EP2603471A1

EP2603471A1 - Concrete article comprising a surface with low open-porosity

Info

Publication number: EP2603471A1
Application number: EP11740942.5A
Authority: EP
Inventors: Jeffrey Chen; Matthieu Horgnies; Mélanie DYKMAN; Patrick Tintillier; Eléonore GUEIT
Original assignee: Lafarge SA
Current assignee: Lafarge SA
Priority date: 2010-08-11
Filing date: 2011-08-09
Publication date: 2013-06-19
Also published as: WO2012020012A4; FR2963789B1; WO2012020012A1; FR2963789A1

Abstract

The invention relates to an article comprising an article comprising an element made of a high performance concrete or an ultra high performance concrete, said element comprising a surface; and particles of calcium dihydroxide covering more than 60 % of the surface. It also relates to a process for the preparation of such an article comprising the coating walls of a mould with a demoulding composition comprising an aqueous solution comprising at least 1 % of a surfactant, introducing the concrete in the fresh state in the mould; and removing the article from the mould after the concrete sets.

Description

CONCRETE ARTICLE COMPRISING A SURFACE WITH LOW OPEN- POROSITY

The present invention relates to the field of concrete articles, in particular concrete articles having a surface which is visible.

The appearance of a visible surface of a concrete article may be defaced when there are stains on the visible walls. The stains may be liquid foods (e.g. coffee, wine, fruit juice), solvents, oil, felt pens, solid powder, etc. These stains may be difficult to remove. Concrete being a material having substantial open porosity, stains such as ink of felt pens, or more generally, a coloured solid or liquid component, tend to penetrate beyond the surface of the concrete and encrust itself in the open pore structure. It is often difficult to completely remove the stain made by such components with typical cleaning techniques.

For this reason, it is generally necessary to cover the concrete surface with a coating or a varnish, which is more easily cleaned. A drawback is that an additional finishing step to apply the coating or the varnish is necessary after making the concrete article. The application of a coating may further not be possible when it is preferred to see the original surface of the concrete part. Another drawback is that the application of a varnish onto the concrete wall may be a difficult operation due to the open porosity of the concrete. It may indeed be necessary to apply several layers of varnish to suitably cover the surface.

The aim of the present invention is therefore to provide a concrete article having a surface which has a low open porosity, said surface being obtained during the production of the concrete article in order to avoid penetration and absorption of liquid or solid components on the surface.

This aim is reached by an article, in particular, an article for the construction field, comprising:

-an element made of a high performance concrete or an ultra high performance concrete, said element comprising a surface; and

-particles of calcium dihydroxide covering more than 60 % of the surface.

The present invention further relates to a process for the preparation of the article described herein above, comprising the steps of:

-coating walls of a mould with a demoulding composition, said composition comprising an aqueous solution comprising at least 1 % of a surfactant; -introducing the high performance concrete or the ultra high performance concrete in the fresh state into the mould;

-allowing said high performance concrete or ultra high performance concrete to set, thereby forming the article; and

-removing the article from the mould.

The present invention also relates to a demoulding composition, for the process described herein above, comprising an aqueous solution comprising at least 1 % of a surfactant.

The invention offers at least one of the following advantages:

-it is more difficult to make stains on the surface covered by the protection layer comprising the particles of calcium dihydroxide (and after carbonation, particles of calcium carbonate or the mix of particles of calcium dihydroxide and particles of calcium carbonate) than on a typical concrete wall;

-cleaning of stains on the surface covered by the protection layer can be carried out easily, for example with a wet sponge;

-the application, when required, of a varnish on the surface covered by the protection layer is facilitated compared to a direct application on a typical concrete wall, the open porosity of the surface covered by the protection layer being reduced compared to a typical concrete wall;

-the development of mould and/or algae (related to the quantity of available water in the surface pores) on the surface covered by the protection layer is reduced compared to a typical concrete wall; and

-in-depth carbonation of the article is reduced.

The expression « hydraulic binder » is understood according to the present invention as a pulverulent material, which, mixed with water, forms a paste which sets and hardens as a result of hydration reactions, and which, after hardening, retains its strength and its stability, even under water.

The term « concrete » is understood according to the present invention as a mix of hydraulic binder, (for example cement), aggregates, water, optionally additives, and optionally mineral additions. Examples of concrete suitable for the present invention are high performance concrete, ultra high performance concrete, self-placing concrete, self-levelling concrete, self-compacting concrete, fibre- reinforced concrete, ready-mix concrete or coloured concrete. This definition also comprises pre-stressed concrete. The term « concrete » comprises mortars, in this specific case, the concrete comprises a mix of hydraulic binder, sand, water, and optionally additives and optionally mineral additions. The term « concrete » according to the invention denotes indistinctly fresh concrete or hardened concrete.

