4 pm or between 6 and 16 pm.

  The average particle size is determined, for large particles (at least 100 μm), preferably according to standard NFX-11507 (December 1970) by dry sieving and determination of the size corresponding to a cumulative refusal of 50%. , and, for particles with a small size (less than 100 μm), preferably by diffraction

  laser on a SYMPATEC granulometer.

  According to a preferred variant of the invention, use is made, as an additive for controlling and / or improving the flow threshold (in particular under low shear) and / or in the viscosity (in particular under high shear). cementitious based on cement and water, at least one precipitated silica, in an amount by weight of between 0.5 and 5% relative to the weight of cement, mixed (or combined) with at least one plasticizer. Thus, said additive formed by precipitated silica can also comprise at least one fluidizer. Said additive can then optionally be prepared by simple mixing of the precipitated silica and the plasticizer and also by prior absorption of said plasticizer on the precipitated silica, this absorption taking place in particular by co-atomization of the precipitated silica and the plasticizer or by placing on support thereof on precipitated silica. The techniques known as co-atomization and placing on a support are well known to those skilled in the art. The expression “placing on a support” in particular means the absorption of a compound in liquid form in the pores of the precipitated silica. In this case, use is preferably made of a precipitated silica having a DOP oil intake of between 180 and 350 ml / 100 g (the DOP oil intake being determined according to ISO standard

  787/5 using dioctylphthalate).

  There may be mentioned, by way of examples of plasticizers, in particular melamine, sodium polynaphthalene sulfonate, sodium polyacrylate, sodium polycarboxylate. The cementitious pastes in which 0.5 to 5% by weight of precipitated silica are used according to the invention may contain, in addition to cement and water, at least one

  plasticizer, this plasticizer having been introduced independently of the precipitated silica.

  It should be noted that the use of precipitated silica according to the invention does not harm the properties of the final compositions after setting; on the contrary, it can decrease their

  permeability, increase their durability and improve their mechanical properties.

  The present invention also relates to cementitious pastes in which is used according to the invention at least one precipitated silica, as well as these

compositions after taking them.

  The precipitated silica is of the type described above, in particular in terms of its method of preparation, its BET specific surface, its average size of the

particles.

  Likewise, the cementitious pastes according to the invention then have a content by weight of precipitated silica of between 0.5 and 5%, for example between 1 and 3% or between

  2.5 and 4.5%, based on the weight of cement.

  These compositions according to the invention can also additionally contain at least

  a fluidizer, in particular as described above.

  They may also contain one or more usual additives, in particular

  especially a setting retarder.

  The invention is not at all limited to a very specific type of cement.

  This must in fact be generally of the hydraulic type, that is to say that it must

  be a material that can harden in the presence of water.

  Some examples of cement will be given below.

  This cement can thus be of the aluminous type, and in particular of the SECAR type.

  The cement can also be of the silicate type and more particularly based on calcium silicate. Examples include PORTLAND cements and, in this type of cements, white cements, those resistant to sulfates as well as those comprising slag (for example CLK cements), fly ash,

pozzolans and / or meta-kaolin.

  It should be noted that fly ash, pozzolans and / or metakaolin

  can be added to cementitious pastes independently of cement.

  The cement can also be based on slag activated by alkalis,

lime or lime sulfate.

  The compositions in which a precipitated silica is used according to the invention

  can be used in any traditional application of cementitious pastes.

  The following examples illustrate the invention without, however, limiting its scope.

Examples

  In these examples, cement pastes are prepared from cement, plasticizer, water and cement pastes from cement, plasticizer, water and either precipitated silica or silica fume. The contents of these raw materials (in parts by weight) are as follows: Cement: 100 Fluidifier: content such that the ratio of fluidizer (dry matter) / cement is 0.012 Water: 30 Precipitated silica: 0 or 4 or silica smoke: 0 , 4 or 10 Raw materials We use PORTLAND cement (referenced CPA 55 HPR from Ciments Lafarge). By way of plasticizer, use is made, according to the examples - of B 6770 (sodium polyacrylate), in the form of a solution, sold by the company Bevaloid; B 36 (sodium polynaphthalene sulfonate), in powder form, sold by the company Bevaloid; - FF 86 (melamine), in the form of a solution, sold by the company Sika;

  - Superplast THP, in the form of a solution, sold by the company Chryso.

  We use: - either a precipitated silica with a BET specific surface area of 250 m2 / g and an average particle size of 3.5 pIm (noted here SP A); - Either a precipitated silica with a BET specific surface of 250 m2 / g and an average particle size of 15 μm (noted here SP B); - or a silica smoke marketed by the company Elkem under the reference 940 U (noted here FS);

  - either neither precipitated silica nor silica smoke.

