Patent Description:
The manufacturing of Portland cement is a complex process that begins with mining and then pre-grinding raw materials that include limestone and clay, to a raw meal which is then heated to a sintering temperature as high as <NUM><NUM> in a cement kiln. In this process, the raw materials chemically react to form new compounds. The result is called clinker, which are rounded nodules typically between around <NUM> and <NUM>. Clinker is then cooled down and ground in large mills. The clinker is ground together with a source of calcium sulphate to produce a Portland cement. Other mineral additions and chemical additives can be added prior to, during or after the grinding to produce different types of Portland cements. For example, granulated blast furnace slag, pozzolans, gypsum are commonly added to the clinker prior to or during the step of grinding of the clinker.

Finally, the Portland cement is either stored in silos or packaged in bags before being transported to the client by road, rail, or on water.

However, a problem encountered with Portland cements is that of their shelf life.

In particular, it is well known known that a Portland cement may be exposed to moisture in the air and begins to partially react, which reduces its reactivity, or could even make it unusable in practice.

There are solutions for limiting water intake from moisture in ambient air which include use of waterproof bags, moisture protection during transport or manipulation of cement and/or use of specific additive to enhance the ageing properties of Portland cement.

For example, <CIT> discloses a hydraulic binder having reduced tendency to form lumps or set during storage, by reducing water intake from ambient air, comprising an organic acid selected from the group consisting of adipic acid and a mixture of adipic, glutaric and succinic acids, wherein the weight concentration of said organic acid is between <NUM> % and <NUM> % relative to the weight of hydraulic binder, and as a main component, a cement selected from the group consisting Portland cement, high-alumina cement, and quick setting cement.

<CIT> discloses a process for manufacturing a Portland cement wherein a Portland clinker is ground in presence of an oleaginous material, preferably oleic acid. It is also disclosed that the proportion of oleaginous material to be used in the grinding step should range between <NUM>% and <NUM>%.

The existing solutions offer interesting results in terms of enhancing the ageing properties of hydraulic binder, but prove to be insufficient with regard to other issues, in particular in terms of preparation and cost and of reduction of the effect of the temperature on the shelf life of the cement.

In particular, when hot freshly ground Portland cement is fed into a container such as a silo and stored in that container, caking may occur and block the exit trap of the container, in particular of the silo, even when the container is protected from moisture of ambient air. For this reason, hot freshly ground Portland cement is usually cooled before feeding the container. However, that intermediate step of cooling is expensive and can be complex to implement in a cement plant. In the context of the invention, the term "hot" refers to a temperature above <NUM>.

Therefore, there is a need for an improved preparation method of Portland cement which is fully integrated in the industrial process and that allows to improve the shelf life of Portland cements.

It has surprisingly been discovered that caking may be induced by gypsum dehydration and that adipic acid at very low dosage may prevent the occurrence of caking.

An aim of this invention is to overcome the drawbacks of the aforementioned prior art and to supply a method for producing and storing Portland cement without any intermediate cooling step prior to feeding the Portland cement into a container, such as a silo, and that can be thus easily integrated into an industrial process and in a cost effective way relative to the methods disclosed in the prior art. Another aim of the invention is to supply a Portland cement obtained according to the method of the invention.

These aims are achieved by the invention, which will be described below.

Therefore, a first aspect of the invention is a method for preparing and storing a Portland cement, wherein the method comprises:.

When Portland cement comprising gypsum is stored at temperatures above <NUM>, dehydration of gypsum occurs and may create caking of the Portland cement.

This problem could be reduced by reducing the temperature at which the clinker and the gypsum are co-ground, for example by cooling the Portland cement down at the exit of the mill before its storage in a container, or by reducing the temperature of the clinker that is introduced into the mill. However, these measures can be complex and costly to implement in the overall production process.

The Applicant unexpectedly discovered that producing a Portland cement by grinding at least a Portland clinker, a source of calcium sulphate comprising gypsum in majority and a very low dosage of an organic compound permitted to significantly decrease the caking of the Portland cement and improve its shelf-life.

Without wishing to be bound by a theory, the Applicant has observed that at a temperature above <NUM>, gypsum tends to dehydrate, and that the slight amount of water released from the gypsum dehydration may partially hydrate the Portland cement, thereby creating caking and hampering final hydraulic properties of the Portland cement. Further, the Applicant discovered that although used at very low dosages, the organic compound allows to significantly and cost-effectively reduce the negative effects of such gypsum dehydration during the period of time when the Portland cement comprising gypsum is at a temperature above <NUM>.