The term « high performance concrete » is understood according to the present invention as a concrete for which the compressive strength at 28 days is from 60 to 100 MPa. The term « ultra high performance concrete » is understood according to the present invention as a concrete for which the compressive strength at 28 days is greater than 100 MPa, generally greater than 120 MPa.

Aggregates suitable for the present invention are gravel, fine gravel and/or sand.

The expression « Portland cement » is understood according to the present invention as a cement of CEM I , CEM II, CEM III, CEM IV or CEM V type according to the NF EN 197-1 « Cement » Standard.

The expression « mineral additions » is understood according to the present invention as a finely divided mineral material used in concrete in order to improve certain properties or to give it particular properties. Examples of mineral additions suitable for the present inventions are fly ash (as defined in the EN 450 Standard), silica fume (as defined in the EN 13263 Standard: 1998 or the NF P 18-502 Standard), slag (as defined in the NF P 18-516 Standard), limestone additions (as defined in the NF P 18-508 Standard) and siliceous additions (as defined in the NF P 18-509 Standard).

The term « setting » is understood according to the present invention as the passage to the solid state by chemical hydration reaction of a hydraulic binder. The setting is generally followed by the hardening period.

The expression « article for the construction field » is understood according to the present invention as any constituent element of a construction. Examples suitable for the present invention include, but are not limited to, a floor, a screed, a foundation, a wall, a partition wall, a ceiling, a beam, a work top, a pillar, a bridge pier, a concrete block, a pipeline, a post, a cornice, an element of road works (for example a border of a pavement), a roof tile, a plaster board, an (acoustic and/or thermal) insulating element.

The surfactant or surface agent suitable for the present invention is a compound which reduces the superficial tension of a liquid and/or which lowers the interface tension between two liquids or between a liquid and a solid. The expression « open porosity » is understood according to the present invention as the pores which communicate with the exterior of the article and which may theoretically be filled with a fluid from the exterior of the article.

The expression « roughness » is understood according to the present invention as irregularities, of the order of the micrometre of a surface, which are defined by comparison with a reference surface, and are classed into two categories: asperities or "peaks" or "protrusions", and cavities or "hollows". The roughness of a given surface may be determined by measuring a certain number of parameters. In the remainder of the description, the parameter R_a, is used, as defined by the NF E 05-015 and ISO 4287 Standards, corresponding to the arithmetic average of all the profile ordinates within a reference length (in our examples, the latter was set at 12.5 mm).

The expression « surface absorption level » of a sample at a given moment is understood according to the present invention, as the mass of water absorbed by the surface of the sample at the said moment.

The present invention relates to an article comprising a concrete element having a surface with more than 60 %, preferably more than 80 %, more preferably more than 90 %, in particular more than 95 % of the surface covered by particles of calcium dihydroxide.

The surface of the article can have any shape or orientation. For example, it can be a plane surface or a curved surface. It can be a horizontal, vertical or inclined surface. It can correspond to a top, bottom or lateral surface of the article.

During the production of the article according to the invention, the protection layer, comprising the particles, is initially a layer of particles of calcium dihydroxide (Ca(OH)₂) mainly constituted (more than 90 %) of particles of Portlandite. These are crystals of calcium hydroxide which grow during the hydration of the concrete.

A carbonation reaction of the particles of Portlandite occurs upon contact with the air typically within a year of production of the article, so that the calcium dihydroxide transforms partially or totally over time into calcium carbonate (CaC0₃). Therefore, over time, the original layer of particles of calcium dihydroxide therefore transforms into a layer of a mix of calcium dihydroxide and calcium carbonate and finally into a layer of calcium carbonate.

During the formation of the layer of particles of calcium dihydroxide, the adjacent particles of calcium dihydroxide on the surface of the concrete article tend to merge and/or to partially cover each other so that the surface of the concrete element is at least partially covered by a continuous layer of particles of calcium dihydroxide. The thickness of the layer varies from 1 to 30 μηι, preferably from 2 to 20 μηι, most preferably from 5 to 15 μηι.

According to an example of embodiment of the invention, the average Ferret diameter of the particles of calcium dihydroxide, considered separately, viewed perpendicularly to the surface of the concrete element, is greater than or equal to 150 μηι, preferably greater than or equal to 200 μηι, more preferably greater than or equal to 250 μηι.

In certain cases, depending on the example of embodiment, the particles may join together and form a continuous film of calcium dihydroxide, in which case it may not be possible to measure the Ferret diameter.

