FR2671342A1 - Hydraulic binder based on a mixture of calcium sulphate and of a cement composition containing calcium aluminates - Google Patents

Abstract

Water-resistant hydraulic binder based on a mixture of calcium sulphate and of a cement composition containing calcium aluminates. The said binder contains, in effective quantity, additives capable, when it sets, of inhibiting the formation of hydrated calcium aluminates at the expense of the crystallisation of ettringite in tabular form.

Description

Hydraulic binder based on a mixture of calcium sulphate and a cementitious composition containing calcium aluminates.

 The present invention relates to a new hydraulic binder based on a mixture of calcium sulfate and a cement composition containing calcium aluminates. It notably includes a new hydraulic binder based on a binary mixture of calcium sulphate and aluminous cement.

 It also relates to the various applications of this new binder, in construction in particular.

 It relates more particularly to concretes or mortars prepared with said binder.

 The binder or cement of the invention has high mechanical strength, regardless of the calcium aluminate content of the cement or the cements used in its composition.

Despite the formation of ettringite when it sets, it is perfectly resistant to water and cracking.

 The terms "aluminous cement", "calcium sulfate", "ettringite", used in the present text, cover concepts perfectly known to those skilled in the art. These concepts are summarized below.

. Aluminous cement is a cement obtained by baking at high temperature (t> 12500C) of a mixture of bauxite (or Bayer alumina) and limestone (or lime). We find in its composition, a majority phase:
CaO-Al203 (identified according to the cement nomenclature by CA) and other phases:
CaO-2Al203 (identified according to the cement nomenclature by CA2),
12CaO-7Al203 (identified according to the cement nomenclature by C12A7),
4CaO-Al203-Fe203 (identified according to the cement nomenclature by C4AF),
2CaO-Fe203 (identified according to the cement nomenclature
C2F).

FONDU LAFARGE cement is an aluminous cement of the registered trademark LAFARGE
Said aluminous cement is therefore rich in calcium aluminates.

On the contrary, a siliceous cement, PORTLAND cement type, is poor in said aluminates. Its majority phases are
3CaO-SiO2 (identified according to the cement nomenclature by C3S),
2CaO-SiO2 (identified according to the cement nomenclature by C2S).

However, they contain other phases:
3caO-Al203 (identified according to the cement nomenclature by C3A),
4CaO-Al203-Fe203 (identified according to the cement nomenclature by C4AF).

 In the composition of the hydraulic binder of the invention, there is at least one cement containing calcium aluminate.

Said invention, as will clearly emerge from the comments below, is of great interest when the cement composition is rich in calcium aluminates, or even when it consists of an aluminous cement.

 Calcium sulfate, CaSO4, exists in different forms. The term used in this text encompasses all of its forms. In anhydrous form, we also speak of anhydrite. In hydrated form, we speak of calcium sulphate semi-hydrate (plaster) or calcium sulphate dihydrate (gypsum). Said calcium sulphate can, according to the invention, intervene pure (synthetic) or in the form of natural ore.

 - Ettringite is a calcium trisulfoaluminate which crystallizes with 32 molecules of water. Its formula is as follows 3CaO-Al203-3CaS04, 32H20. It is written according to the cement nomenclature: C6A S3 H32.

It is well known that said ettringite can form during the hydraulic setting of PORTLAND cements, cements of
Dairy or calcium fly ash. It is also well known that cements with a high alumina content (FONDU or other) cannot be used in the presence of an excess of internal sulphate sources (gypsum added) or external sources (sea water). The formation of ettringite, under these conditions, is harmful because it is accompanied by a strong increase in volume (expansion) which generates cracks and sometimes even, a total disintegration of the material. It is also known that the expansive nature of said ettringite can manifest itself in the long term in conventional cements (PORTLAND, for example) and lead to unpredictable deterioration of works.

We have therefore sought, according to the prior art, to avoid the significant formation of ettringite or to minimize its effects.
- we have notably described in the patent application
FR-A-2 527 107 compositions of aluminous cement and of purified desulfurization plaster, which have insufficient resistance to water and it has been shown that said water resistance could however be improved by the addition within said compositions of a non-negligible amount of fly ash. Reference will be made in particular to Table 2 of this document.

 - We have also described in the literature, for example in applications FR-A-2 289 460 and WO 86/04889, ternary mixtures of the type: aluminous cement - PORTLAND cement - calcium sulfate and, for example in EP application -AO 143 633, binary mixtures of aluminous cement-gypsum additionally containing a latex and salts.

 There have never been described, according to the prior art, mixtures containing calcium aluminates and calcium sulphate, during the setting of which it would have avoided the formation of solid intermediates, capable of expanding; said solid intermediates consisting of hydrated calcium aluminates.

