WO2013001038A1

WO2013001038A1 - Skin layer reduction in calcium sulfate based binder compositions

Info

Publication number: WO2013001038A1
Application number: PCT/EP2012/062638
Authority: WO
Inventors: Pascal Bailleul; Christophe Guyot
Original assignee: Sika Technology Ag
Priority date: 2011-06-29
Filing date: 2012-06-28
Publication date: 2013-01-03

Abstract

An acidic compound is used for reducing or preventing the formation of a skin layer on a surface of a calcium sulfate based binder composition.

Description

Skin layer reduction in calcium sulfate based binder compositions

Technical field

The invention relates to the use of a compound for reducing or preventing the formation of a skin layer on the surface of a calcium sulfate based binder composition. Additionally, the invention deals with a calcium sulfate based binder composition as well as a method for the production of such a binder composition. Further aspects of the invention relate to a building element and an admixture for preparing a calcium sulfate based binder composition.

Background art

Binder compositions or mortars based on calcium sulfate as binder are widely used for making coating layers or screeds in building industry. A screed is usually a thin and smooth layer of material put on a structural building element, such as e.g. a floor. On top of the screed, a finishing material or cover layer can be applied. Thus, the screed acts as an interface layer between the building element and the finishing material or cover layer.

A common problem encountered in this field is the formation of a thin glassy or powdered boundary layer at the surface of the screed upon setting of the fresh binder composition. This boundary layer, usually referred to as skin layer or laitance, has a thickness of a few hundredths of millimeters and is formed within the first few hours during drying of the binder composition.

The formation of the skin layer is mainly caused by lime or Ca(OH)₂ often being present in calcium sulfate binders. This is because calcium sulfate binders are often by-products of hydrofluoric acid production or of desulphurization of flue gases from thermal power plants. Thereby, lime is generally put in slight excess in order to assure proper neutralization. This excess lime, typically about 1 wt.-%, remains in the calcium sulfate binder.

When using such kind of calcium sulfate binder for the preparation of screeds, the water soluble lime follows the moving water which migrates towards the surface of the drying screed. At the interface between the surface of the screed and ambient air, the lime precipitates or deposits because it is too heavy to evaporate from the surface. The precipitated lime might also react with C0₂ contained in air and generate calcium carbonate or CaC0₃. The later is a white powder, visible at the surface and linked to the known problem called "efflorescence".

Apart from aesthetic considerations, the skin layer of precipitated lime at the surface of the screed features only low cohesion and very low adherence to the surface of the screed. This in turn negatively affects the adherence of the finishing material or the cover layer that is put on top of the screed. In particular, if glue is applied on the surface of the screed in order to fix a finishing material, the skin layer will prevent a strong adherence between screed and glue. Thus, when submitting the glue to a strong traction, rupture will appear within the skin layer or in between the skin layer and the screed body. Such kind of rupture is also called an adhesive rupture.

In order to obtain good adherence, extensive brushing or sanding of the screed surface is required before applying the glue. In this case, when submitting the glue to a strong traction, rupture will appear within the screed itself. This is also called a cohesive rupture. However, this additional brushing and sanding step is

impractical, time-consuming and costly. Moreover, it also appears not to be fully satisfactory because making sure that the skin has been correctly disposed is difficult to check in the context of routine building work.

Several chemical methods aiming at solving the problems associated with the presence of a skin layer on top of screeds have been described in the prior art. Nevertheless, these known methods all have disadvantages.

For example, FR 2 928 915 (Lafarge Platres) describes the use of an ethoxylated or non ethoxylated fatty alcohol having 12 to 40 carbon atoms, as additive to avoid the formation of a skin on the surface of the screed. However, these fatty alcohols are not liquid at room temperature. Therefore, a complicated process (heating) is needed to mix them with a mineral powder before being introduced in the formulation of the screed on the field.

EP 1 801 085 (Lafarge Platres) relates to the use of 0.3 to 3.0% of aluminum sulfate as an additive in the formulation of a floor screed based on calcium sulfate. The so formed screed is essentially skin-free. But, aluminum sulfate cannot be used alone. To achieve suitable workability, the formulation of the calcium sulfate screed must additionally include well chosen and specific additives. Nevertheless, acceptable fluidity for self-leveling screeds is hardly achievable with this solution. There is thus a need to develop improved or alternative methods for solving the problems associated with the formation of a skin layer on the surface of calcium sulfate based binder compositions.

Disclosure of the invention

It is an object of the present invention to provide improved methods or

compositions for reducing or preventing the formation of a skin layer, especially comprising lime and/or carbonate, on a surface of a calcium sulfate based binder composition or screed, respectively. Another object is to enhance the adhesion behavior of a surface of a calcium sulfate based binder composition.

Surprisingly, it has been found that these objects are achieved by the features of independent claim 1 . Thereby, the core of the invention is the use of an acidic compound as additive for calcium sulfate based binder compositions. This allows for reducing or even preventing the formation of a skin layer. As well, the use of an acidic compound enhances the adhesion behavior of a surface of a calcium sulfate based binder composition. These advantageous effects can essentially be achieved without significantly affecting the flowability or workability of freshly prepared binder compositions.

As could be shown, the use of an acidic compound allows preparing self-leveling calcium sulfate based binder compositions and/or screeds which are essentially skin free after drying or hardening. Consequently, it is not anymore necessary to sand or brush the surface of the hardened calcium sulfate based binder composition or the screed, respectively. Thus, it is possible to obtain good adhesion between the hardened calcium sulfate based binder composition and a finishing material or cover layer put on top of it. This even without the need for any further surface treatment of the calcium sulfate based binder composition. As a result, there is essentially no risk of delamination of the finishing material or cover layer put on top of the calcium sulfate based binder composition. Put differently, the inventive use of an acidic compound enhances the adhesion behavior of a surface of a calcium sulfate based binder composition and/or a screed. Thus, the acidic compound can be used for enhancing the adhesion behavior of a surface of a calcium sulfate based binder composition as well.