Each particle of calcium dihydroxide corresponds to a crystal. The base of the crystal corresponds to the surface of the crystal, having the highest area, in contact with the surface of the concrete element. The orientation of the base of each crystal can be measured with respect to a plane corresponding locally to the surface of the concrete element. According to an example of embodiment, at least 50 %, preferably at least 70 %, more preferably at least 80 %, of the particles of calcium dihydroxide correspond to a crystal having a base in contact with the concrete article which is substantially parallel to the plane formed by the surface of the concrete element. The local difference between the direction of the base of the crystal and the plane formed by the surface of the concrete element is less than 20 degrees, preferably less than 15 degrees, more preferably less than 10 degrees.

The roughness R_a of the layer formed by the particles of calcium dihydroxide may be from 1 to 10 μηι, preferably from 1 to 5 μηι, more preferably from 1 to 2 μηι.

The surface water absorption level of the concrete article covered by the protection layer of calcium dihydroxide, measured 90 minutes after putting the concrete article in contact with the water, is less than or equal to 300 g/m², preferably less than or equal to 275 g/m², the said article having been kept for one month at 25°C in an atmosphere of 25 % humidity, then for 6 days at 45°C.

The concrete used in the present invention is a high performance concrete. The compressive strength of the high performance concrete at 28 days is greater than or equal to 60 MPa. Preferably, the concrete is an ultra high performance concrete. The compressive strength of the ultra high performance concrete at 28 days is greater than or equal to 100 MPa. The ultra high performance concrete may for example be an ultra high performance fibre-reinforced concrete.

Ultra high performance fibre-reinforced concretes are concretes having a cement matrix comprising fibres as described for example in the document entitled « Betons fibres a ultra-hautes performances » [Ultra High Performance Fibre- Reinforced Concrete] by the Service d'etudes techniques des routes et autoroutes - Setra [Technical Study Department of Roads and Highways] and by the Association Francaise de Genie Civil - AFGC [French Association of Civil Engineering]. The fibres are suitably metallic, organic, mineral or mixture thereof. The amount of fibres is generally low, for example from 1 to 8 % by volume.

The concrete may comprise a hydraulic binder and the water/cement ratio (W/C) of the concrete in the fresh state may be suitable less than or equal to 0.34, preferably less than or equal to 0.28, more preferably less than or equal to 0.26.

The cement matrix may comprise cement (Portland), an element having a pozzolanic reaction (in particular silica fume) and a fine sand. For example, the cement matrix may comprise:

Portland cement;

fine sand;

an element of silica fume type;

optionally silica flour;

the amounts being variable and the dimensions of the different elements being selected between the range of the micron or submicron and the millimetre, with a maximum dimension generally not exceeding 5 mm; and

a superplasticizer being generally added with the mixing water. Examples of a cement matrix are described in patent applications EP-A- 518777, EP-A-934915, WO-A-9501316, WO-A-9501317, WO-A-9928267, WO-A- 9958468, WO-A-9923046, WO-A-0158826, WO2008/090481 , WO2009/081277.

Examples of a matrix are the RPCs, Reactive Powder Concretes, whilst examples of UHPFC, Ultra High Performance Fibre-reinforced Concretes are the BSI concretes from Eiffage, Ductal® from Lafarge, Cimax® from Italcementi and BCV from Vicat. A thermal cure may be applied on these concretes. For example, after the hydraulic setting, the thermal cure comprises, heating to a temperature of 60°C or more for several hours, typically 90°C for 48 hours.

The present invention also relates to a process for the preparation of the article as described herein above, comprising the following steps:

- coating walls of a mould with a demoulding composition comprising an aqueous solution comprising at least 1 % of a surfactant;

- introducing the concrete in the fresh state in the mould; and

- removing the article from the mould after the concrete sets.

According to an example of embodiment, the mould has a roughness R_a less than or equal to 10 μηι, preferably less than or equal to 5 μηι, more preferably less than or equal to 1 μηι.

The mould may be of polyvinyl chloride. Advantageously, the parts of the mould in contact with the demoulding composition do not comprise steel, silicone, polyurethane or polyoxymethylene.

The coating of the mould with the demoulding composition may be carried out by known methods, for example by application with a brush, with a rag cloth or roller, by dipping or by spraying, the last application mode being preferred.

The quantity of composition to be applied is selected so as to be sufficient to form a continuous film on the entire surface of the mould intended to be put in contact with the concrete. The thickness of the formed film of the demoulding composition is generally by the order of 1 to 15 micrometres.

Therefore, the quantity of the demoulding composition depends on its viscosity and hence on its formulation. The material and the topology of the mould may also be factors to consider.