Indeed, the method of the invention advantageously permits to obtain a Portland cement comprising gypsum whose mechanical strength is largely preserved even when it is exposed to temperatures above <NUM>, and during long term storage in standard conditions.

Advantageously, the Portland cement obtained according to the method of the invention maintain its mechanical performance even when stored in a container directly after the grinding step, meaning that the Portland cement may be stored in a container directly from the cement mill output, the Portland cement having a temperature above <NUM> at the entry of the container. This is particularly advantageous in the framework of an industrial production, as the method of the invention avoids having the Portland cement to be cooled down at the output of the cement mill before its storage in a container.

Further, the method of the invention preserves the mechanic performance of the Portland cement obtained even after a long storage in standard conditions and in a container whose initial internal temperature is above <NUM>. This is particularly advantageous in warm countries, where it provides for the possibility of storing the Portland cement for a long time in a container, such as a silo, with no loss of quality or performance of the Portland cement.

Thus, in the method of the invention, the grinding of a Portland clinker, a source of calcium sulphate comprising gypsum in majority and a very low dosage of organic compound allows to avoid the adverse effects mentioned above and to preserve the hydraulic properties of the Portland cement obtained thereof.

Also, in the method of the invention, the grinding of a Portland clinker, a calcium sulphate source comprising gypsum in majority and an organic compound allows an optimal distribution of the organic compound in the Portland cement obtained without any need of a subsequent homogenization step.

Importantly, the Applicant further observed that the organic compound did not negatively impact the operation of the mill, as no dust was observed, as well as no flushing of the mill when using the organic compound at dosages of less than <NUM>%, in % expressed by weight relative to the weight of the Portland cement.

Thus, the method of the invention may easily be carried out in a cement manufacturing plant.

In particular, the method of the invention avoids the need to cool down the Portland cement at the exit of the mill before its storage in a container, or to reduce the temperature of the Portland clinker that is introduced into the mill.

Surprisingly, the Applicant further observed that the method of the invention results in a reduction of soluble Cr(VI) content in the cement. Accordingly, adding the organic molecule described below may allow to reduce the content of metal sulphate additive usually introduced in the cement to reduce soluble Cr(VI) content.

The method of the invention is thus both cost and energy effective and in addition decreases the processing time when compared to processes involving a step of mixing a ground clinker with an anti-ageing additive.

The method of the invention comprises a step (i) of providing a Portland cement comprising:.

In the present invention, a Portland cement is a material in powder form that chemically reacts with water, by converting the water-binder system with a plastic consistency into a solid matrix that has the ability to agglomerate other solid materials. This hardening process takes place spontaneously at room temperature, underwater or exposed to wet air conditions.

According to the present invention, the Portland cement comprises a Portland clinker and a source calcium sulphate. The Portland cement may contain mineral addition(s) and other additive(s) as detailed below.

The clinker has advantageously a mean diameter, D50, comprised from <NUM> to <NUM>. The Portland cement according to the invention is a Portland cement as defined in the cement standard NF EN <NUM>-<NUM> of April <NUM>.

A Portland cement comprises Portland clinker, calcium sulphate, and optional mineral components, such as those described in the cement standard NF EN <NUM>-<NUM> published in April <NUM> and further defined below.

Portland clinker is obtained by clinkering at high temperature a mixture comprising limestone and, for example, clay.

Preferably, Portland clinker has preferentially the following mineralogical composition, in % expressed by weight relative to the weight of clinker:.

The chemical components of Portland clinker may be noted according to the common cement industry notation: C represents CaO; A represents Al<NUM>O<NUM>; F represents Fe<NUM>O<NUM>, and S represents SiO<NUM>.

Preferentially, the Portland cement is selected from a CEM I, CEM II, CEM III, CEM IV, or CEM V as described in the cement standard NF EN <NUM>-<NUM> of April <NUM>.

A CEM I comprises at least <NUM>% of a Portland clinker, in % expressed by weight relative to the weight of CEM I.

The other cement types described in the cement standard NF EN <NUM>-<NUM> of April <NUM> (CEM II, CEM III, CEM IV, CEM V) comprise Portland clinker, calcium sulphate, and mineral addition(s) as defined below.