Moreover, the inventive use of an acidic compound significantly reduces time and costs in the preparation of screeds, in particular in the preparation of floor screeds on building elements. Firstly, this is because no sanding or brushing is required. Secondly, the use of an acidic compound allows the preparation of binder compositions with high flowability or workability being essentially self-leveling.

Additional aspects of the invention are subject of further independent claims.

Particularly preferred embodiments are outlined throughout the description and the dependent claims.

Ways of carrying out the invention A first aspect of the invention relates to the use of an acidic compound for reducing or preventing the formation of a skin layer on a surface of a calcium sulfate based binder composition.

In the present context, the expression "skin layer" in particular stands for a layer comprising or consisting of lime and/or calcium carbonate. In particular, an amount of lime and calcium carbonate together is at least 5 wt.-%, in particular at least 10 wt.-%, especially at least 25 wt.-% or at least 50 - 100 wt.-% with respect to the total weight of the skin layer. Especially, the skin layer has a thickness of 0.001 - 1 .0 mm, in particular 0.01 - 0.1 mm.

The term "lime" stands for Ca(OH)₂, whereas "calcium carbonate" stands in particular for CaC0₃. An "acidic compound" is in particular a compound that reduces the pH of water or increases the acidity of water when added to pure water. The acidic compound can be a pure or single chemical compound as well as a mixture of two or more acidic compounds. Generally, the acidic compound can be an organic acid, e.g. a carboxylic acid, and/or an inorganic acid, e.g. a mineral acid.

A "binder component" represents a mineral material being capable of undergoing a hydration reaction with water, resulting in the formation of solid hydrates or hydrated phases.

With the expression "calcium sulfate based binder composition" is meant a composition comprising calcium sulfate as a main binder component. Thereby, calcium sulfate forms the major part of all binder components of the composition. However one ore more other components, e.g. potassium sulfate, aluminium sulfate, calcium aluminate cements, calcium sulfo-aluminate cements, can be present additionally. Preferably, an amount of calcium sulfate in the calcium sulfate based binder composition is at least 50 wt.-%, in particular at least 75 wt.- %, especially at least 90 wt.-%, with respect to the total weight of all binder

components in the calcium sulfate based binder composition. Especially, calcium sulfate is the only binder component of the calcium sulfate based binder composition. Apart from binder materials, further components, such as e.g.

aggregates, additives and/or water, can optionally be present in the calcium sulfate based binder composition. A calcium sulfate based binder composition comprising optional aggregates can also be called a calcium sulfate based mortar

composition.

The calcium sulfate may in principle be any type of calcium sulfate being capable of undergoing a hydration reaction with water. Thus, the calcium sulfate can be hemi-hydrated calcium sulfate, in particular of alpha and/or beta type, coming from natural and/or synthetic sources. As well, the calcium sulfate can be anhydrous calcium sulfate or anhydrite, for example type II or III anhydrite, obtained from synthetic sources, natural quarries and/or by calcination of natural or synthetic gypsum, respectively. Although calcium sulfate can be used in various forms described above, anhydrous calcium sulfate or anhydrite is the preferred form, in particular thermal anhydrite or synthetic anhydrite or a mixture of both. Since thermal and synthetic anhydrites usually comprise a certain amount of lime originating from the manufacturing process of said anhydrites, carbonation and therefore the formation of a skin layer on top of the setting calcium sulfate based binder composition or mortar is particularly problematic with these forms of calcium sulfate. Thus, according to a preferred embodiment, the calcium sulfate in the binder composition comprises or consists of thermal or synthetic anhydrite. In particular, a weight proportion of anhydrite to all other modifications of calcium sulfate is at least 50 wt.-%, preferably at least 75 wt.-%, more preferably at least 90 wt.-%.

Particularly, the calcium sulfate and/or the calcium sulfate based binder

composition comprises lime or Ca(OH)₂. Especially, an amount of lime or Ca(OH)₂ with respect to the calcium sulfate is 0.01 - 5 wt.%, preferably 0.5 - 3 wt.-%, more preferably 0.7 - 2 wt-%, in particular 1 - 1 .5 wt.-%. Especially, the amount of lime or Ca(OH)₂ with respect to the calcium sulfate is about 1 wt.-%.

In particular the calcium sulfate based binder composition is used for preparation of a screed on a building element. For example, the building element is a basement, a floor, of a building. Most preferably, the calcium sulfate based binder composition is used in preparation of a floor screed.

In particular, the acidic compound is intermixed with the calcium sulfate based binder composition being present in fluid and/or pourable state. For example, a wet calcium sulphate binder directly after preparation with water is in a fluid state. A dry calcium sulphate binder in powder form is regarded as pourable. Thereby, the acidic compound is essentially homogeneously distributed within the calcium sulfate based binder composition resulting in an optimal effect of the acidic compound.

In particular, the acidic compound is water soluble. Advantageously the solubility of the acidic compound in pure water at 20^QC is at least 40 g/l (grams per liter). Such kind of acidic compounds can easily be intermixed with the calcium sulphate binder resulting in a more uniform distribution. However, acidic compounds with a lower solubility might be used as well, if additional measures are taken in order to achieve a homogeneous distribution of the acidic compound. Preferably, the acidic compound is a rather small compound. Especially, a molecular weight of the acidic compound of 30 - 200 g/mol, in particular of 50 - 100 g/mol, turned out to be advantageous. Nevertheless, smaller or larger compounds are possible as well.

According to an advantageous embodiment, a pK_s or -log-i₀K_s of the acidic compound is less than 10, preferably, less than 5, most preferably less than 4. Such kind of acidic compounds have shown to effectively reduce or prevent the skin layer formation. In this context, "pK_s" stands for the negative logarithm to the base 10 of the acid dissociation constant K_s. The acid dissociation constant K_s (also known as acidity constant or acid-ionization constant) represents a well known quantitative measure of the strength of an acidic compound in water under standard conditions.