By way of example, it is generally sufficient to apply 5 to 15 g/m² of a demoulding composition having a viscosity of approximately 50 mPa.s on a PVC mould. The quantity applied will be greater in absorbent moulds or for a formulation with a higher viscosity.

The present invention also relates to a demoulding composition, for use of the process described herein above, comprising an aqueous solution comprising at least 1 % of the surfactant, preferably at least 5 % of the surfactant, more preferably at least 10 % by weight of the surfactant, for example at least 15 % of the surfactant. The demoulding composition may comprise less than 50 % by weight of the surfactant.

Preferably, the demoulding composition does not comprise oil.

The surfactant may be an electrically neutral compound.

The hydrophilic-lipophilic ratio of a surfactant may be given as the value or

HLB ratio (hydrophilic-lipophilic balance), which is determined according to the Griffin method described in the document « Calculation of HLB Values of Non-Ionic Surfactants » Journal of the Society of Cosmetic Chemists 5 (1954): 259. The HLB ratio of a molecule of a non ionic surfactant is given by the following relation:

HLB = M_h/M x 20

where M is the mass of the molecule of the surfactant and M_h is the mass of the hydrophilic part of the molecule of the surfactant.

According to an example of embodiment of the invention, the surfactant has a hydrophilic-lipophilic ratio less than or equal to 16, preferably less than or equal to 1 1 , more preferably less than or equal to 8,

Examples of surfactants adapted to the demoulding composition according to the invention are alkoxy derivatives based on chains, for example:

- linear or branched, saturated, unsaturated or polysaturated fatty alcohols, (for example lauryl alcohol polyglycol ether 8 EO, tridecyl alcohol polyglycol ether 5 EO, oleic alcohol polyglycol ether 10 EO and C10 Guerbet alcohol polyglycol ether 7 EO);

- linear or branched, saturated, unsaturated or polysaturated fatty acids comprising 6 to 32 carbon atoms (for example oleic acid polyglycol ether 6 EO);

- diesters of fatty acid and polyglycol ether (for example polyethylene glycol 600 dioleate);

-aromatic derivatives, for example tristyryl phenol, phenol and alkyl aryl phenols (for example tristyryl phenol 10 EO and nonyl phenol 8 EO, octyl phenol 7 EO);

- sugar esters and sugar derivatives (for example sorbitan monooleate polyglycol ether 20 EO and sorbitan trioleate polyglycol ether 20 EO);

- polypropylene glycol and polybutylene glycol (for example polymers by EO- PO-EO blocks and the PO-EO polymers);

- polyamines and fatty amines (for example oleylamine polyglycol ether 12

EO); - fatty amides (for example polyglycol coconut amide 7 EO); and

- triglycerides (for example ethoxylated castor oil 40 EO).

In the examples herein above, the symbol EO is understood as ethylene oxide and the symbol PO is understood as propylene oxide.

All the families and products may have an alkyl group or a proplyene oxide group or a butylene oxide group on the terminal hydroxyl group (for example: alcohol C12-14 polyglycol ether (8EO) ter-butyl ether).

The demoulding composition may comprise a mix of at least two different surfactants.

Advantageously, the surfactant may be a ethoxylated fatty alcohol or a mix of ethoxylated fatty alcohols. The ethoxylated fatty alcohol may comprise a lipophilic portion comprising from 6 to 32 carbon atoms, preferably from 8 to 22 carbon atoms, more preferably from 8 to 18 carbon atoms. The ethoxylated fatty alcohol may comprise a hydrophilic portion comprising from 1 to 100 ethoxy groups, preferably from 3 to 30 ethoxy groups, more preferably from 4 to 20 ethoxy groups.

The demoulding may further comprise one or more compounds selected from a stabilizer, a dispersant, a preservative, a thickener and a thixotropic agent.

The invention will be described in more detail in the following examples, given for non-restricting purposes, related to Figure 1 , which illustrates the measurements of the water absorption rates by surfaces of several concretes.

EXAMPLES

The products and materials used in the examples are available from the following suppliers:

Product or material Supplier

(1) White Portland Cement Lafarge-France Le Teil

(2) Grey Portland Cement Lafarge Val d'Azergues

(3) Standard Sand Societe Nouvelle du Littoral

(4) BE01 Sand (D50 at 307 μι ι) Sibelco France

(Bedoin quarry)

(5) DURCAL 1™Limestone filler OMYA

(6) MST Silica fume SEPR (Societe Europeenne

des Produits Refractaires)

(7) Superplasticizer F2 Chryso

(8) Admixture CHRYSO™ Fluid Optima 203 Chryso Formulation of ultra high performance concrete

The formulation (1) of ultra high performance concrete used to carry out the tests is described in the following Table 1 :

Table 1 : Formulation (1) of ultra high performance concrete

The cement used was a white cement produced by Lafarge coming from the Le Teil site. It was of the CEM I 52.5 PMES type according to the EN 197-1 Standard. The superplasticizer F2 was a polycarboxylate polyoxyalkylene in aqueous phase. The water/cement ratio was 0.26. The concrete was an ultra high performance concrete that was not fibre-reinforced.