The mineral addition, if any, is preferably selected from slags (for example, as defined in the European NF EN <NUM>-<NUM> Standard of April <NUM>, paragraph <NUM>. <NUM>) such as granulated slags, pozzolanic materials (for example as defined in the European NF EN <NUM>-<NUM> Standard of April <NUM>, paragraph <NUM>. <NUM>), fly ash (for example, as described in the European NF EN <NUM>-<NUM> Standard of April <NUM>, paragraph <NUM>. <NUM>), calcined schists (for example, as described in the European NF EN <NUM>-<NUM> Standard of April <NUM>, paragraph <NUM>. <NUM>), material containing calcium carbonate, for example limestone (for example, as defined in the European NF EN <NUM>-<NUM> Standard paragraph <NUM>. <NUM>), silica fume (for example, as defined in the European NF EN <NUM>-<NUM> Standard of April <NUM>, paragraph <NUM>. <NUM>), siliceous additions (for example, as defined in the "Concrete" NF P <NUM>-<NUM> Standard), metakaolin or mixtures thereof.

A fly ash is generally a powdery material comprised in the fumes from coal-fired thermal power stations. It is generally recovered by electrostatic or mechanical precipitation. The chemical composition of a fly ash mainly depends on the chemical composition of the coal burned and of the method used in the power plant from which it comes. The same is true for its mineralogical composition. The fly ashes used according to the invention may be of siliceous or calcic nature.

Blast furnace slags are generally obtained by rapid cooling of the molten slag coming from the melting of iron ore in a blast furnace. Slags may be selected from granulated blast furnace slags according to the European standard NF EN <NUM>-<NUM> of April <NUM> paragraph <NUM>.

Silica fumes may be a material obtained by reduction of high purity quartz by carbon in electric arc furnaces used for the production of silica and ferrosilica alloys. Silica fumes are generally formed of spherical particles comprising at least <NUM> % by weight of amorphous silica. Preferably, the silica fumes are selected from silica fumes according to the European standard NF EN <NUM>-<NUM> of April <NUM> paragraph <NUM>.

Pozzolanic materials may be natural siliceous or silico-aluminous substances, or a combination thereof. Among pozzolanic materials may be cited natural pozzolans, which are in general materials of volcanic origin or sedimentary rocks, and natural calcinated pozzolans, which are materials of volcanic origin, clays, schists or sedimentary rocks, thermally active.

Preferably, the pozzolanic materials may be selected from pozzolanic materials according to the European standard NF EN <NUM>-<NUM> of April <NUM> paragraph <NUM>.

Calcium sulphate used according to the present invention includes gypsum (calcium sulphate dihydrate, CaSO<NUM>. <NUM><NUM>O), hemi-hydrate (CaSO<NUM>. <NUM>/<NUM><NUM>O), anhydrite (anhydrous calcium sulphate, CaSO<NUM>) or a mixture thereof. The gypsum and anhydrite exist in the natural state. Calcium sulphate produced as a by-product of certain industrial processes may also be used.

In the present invention, the expression "a source of calcium sulphate comprising gypsum in majority" means a source of calcium sulphate wherein gypsum predominate by weight, i.e. means a source of calcium sulphate containing at least <NUM>% of gypsum, preferably more than <NUM>% of gypsum, in % expressed by weight relative to the weight of the source of calcium sulphate. The source of calcium sulphate may further contain impurities, preferably in a content ranging from <NUM>% to <NUM>% in weight, compared to the total weight of the source of calcium sulphate.

Advantageously, the source of calcium sulphate further comprises calcium sulphate hemihydrate, anhydrite, or mixtures thereof.

Advantageously, the Portland cement comprises from <NUM> to <NUM>%, preferably from <NUM> to <NUM>%, preferably from <NUM> to <NUM>%, of a source of calcium sulphate comprising gypsum in majority, in % expressed by weight relative to the weight of clinker.

The organic compound is selected from adipic acid, stearic acid, <NUM>,<NUM>,<NUM>,<NUM>-tetramethyl-<NUM>-decyne-<NUM>,<NUM>-diol, mixture of adipic, glutaric and succinic acids, salts thereof, and mixtures thereof. The organic compound is preferably selected from adipic acid, <NUM>,<NUM>,<NUM>,<NUM>-tetramethyl-<NUM>-decyne-<NUM>,<NUM>-diol, mixture of adipic, glutaric and succinic acids, salts thereof, and mixtures thereof.

<NUM>,<NUM>,<NUM>,<NUM>-Tetramethyl-<NUM>-decyne-<NUM>,<NUM>-diol is commercially available under the name Surfynol <NUM>.

Mixture of adipic, glutaric and succinic acids preferably comprises at least <NUM>% by weight, compared to the total weight of the mixture, of adipic acid, more preferably at least <NUM>% by weight of adipic acid.

