FR2707627A1 - Process and mix for preparing a concrete which has a remarkable compressive strength and fracturing energy, and concretes obtained - Google Patents

Abstract

Process for obtaining a concrete which has a compressive strength of at least 400 MPa and a fracturing energy of at least 1000 J/m<2>, characterised in that it comprises a transformation of the products of hydration of the cement to crystalline hydrates of xonotlite type, so as to remove from the concrete, after its setting, virtually all of the free water and at least 50 %, preferably at least 70 % and, still better, at least 80 % of the chemically bound water. Figure 1 shows the influence of the cure temperature on the drying. Application to the production of prefabricated articles and of moulded articles.

Description

The invention relates to a concrete having a compressive strength of at least 400 MPa and a fracturing energy of at least 1000 J / m2, which can be used for the production of prefabricated parts and molded parts.

According to the invention, to obtain such a concrete, the hydration products of the cement are transformed into crystalline hydrates of the xonotlite type so as to remove from the concrete, after setting, almost all of the free water and at least 50%, preferably. at least 70% and even better at least 80%, chemically bound water.

It is known to heat concrete in an autoclave to transform the amorphous hydrates of concrete of the [CaO SiO2, H2O] type, into a crystalline hydrate called tobermorite, the formula of which is [CaO] 5 [SiO2] 6, [H2OJ5, but this technique does not constitute a satisfactory solution to the present problem because tobermorite contains five times more water than xonotlite whose formula is [CaO] 6 [SiO2] 6 [H20] 1 -
In addition, the invention aims to provide a process such that the transformation of the hydration product of the cement can be obtained by simple heating, under ambient conditions of pressure and hygrometry.

In a particular embodiment of the process of the invention, a concrete is prepared with a cumulative porosity of less than 0.01 cm3 / gram (as measured with a mercury porosimeter), containing ground quartz and steel wool and , after setting, this concrete is subjected to a temperature of at least 250 "C, preferably at least 400" C, under ambient conditions of pressure and hygrometry, for a time sufficient to obtain and transform cement hydration products to crystalline hydrates of the xonotlite type.

The water vapor released during heating remains confined to the core of the concrete due to the low porosity of the latter; ground quartz promotes the formation of crystalline hydrates richer in CaO than amorphous hydrates and steel wool gives the matrix sufficient strength in the transient phase during which the water vapor pressure in the pores is maximum.

Under these conditions, the trapped water vapor achieves within the concrete the hydrothermal conditions necessary for the transformation of amorphous or semi-crystalline hydrates into xonotlite crystals.

The duration of the cure is generally several hours.

In preferred embodiments, the method of the invention also has one or more of the following characteristics: the fresh concrete is subjected, during its setting, to a clamping pressure of at least 5 MPa or better of at least 50 MPa; - the concrete is prepared by kneading a mixture comprising the following proportions, expressed in parts by weight (p): a) 100 p of Portland cement, b) 30 to 100 p, or better 40 to 70 p, of fine sand having a grain size 150 to 400 microns, c) 10 to 40 p, or better 20 to 30 p, of amorphous silica having a grain size of less than 0.5 microns, d) 20 to 60 p, or better 30 to 50 p, of ground quartz having a grain size of less than 10 microns, e) 25 to 100 p, or better 45 to 80 p, of steel wool, f) optionally other additives, g) 13 to 26p, or better 15 to 22p, of water; - 0.6 to 3.0 p, or better still 1.4 to 2.6 p, of a superplasticizer are incorporated into the mixture; - ground steel wool is used from 1 to 5 mm (size of the cutting grid at the grinder).

An example of the preparation of a concrete in accordance with the present invention will be described below.

Example
Concrete samples are prepared by mixing a mixture containing substantially, per 100 p by weight of Portland cement: 50 p. fine sand (grain size 150-400 microns), 23 p. of amorphous silica (grain size less than 0.5 microns), 39 p. of crushed quartz (grain size less than 10 microns), 2p. of superplasticizer (dry extract), 63p. of AISI 430 stainless steel wool ground to 3 mm (size of the cutting grid on the grinder), marketed by the company GERVOIS, 18 p of water.

The superplasticizer is, for example, of the polyacrylate, melamine or naphthalene type.

These samples are fired at different temperatures and the desiccation of the test pieces is measured. It can be seen (fig. 1) that this desiccation increases slightly with the curing temperature up to 220 "C and above 250" C. On the other hand, this desiccation is intense at around 230-240 "C. This temperature corresponds to the transformation of amorphous or semi-crystalline hydrates into xonotlite.

