The present invention relates to the pre-cast industry and more particularly to thermal treatments used in this field. The present invention relates to a process to improve the mechanical strength 28 days after mixing of a hydraulic composition submitted to a thermal treatment.

Thermal treatments are often used in the field of the pre-cast industry to accelerate the hydration of hydraulic compositions, in particular with the aim of obtaining high early-age mechanical strength (for example 8 hours after mixing a hydraulic composition). The aim of the thermal treatment is to speed up the removal of the formwork and/or the handling of the moulded objects for the construction field obtained, and thus to increase productivity.

However, thermal treatments are known to present the disadvantage of decreasing the long-term mechanical strength, in particular 28 days after the mixing.

In order to meet the requirements of the pre-cast industry, it has become necessary to find another means of increasing the early age mechanical strength (for example, 8 hours after the mixing) whilst keeping and even improving the long-term mechanical strength (for example 28 days after the mixing) of a hydraulic composition which is submitted to a thermal treatment.

Therefore, the problem which the invention intends to solve is to provide a new means to improve the mechanical strength 28 days after mixing of a hydraulic composition which is submitted to a thermal treatment.

Unexpectedly, the inventors have found that the addition of a calcium salt, in particular calcium nitrite or calcium nitrate, and of an optimised quantity of sulphate to a hydraulic composition submitted to a thermal treatment can results in the improvement of the mechanical strength 28 days after mixing.

Accordingly, the present invention provides a process to improve the mechanical strength 28 days after mixing of a hydraulic composition submitted to a thermal treatment, the hydraulic composition comprising a hydraulic binder and optionally a mineral addition, wherein the process comprises the addition of a calcium salt and an optimized quantity of sulphate to the hydraulic composition.

The invention may present one of the advantages described herein below.

It is not necessary to reduce the water/cement ratio. This solution is indeed known to increase the mechanical strength 28 days after mixing, but it may present disadvantages, in particular from a rheological point of view of the hydraulic composition. The use of a calcium salt and an optimised quantity of sulphate can make it possible to avoid a supplementary addition of alkaline salts, known as accelerators, which may provoke negative effects on the finished product.

The use of a calcium salt and an optimised quantity of sulphate makes it possible to avoid a supplementary addition of chlorides, which are known to be counter ions for accelerating salts and which are responsible for the corrosion of metal reinforcements within hydraulic compositions.

The nitrites used according to the present invention may also be used as corrosion inhibitors.

The use of the calcium salt and of the optimised quantity of sulphate may result in a reduction of the duration of the thermal treatment. A thermal treatment, generally lasting 14 hours, may be reduced to 3 and a half hours, more preferably to 6 hours. Thus the hydraulic composition may be heated earlier than what is typically done in the field.

The process of the present invention may shorten the pre-curing time. The procuring time is the first phase of the thermal treatment cycle, during which the hydraulic composition is maintained at ambient temperature. The pre-curing time precedes the phase during which the temperature increases to reach the target temperature of the thermal treatment. Typically, for a pre-curing time of four hours, it may be possible to shorten this pre-curing time by two hours.

In the present description and associated claims the term « one » refers to « one or more ».

The process of the present invention uses a thermal treatment. Thermal treatment generally comprises heating a hydraulic composition poured into a mould or a formwork, at a temperature typically from 40°C to 90°C. The thermal treatment may suitably be carried out at atmospheric pressure.

The process of the present invention requires the addition of a calcium salt to the hydraulic composition. The quantity of calcium salt may be from 0.5 to 3.5 %, more preferably from 0.5 to 3 %, most preferably from 0.5 to 2 % by mass relative to the mass of hydraulic binder and optional mineral additions.

Preferably, the calcium salt is calcium nitrite or calcium nitrate, more preferably calcium nitrite.

The process of the present invention requires the addition of an optimised quantity of sulphate. The optimised quantity of sulphate may vary according to the raw materials of the hydraulic composition and can be determined by the person skilled in the art according to known methods. Preferably, the quantity of sulphate is from 2 to 5.5 %, more preferably from 2 to 4 %, percentage expressed by mass of S0 ₃ relative to the mass of hydraulic binder and optional mineral additions.