 It has indeed been demonstrated, according to the invention, that the harmful, even catastrophic effects of the expansion during the formation of ettringite - during the hydraulic setting of a mixture containing the ingredients capable of generating said ettringite: mixture of cement (s) containing calcium aluminates and calcium sulphate or even mixture of aluminous cement and calcium sulphate- were due to the direct transformation of hydrated calcium aluminates, solid initially formed, into ettringite; said ettringite then having a crystal structure of the acicular type. Fine needles of hexagonal section were observed under the scanning electron microscope.

 It is therefore proposed, according to the invention, to minimize or even eliminate said harmful effects, by inhibiting the formation of said hydrated calcium aluminates, while accelerating the direct formation of ettringite in a suitable crystalline form.

To this end, it is advisable to bring in, for the hydraulic setting of the mixture: calcium sulphate / cement composition containing calcium aluminates, suitable additives.

 Therefore, according to the invention, a new hydraulic binder is proposed, which is resistant to water, although it is based on ingredients - calcium sulphate + cement composition containing calcium aluminates - capable of generating, with expansion, of L'ettringite.

 Said new hydraulic binder contains, in addition to the mixture of calcium sulphate and a cementitious composition containing calcium aluminates, in an effective amount, additives capable, during its setting, of inhibiting the formation of hydrated calcium aluminates for the benefit of crystallization, in tabular form of ettringite.

 The additives which are suitable for the purposes of the invention consist of any product or mixture of products responsible for direct crystallization of ettringite from a mixture of calcium sulphate and a cement composition containing aluminates of calcium.

 Such direct crystallization - without intermediate formation of hydrated, solid calcium aluminates - generates ettringite, in the form of tabular crystals.

 It is easily understood that said additives are used in greater or lesser quantity depending on the composition of the base mixture (calcium sulphate + cement (s) containing calcium aluminates).

 As specified above, the calcium sulphate - basic constituent of the hydraulic binder according to the invention - can be synthetic or natural, anhydrous or more or less hydrated.

Thus, according to one of its variants, the invention offers an interesting outlet for natural anhydrite.

 It is not excluded, according to the invention, to include in the composition of a hydraulic binder, several types of calcium sulfate.

The other basic constituent of the hydraulic binder of
The invention therefore consists of a cementitious composition containing calcium aluminates. It is the presence of said aluminates which creates the technical problem to which the present invention provides a solution. Said cement composition can consist of a cement or a mixture of cements, more or less rich in said aluminates.

It can consist of an aluminous cement.

 According to this variant, the invention reveals all its interest. The hydraulic binder of the invention can therefore be the basis of a binary mixture: aluminous cement / calcium sulfate.

According to this variant, each of said constituents can intervene in all proportions. Advantageously, within said binary mixture, the two constituents are involved in a weight ratio of aluminous cement of between 0.25 and 4.

calcium sulfate
In the mixtures: calcium sulphate / cement composition containing calcium aluminates (such as aluminous cement), there are therefore introduced, according to the invention, additives capable of leading to the expected result, developed above, when they are taken. hydraulic.

 Said additives advantageously consist of at least one setting retarder and at least one modifier of the crystalline habit of ettringite.

 Said setting retarder is chosen from products known to perform this function during the hydraulic setting of cements, in particular siliceous. A product is generally involved, but the use of several, in a mixture, is not excluded from the scope of the invention.

The most commonly used setting retarders according to the prior art are:
- lignosulfonates (calcium, sodium, ammonium),
- the acids and salts of hydroxycarboxylic acids,
- oxalic and gluconic acids,
- sodium and calcium gluconates,
- Citric acid, sodium citrate,
- hydrofluoric, phosphoric, humic, tetraboric acids,
- carbohydrates and in particular sucrose, glucoses, starch and cellulose,
- alkaline fluorides (in particular FNa),
- phosphates (in particular sodium, aluminum),
- boron compounds, such as boric acid, borax, meta- and sodium orthoborate,
- certain metallic oxides (in particular PbO and ZnO),
- lead nitrate, zinc acetate, copper acetate,
- glycerin,
- alcohols, amines, urea,
- magnesium salts ...

 Such setting retarders are suitable for the purposes of the invention.

 Preferably, a stearate, a borate, a citrate, a lignosulfate, a gluconate or a carbohydrate (glucose, sucrose, arabinose, xylose) are used within the mixture of the invention, as setting retarder. mannose ...).

The use of sucrose is particularly preferred.

 Said setting retarder intervenes in the hydraulic binder of the invention, in the same quantities as those in which the skilled person uses it, in hydraulic binders of the prior art. It is generally added from 0.01 to 5 parts by weight in 100 parts by weight of the basic mixture (calcium sulphate / cement composition containing calcium aluminates; and in particular calcium sulphate / luminous cement).

 The modifier of the crystalline habit of ettringitis induces the direct formation of tabular ettringitis. It is advantageously a soluble metal sulfate, single or double.