Method for preparation of the ultra high performance concrete

The ultra high performance concrete according to formulation (1) was produced with a mixer of the RAYNERI type. The entire operation was carried out at 20°C. The method of preparation comprised the following steps:

• At T = 0 second: put the cement, the limestone fillers, the silica fume and the sand in the bowl of the mixer and mix for 7 minutes (15 rpm);

• At T = 7 minutes: add the water and half the mass of admixture and mix for 1 minute (15 rpm);

· At T = 8 minutes: add the remaining admixture and mix for 1 minute (15 rpm);

• At T = 9 minutes: mix for 8 minutes (50 rpm); and

• At T = 17 minutes: mix for 1 minute (15 rpm). • Beginning at T = 18 minutes: pour the concrete horizontally in the mould(s) intended for this operation.

Formulation of a mortar

The formulation (2) of the mortar used to carry out the tests is described in the following Table 2:

Table 2: Formulation (2) of mortar

The cement used was a grey cement produced by Lafarge coming from the Val d'Azergues site. It was of the CEM I 52.5 PMES type according to the EN 197-1 Standard. The sand was a 0/4 mm siliceous sand according to the standard sand disclosed in the standard EN 196-1 entitled « Methods of testing cements - Part 1 : Determination of strenght ». The superplasticizer CHRYSO™ Fluid Optima 203 was a polycarboxylate polyoxyalkylene in aqueous phase. The water/cement ratio was 0.496.

Method for preparation of the mortar

The mortar according to formulation (2) was produced with a mixer of the Perrier type. The entire operation was carried out at 20°C. The method of preparation comprised the following steps:

· put the sand in the bowl of the mixer;

• At T = 0 second: put the prewetting water and mix for 1 minute (140 rpm); • At T = 1 minute: stop the mixing for 4 minutes (140 rpm);

• At T = 5 minutes: add the cement;

• At T = 6 minutes: mix for 1 minute (140 rpm);

• At T = 7 minutes: add the water and mix for 30 seconds (140 rpm); · At T = 7 minutes and 30 seconds: mix for 2 minutes (140 rpm);

• At T = 9 minutes and 30 seconds: pour the mortar in the mould(s) intended for this operation.

Formulation of a cement paste

The formulation (3) of the mortar used to carry out the tests is described in the following Table 3:

Table 3: Formulation (3) of cement paste

The cement used was a grey cement produced by Lafarge coming from the Val d'Azergues site. It was of the CEM I 52.5 PMES type according to the EN 197-1 Standard. The water/cement ratio was 0.388.

Method for preparation of the cement paste

The cement paste according to formulation (3) was produced with a mixer of the Perrier type. The entire operation was carried out at 20°C. The method of preparation comprised the following steps:

• put the cement in the bowl of the mixer;

• At T = 0 second: put the water and mix for 2 minutes (140 rpm);

• At T = 2 minutes: pour the cement paste in the mould(s) intended for this operation.

Method for measurement of the average Ferret diameter of the particles of Ca(OH)? and/or CaCO¾ and of the total surface covered by the particles of Ca(OH)? and/or CaCOs on the surface of a sample made of a hydraulic composition The sample was cut in a cubic specimen the height of which was 1 cm. The surface of the specimen corresponding to the surface of the sample, designated as measurement surface, was covered by black ink to increase the contrast between the particles of Ca(OH)₂ and/or CaC0₃ and the cement paste, the particles of Ca(OH)₂ and/or CaC0₃ appearing lighter than the cement paste. The measurement surface was photographed under a binocular magnifying glass. The particles of Ca(OH)₂ and/or CaC0₃ appeared lighter in the photographs. The image was analyzed using the ImageJ software, which can be downloaded free of charge from the website of the National Centre for Biotechnology Information. Only the particles of Ca(OH)₂ and/or CaC0₃ with a surface greater than 0.1 mm² were taken into account. The number of particles of Ca(OH)₂ and/or CaC0₃ per mm² of the surface of the specimen was measured. The Ferret diameter was determined for each particle, the Ferret diameter corresponding to the greatest distance between two parallel straight lines which were tangent to the contour of the particle. The average Ferret diameter of the particles of Ca(OH)₂ and/or CaC0₃ of the specimen was determined as the average of the Ferret diameters of the particles of Ca(OH)₂ and/or CaC0₃.