Preferably, the sulphate used according to the present invention is added in the form of calcium sulphate selected from gypsum (dehydrated calcium sulphate, CaS0 ₄ .2H ₂ 0), hemi hydrate (CaS0 ₄ .1/2H ₂ 0), anhydrite (anhydrous calcium sulphate, CaS0 ₄ ) and mixtures thereof. Gypsum and anhydrite exist in their natural state. The calcium sulphate also exists in the form of a by-product of certain industrial processes. These two sources of calcium sulphate may be used in the process of the present invention.

The process of the present invention requires the thermal treatment of a hydraulic composition. The hydraulic composition comprises a hydraulic binder and optionally mineral additions. A hydraulic binder is a material which sets and hardens by hydration. The hydraulic binder is suitably a cement. Suitable cements include Portland cement.

The mineral additions may be selected from, for example, slags (for example as defined in the "cement" standard NF EN 197-1 standard, paragraph 5.2.2), pozzolans (for example as defined in the "cement" standard NF EN 197-1 standard, paragraph 5.2.3), fly ash (for example as defined by the "cement" NF EN 197-1 standard, paragraph 5.2.4), calcined shales (for example as defined by the "cement" NF EN 197-1 standard, paragraph 5.2.5), calcium carbonate (for example limestone as defined by the "cement" NF EN 197-1 standard, paragraph 5.2.6), silica fume (for example as defined by the "cement" NF EN 197-1 standard, paragraph 5.2.7), metakaolin or mixtures thereof.

The hydraulic composition may also include, in addition to the hydraulic binder and mineral additions, water, aggregates and admixtures. The hydraulic composition according to the invention may be a cement slurry, a mortar or a concrete. The term hydraulic composition includes both fresh and hardened concrete.

Aggregates used in the compositions of the invention include sand (whose particles generally have a maximum size (Dmax) of less than or equal to 4 mm), and gravel (whose particles generally have a minimum size (Dmin) greater than 4 mm and preferably a Dmax less than or equal to 20 mm or more).

The aggregates include calcareous, siliceous, and silico-calcareous materials. They include natural, artificial, waste and recycled materials. The aggregates may also comprise, for example, wood.

The hydraulic composition may also comprise an admixture, for example in accordance with the EN 934-2, EN 934-3 or EN 934-4 standards, for example an air- entraining agent, a viscosity modifying agent, a retarder, a clay inertant, a plasticizer and/or a superplasticizer. In particular, it is useful to include a polycarboxylate superplasticizer, in particular from 0.05 to 1.5%, preferably from 0.1 to 0.8%, by dry mass relative to the mass of hydraulic binder.

Clay inertants are compounds which permit the reduction or prevention of the harmful effect of clays on the properties of hydraulic binders. Clay inertants include those described in WO 2006/032785 and WO 2006/032786.

The term superplasticizer as used in this specification and the accompanying claims is to be understood as including both water reducers and superplasticizers as described in the Concrete Admixtures Handbook, Properties Science and Technology, V.S. Ramachandran, Noyes Publications, 1984.

A water reducer is defined as an additive which reduces the amount of mixing water of concrete for a given workability by typically 10 - 15%. Water reducers include, for example lignosulphonates, hydroxycarboxylic acids, carbohydrates, and other specialized organic compounds, for example glycerol, polyvinyl alcohol, sodium alumino-methyl-siliconate, sulfanilic acid and casein.

Superplasticizers belong to a new class of water reducers chemically different from the normal water reducers and capable of reducing water contents by about 30%. The superplasticizers have been broadly classified into four groups: sulphonated naphthalene formaldehyde condensate (SNF) (generally a sodium salt); sulphonated melamine formaldehyde condensate (SMF); modified lignosulfonates (MLS); and others. More recent superplasticizers include polycarboxylic compounds such as polycarboxylates, e.g. polyacrylates. The superplasticizer is preferably a new generation superplasticizer, for example a copolymer containing polyethylene glycol as graft chain and carboxylic functions in the main chain such as a polycarboxylic ether. Sodium polycarboxylate-polysulphonates and sodium polyacrylates may also be used. Phosphonic acid derivatives may also be used. The amount of superplasticizer required generally depends on the reactivity of the cement. The lower the reactivity the lower the amount of superplasticizer required. In order to reduce the total alkali content the superplasticizer may be used as a calcium rather than a sodium salt.