Furthermore, advantageously, the pK_s of the acidic compound is at least 1 , preferably at least 2, more preferably at least 3. Such kind of acidic compounds are easier and safer to handle as well as more convenient to dose.

According to a preferred embodiment, the acidic compound has a pK_s in the range of 1 - 10, especially 2 - 5, in particular 3 - 5, preferably 3 - 4.

Advantageously, the acidic compound is a non-chelating compound for calcium ions or does not form chelates with calcium ions. Furthermore, preferably, the acidic compound is not used in combination with a chelating compound for calcium ions. Chelates are polydentate coordination complexes between a metal ion and one or more ligands. In particular, the acidic compound is a compound that does not comprise a carboxylic acid group in β-position to another carboxylic acid group. Specifically, the acidic compound does not comprise citric acid, tartaric acid, succinic acid, maleic acid and/or fumaric acid. Moreover, the acidic compound in particular is not used in combination with these substances. As well, the acidic compound is preferably free of an amine group and/or a salt thereof. Especially, the acidic compound does not comprise or consist of an amino acid and/or is not used in combination with such substances. Such kind of substances may induce too much delay in setting of calcium sulfate based binder compositions and/or reduce workability of the screed. Preferably, the acidic compound is selected amongst carboxylic acids and/or inorganic acids. Highly preferred carboxylic acids are monocarboxylic acids. These are carboxylic acids bearing exactly one carboxylic group per molecule. Especially, the acidic compound is an cc-hydroxy carboxylic acid. These kind of carboxylic acids are naturally available and usually environmental friendly.

Preferably, the acidic compound is glycolic acid, lactic acid, acetic acid and/or sulfuric acid. In particular, the acidic compound is lactic acid.

As could be shown, the acidic compound is advantageously used in an amount of 0.01 - 5 wt.-%, especially 0.05 - 1 wt.-%, preferably 0.1 - 0.5 wt.-%, with respect to the calcium sulfate in the binder composition. Such dosages have been found optimal in view of the reduction or prevention of the formation of skin layers. This even without significantly affecting the functionality of other additives, such as e.g. plasticizers. Nevertheless, for specific purposes, lower or higher dosages might be useful as well.

Particularly, the amount of acidic compound used is chosen such that the formation of a skin layer on the surface of the calcium sulphate based binder composition is reduced or prevented. Especially, the amount of acidic compound used is selected depending on the amount of lime or Ca(OH)₂ present in the calcium sulphate based binder composition.

Preferably, the acidic compound is used in an amount of at least 10 wt.-%, preferably at least 15 wt.-%, especially at least 20 wt.-%, with respect to the amount of lime or Ca(OH)₂ present in the calcium sulphate based binder composition. Advantageously, the acidic compound is used in an amount of 10 - 150 wt.-%, preferably 10 - 1 10 wt.-%, more preferably 15 - 50 wt.-%, especially 20 - 25 wt.-%, with respect to the amount of lime or Ca(OH)₂ present in the calcium sulphate based binder composition.

According to a further advantageous embodiment, the acidic compound is used in combination with a plasticizer. With the help of a plasticizer, the workability of the calcium sulfate based binder composition directly after preparation with water can be improved. As well, a plasticizer allows reducing the amount of water required. In a further preferred embodiment, the acidic compound is premixed with the plasticizer before mixing with the calcium sulfate binder.

Preferably, the plasticizer is a comb polymer, in particular a comb polymer with polycarboxylate backbone and polyether side chains. The polyether side chains, which are in particular hydrophilic, are preferably connected to the backbone via ester, ester, amide and/or imine groups. These kinds of plasticizers have been proven to be highly compatible with the acidic compounds and allow for an effective water reduction in calcium sulfate based binder compositions.

Especially, the comb polymer comprises or consists of:

a) a molar parts of a structural unit S1 of formula I

b) b molar parts of a structural unit S2 of formula II

molar parts of a structural unit S3 of formula III

d) d molar parts of a structural unit S4 of formula IV

each M independently of the others represents H⁺, an alkali metal ion, an alkaline earth metal ion, a di- or trivalent metal ion, an ammonium ion or an organic ammonium group,

each R^u independently of the others represents Hydrogen or a methyl group, each R^v independently of the others represents Hydrogen or COOM, m = 0, 1 or 2,

p = 0 or 1 ,

each R¹ and each R² independently of the others stand for d- to C₂o-alkyl, - cycloalkyl, -alkylaryl or for -[AO]_n-R⁴,

whereby A = C₂- to C₄-alkylene, R⁴ represents H, C to C₂o-alkyl, - cyclohexyl or -alkylaryl,

and n = 2 - 250,

each R³ independently of the others represents NH₂, -NR⁵R⁶, -OR⁷NR⁸R⁹, with R⁵ and R⁶ independetly of each other stand for d- to C₂o-alkyl, -cycloalkyl

-alkylaryl or -aryl,

or for a hydroxyalkyl- or acetoxyethyl- (CH₃-CO-0-CH₂-CH₂-) or hydroxyisopropyl- (HO-CH(CH₃)-CH₂-) or acetoxyisopropyl group (CH₃-CO-0-CH(CH₃)-CH₂-);

or R⁵ and R⁶ together form a ring of which the nitrogen is part, to form a morpholine or imidazoline ring,

R⁷ is a C₂ -C₄-alkylene group,

each R⁸ and R⁹ independently of each other represent d- to C₂₀- alkyl, -cycloalkyl, -alkylaryl,

-aryl or a hydroxyalkyl group,

and whereby a, b, c and d stand for the molar parts of the structural units S1 , S2, S3 und S4, with

a/b/c/d/ = (0.1 - 0.9) / (0.1 - 0.9) / (0 - 0.8) / (0.0 - 0.8),

in particular a/b/c/d = (0.3 - 0.9) / (0.1 - 0.7) / (0 - 0.6) / (0.0 - 0.4) and with the provision that a + b + c + d = 1 .