Method for measurement of the surface water-absorption rate of a sample made of a hydraulic composition

The test consisted of measuring, over time, the mass of water absorbed by the surface of a sample. This method is described in the documents « Standard Test Method for Measurement of Rate of Absorption of water by Hydraulic-Cement Concretes » ASTM C 1585-04 and « Water Sorptivity of Mortars and Concretes - a Review » Magazine of Concrete Research 41 (147) (1989) : 51-61. The analyzed samples were in the form of parallelepiped plates, having a length of 150 mm, a width of 100 mm and a thickness of 10 mm. The surface of the plate corresponded to the surface to be tested. The other sides of the sample were covered with a waterproof layer, for example a layer of epoxy resin. The tests took place below a temperature of 20°C and relative humidity of 50 %. The dry mass of the sample also had to be constant to carry out the test. The sample was stored for one month at 25° C and at 50 % relative humidity, then was dried in a ventilated drying oven at 45°C for 6 days, until reaching a dry state, then left 30 minutes in the laboratory at ambient temperature before carrying out the test. The initial mass (m₀) of the sample was measured after application of the water-proof layer on the other sides of the sample. The area (a) of the surface of the sample in contact with the water was measured. The sample was overturned and the surface to be tested was immersed in water contained in a receptacle. With this aim, the sample was plunged 3mm into the water in the receptacle. Measurements of the surface water-absorption rate of the sample were carried out at successive time periods. With this aim, at each time period, the sample was removed from the receptacle, dried using a damp cloth, then weighed and replaced in the receptacle. The time periods selected for the tests were the following: 1 , 4, 9, 18, 25, 36, 49, 64, 90 and 230 minutes. The rate of surface water absorption (i) of the sample in water (given in g/m²) was then defined by the following relation:

i = (m_x - m₀ ) / a

where m₀ is the initial mass of the sample with the resin (g),

m_x is the mass of the sample at a given time period (g), and

a is the area of the surface of the sample in contact with the water (m²).

Method for measurement of the surface roughness R₂ of a concrete sample or on the surface of a mould

The measurement was carried out with a roughness meter with a sensor, commercialised by MITUTOYO under the brand name of SURFTEST SJ-201 M. The average roughness parameter (R_a) was measured five times over a distance of 12.5 mm and the roughness value R_a was equal to the average of these five measurements.

Method for measurement of the inclination of the particles of Ca(OH)? and/or CaCO¾

The concrete sample was cut in a cubic specimen the height of which was 1 cm. A surface of the specimen corresponding to the surface of the concrete sample, called measurement surface, was covered by a layer of carbon, then was observed by Scanning Electron Microscopy (SEM) using a Scanning Electron Microscope commercialised by the FEI COMPANY under the brand name of FEI Quanta 400 FEG™ with a 15 kV voltage and a current with 1 mA intensity. The inclination of the particles of Ca(OH)₂ and/or of CaC0₃ was estimated by visual inspection of the obtained images. EXAMPLE 1

The following demoulding compositions were tested.

Demoulding Composition

The percentages were given by mass relative to the total mass of the demoulding composition.

The compound Z was distilled water. The surfactants A, B, C, D, E, F and G were ethoxylated fatty alcohols of formula R-(OC₂H₄)_nOH where R was an aliphatic hydrocarbonated chain comprising m carbon atoms. The values of the numbers m and n are given in Table 5 herein below Table 5 - Surfactant

A concrete sample was made and tested for each demoulding composition. With this aim, a concrete according to formulation (1) was prepared. A mould in polyvinyl chloride (PVC) was used. The mould had a roughness of approximately 1 μηι. The demoulding composition was applied on the internal walls of the mould with 15 grams of the demoulding composition per square metre of the mould. The concrete was poured in the PVC mould approximately 15 minutes after the application of the demoulding composition without stirring. The mould containing the concrete was kept for 20 hours, then the concrete sample was removed from the mould. The sample corresponded to a parallelepiped having a length of 150 mm, a width of 100 mm and a thickness of 10 mm. The sample was stored for 7 days at 25°C and 50 % relative humidity.

The average Ferret diameter and the number of particles of Ca(OH)₂ and/or CaC0₃ per surface unit of the surface of the concrete were measured for each sample according to the methods described herein above. The results are grouped together in Table 6 herein below. Table 6

For the demoulding compositions A10 and A20, the particles of Ca(OH)₂ and/or CaC0₃ covered the surface of the concrete sample quite uniformly in such a way that they could not be counted separately. The thickness of the layer was about 10μηι. For the demoulding composition C20, the roughness of the sample was too high to make a reliable measurement.