The organic compound is preferably selected from adipic acid, mixture of adipic, glutaric and succinic acids, salts thereof, and mixtures thereof. The organic compound is more preferably adipic acid or a salt thereof. The sodium salt of adipic acid is preferred.

Advantageously, the Portland cement comprises from <NUM> to less than <NUM>% of organic compound, preferably from <NUM> to <NUM>%, more preferably from <NUM> to <NUM>%, in % expressed by weight relative to the weight of Portland cement. Accordingly, at step (i) one provides from <NUM>% to less than <NUM>% of organic compound, preferably from <NUM>% to <NUM>%, in % expressed by weight relative to the weight of the Portland cement.

Salts of organic compounds can also be used, preferably sodium salt, calcium salt, copper salt, ammonium salt, nickel salt, or potassium salt. Sodium salt is preferred.

In the present invention, the organic compound may be supported by inorganic particles.

The inorganic particles are preferably made of an inorganic material suitable to support the organic compound, meaning that the material forming the inorganic particle is inert with regard to the organic compound.

In other terms, the inorganic material forming the inorganic particles is not likely to be attacked by the organic compound or to modify the chemical structure of the organic compound. Preferably, the inorganic particles are not particles based on materials that are capable of forming salts (adipates) with adipic acid, such as highly basic materials. Preferably, the particles used as support of the organic compound are particles based on silica or on alumina.

Silica-based particles comprise at least <NUM>% of silica, preferably at least <NUM>% of silica, and even more preferably at least <NUM>% of silica, in % expressed by weight relative to the weight of inorganic particles.

The silica-based particles may contain precipitated silica, siliceous stone, or mixtures thereof. Precipitated silica is silica obtained by a precipitation reaction of a silicate, in particular an alkali metal silicate such as sodium silicate with an acid such as sulfuric acid.

Siliceous stone may in particular be diatomite. Diatomite is a naturally occurring, soft, siliceous sedimentary rock that is usually crumbled into a fine white to off-white powder. It has a particle size ranging from less than <NUM> to more than <NUM>, typically from <NUM> to <NUM>. Diatomite typically comprises from <NUM> to <NUM>% of silica, <NUM> to <NUM>% of alumina and <NUM> to <NUM>% of iron oxide, in % expressed by weight relative to the weight inorganic particles.

The preparation method of inorganic particles as mentioned above is well known in the art.

The method comprises a step (ii) of grinding the components of step (i).

The grinding can be performed by any know means and is preferably performed at a temperature of at least <NUM>.

Step (ii) is preferably carried out a temperature of at least <NUM>, preferably of at least <NUM>, more preferably of at least <NUM>. Indeed, the Applicant has shown that the gypsum dehydration significantly impedes the properties of the Portland cement when step (ii) is carried out at temperatures exceeding <NUM>. Advantageously, the temperature of step (ii) is below the decomposition temperature of the organic compound.

Step (ii) is preferably carried out a temperature ranging from <NUM> to <NUM>, and more preferably from <NUM> to <NUM>.

Step (ii) is preferably carried out in a milling unit.

In the present invention, the expression "milling unit" refers to units comprising a mill suitable to co-grind a clinker and a source of calcium sulphate, optionally together with at least one mineral addition. The milling unit may further comprise a particle separator to normalize the particle size of the ground mixture.

In the present invention, the mill may be any mill which can be used for grinding clinker, especially at temperature above <NUM>, such as a ball mill or a roll mill.

Preferably, the grinding step (ii) is carried out in a ball mill or a vertical roll mill.

The grinding step (ii) is advantageously conducted so that the Blaine specific surface of the clinker at the end of step (ii) is comprised between <NUM><NUM> and <NUM><NUM><NUM>/g. Preferably, the Blaine specific surface of the clinker at the end of step (ii) is comprised between <NUM><NUM> and <NUM><NUM><NUM>/g.

The fineness of the ground clinker at the output of step (ii) may be expressed in terms of Blaine Specific Surface, as determined according to the determination of fineness standard NF EN <NUM>-<NUM> of December <NUM>.

The use of additives commonly referred to as grinding aids or grinding agent to facilitate the grinding of the clinker is known in the art. Their purpose is mainly to reduce the energy required to grind the Portland cement to a given fineness.

In the invention, the grinding aid may be added to the components of step (i) or during step (ii), preferably at a dosage of from <NUM> to <NUM>%, advantageously of from <NUM> to <NUM>%, in % expressed by weight relative to the weight of the components of step (i).