The setting is generally the passage of the hydraulic binder to the solid state by hydration reaction. The setting is generally followed by a hardening period.

The hardening is generally the development of mechanical strength of a hydraulic binder. The hardening generally occurs after the end of the setting.

The process of the present invention requires the presence of a calcium salt and an optimized quantity of sulphate. Mixing may be effected, for example, by known methods. The calcium salt and the sulphate may be added to the hydraulic composition separately or at the same time. The calcium salt and the sulphate may be added to the hydraulic composition at any suitable stage. The calcium salt and the sulphate may be introduced:

- at the same time as and/or in the mixing water;

- directly to at least one of the components of a hydraulic composition before adding the mixing water; or

- during the mixing process.

According to a variant, the calcium salt and/or the sulphate may be directly added to a hydraulic binder, for example a celent, before the production of a hydraulic composition.

The hydraulic composition used according to the present invention may be shaped to produce, after hydration and hardening a shaped article for the construction field. Shaped articles for the construction field include, for example, a slab, a floor, a screed, a foundation, a base, a shear wall, a beam, a work top, a pillar, a bridge pier, a block of foamed concrete, a pipe, a conduit, a post, a stair, a panel, a cornice, a mold, a road system component (for example a border of a pavement), a roof tile, a surfacing (for example of a road), a jointing plaster (for example for a wall) and an insulating component (acoustic and/or thermal).

Figure 1 represents the results of 6-hour compressive mechanical strength tests obtained for Example 1 .

Figure 2 represents the evolution of the mechanical strength as a function of the quantity of sulphate in Cement 1 with 2 % of calcium nitrite at the 6-hour and 8-hour time periods, WC=0.5 (l-a) and W/C=0.35 (l-b) and the evolution of the mechanical strength as a function of the quantity of sulphate in Cement 2 with 2 % of calcium nitrite at the 6- hour and 8-hour time periods, W/C=0.5 (I l-a) and W/C=0.35 (I l-b), for Example 2.

Measurement method of the Blaine specific surface:

The Blaine specific surface is determined according to the EN 196-6 Standard, paragraph 4.

The following examples illustrate the invention without limiting its scope.

EXAMPLES

Example 1 : Example of a method to optimise together the quantity of the sulphate and the quantity of the nitrite

Raw materials

Cements: the chemical and mineralogical composition of the studied cement is presented in Table 1 herein below: Table 1 : chemical and mineralogical composition of the cement used in Example 1

Additions: Three additions were used:

5 - Calcium nitrite: Ca(N0 ₂ ) ₂ n° CAS 13780-06-8 (Supplier: Wintersun Chemical);

Inert silica (Supplier: Sifraco);

Gypsum.

The mix comprising the inert silica, the calcium nitrite and the gypsum represented 10 % by mass relative to the mass of cement.

0 The gypsum was used to increase the quantity of sulphate:

- % total initial S0 ₃ in the cement = 3.33 %

The required quantity of added sulphate to obtain the required quantity of S0 ₃ was:

% added S0 ₃ = % required S0 ₃ - % initial S0 ₃

5 The quantity of sulphate was added by gypsum:

% added gypsum = % added S0 ₃ x (M(gyp _S um) M(so3)) x (100/pureness of the gypsum expressed in percentage)

= % added S0 ₃ x (172.1 +80) x (100+97.16)

wherein M(gyp _S um) is the molar mass of the gypsum and M _(S o3) is the 0 molar mass of the S0 ₃ .

Superplasticizer: Glenium ACE 43: non-chloride polycarboxylate (supplier: BASF). The superplasticizer was used in the form of an aqueous solution, and the water contained in the solution of superplasticizer was taken into account in the total water content.

5 Standardized sand: siliceous sand according to the EN 196-1 Standard (supplier:

Societe Nouvelle du Littoral).