When the demoulding composition only comprised distilled water, there was no formation of particles or Ca(OH)₂ and/or CaC0₃ on the surface of the sample. For each demoulding composition, the number of particles of Ca(OH)₂ and/or CaC0₃ per mm² of concrete increased when the concentration of surfactant increased. Furthermore, the number of particles of Ca(OH)₂ and/or CaC0₃ per mm² of concrete increased, for a same concentration of surfactant, when the hydrophobicity of the surfactant increased. For a same concentration of surfactant, the average angle of inclination of the particles of Ca(OH)₂ and/or CaC0₃ relative to the measurement surface decreased when the hydrophobicity of the surfactant increased. This means that the basal sides of the particles of Ca(OH)₂ and/or CaC0₃ tend to be parallel to the measurement surface when the hydrophobicity of the surfactant increases.

EXAMPLE 2

A demoulding composition comprising 20 % by mass of the Lutensol T03 surfactant from BASF relative to the total mass of the demoulding composition was used.

A concrete sample according to formulation (1) was made. A mould of polyvinyl chloride (PVC) was used. The mould had a roughness R_a of approximately 1 μηι. The demoulding composition was applied on the internal walls of the mould with 15 grams of the demoulding composition per square metre of the mould. The concrete was poured in the PVC mould approximately 15 minutes after the application of the demoulding composition. The mould containing the concrete was kept 20 hours, then the concrete sample was removed from the mould. The sample corresponded to a parallelepiped having a length of 150 mm, a width of 100 mm and a thickness of 10 mm. The sample was stored for 7 days at 25°C and 50 % relative humidity.

The following stains were made on the surface of the sample:

-felt pen;

-pen;

-red marker pen;

-green marker pen; and

-pencil.

The surface was then cleaned with a damp cloth. All the stains disappeared. EXAMPLE 3

Four samples of concrete according to formulation (1) were made. The first of them (sample 3A) was moulded with a mould in silicone without using a demoulding agent (the silicone mould had a roughness R_a of approximately 2 μηι). In the three other cases (samples 3B, 3C and 3D), a polyvinyl chloride mould (PVC) was used. The PVC mould had a roughness R_a of approximately 1 μηι. No demoulding agent was applied on the PVC mould in sample 3B. An aqueous demoulding composition comprising 20 % by mass of the Lutensol T03 surfactant from BASF (relative to the total mass of the demoulding composition) was applied on the internal walls of the mould of sample 3C with 15 grams of the demoulding composition per square metre of the mould. An aqueous demoulding composition comprising 20 % by mass of the Lutensol T03 surfactant from BASF and 50% by mass of EC02 oil from Chryso (relative to the total mass of the demoulding composition) was applied on the internal walls of the mould of sample 3D with 15 grams of the demoulding composition per square metre of the mould.

The concrete was then poured without stirring in the different moulds approximately 15 minutes after the application of the demoulding composition. The moulds containing the concrete were kept 20 hours, then the concrete samples 3C and 3D were removed from the mould. Each sample corresponded to a parallelepiped having a length of 150 mm, a width of 100 mm and a thickness of 10 mm.

The surface water-absorption rate was measured for each concrete sample as a function of time according to the previously described method. The results are grouped together in Table 7 herein below.

Table 7

The surface absorption rate of sample 3C, obtained with a PVC mould and with a demoulding composition comprising a mix of water and surfactant, was less than the surface absorption rate of sample 3B obtained with a PVC mould and without using a demoulding agent. This means that sample 3C had reduced open porosity and would absorb less quantity of staining liquid or solid than sample 3B.

The surface absorption rate of sample 3D, obtained with a PVC mould and with a demoulding composition comprising a mix of water, surfactant and oil was higher than the surface absorption rate of sample 3C, obtained with a demoulding composition comprising a mix of water and surfactant. This means that sample 3C had reduced open porosity and would absorb less quantity of staining liquid or solid than sample 3D.

The surface absorption rate of sample 3A, obtained with a silicone mould and without using a demoulding agent was higher than the surface absorption rate of sample 3C, obtained with a demoulding composition comprising a mix of water and surfactant. This means that sample 3C had reduced open porosity and would absorb less quantity of staining liquid or solid than sample 3A.

Figure 1 represents the evolution curves of the water-absorption rates for the surfaces of samples 3A, 3B, 3C and 3D during the measurement period.