The grinding aid is preferably selected from polycarboxylate ethers, paraffin oil, rape seed oil, glycerin, dioxan, acetone, ethyl acetate, isopropanol, diethylene glycol, polyglycerol, diols (e.g. <NUM>,<NUM>-propanediol, <NUM>,<NUM> hexane-diol, <NUM>,<NUM> butane-diol), acetic acid, alkanolamines (e.g. triethanolamine, diisopropanolamine, triisopropanolamine), lignosulfonates, and mixtures thereof.

Chromium is an unavoidable trace element of the raw material used in the manufacture of Portland clinker. In particular, hexavalent chromium (Cr(VI)) may be formed in the oxidizing and alkaline burning condition of the cement kiln. Cr(VI) compounds are toxic because of their high solubility and oxidation potential and their ability to penetrate biologic tissues. The European Union legislation provides that cements should have levels of soluble Cr(VI), when water is added to the cement, of no more than <NUM> ppm (<NUM>%) by weight of the dry cement.

The use of additives commonly referred to as Cr(VI) reducers or chromium reducers is known in the art. Their purpose is mainly to reduce the amount of water-soluble hexavalent chromium to trivalent chromium form because the trivalent form tends to precipitate from solution as a stable complex, thereby limiting the amount of soluble Cr(VI).

The chromium reducer is preferably sulphate of a metal, more preferably comprises a sulphate of iron(II), tin(II), antimony(III), or manganese(II). The chromium reducer preferably comprises iron (II) sulphate such as iron(II) sulphate heptahydrate or iron(II) sulphate monohydrate.

In the invention, the chromium reducer may be added to the components of step (i), during step or after (ii), preferably at a dosage of from <NUM> to <NUM>%, advantageously of from <NUM> to <NUM>%, in % expressed by weight relative to the weight of the Portland cement.

Indeed, the Applicant has shown the method of the invention allows a significant increase of the duration of the effectiveness of the Cr(VI) reducer. This effect may be used to reduce the amount of Cr(VI) reducer needed in the Portland cement.

The method can comprise a step of adding to the Portland cement additional component such as a mineral addition after step (ii) or during step (ii) and before step (iii).

The method comprises a step (iii) of feeding a container with the Portland cement obtained after step (ii), wherein the Portland cement has a temperature above <NUM> when entering the container.

Preferably, the Portland cement has a temperature above or equal to <NUM>, preferably above or equal to <NUM>, when feeding the container. The Portland cement may be at a temperature up to <NUM>, advantageously up to <NUM> when feeding the container.

In the method, the Portland cement obtained after step (ii) is advantageously stored directly in a container at the end of step (ii), meaning that, except optional step of adding a mineral addition or additive, the method does not comprise intermediate step between step (ii) and step (iii). In particular, the Portland cement obtained after step (ii) is not cooled or let to be cooled before step (iii) of feeding the Portland cement in a container.

A second aspect of the invention relates to a Portland cement obtained according to the method as defined in the first aspect of the invention, wherein the Portland cement comprises:.

The Portland cement is as defined in the first object of the invention, in particular it advantageously comprises Portland clinker and optionally at least one mineral addition(s).

In particular, the Portland cement comprises a chromium reducer at a dosage of from <NUM> to <NUM>%, advantageously of from <NUM> to <NUM>%, in % expressed by weight relative to the weight of the Portland cement.

A third aspect of the invention also relates to the use of an organic compound selected from adipic acid, stearic acid, <NUM>,<NUM>,<NUM>,<NUM>-tetramethyl-<NUM>-decyne-<NUM>,<NUM>-diol, mixture of adipic, glutaric and <NUM> succinic acids, salts thereof, and mixtures thereof during the grinding of a Portland cement comprising a Portland clinker and a source of calcium sulphate comprising gypsum in majority so as to allow to store the Portland cement obtained therefrom in a container directly after its hot grinding in a cement mill, while maintaining its mechanic properties during its subsequent storage, wherein the Portland cement is fed into the container at a temperature above <NUM>, preferably at a temperature above <NUM>.

In particular, the organic compound is preferably used in a weight concentration comprised between <NUM> to <NUM>%, in % expressed in weight relative to the weight of Portland cement. Advantageously, the organic compound is preferably used in a weight concentration from <NUM> to <NUM>% of organic compound, preferably from <NUM> to less than <NUM>%, more preferably from <NUM> to <NUM>%, more preferably from <NUM> to <NUM>%, in % expressed by weight relative to the weight of Portland cement.