Thereby, comb polymers with n = 8 - 200, more preferably n = 1 1 - 150, most preferably n = 20 - 70, turned out to be highly advantageous.

In particular, the molar parts of the structural units S1 , S2, S3 und S4 are chosen as follows: a/b/c/d = (0.1 -0.9)/(0.1 -0.9)/(0-0.5)/(0-0.1 ), preferably a/b/c/d = (0.1 - 0.9)/(0.1 -0.9)/(0-0.3)/(0-0.06). Furthermore, comb polymers with c + d > 0 have proven to be highly suitable in the present context.

According to an especially advantageous embodiment R^u represents a methyl group, R^v stands for Hydrogen, m = 0, p = 1 and R¹ represents -[AO]_n-R⁴ with n = 20 - 70.

Preferably, A in -[AO]_n-R⁴ represents C₂-alkylene.

Especially, a weight average molecular weight (Mw) of the comb polymer is 5Ό00 - 150Ό00 g/mol, in particular 10Ό00 - 100Ό00 g/mol, most preferably 20Ό00 - 40Ό00 g/mol. The weight average molecular weight is in particular measured using gel permeation chromatography (GPC) with polyethylenglycol (PEG) as standard.

In particular, the structural units S1 , S2, S3, and S4 together have a combined weight of at least 50 wt.-%, in particular at least 90 wt.-%, preferably at least 95 wt.-%, of the total weight of the comb polymer.

A second aspect of the present invention deals with a calcium sulfate based binder composition, comprising calcium sulfate binder and an acidic compound. The calcium sulfate is in particular calcium sulfate as described above.

As well, the acidic compound is in particular an acidic compound as described above. Most preferred, the acidic compound is glycolic acid, lactic acid, acetic acid and/or sulfuric acid, preferably lactic acid.

Especially, the acidic compound is present in an amount of 0.01 - 5 wt.-%, especially 0.05 - 1 wt.-%, preferably 0.1 - 0.5 wt.-%, with respect to the calcium sulfate.

Preferably, the calcium sulfate based binder composition comprises a plasticizer. The plasticizer is preferably a plasticizer as described above. A highly preferred plasticizer is a comb polymer with polycarboxylate backbone and polyether side chains, in particular comprising two or more of the structural units S1 - S4

described above.

Advantageously, an amount of plasticizer is 0.1 - 10 wt.-%, especially, 0.5 - 5 wt.- %, with respect to the total amount of binder material in the binder composition.

Furthermore, the calcium sulfate based binder composition can optionally contain aggregates, in particular sand. In this case, the calcium sulfate based binder composition can e.g. be used as mortar. Advantageously, an amount of

aggregates is about 100 - 300 wt.-%, in particular 150 - 250 wt.-%, with respect to the total binder content of the calcium sulfate based binder composition.

The calcium sulfate based binder composition, with or without aggregates, can be present as a storage stable dry mixture. As well, the calcium sulfate based binder composition, with or without aggregates, can be in the form of a humid or wet mixture. Especially, the calcium sulfate based binder composition is a flowable mixture, in particular a self levelling mixture.

Thus, according to a preferred embodiment, the calcium sulfate based binder composition comprises water, in particular with a weight ratio of water to calcium sulfate binder of 0.3 - 0.7, preferably 0.4 - 0.5.

A third aspect of the present invention deals with a building element comprising a hardened calcium sulfate based binder composition. The building element can e.g. be a base or a floor. Preferably, the building element is a floor and/or a floor screed. The hardened calcium sulfate based binder composition is in particular a calcium sulfate based binder composition as described above which has hardened due to hydration reactions after mixing with water. Especially, the hardened calcium sulfate based binder composition additionally comprises a plasticizer, in particular a comb polymer, and aggregates, especially sand.

A forth aspect of the present invention deals with a method for preparing a calcium sulfate based binder composition as described above. Thereby an acidic compound is intermixed with calcium sulfate. Optionally, aggregates, plasticizer and/or water are admixed as well. The acidic compound and the calcium sulfate are in particular the acidic compound and the calcium sulfate as described above. According to an especially advantageous embodiment, the acidic compound is premixed with a plasticizer and subsequently intermixed with the calcium sulfate. Thereby, the plasticizer is in particular the plasticizer as described above.

A fifth aspect of the present invention relates to an admixture for a calcium sulfate based binder composition, comprising a plasticizer and an acidic compound. The admixture is in particular used for preparing a screed, especially a floor screed, most preferably a self-leveling floor screed. Thereby, the acidic compound and the plasticizer are in particular the acidic compound and the plasticizer as described above. Such kind of admixtures can be used as easy to use additives for calcium sulfate based binder composition. In particular, the preparation of calcium sulfate based binder composition can be simplified with such an admixture. Thereby, in particular, the plasticizer is a comb polymer with polycarboxylate backbone and polyether side chains and the acidic compound is glycolic acid, lactic acid, acetic acid and/or sulfuric acid, preferably lactic acid. Highly preferred, the comb polymer is the one described above.

Especially, a weight ratio of plasticizer to acidic compound in the additive is 20:1 - 1 :20, preferably, 5:1 - 1 :5, more preferably, 2:1 - 1 :2, in

particularl .5:1 - 1 :1 .5. However, the ratio might e.g. be different if the plasticizer and/or the acidic compound is used in diluted form.

Advantageously, the admixture is in the form of a solution or dispersion, in particular in water. This helps in obtaining a homogeneous mix with the calcium sulfate based binder. Preferably, the admixture contains water, especially 40 - 95 wt.-%, preferably 50 - 90 wt.-%, most preferably 60 - 80 wt.-%.