The mechanical strength were measured on 1.6-litre batches of mortar comprising two bags of standardized sand (1 ,350 kg per bag), the composition of which is presented in Table 2 below: Table 2: Formulations of tested mortar for Example 1

Mixing procedure for the mortar

Preparation of the « powder » mix

5 - Weigh the different products: cement, calcium nitrite, potassium sulphate and gypsum.

Introduce the mix in a metal container (approximately 3 litres capacity).

Homogenise the powder mix using a blade (10-cm diameter) with a TURBOTEST stirrer for approximately thirty seconds making sure that the 0 bottom and edges of the container are scraped by the blade.

Mixing procedure

Mix the standardized sand at low speed (140 rpm) for one minute whilst pouring the wetting water in thirty seconds.

Let rest for three minutes covering the bowl with a damp cloth.

5 - Introduce the mix of binders in the bowl and set the stop watch at 0. This is the

T ₀ time for the batch.

At T ₀ , mix at low speed for one minute.

At T ₀ + 1 minute, introduce the mixing water in thirty seconds whilst mixing at slow speed.

0 At T ₀ + 1 minute and 30 seconds, mix at high speed (280 rpm) for one minute.

At T ₀ + 2 minutes and 30 seconds, stop the mixing.

Production of mortar specimens

The mechanical strength were measured on 4 cm x 4 cm x 16 cm specimens: Fill the first half of the mould.

5 - Cover the mould with a plastic film and tap it twenty times energetically.

Finish filling the mould, re-cover it with a plastic film and tap twenty times energetically again. Level using a ruler.

Cover the mould with a sealer and a glass plate.

Thermal cycle

Keep the mortar specimens at 20°C for 150 minutes (2.5 hours);

Increase the temperature from 20°C to 55°C for 152 minutes (2.53 hours);

Remain at 55°C for 30 minutes;

Remove the mortar specimens from the drying oven at T ₀ + 5.5 hours;

Keep the mortar specimens at 20°C until T ₀ + 6 hours;

Measure the compressive mechanical strength, according to the procedure described in the EN 196-2 Standard.

The relative humidity is monitored at 100% during the cycle.

Results:

The results of the compressive mechanical strength at 6 hours are represented in Figure 1 . The mechanical strength vary depending on the quantity of calcium nitrite (y axis) and the quantity of total S0 ₃ (x axis). Each curve represents a level of mechanical strength in MPa. The points represent experimental data (B1 to B7). The circle limits the zone inside which the mechanical strength may be predicted.

The measured mechanical strength for formulations B1 to B7 were the following:

- 31 MPa for B1 ;

- 21 .8 MPa for B2;

- 32.5 MPa for B3;

- 7 MPa for B4;

- 3.7 MPa for B5;

- 19.2 MPa for B6; and

- 26 MPa for B7.

According to Figure 1 :

For a quantity of calcium nitrite less than 1 .8 %, the increase of the quantity of total S0 ₃ had little effect, or even a negative effect on the 6-hour compressive mechanical strength. For example, for a mortar comprising 0.40 % of calcium nitrite of, a quantity of 3.60 % of total S0 ₃ gave a 6-hour compressive mechanical strength of 7 MPa, whilst a quantity of 4.20 % of total S0 ₃ gave a 6-hour compressive mechanical strength of 4 MPa. It would therefore appear that no real optimum of the quantity of total S0 ₃ exists for these formulations (refer to points B4 and B5);

For a quantity of calcium nitrite greater than 1 .8 %, the 6-hour compressive mechanical strength increased with the quantity of total S0 ₃ . For example, for a mortar comprising 3.20 % of calcium nitrite, a quantity of 3.60 % of total S0 ₃ gave a 6-hour compressive mechanical strength of 16 MPa (refer to point B6), whilst a quantity of 4.20 % of total S0 ₃ gave a 6-hour compressive mechanical strength of 31 ,5 MPa (refer to point B3). The optimum quantity of total S0 ₃ did not appear to have been reached in these tests. Complementary tests would have to be carried out using greater quantities of total S0 ₃ to observe this optimum.