The Portland cement is obtained by the method of the first object of the invention or as defined in the second object of the invention.

Further aspects and advantages of the present invention will be disclosed in the following experimental section, which should be regarded as illustrative and not limiting the scope of the present application.

CEM II/B-L cements (composition according to the standard EN <NUM>-<NUM> of April <NUM>) from the plant of Surma (Bangladesh) were produced by co-grinding Portland clinker, limestone, and gypsum.

Adipic acid was added to the components of the CEM II/B-L cement. The mixes were co-ground to a surface specific Blaine of <NUM><NUM> +/- <NUM><NUM>/g.

A cement CEM II/B-L whose components were co-ground with <NUM>% of adipic acid by weight of cement and with <NUM>% of grinding aid by weight of cement is referred as "Cement A" in the following. The grinding step lasted <NUM> hours.

A cement CEM II/B-L whose components were co-ground with <NUM>% of adipic acid of by weight cement and with <NUM>% of grinding aid by weight cement is referred as "Cement B" in the following. The grinding step lasted <NUM> hours.

The results are detailed in Table <NUM>.

Cement B (<NUM>% adipic acid addition) showed no significant changes in industrial operation. With Cement A (<NUM>% adipic acid addition), risks of mill flushing appear.

The aging test is done by storing the ground cements at a depot located in a place where ambient temperature is above <NUM>.

A cement CEM II/B-L whose components were co-ground with <NUM>% of grinding aid by weight of cement but without any adipic acid addition is referred as "Ref. A" in the following. The grinding step lasted <NUM> hours.

A cement CEM II/B-L whose components were co-ground with <NUM>% of grinding aid by weight of cement but without any adipic acid addition is referred as "Ref. B" in the following. The grinding step lasted <NUM> hours.

Ref. A and Ref. B are used as references.

The results detailed in Table <NUM> show a visible effect of acid adipic at <NUM>% and <NUM>% on lump formation after <NUM> weeks.

In this example, an industrial CEM III/A <NUM> N-LH CE PM-ES-CP1 NF containing less than <NUM>% by weight of gypsum is used. This cement is referred as "CEM III" in the following. More specifically, CEM III contains a source of calcium sulphate mainly comprising hemihydrate. The quantity of gypsum provided by the source of calcium sulphate is below <NUM>% by weight of the CEM III.

In a first set of experiments, the CEM III cement is co-ground with <NUM>% or <NUM>% of adipic acid in % by weight of cement weight into a heating ball mill at <NUM>.

In a second set of experiments, the CEM III cement is co-ground with <NUM>% or <NUM>% of adipic acid by weight of cement and with <NUM>% of gypsum by weight of cement into a heating ball mill at <NUM>.

In this second set of experiment, the grinding step increases the Blaine Specific Surface of CEM III cement by around +<NUM><NUM>/g.

The ground mixes are then placed in sealed boxes at <NUM> or <NUM> during <NUM> days. These conditions simulate a long-term storage of those mixes in containers such as silos at high temperature, especially at ambient temperature above <NUM>.

The cements are placed in sealed boxes at <NUM> and <NUM> (or at ambient temperature, <NUM>, for reference).

After <NUM> days of storage at <NUM> or <NUM> (ambient for Reference), the sealed boxes are placed at ambient temperature for <NUM> days.

The cements are then mixed with water and the mechanical resistances of the mortars obtained are measured according to the standard EN196-<NUM> of September <NUM>.

To allow for a relevant comparison between the samples, the results are corrected for the air amount, meaning that the values of compressive strength are recalculated for a zero porosity by linear extrapolation.

The results recalculated for a zero porosity are shown on <FIG>. The compressive strength at <NUM> days of MReference1, Mcontrol1(<NUM>), Mcontrol1(<NUM>), CM1(<NUM>), CM(<NUM>), CM2(<NUM>) and CM2(<NUM>) are equivalent. This means that the mechanical resistance of the mortar at <NUM> days does not appear to be altered by a storage at <NUM> or <NUM> of the cement.

The results recalculated for a zero porosity are shown on <FIG>.

The compressive strength at <NUM> days of MReference1, Mcontrol1(<NUM>), Mcontrol1(<NUM>), CM1(<NUM>), CM1(<NUM>), CM2(<NUM>) and CM2(<NUM>) are equivalent. This means that the mechanical resistance of the mortar at <NUM> days does not appear to be altered by a storage of the cement at <NUM> or <NUM>.