Several of said sulphates can intervene within the basic mixture: cement composition containing calcium aluminates / calcium sulphate. By way of examples, mention may be made, among the simple sulfates suitable for the purposes of the invention, of alkali, alkaline-earth or transition metal sulfates (such as sodium, zinc, iron sulfates, etc.) among the double sulfates suitable for the purposes of the invention, aluminum or iron alums, jarosites, Tutton salts. Said double sulfates combine a metallic element and an alkali or an alkaline earth
Said modifier of the crystalline habit of the ettringite intervenes in greater or lesser quantity, depending on the composition of the basic mixture (calcium sulphate / cement composition containing calcium aluminates and in particular calcium sulphate / aluminous cement). It generally occurs in an amount of 0.1 to 10 parts by weight per 100 parts by weight of said base mixture.

Thus, the hydraulic binder of the invention advantageously contains, per 100 parts by weight of said base mixture:
- 0.1 to 10 parts by weight of metal sulfate (s),
- and 0.01 to 5 parts by weight of setting retarder (s).

 Said additives can be found, in a smaller quantity, in hydraulic binders of the invention, poor in ingredients capable of generating, with expansion, ettringite.

 The presence within said binders of other additives commonly used in the field is not in any way excluded according to the invention.

 The hydraulic binder of the invention sets by adding water, generally from 30 to 60 parts by weight of water per 100 parts by weight of said binder.

 Observation with a scanning electron microscope of such a binder after setting, shows the presence of tabular ettringite crystals, crystals whose structure does not change over time, even in immersion.

 The material obtained by taking the hydraulic binder of the invention has high mechanical strength and is completely waterproof.

 The hydraulic binder of the invention allows the production of very efficient concretes or mortars. Another object of the invention is such concretes or mortars.

 The invention is illustrated, without limitation, by the two examples below.

Example 1
A hydraulic binder is prepared by mixing:
- 50 parts by weight of FUSED cement (LAFARGEQR (aluminous cement at 50% by weight of monocalcic aluminate)
- 50 parts by weight of anhydrous calcium sulfate (natural Folquemont anhydrite)
- 1.4 parts by weight of sucrose
- 2.9 parts by weight of hydrated zinc sulphate.

 The hydraulic setting of this mixture is obtained by adding 35 parts by weight of water.

 The material obtained has a water resistance which is excellent over time.

Example 2
A hydraulic binder of the invention has the following composition: - 58.4% of FUSED cement (LAFARGE @ (aluminous cement at 50% by weight of monocalcium aluminate)
- 39% natural anhydrite from Folquemont
- 1.04% sucrose
- 1.56% Na2SO4; the percentages being by weight.

 A mortar is made by mixing 1 part by weight of said binder with 3 parts of sand and 0.5 part of water.

The compressive strengths of such a mortar are
- 422 bars at 24 hours,
- 535 bars at 3 days,
- 540 bars at 7 days.

 These values are of the same order of magnitude as those obtained with a conventional high performance mortar.

Claims (10)

1. Hydraulic binder resistant to water, based on a mixture of calcium sulphate and a cementitious composition containing calcium aluminates, characterized in that it contains, in an effective quantity, suitable additives, tors de when taken, to inhibit the formation of hydrated calcium aluminates in favor of crystallization, in tabular form, of ettringite. 2. Hydraulic binder according to claim 1, characterized in that the calcium sulphate, synthetic or natural, intervenes in anhydrous or, more or less hydrated form. 3. Hydraulic binder according to one of claims 1 or 2, characterized in that it is based on a binary mixture: calcium sulfate / aluminous cement. 4. Hydraulic binder according to claim 3, characterized in that the weight ratio of the constituents of said binary mixture: aluminous cement is between 0.25 and 4. The crystalline habit of L'ettringite.  calcium sulfate 5. Hydraulic binder according to any one of claims 1 to 4, characterized in that said additives consist of at least one setting retarder and at least one modifier of 6. Hydraulic binder according to any one of claims 1 to 5, characterized in that it contains a setting retarder chosen from carbohydrates, advantageously consisting of sucrose. 7. Hydraulic binder according to any one of claims 1 to 6, characterized in that it contains a modifier The crystalline habit of ettringite chosen from soluble metallic sulfates, single or double, and their mixtures. 8. Hydraulic binder according to claim 7, characterized in that said modifier of the crystalline habit of The ettringite is chosen from sodium, zinc, iron sulphates, aluminum or iron alums, Jarosites, Tutton and their mixtures. 9. Hydraulic binder according to any one of claims 1 to 8, characterized in that it contains per 100 parts by weight of said base mixture, of calcium sulphate and of a cementitious composition containing calcium aluminates:  - 0.1 to 10 parts by weight of soluble metal sulphate (s), and  - 0.01 to 5 parts by weight of setting retarder (s). 10. Concrete or mortar prepared with a binder according to any one of claims 1 to 9.