Example 2: Effect of the calcium nitrite on the mechanical strength at 28 days for a hydraulic composition submitted to thermal treatment

Raw materials

Cement: two cements were used, the chemical compositions of which are given in Table 3a and their mineralogical compositions are given in Table 3b below. Table 3a: Chemical composition of the cements used in Example 2

Table 3b: Mineralogical composition of the cements used in Example 2

- Calcium nitrite: Ca(N0 ₂ ) ₂ n° CAS 13780-06-8 (supplier : Wintersun Chemical);

Potassium sulphate;

Gypsum;

Standardised sand: siliceous sand according to the EN 196-1 Standard (supplier: Societe Nouvelle du Littoral).

Superplasticizer. Glenium ACE 43: non-chloride polycarboxylate (supplier: BASF).

Water/Cement ratio (W/C): 0.35 and 0.50.

Parameters of the steam curing cycle:

o Pre-curing time: 1 hour and 2 hours

o Duration of the cycle (not including the cooling time): 3h00, 5h30 and 7h30

o Plateau temperature: 60°C and 80°C

Complementary conservations were made at 20°C in a room at 100% hygrometry for the 1 -day and 28-day time periods. Control steam curing cycle

• Initial conservation time: twenty minutes at 20°C, then forty minutes or a hundred minutes at 40°C, respectively, for a pre-setting time of one or two hours.

• Temperature increase ramp: 1/3°C per minute until reaching the temperature set for the steam curing plateau, i.e. one or two hours respectively for 60°C and

80°C.

• Plateau temperature at the selected temperature (60°C or 80°C) determined by the total duration of the cycle.

• Cooling time: thirty minutes at 20°C in the laboratory.

During the cycle, the relative humidity was monitored at 100%.

The longer pre-setting time induced a shorter plateau time.

The plateau time (in minutes) was calculated according to the formula:

Plateau time = total cycle time (not including the cooling time) - (pre-setting time + temperature increase ramp time)

The temperature cycles at 60°C and 80°C for one or two hours of initial conservation time are presented in Tables 4a and 4b below.

Table 4a: Recapitulative table of the steam curing cycles at 60°C and 80°C for a one- hour pre-setting time

Table 4b: Recapitulative table of the steam curing cycles at 60°C and 80°C for a two- hour pre-setting time

The plateau time was nil for the 3h30 cycle at 80°C. For an initial conservation time of two hours, the plateau time for each cycle was reduced by one hour. The plateau time was therefore nil for the 3h30 cycle at 60°C and it was no longer possible to carry out the 3h30 cycle at 80°C (duration: - 60 minutes). Reference mortar formulae

The tested mortar formulae at W/C=0.35 and W/C=0.50 had a volume of 0.877 litre and were respectively carried out according to Table 5 herein below.

Table 5: Formulation of the mortars used in Example 2

The binder comprised cement and optional additions of calcium nitrite (w = 0 or 2, % expressed by mass relative to the total binder) and additional calcium sulphate in the form of gypsum (y), expressed in grams. These additions were mixed beforehand with the cement according to the procedure described below.

The water provided by the plasticizer (z g of Glenium ACE 43) was subtracted from the total water content. The wetting water was 6 % relative to the mass of standardized sand. The quantity of superplasticizer (z) was adjusted for each formulation in order to obtain a 270 mm spread, according to a practice known to the person skilled in the art.

The mechanical strength tests were carried out using two bags of standardized sand (1 ,350 kg per bag) and a double volume of mix, i.e. 1 ,754 litres.

Procedures

Preparation of the « powder » mix

Weigh the different products: cement, calcium nitrite, potassium sulphate and gypsum.

Introduce the mix in a metal container (approximately 3 litres capacity).

Homogenise the powder mix using a blade (10-cm diameter) with a TURBOTEST stirrer for approximately thirty seconds making sure that the bottom and edges of the container are scraped by the blade.

Mixing procedure

Mix the standardized sand at low speed (140 rpm) for one minute whilst pouring the wetting water in thirty seconds.

Let rest for three minutes covering the bowl with a damp cloth.

Introduce the mix of binders in the bowl and set the stop watch at 0. This is the T ₀ time for the batch.

At T ₀ , mix at low speed for one minute. At To + 1 minute, introduce the mixing water in thirty seconds whilst mixing at slow speed.