This is coherent with the fact that CEM III cement assayed here does not contain significant amount of gypsum. This cement has a low sensitivity to a storage at high temperature, and the adipic acid treatment does not change its mechanical performances.

Control <NUM>, IHB1 and IHB2are placed in sealed boxes at <NUM> and <NUM>. Reference <NUM> is placed in sealed boxes at ambient temperature (<NUM>).

After <NUM> days of storage at <NUM> or <NUM>, the sealed boxes are placed at ambient temperature for <NUM> days.

These cements are then mixed with water and the mechanical resistances of the mortars obtained are determined. The results are corrected for the same air amount (the values of mechanical resistances are given at zero porosity).

The compressive strength of MReference2, Mcontrol2(<NUM>), IM1(<NUM>) and IM2(<NUM>) are equivalent.

The compressive strength of Mcontrol2(<NUM>) is altered and fell by about <NUM>% (from <NUM> MPa to <NUM> MPa), in comparison to MReference <NUM>. IM1(<NUM>) and IM2(<NUM>) have a compressive strength above <NUM> Mpa, and thus greater than Mcontrol2(<NUM>). IM2(<NUM>) has a compressive strength (<NUM> MPa) equivalent to MReference2 (<NUM> MPa).

The compressive strength of MReference2, Mcontrol2(<NUM>), IM1(<NUM>) and IM2(<NUM>) are equivalent. The results found at <NUM> days are confirmed for Mcontrol2(<NUM>), IM1(<NUM>) and IM2(<NUM>). At <NUM> days, the compressive strength of Mcontrol2(<NUM>) is altered and fell by <NUM>% (from <NUM> MPa to <NUM> MPa) whereas IM1(<NUM>) and IM2(<NUM>) present very good compressive strength (<NUM> MPa for IM1(<NUM>) and <NUM> MPa for IM2(<NUM>), to be compared with a value of <NUM> MPa for MReference2.

These results show a phenomenon of pre-hydration of a cement co-ground with <NUM>% gypsum (% by weight of cement) during its storage at temperatures above <NUM> when no adipic acid is added.

Thermogravimetric analyses are performed on Reference2, control2(<NUM>) and control2(<NUM>) cements.

On the TGA curves (<FIG>), one can observe that between <NUM> and <NUM>, the loss in mass is less important in control2(<NUM>), meaning that there is less constituting water in that sample. This highlights the loss of water of crystallization of the hydrated calcium sulphates during the treatment at <NUM>.

The derivated TGA curves i.e. the DTG curves (<FIG>), illustrate the residual quantity of gypsum and/or of hemihydrate, and support the fact that the higher the storage temperature, the more those two hydrated calcium sulphates are dehydrated and thus become visible in TGA/DTG. The DTG curves (<FIG>) of control2(<NUM>) are similar to Reference2 meaning that when treated at a temperature <NUM> the alteration of the cement is hardly discernible. In contrast, the DTG curves (<FIG>) of control2(<NUM>) clearly show an alteration of the cement. The symbol * illustrates the apparition of a hydrated phase.

Thermogravimetric analyses are performed on Reference2, control2(<NUM>), IHB1(<NUM>) and IHB2(<NUM>).

On <FIG>, between <NUM> and <NUM>, the DTG curves illustrate the residual quantity of gypsum and/or hemihydrate in the samples (in connection with the peak area).

The clear signal reduction observed on the curve control2(<NUM>) is significantly reduced when the cement CEM III is co-ground in presence of <NUM>% or <NUM>% of adipic acid. The curves of the cements treated with <NUM>% and <NUM>% adipic acid and stored at <NUM>, IHB1(<NUM>) and IHB2(<NUM>), have a profile similar to that of the reference cement, Reference2.

This means that the adipic acid protect the calcium sulphates and limits their dehydration at <NUM>.

A CEMII/B M(LL-S) <NUM>. 5R cement has been used in these studies, hereafter called "CEMII". The relative composition of cements (at the grinder entry - at step (i) of the method) is given in the following table.

After grinding, the cements have the same grain size curves.

Cement aging is monitored under identical storage conditions which are as follows:.

Measurements of soluble Cr(VI) contents on cement are carried out initially and then monthly check of <NUM> to <NUM> months of cement age, according to the standard EN <NUM>-<NUM> of June <NUM>.

For bags with plastic foil (<FIG>), we see that addition of adipic acid significantly reduces the soluble Cr(VI) content in the cement.