EXAMPLE 4

Two samples of mortar according to formulation (2) were made. For the two samples 4A and 4B, a polyvinyl chloride mould (PVC) was used. The PVC mould had a roughness R_a of approximately 1 μηι. No demoulding agent was applied on the PVC mould in sample 4A. An aqueous demoulding composition comprising 20 % by mass of the Lutensol T03 surfactant from BASF (relative to the total mass of the demoulding composition) was applied on the internal walls of the mould of sample 4B with 15 grams of the demoulding composition per square metre of the mould.

The mortar was then poured without stirring in the different moulds approximately 15 minutes after the application of the demoulding composition. The moulds containing the mortar were kept 18 hours, then the mortar samples 4A and 4B were removed from the mould. Each sample corresponded to a parallelepiped having a length of 150 mm, a width of 100 mm and a thickness of 10 mm.

The surface water-absorption rate was measured for each mortar sample as a function of time according to the previously described method. The results are grouped together in Table 8 herein below. Table 8

The surface absorption rates of mortar samples 4A and 4B were superior to the surface absorption rate of sample 3C of Example 3 (table 7) obtained with a ultra high performance concrete according to formulation (1) and with a demoulding composition comprising a mix of water and surfactant. This means that sample 3C had reduced open porosity and would absorb less quantity of staining liquid or solid than mortar samples 4A and 4B.

EXAMPLE 5

Two samples of cement paste according to formulation (3) were made. For the two samples 5A and 5B, a polyvinyl chloride mould (PVC) was used. The PVC mould had a roughness R_a of approximately 1 μηι. No demoulding agent was applied on the PVC mould in sample 5A. An aqueous demoulding composition comprising 20 % by mass of the Lutensol T03 surfactant from BASF (relative to the total mass of the demoulding composition) was applied on the internal walls of the mould of sample 5B with 15 grams of the demoulding composition per square metre of the mould.

The cement paste was then poured without stirring in the different moulds approximately 15 minutes after the application of the demoulding composition. The moulds containing the cement paste were kept 18 hours, then the mortar samples 5A and 5B were removed from the mould. Each sample corresponded to a parallelepiped having a length of 150 mm, a width of 100 mm and a thickness of 10 mm.

The surface water-absorption rate was measured for each cement paste sample as a function of time according to the previously described method. The results are grouped together in Table 9 herein below. Table 9

The surface absorption rates of cement paste samples 5A and 5B were superior to the surface absorption rate of sample 3C of Example 3 (table 7) obtained with a ultra high performance concrete according to formulation (1) and with a demoulding composition comprising a mix of water and surfactant. This means that sample 3C had reduced open porosity and would absorb less quantity of staining liquid or solid than cement paste samples 5A and 5B.

Claims

1. An article comprising:

-particles of calcium dihydroxide covering more than 60 % of the surface.

The article according to claim 1 , wherein more than 80 % of the surface is covered by the particles.

The article according to claim 1 or claim 2, wherein the water absorption rate of the surface covered by the particles is less than 300 g/m² measured 90 minutes after putting the article in contact with the water.

The article according to any one of claims 1 to 3, wherein the average Ferret diameter of the particles is greater than or equal to 150 μηι.

The article according to any one of claims 1 to 4, wherein at least 50 % of the particles correspond to a crystal whose base is inclined by less than 20 degrees relative to the surface.

The article according to any one of claims 1 to 5, wherein the high performance concrete or the ultra high performance concrete is made by mixing cement and water and wherein the water to cement ratio of the high performance concrete or the ultra high performance concrete in the fresh state is less than or equal to 0.34.

A process for preparation of an article according to claims 1 to 6, comprising the following steps:

-coating walls of a mould with a demoulding composition comprising an aqueous solution comprising at least 1 % of a surfactant;

-introducing the high performance concrete or the ultra high performance concrete in the fresh state into the mould; -allowing said high performance concrete or ultra high performance concrete to set thereby forming the article;

-removing the article from the mould.

8. The process according to claim 7, wherein the mould has a roughness R_a less than or equal to 10 μηι.

9. The process according to claim 7 or claim 8, wherein the mould is made of polyvinyl chloride material.

10. The process according to any one of claims 7 to 9, wherein the parts of the mould in contact with the demoulding composition do not comprise steel, silicone, polyurethane or polyoxymethylene.

1 1. The process according to any one of claims 7 to 10, wherein the surfactant is an electrically neutral compound.

12. The process according to any one of claims 7 to 1 1 , wherein the surfactant has a hydro-lipophilic ratio less than or equal to16.

13. The process according to any one of claims 7 to 12, wherein the surfactant is an ethoxylated fatty alcohol.

14. The process according to any one of claims 7 to 13, wherein the demoulding composition comprises more than 10 % by weight of the surfactant.

15. The process according to any one of claims 7 to 14, wherein the demoulding composition does not comprise oil.