  Mixing protocol The cement paste is mixed in a planetary mixer according to the following protocol: - introduction into the mixer of cement and precipitated silica or silica smoke (according to the examples); - homogenization (by kneading) of the dry mixture obtained for 30 seconds at rpm; - Addition of water and thinner under mixing at 50 rpm for 30 seconds;

  - continued mixing for 5 minutes at 250 rpm.

  Measurement of rheological characteristics We measure, for each of the cementitious pastes obtained after mixing, their viscosity (V1) and their flow threshold (S1) under low shear and their viscosity

  (V2) and their flow threshold (S2) under high shear.

  For this, each composition is introduced into a measuring cell.

  Two high-speed rheograms and one

low speed rheogram.

  The values of the viscosities (expressed in centipoises) and the flow thresholds (expressed in Pa) are calculated according to the BINGHAM model on the descending curves of the rheograms: - for the high speed, the descending curve (from 1000 s-1 to 1 s-1) of the second high speed rheogram; - for low speed, the downward curve (from 50 s-1 to 1 s-1) is used

low speed rheogram.

  Protocol for recording the rheogram at high speed: The stress (Pa) induced by the cement paste is recorded on the axis of the cylinder of the MSDIN125 measuring cell, during the imposition of the following shear profile: - 10 seconds at a shear of 1 s-1; - rise in 60 seconds from a shear of 1 s-1 to a shear of 1000 s-1; step of 60 seconds at a shear of 1000 s-; - descent in 60 seconds from a shear of 1000 s-1 to a shear of

1 S-1.

  Protocol for recording the rheogram at low speed: The stress (Pa) induced by the cement paste is recorded on the axis of the cylinder of the MSDIN125 measuring cell, during the imposition of the following shear profile: - 10 seconds at a shear of 1 s-1; - rise in 60 seconds from a shear of 1 s-1 to a shear of 50 s-1; 60 second step at 50 s-1 shear;

  - descent in 60 seconds from a shear of 50 s- "to a shear of 1 s-1.

  The results are listed in Tables 1 to 5 below; the first column indicates whether the composition contains precipitated silica or silica fume

  (weight content in parentheses), or neither ("White").

Example 1

  The fluidizer used is B 6770.

VI If V2 S2 "White" 220 0.1 230 0

SPA (4%) 350 2.7 310 4, 6

SP B (4%) 560 3.5 460 12.0

FS (4%) 230 2.6 230 2.6

FS (10%) 190 0.6 160 1.8

Table 1

Example 2

  The fluidizer used is B 36.

Vi S1i V2 S2 "White" 1200 1.2 170 0

SP A (4%) 5510 51.7 300 360

SP B (4%) 3540 67.7 650 463

FS (4%) 130 1.4 150 0

FS (10%) 160 0.3 160 0

Table 2

Example 3

  The fluidizer used is FF 86.

  | _____ Vi If V2 S2 "White" 110 140 0 0

SP A (4%) 6940 27.6 290 403

SP B (4%) 3290 17.3 620 436

FS (4%) 200 13.7 120 22.4

FS (10%) 19 2.4 90 0.8

Table 3

Example 4

  The plasticizer used is Superplast THP.

  ____ _Vl _ | VISi V2 S2 "White" 180 0 160 0 SPA (4%) 10 100 158 nm nm SP B (4%) 9830 43.8 nm nm

FS (4%) 140 0 160 0

FS (10%) 210 10.4 160 6.5

(nm = not measured)

Table 4

  A test carried out on a cementitious paste containing neither a thinner, nor precipitated silica, nor silica fume, a composition which is heterogeneous after mixing, leads to the following results:

V1: 1200; S1: 8.9; V2: 190; S2: 102.

  The results included in the preceding tables show that the use of precipitated silica makes it possible to modulate the rheological characteristics (flow threshold and viscosity) of the cementitious paste, in order to obtain in particular a relatively low viscosity under high shear associated with either high flow threshold under low

  shear, ie at a fairly low flow threshold under low shear.

  The compositions containing silica smoke are all, whatever the

  fluidizer, characterized by a very low or even zero flow threshold.

Claims (8)

  1. Use, as additive for controlling the flow threshold and / or viscosity, of at least one precipitated silica in a cementitious paste, in an amount by weight   between 0.5 and 5% relative to the weight of cement of said paste.   2. Use according to claim 1 characterized in that said additive comprises in in addition to at least one fluidizer.   3. Use according to one of claims 1 and 2 characterized in that the content of   weight of silica precipitated in said paste is between 2.5 and 4.5% relative to by weight of cement in the dough.   4. Use according to one of claims 1 to 3 characterized in that said silica   precipitate has a BET specific surface of at least 40 m2 / g.   5. Use according to one of claims 1 to 4 characterized in that said silica   precipitate has an average particle size of at least 0.5 µm.   6. Cement paste based on cement and water in which at least one is used   precipitated silica according to one of claims 1 to 5.   7. Cement paste according to claim 6 after setting.   8. Cement paste according to one of claims 6 and 7 characterized in that it   additionally contains at least one plasticizer.