Further advantageous configurations of the invention are evident from the exemplary embodiments. Exemplary embodiments

1 . Preparation of binder compositions and screeds

All the examples are using the following binder composition or self-levelling screed formulation according to table 1 :

Table 1

The anhydrite used is from an industrial source and contains around 1 wt.-% of lime or Ca(OH)₂, respectively. As superplasticizer, Viscocrete TEMPO 1 2, available from Sika (France), was used. This is a plasticizer based on a comb copolymer with polycarboxylate backbone and polyether side chains. The amount of superplasticizer is adjusted such as to provide an initial spread of 250 mm. Its dosage is expressed in wt.-% relative to the amount of anhydrite.

Fresh screeds are produced in batches of 20 liters in a usual concrete drum mixer of the type used in practice, run with a rotary speed is 25 rounds per minute.

Thereby, the acidic compound and the superplasticizer are first mixed with the water (either separately or in premixed form). Then, the anhydrite is added under agitation, followed by the addition of the sand over a period of 2 minutes. After 8 minutes of agitation (i.e. 1 0 minutes after addition of the anhydrite), the initial flow table spread value and amount of entrapped air are measured.

2. Testing procedures

To assess the performance of the calcium sulfate binder compositions or screed formulations, the following parameters were determined:

- Flow table spread values were assessed according to the CSTB technical approval n °1 3/09-1 063 at different times after preparation of screeds. These values characterize the ability of the fresh screed to retain its fluidity over time (evolution of the spread over time)

- Compressive strength of the screed at 7 and 28 days were measured on

4x4x 1 6 cm prisms according to the standard EN 1 96-1

- Dimensional variation evolution (shrinkage/expansion evolution) up to 28 days have been measured on 4x4x 1 6 cm prisms according to standard NF P 1 5-433

- Pull-off strengths at 42 days (= 28 days + 1 4 days) have been measured

according to EN 1 348 with tiles glued on a screed surface with a standard tile adhesive for anhydrite. The first period of 28 days is for drying/hardening of the formed screed, the second period of 1 4 days is for the drying of the anhydrite tile adhesive. In practice, the bond strength should be over 1 MPa, with a cohesive rupture in the screed. Residual humidity in the screed should be not more than 1 % to carry out the bond strength measurement - The surface of the hardened binder composition or screed has been checked visually. In particular, the surface should be free of cracks or deposits.

All measurements were done at 21 °C and 60% relative humidity.

2. Examples with separate addition of additives 2.1 Lactic acid at different dosages

Table 2 gives an overview of the properties of calcium sulfate binder compositions and screeds produced thereof with direct addition of the acidic compound into the water for preparing the binder composition at different dosages. In this set of experiments, the acidic compound is not premixed with e.g. the plasticizer before intermixing with the water. That is, the acidic compound and the plasticizer are separately put into the water for preparing the binder composition. The acidic compound used in this series is pure lactic acid (commercially available from different suppliers). The calcium sulfate is of the anhydrite type (> 95 wt.-% anhydrite) and contains around 1 wt.-% of lime.

Table 2

Unit Ref1 A1 A2 A3 A4

SIKA wt.-% on 0.90 0.80 0.80 0.80 1 .10

Viscocrete anhydrite

TEMPO 12

Lactic acid wt.-% on 0 0.10 0.15 0.20 0.24 (pure) anhydrite

Flow Table mm

Spread

0' 250 230 235 235 265

30' 255 235 240 245 265

60' 250 230 240 230 265

90' 245 220 225 220 260

120' 230 205 220 220 260

150' 220 210 210 250

180' 210 245

Entrapped air Volume %

0' 1 .4 1 .2 1 .3 1 .2 1 .1

Compressive MPa

strength

7 days 14.5 22.7 22.5 20.6 18.3

28 days 21 .4 33.2 33.1 33.0 31 .0

Dimensionnal μηι/ηι

variations

7 days -108 48 52 27 -19

14 days -290 -33 19 23 -88

21 days -271 -60 56 4 -60

28 days -306 -75 0 6 -75

Residual wt.-%

humidity

28 days 1 0.6 1 1 1

Bond strength MPa

28+14 days 1 .2 1 .4 1 .7 1 .5 1 .15

Failure mode adhesive cohesive cohesive cohesive cohesive

Skin layer visible not not not not visible visible visible visible

These results clearly show the benefit of using lactic acid in the formulation of a calcium sulfate based binder composition or skin-free screed. After 28 + 14 days of ageing, the reference formulation (Rett ) displays a bad behavior in terms of adherence. When pulling out the tile which was glued onto the surface of the screed, an adhesive failure is observed. This is indicative of the presence of a weak boundary layer, a skin, at the surface of the screed. The formation of a skin on a free surface of the reference formulation Rett can also visually be observed by eye in the form of a smooth and glossy layer being present on the surface. In contrast to the reference formulation, formulations A1 - A4 with 0.1 to 0.24 wt.- % lactic acid with respect to the total amount of the anhydrite only show the desired cohesive failure. Visual inspection did not reveal skin layer formation. The surface is matt and rough. Furthermore, the retention of fluidity over time (spread measurements) for formulations A1 - A4 is very good and remains similar to the reference (Ref1 ) at a high level. As can be concluded from the experiments, lactic acid dosages in the range of 0.1 -0.24 wt.-% on binder do not negatively impact the effect of the superplasticizer. Thus, there is no need to increase the dosage of the

superplasticizer in order to keep the same level of fluidity.

2.2 Different acidic compounds

Table 3 gives an overview of the properties of calcium sulfate binder compositions and screeds produced thereof with direct addition of different acidic compounds into the water for preparing the binder composition. The acidic compounds used in this series are pure acetic acid (formulation B1 ), sulfuric acid (formulation B2), tartaric acid (formulation B3) and citric acid (formulation B4). All of these acidic compounds are commercially available from different suppliers. The calcium sulfate or anhydrite is the same as used for the formulations A1 - A4 of table 2.