In bag with plastic foil on the pallet <NUM>st row, soluble Cr(VI) content in comparative cement is initially at about <NUM> ppm, about <NUM> ppm after <NUM> month storage, about <NUM> ppm after <NUM> months storage, about <NUM> ppm after <NUM> months storage while the initial soluble Cr(VI) content is only at about <NUM> ppm in invention cement, about <NUM> ppm after <NUM> month storage, about <NUM> ppm after <NUM> months storage, about <NUM> ppm after <NUM> months storage.

In bag with plastic foil in "dry" room, soluble Cr(VI) content in comparative cement is initially at about <NUM> ppm, about <NUM> ppm after <NUM> month storage, about <NUM> ppm after <NUM> months storage, about <NUM> ppm after <NUM> months storage while the initial soluble Cr(VI) content is only at about <NUM> ppm in invention cement, about <NUM> ppm after <NUM> month storage, about <NUM> ppm after <NUM> months storage, about <NUM> ppm after <NUM> months storage. In bag with plastic foil on the pallet <NUM>th row, soluble Cr(VI) content in comparative cement is initially at about <NUM> ppm, about <NUM> ppm after <NUM> months storage, about <NUM> ppm after <NUM> months storage while the initial Cr(VI) content is only at about <NUM> ppm in invention cement, about <NUM> ppm after <NUM> month storage, about <NUM> ppm after <NUM> months storage, about <NUM> ppm after <NUM> months storage.

For bags with pure paper bag (<FIG>), we see that addition of adipic acid significantly reduce the soluble Cr(VI) content in the cement.

In paper bags of the <NUM>st row of a pallet covered with plastic foil , soluble Cr(VI) content in comparative cement is initially at about <NUM> ppm, about <NUM> ppm after <NUM> month storage, about <NUM> ppm after <NUM> months storage, about <NUM> ppm after <NUM> months storage while the initial soluble Cr(VI) content is only at about <NUM> ppm in invention cement, about <NUM> ppm after <NUM> month storage, about <NUM> ppm after <NUM> months storage, about <NUM> ppm after <NUM> months storage.

In paper bags covered with plastic foil in a "dry" room, soluble Cr(VI) content in comparative cement is initially at about <NUM> ppm, about <NUM> ppm after <NUM> month storage, about <NUM> ppm after <NUM> months storage, about <NUM> ppm after <NUM> months storage while the initial soluble Cr(VI) content is only at about <NUM> ppm in invention cement, about <NUM> ppm after <NUM> month storage, about <NUM> ppm after <NUM> months storage, about <NUM> ppm after <NUM> months storage.

In paper bags of the <NUM>th row of a pallet covered with plastic foil , soluble Cr(VI) content in comparative cement is initially at about <NUM> ppm, about <NUM> ppm after <NUM> month storage, about <NUM> ppm after <NUM> months storage, about <NUM> ppm after <NUM> months storage while the initial soluble Cr(VI) content is only at about <NUM> ppm in invention cement, about <NUM> ppm after <NUM> month storage, about <NUM> ppm after <NUM> months storage, about <NUM> ppm after <NUM> months storage.

Accordingly, adipic acid can also be used as further reducing soluble Cr (VI) contents, which in turn allows to reduce the iron sulphate dosage usually needed.

In the current example, a CEM I <NUM>. 5R from the cement plant of Le Teil was ground at the laboratory at a temperature of <NUM>. This cement contains <NUM>% gypsum and <NUM>% of hemihydrate, expressed in weight percentage of the weight of CEM I, measured by Differential Scanning Calorimetry (DSC).

The cement was further ground at <NUM> for <NUM> rounds of the ball mill without addition or with <NUM>%, expressed in weight percentage of the weight of CEM I, of adipic acid. The results summarised in the table below show that the addition of <NUM>% adipic acid during the grinding step reduces the dehydration of the gypsum into hemihydrate.

Claim 1:
A method for preparing and storing a Portland cement, wherein the method comprises:
(i) providing at least:
- a Portland clinker,
- a source of calcium sulphate comprising gypsum in majority,
- from <NUM> to <NUM>% of an organic compound selected from adipic acid, stearic acid, <NUM>,<NUM>,<NUM>,<NUM>-tetramethyl-<NUM>-decyne-<NUM>,<NUM>-diol, mixture of adipic, glutaric and succinic acids, salts thereof, and mixtures thereof, in % expressed by weight relative to the weight of the Portland cement,
(ii) grinding the components of step (i) to produce a Portland cement;
(iii) feeding a container with the Portland cement obtained after step (ii), wherein the Portland cement has a temperature above <NUM> when feeding the container.