At T ₀ + 1 minute and 30 seconds, mix at high speed (280 rpm) for one minute. At T ₀ + 2 minutes and 30 seconds, stop the mixing.

Spread measurement procedure

The spread was measured at T ₀ + 5 minutes after the end of the mixing, just before using the mortar. The spread measurement was carried out using a bottomless mould with a truncated shape, which is a reproduction at the scale ½ of the Abrams cone (refer to the NF P 18-451 Standard of 1981 ):

- Top diameter: 50 +/- 0.5 mm;

Bottom diameter: 100 +/- 0.5 mm;

Height: 150 +/- 0.5 mm.

The other pieces of equipment required for this measurement are a steel tapping rod (diameter of 6 mm and length of 300 mm), and a plate of glass.

The following steps were carried out:

- Dampen the glass plate and the cone (moisten using a sponge and wipe with paper);

- Fill the first half of the cone and tap twenty times with the tapping rod;

- Finish filling the cone and tap again twenty times;

- Level the cone and lift it at T ₀ + 5 minutes by the stop watch;

- Measure the spread, by measuring the diameter of the obtained disk of mortar. Implementation of the mortar

• Produce specimens

The mechanical strength were measured on 4 cm x 4 cm x 16 cm specimens: - Fill the first half of the mould.

- Cover the mould with a plastic film and tap it twenty times energetically.

- Finish filling the mould, re-cover it with a plastic film and tap twenty times energetically again.

- Level using a ruler.

- Cover the mould with a sealing liner and a glass.

• Conservation and treatment of the specimens

For the thermal cycle tests:

Introduce the mould in a drying oven stabilised at 40°C and 100 % hygrometry at T ₀ + twenty minutes.

- Remove the mould from the drying oven at the selected cycle plateau time period. Immediately demould the specimens and leave to cool at 20°C for thirty minutes. Weigh the specimens at the end of cycle time period + twenty-five minutes Break the specimens at the end of cycle time period + thirty minutes.

For the tests at 20°C:

- Keep the mould out of the drying oven at 20°C for one day.

At T ₀ + one day, demould, weigh and break the specimens for the 1 -day time period 20°C.

Continue to keep the specimens for the 28-day time period at 20°C in a humid room at 100 % hygrometry.

- At T ₀ + 28 days, break the specimens for the 28-day time period at 20°C.

Measurement procedure of the mechanical strength

The procedure used to measure the mechanical strength was the procedure described in the EN196-2 Standard.

Results

Optimum sulphate addition

The sulphate addition of the binders was optimised by a supplementary addition of gypsum to the calcium sulphate already present in the industrial cement. This optimisation was carried out according to the W/C ratio and to the different cycle profiles studied after one hour of initial conservation time.

The main results obtained for the control cements without an addition of calcium nitrite are given in Table 6 below.

Table 6: Results of the optimum sulphate addition for the control cements without calcium nitrite in Cement 2

Duration of the Total S0 ₃

W/C Temperature Mechanical strength (MPa) thermal cycle (mass %)

2.36 0.4

6 h 3.07 0.6

4.49 <1

80°C

2.36 18.3

8 h 3.07 24.0

0.35 4.49 23.3

2.36 4.9

6 h

3.07 4.1

60°C 2.36 22.0

8 h 3.07 26.1

4.49 25.5 w/c Duration of the Total S0 ₃

Temperature Mechanical strength (MPa) thermal cycle (mass %)

2.36 3.2

6 h

3.00 3.1

80°C 2.36 14.4

8 h 3.00 17.9

3.50 16.3

0.5

2.36 3.0

6 h

3.00 2.3

60°C 2.36 11.6

8 h 3.00 13.6

3.50 13.9

The results obtained for Cement 1 are not represented here but were equivalent.

According to Table 6, there was no optimum for the supplementary addition of sulphate in the control cements without an addition of calcium nitrite, whatever the quality of the cement, the steam curing cycle and the W/C ratio, except for the long cycles (broken at eight hours) for Cement 2 (3% instead of 2.36 % of total S0 ₃ ).