Table 3

Unit Ref1 B1 B2 B3 B4 wt.-% on 0.90 1 .15 1 .30 0.90 0.95

SIKAVISCOCRETE anhydrite

TEMPO 12

Acid wt.-% on 0 0.1 % 1 .32 1 .0 0.17 anhydrite

Acetic Sulfuric Tartaric Citric acid, acid, acid, acid, pure pure pure pure

Flow Table Spread mm Too Too viscous viscous

0' 250 235 227 145 n.m.

30' 255 235 235 205 n.m.

60' 250 230 255 192 n.m.

90' 245 230 254 n.m.

120' 230 225 240 n.m.

150' 220 215 230 n.m.

180' 210 225 Entrapped air Volume

%

0' 1 .4 1 .3 1 .0 1 .0 n.m.

Compressive MPa

strength

7 days 14.5 15.2 13.3 n.m. n.m.

28 days 21 .4 22.2 24.3 n.m. n.m.

Dimensional μηι/ηι

variations

7 days -108 108 -433 n.m. n.m.

14 days -290 8 -515 n.m. n.m.

21 days -271 -44 n.m. n.m. n.m.

28 days -306 -17 -560 n.m. n.m.

Residual humidity wt.-%

28 days 1 0.7 0.9 n.m. n.m.

Bond strength MPa

28+14 days 1 .2 1 .05 1 .1 n.m. n.m.

Failure mode adhesive cohesive cohesive

Skin layer visible not not

visible visible

n.m. = not measured

As can been deduced from the results presented in table 3, acetic acid

(formulation B1 ) as well as sulfuric acid (formulation B2) can be used as well as acidic compounds in order to achieve an essentially skin-free anhydrite screed. With both formulations, the failure mode can be changed from adhesive to cohesive.

On the other hand, tartaric acid and citric acid as acidic compounds are detrimental to the behavior of the fresh binder compositions or screed

formulations, respectively. In particular, formulations with tartaric acid (formulation B3) and citric acid (formulation B4) (both of them being dicarboxylic acids comprising carboxylic acid groups in β-position to another carboxylic acid group), are too stiff for screed applications and can not be used as self-levelling screeds.

3. Examples with addition of premixed additives 3.1 Admixture

An admixture AM has been prepared by premixing the plasticizer and the acidic compound. Specifically, 76 wt.-% Sika Viscocrete TEMPO 12 has been mixed with 24 wt.-% a lactic acid solution (80 wt.-% lactic acid in water).

The admixture AM has turned out to be storage stable for several months.

3.2 Admixture AM at different concentrations

Table 4 gives an overview of the properties of calcium sulfate binder compositions and screeds produced thereof. Hereby, admixture AM (see chapter 3.1 ) is added to the water for preparing the binder composition. Thus, in this set of experiments, the premixed acidic compound and the plasticizer are put together into the water for preparing the binder composition. The anhydrite is the same as used in the previous formulations.

Table 4

Unit Ref1 A3 A4 G1 G2

SIKA wt.-% on 0.90 0.8 1 .10

VISCOCRETE anhydrite

TEMPO 12

Lactic acid wt.-% on 0 0.2 0.24

(pure) anhydrite

Admixture AM wt.-% on 1 .0 1 .25 anhydrite

Flow Table mm

Spread

0' 250 235 265 240 250

30' 255 245 265 240 260

60' 250 230 265 230 260

90' 245 220 260 225 255

120' 230 220 260 225 255

150' 220 210 250 205 250

180' 210 245 245

Entrapped air Volume

0' 1 .4 1 .2 1 .1 1 .1 1 .0 Compressive MPa

strength

7 days 14.5 20.6 18.3 22.1 20.7

28 days 21 .4 33.0 31 .0 29.9 32.0

Dimensional μηι/ηι

variation

7 days -108 27 -19 +81 +65

14 days -290 23 -88 -75 -40

21 days -271 4 -60 -98 -71

28 days -306 6 -75 -152 -96

Residual wt.-%

humidity

28 days 1 1 1 0.8 0.6

Bond strength MPa

28 days 1 .24 1 .47 1 .15 1 .2 1 .6

Failure mode adhesive cohesive cohesive cohesive cohesive

Skin layer visble not not not not visible visible visible visible

Ref1 and formulations A3 and A4 (cf. table 2) have been included into table 4 for reasons of clarity.

It can be seen from the results in table 4 that preblending of lactic acid with the superplasticizer prior to mixing the blend with the other ingredients of the calcium sulfate compositions or screed (formulations G1 and G2) leads to similar performances than the separate addition of lactic acid and superplasticizer into the screed mixer (formulations A3 and A4).

Increasing the dosage of admixture AM enhances the properties of the binder composition or screed (cf. formulation G1 versus formulation G2).

3.3 Admixture AM with different anhydrites

Table 5 gives an overview of the properties of calcium sulfate binder compositions and screeds produced thereof. As in chapter 3.2, admixture AM (see chapter 3.1 ) is added to the water for preparing the binder composition. As well, the premixed acidic compound and the plasticizer are put together into the water for preparing the binder composition. In this series, the calcium sulfate is of the anhydrite type (> 95 wt.-% anhydrite) but with lime in different concentrations of 1 and 1 .5 wt.-%. Moreover, the calcium sulfate used in this series is from a different batch than the calcium sulfate used for the previous experiments. Thus, mechanical properties are not directly comparable.