For this transformation to take place completely, the sample should therefore be fired at a temperature equal to or greater than 250 ° C.

The curing conditions lead to exposing the steel wool to high temperature conditions associated with high humidity.

Despite confinement in the cementitious matrix, regular steel wool is severely corroded. The iron oxides resulting from this corrosion are visible on the facings of the sample. In the case of stainless steel use, corrosion is much more limited and no trace of rust appears on the facing.

The mechanical performance of the concrete of the invention can be improved by applying to the sample of fresh concrete, a clamping pressure of between 5 and 50 MPa. The purpose of this pressure is to eliminate the porosity of the sample due to the occluded air, and to reduce the water content of the fresh concrete by dewatering.

For example, the results indicated in the table below are observed:
COMPRESSION RESISTANCES

<tb><SEP> TEMPERATURE <SEP> DE <SEP> CURE
<tb><SEP> 250 "C <SEP>400" C <SEP>
<tb> Sample <SEP> standard <SEP> 488 <SEP> MPa <SEP> 524 <SEP> MPa
<tb> Sample <SEP> compressed
<tb> during <SEP> the <SEP> take <SEP> 631 <SEP> MPa <SEP> 673 <SEP> MPa
<tb>
Traditional concrete is characterized by its resistance to 28 days, measured on a cylinder. Common concretes have compressive strengths of between 25 and 45 MPa. So-called high performance concretes have strengths of 50 to 60 MPa. So-called very high performance concretes have strengths which can slightly exceed 100 MPa.

Reactive powder concrete has a compressive strength which can reach 250 MPa.

The strengths obtained with the concrete of the invention are between 400 and 680 MPa.

Three-point bending tests on notched 4 x 4 x16 specimens made it possible to measure fracturing energies ranging from 1,200 J / m2 to 1,800 J / m2, while common concretes, high-performance concretes and high-performance concretes. very high performances all have fracturing energies of less than 150 J / m2.

Claims (11)

1. Process for obtaining a concrete having a compressive strength of at least 400 MPa and a fracturing energy of at least 1000 J / m2, characterized in that it comprises a transformation of the hydration products of the cement crystalline hydrates of the xonotlite type, so as to remove from the concrete, after setting, almost all of the free water and at least 50%, preferably at least 70% and even better, at least 80% of the chemically bound water. 2. Method according to claim 1, characterized in that one prepares a concrete of cumulative porosity of less than 0.01 cm3 / gram, containing crushed quartz and steel wool, and that this concrete is heated. , after setting, at a temperature of at least 250 "C. 3. Method according to claim 2, characterized in that use is made of ground steel wool from 1 to 5 mm (size of the cutting grid at the grinder). 4. Method according to one of claims 2 and 3, characterized in that stainless steel wool is used. 5. Method according to one of claims 1 to 4, characterized in that the concrete cures at a temperature at least equal to 250 C, or better, at least equal to 400 "C, at ambient pressure and at l ambient humidity. 6. Method according to one of claims 1 to 5, characterized in that the fresh concrete is subjected during setting to a pressure of at least 5 MPa, or better still, of at least 50 MPa. 7. Method according to one of claims 1 to 6, characterized in that the concrete is prepared by kneading a mixture comprising the following proportions, expressed in parts by weight (p): a) 100 p. of Portland cement, b) 30 to 100 p., or better 40 to 70 p., of fine sand having a grain size of 150 to 400 microns, c) 10 to 40 p. or better 20 to 30 p., of amorphous silica having a grain size of less than 0.5 microns, d) 20 to 60 p., or better 30 to 50 p, of ground quartz having a grain size of less than 10 microns , e) 25 to 100 p., or better 45 to 80 p. of steel wool, f) optionally a superplasticizer, g) optionally other adjuvants, h) 13 to 26 p., or better 15 to 22 p., water. 8. Method according to claim 5, characterized in that said mixture contains 0.6 to 3.0 p., Or better 1.4 to 2.6 p., Of a superplasticizer. 9. Method according to one of claims 7 to 8, characterized in that said mixture contains substantially, for 100 p. cement Portland, 50 p. of sand, 23 p. silica, 39 p. quartz, 2 p. superplasticizer, 63 p. of steel wool, 18 p. of water. 10. Mixtures for preparing a concrete, as defined in one of claims 7 and 8. 11. Concretes prepared by a process according to one of claims 1 to 9.