Table 5

Unit Ref2 C1 Ref3 C2

Ca(OH)₂ in wt.-% on 1 1 1 .5 1 .5

anhydrite anhydrite

SIKA wt.-% on 0.80 0.80

VISCOCRETE anhydrite

TEMPO 12

Admixture AM wt.-% on 1 .0 1 .0

anhydrite

Flow Table mm

Spread

0' 235 230 235 230

30' 240 230 245 235

60' 240 230 250 235

90' 235 225 240 230

120' 220 225 235 220

150' 215 210 220 210

180' 200 200 205 205

Entrapped air Volume

%

0' 1 .4 1 .1 1 .1 1 .2

Compressive MPa

strength

7 days 22.0 29.8 22.9 30.7

28 days 32.1 45.2 36.7 45.7

Dimensional μηι/ηι

variation

7 days -81 -15 -48 +21

14 days -102 -52 -156 -40

21 days -125 -94 -140 -225

28 days -306 -65 -219 -231 Residual wt.-%

humidity

28 days 0.6 0.8 0.9 0.7

Bond strength MPa

28+14 days 1 .1 1 .7 1 .2 1 .7

Failure mode adhesive cohesive adhesive cohesive

Skin layer visible not visible not

visible visible

Ref2 and Ref3 are reference formulations containing only superplasticizer but no admixture AM or acidic compounds. Formulations C1 and C2 comprise admixture AM which in turn contains the acidic compound (lactic acid) as well as

superplasticizer (Sika Viscocrete TEMPO 12).

As can be seen from table 5, lactic acid (originating from admixture AM) in formulations C1 and C2, leads to a cohesive mode of rupture (desired). As well, the bond strength is clearly enhanced in this case. These findings are even true at high level of Ca(OH)₂ in the anhydrite (formulation C2). If lactic acid is missing, the mode of rupture is adhesive (undesired) and the bond strength is remarkably lower.

4. Comparative examples based in other additives

For reasons of comparison, several formulations using other additives instead of an acidic compound have been prepared. The mode of preparation as well as the calcium sulfate used in this series is essentially the same as with formulations A1 - A4.

4.1 Aluminum sulfate

Table 6 gives an overview of different formulations D1 - D4 using aluminium sulfate instead of an acidic compound.

Table 6

Unit Ref1 D1 D2 D3 D4

Aluminium wt.-% 0 0.3 0.5 0.7 1 .0 sulfate on

(expressed in anhydr

dry AI₂(SO₄)₃ ite )

Sika wt.-% 0.90 1.10 1.20 1.40 1.40

Viscocrete on

TEMPO 12 anhydr

ite

Total Water Liter/m 295 305 310 320 335

3

Flow Table mm

Spread

0' 250 229 226 253 207

30' 255 222 220 227 200

60' 250 210 214 223 189

90' 245 204

120' 230

150' 220

180' 210

Entrapped air Volum

e%

0' 1.4 1.2 1.5 1.5 1.0

Compressive MPa

strength

7 days 14.5 20.0 17.7 15.6 16.4

28 days 21.4 24.2 26.0 21.3 19.1

Dimensional μηι/ηι

variation

7 days -108 -6 -37 -25 -223

14 days -290 -135 -109 -123 -212

21 days -271 -200 -272

28 days -306 -158 -175 -179 -308

Residual % wbw

humidity

28 days 1 1 1 1 1

Bond MPa

strength

28+14 days 1.2 1.2 1.3 1.1 1.4

Failure mode adhesive adhesive adhesive cohesive cohesive The results show that the addition of aluminium sulfate to the formulation of a calcium sulfate binder composition or screed containing lime might be a solution to move from a adhesive mode of failure to a cohesive mode of failure. However, aluminium sulfate has a strong negative impact on the action of the

superplasticizer and on the fluidity of the fresh screed.

In particular, more superplasticizer and more water are needed. Nevertheless, the initial spreads are mainly not at a good level. Such properties are neither acceptable for a self-levelling screed nor convenient for a standard screed.

4.2 Aluminum sulfate in combination with additional additives

Table 7 gives an overview of different formulations E1 - E4 using aluminium sulfate in combination with commercial retarders, potassium sulfate and

hemihydrate instead of an acidic compound. As retarders, Retardan P and Retardan L (commercially available from Tricosal Bauabdichtungs-GmbH, Germany) were used.

Table 7

Unit Ref1 D3 E1 E2 E3 E4

Aluminium wt.-% 0 0.7 0.7 0.7 0.5 0.5 sulfate

on binder

(expressed in

dry AI₂(S0₄)₃ )

Retardan P wt.-% 0.01 1 0.01 1 0.03

on binder

Retardan L wt.-% 0.03 on binder

Potassium wt.-% 0.5

sulfate

on binder

Hemihydrate kg/m³ 6.2

taken

out

from

the

anhydri

te

Sika wt.-% 0.90 1 .40 1 .20 1 .40 1 .20 1 .20

Viscocrete TEMPO 12 on binder

Total Water Liter/m³ 295 320 310 315 305 305

Flow Table mm

Spread

0' 250 253 222 200 230 229

30' 255 227 200 205 232 233

60' 250 223 199 170 219 218

90' 245 214 205

120' 230

150' 220

180' 210

Entrapped air Volume

%

0' 1 .4 1 .5 1 .4 1 .1 1 .5 1 .3

Compressive MPa

strength

7 days 14.5 15.6 19.8 21 .7 18.2 17.3

28 days 21 .4 21 .3 27.9 29.3 25.0 26.7

Dimensional μηι/ηι

variation

7 days -108 -25 -1 10 -165 -35 -40

14 days -290 -123 -137 -171 -142 -138

21 days -271 -158 -192 -176 -160

28 days -306 -179 -177 -213 -176 -160

Residual wt.-%

humidity

28 days 1 1 0.3 0.4 1 1

Bond strength MPa

28+14 days 1 .2 1 .1 1 .5 1 .6 1 .6 1 .6

Failure mode adhesiv cohes cohesi cohesi cohesi cohesi e ive ve ve ve ve

Despite the use of additional additives, in particular retarders, it is not possible to achieve an acceptable fluidity with aluminium sulfate.

4.3 Fatty alcohols Table 8 shows the results obtains with a formulation F1 using a fatty alcohol instead of an acidic compound. Specifically, the fatty alcohol is cetyl alcohol which is a saturated and linear alcohol with 16 carbon atoms. Cetyl alcohol is

commercially available from different suppliers.

Table 8

Unit Ref1 F1

SIKA wt.-% on 0.90 0.95

VISCOCRETE anhydrite

TEMPO 12

Cetyl alcohol kg/m^a 0.3

Flow Table mm

Spread

0' 250 245

30' 255 250

60' 250 250

90' 245 235

120' 230 235

150' 220 225

180' 210

Entrapped air Volume

%

0' 1 .4 1 .6

Compressive MPa

strength

7 days 14.5 15.0

28 days 21 .4 24.4

Dimensional μηι/ηι

variation

7 days -108 +31

14 days -290 -69

21 days -271 -144

28 days -306 -94

Residual wt.-%

humidity

28 days 1 0.6 Bond strength MPa

28 + 14 days 1 .2 1 .7

Failure mode adhesive adhesive

As can be deduced from table 8, it is possible to improve the bond strength with help of a fatty alcohol. Nevertheless, the mode of failure is still adhesive

(undesired).

Thus, it will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted.

For example, the plasticizer used in the above mentioned experiments can e.g. be omitted or replaced by other substances.

As well, lactic acid, acetic acid and/or sulfuric acid can e.g. be replaced by other acidic compounds. It is also possible to use mixtures of different acidic

compounds.

In addition, the anhydrite used as calcium sulfate binder can for example at least partially be replaced by other binder materials, such as e.g. hemihydrate and/or cement.

Claims

1 . Use of an acidic compound for reducing or preventing the formation of a skin layer on a surface of a calcium sulfate based binder composition.

2. Use according to claim 1 , characterized in that the calcium sulfate based binder composition is used for preparation of a screed on a building element, in particular a floor screed.

3. Use according to at least one of preceding claims, characterized in that, the acidic compound is selected amongst monocarboxylic acids and/or inorganic acids.

4. Use according to at least one of preceding claims, characterized in that the acidic compound is glycolic acid, lactic acid, acetic acid and/or sulfuric acid, preferably lactic acid.

5. Use according to at least one of preceding claims, characterized in that, the acidic compound is used in an amount of 0.01 - 5 wt.-%, especially 0.05 - 1 wt.-%, preferably 0.1 - 0.5 wt.-%, with respect to the calcium sulfate in the binder composition.

6. Use according to at least one of the preceding claims, characterized in that, the acidic compound is used in combination with a plasticizer.

7. Use according to claim 6, characterized in that the plasticizer is a comb

polymer with polycarboxylate backbone and polyether side chains, in particular the comb polymer comprises:

a) a molar parts of a structural unit S1 of formula I

b) b molar parts of a structural unit S2 of formula II

molar parts of a structural unit S3 of formula III

d) d molar parts of a structural unit S4 of formula IV

wherby

p = 0 or 1 ,

whereby A = C₂- to C₄-alkylene, R⁴ represents H, d- to C₂o-alkyl, - cyclohexyl or -alkylaryl,

and n = 2 - 250, each R³ independently of the others represents NH₂, -NR⁵R⁶, -OR⁷NR⁸R⁹, with R⁵ and R⁶ independetly of each other stand for

C-i - to C₂₀-alkyl, -cycloalkyl

-alkylaryl or -aryl,

R⁷ is a C₂ -C₄-alkylene group,

-aryl or a hydroxyalkyl group, and whereby a, b, c and d stand for the molar parts of the structural units S1 , S2, S3 und S4, with

a/b/c/d/ = (0.1 - 0.9) / (0.1 - 0.9) / (0 - 0.8) / (0.0 - 0.

8),

Use according to at least one of preceding claims, characterized in that the calcium sulfate in the binder composition comprises anhydrite, in particular a weight proportion of anhydrite to all other modifications of calcium sulfate is at least 50 wt.-%, preferably at least 75 wt.-%, more preferably at least 90 wt.-

9. Calcium sulfate based binder composition, comprising calcium sulfate and an acidic compound, whereby the acidic compound is an acidic compound as described in at least one of claims 1 - 8.

10. Calcium sulfate based binder composition according to claim 9, characterized in that the acidic compound is glycolic acid, lactic acid, acetic acid and/or sulfuric acid, preferably lactic acid, and in that the acidic compound is present in an amount of 0.01 - 5 wt.-%, especially, 0.05 - 1 wt.-%, preferably 0.1 - 0.5 wt.-%, with respect to the calcium sulfate.

1 1 . Calcium sulfate based binder composition according to at least one of claims 9 - 10, characterized in that a plasticizer is present, whereby the plasticizer is a comb polymer with polycarboxylate backbone and polyether side chains, in particular a comb polymer as described in claim 7, and characterized in that the plasticizer is present an amount of 0.1 - 10 wt.-%, especially, 0.5 - 5 wt.-

%, with respect to the total amount of binder material in the binder

composition.

12. Building element, in particular a floor and/or a floor screed, comprising a

hardened binder composition according to at least one of claims 9 - 1 1 .

13. Method for preparing a calcium sulfate based binder composition according to any of claims 9 - 1 1 , whereby an acidic compound is intermixed with hydratable calcium sulfate.

14. Admixture for preparing a calcium sulfate based binder composition, in

particular for preparing a self-leveling floor screed, comprising a plasticizer and an acidic compound, whereby the plasticizer and the acidic compound are in particular defined according to at least one of claims 1 - 8.

15. Admixture according to claim 14, characterized in that the plasticizer is a

comb polymer with polycarboxylate backbone and polyether side chains and the acidic compound is lactic acid, acetic acid and/or sulfuric acid, preferably lactic acid.

16. Admixture according to at least one of claims 14 - 15, characterized in that a weight ratio of plasticizer to acidic compound is 20:1 - 1 :20, preferably, 5:1 -

1 :5, more preferably, 2:1 - 1 :2, in particular 1 .5:1 - 1 :1 .5.

17. Admixture according to at least one of claims 14 - 16, characterized in that the admixture is in the form of a solution or dispersion containing 40 - 95 wt.-

%, preferably, 50 - 90 wt.-%, most preferably 60 - 80 wt.-%, of water.