The main results obtained for the cements with 2 % of calcium nitrite are presented in Figure 2. According to Figure 2, for the cements with 2 % of calcium nitrite, the optimum for the sulphate addition increased within the studied area. The increase of the optimum expressed in total S0 ₃ was higher for the short steam curing times (three hours). To conclude, the optimum for the sulphate addition increased by approximately 2 % of total S0 ₃ in all the tested cements. This increase of the optimum sulphate addition induced an increase of the mechanical strength from approximately 1 MPa. (for the l-b curves) to approximately 20 MPa (for the ll-b curves).

The impact was studied of the addition of 2 % of calcium nitrite on the mechanical strength of the specimens kept for 28 days after mixing, at the optimum values of the sulphate additions determined in the first step, described herein above (Figure 2). The optimum sulphate addition corresponds, on each curve, to the quantity of total S0 ₃ for which the mechanical strength were the highest.

Mechanical strength obtained at 20°C

The mechanical strength obtained at 20°C 28 days after mixing are presented in Table 7 below.

Table 7: Mechanical strength obtained at 20°C at 24 hours and at 28 days for Example 2

Mechanical strength at Mechanical strength at

24 hours 28 days

W/C = 0.35 W/C = 0.50 W/C = 0.35 W/C = 0.50

Cement 1 48 26 87 62

Cement 2 29 15 86 61 The values presented in Table 7 herein above, were used as a control to illustrate the drop of mechanical strength at 28 days for hydraulic compositions submitted to a thermal treatment.

Mechanical strength obtained at the end of the steam curing

The results of the compressive mechanical strength obtained at the end of the steam curing are presented in Table 8 below:

Table 8: Results of the compressive mechanical strength obtained at the end of the steam curing in Example 2

According to Table 8 herein above:

- For Cement 1 , the calcium nitrite had a positive impact whatever the temperature and the duration of the thermal treatment. For example, at 60°C, for a 6-hour thermal treatment and a W/C of 0.35, the mechanical strength increased from 46 MPa without calcium nitrite to 57 MPa with calcium nitrite.

- For Cement 2, the calcium nitrite had a strong positive impact on the entire range, in particular for the 6-hour thermal treatment. For example, at 80°C, for a 6-hour thermal treatment and a W/C of 0.50, the mechanical strength increased from 3 MPa without calcium nitrite to 21 MPa with calcium nitrite.

Mechanical strength obtained 28 days after the steam curing

The results of the compressive mechanical strength obtained 28 days after the steam curing are presented in Table 9 below: Table 9: Results of the compressive mechanical strength obtained 28 days after the steam curing in Example 2

According to Table 9 herein above, 28 days after the steam curing, the calcium nitrite improved the mechanical strength, in contrast to the generally accepted idea that an accelerated system has degraded long-term performances.

On another hand, according to Table 9 herein above:

• Cement 1

The negative effect of the steam curing on the 28-day performances decreased by the presence of the calcium nitrite. For example, at 60°C for a 3h30-thermal treatment and a W/C of 0.50, the 28-day mechanical strength increased from 50 MPa without calcium nitrite to 61 MPa with calcium nitrite, whilst the 28-day mechanical strength of the control specimen at 20°C were 62 MPa.

The 28-day mechanical strength of a specimen submitted to a thermal treatment and with an addition of calcium nitrite were higher than the 28-day mechanical strength of the control specimen at 20°C, whilst they were always lower than those of the control specimen at 20°C when calcium nitrite was absent. For example, at 80°C for an 8-hour thermal treatment and a W/C of 0.35, the 28-day mechanical strength increased from 77 MPa without calcium nitrite to 95 MPa with calcium nitrite, whilst the 28-day mechanical strength of the control specimen at 20°C were 87 MPa.

• Cement 2

The addition of calcium nitrite substantially reduced the negative impact of the steam curing, in particular for the longer steam curing times. For example, at 80°C for an 8-hour thermal treatment and a W/C of 0.35, the 28-day mechanical strength increased from 55 MPa without calcium nitrite to 86 MPa with calcium nitrite, whilst the 28-day mechanical strength of the control specimen at 20°C were 86 MPa. The addition of calcium nitrite made it possible to increase the 28-day strength of a specimen submitted to a thermal treatment to approximately the same level as those measured after 28 days on the control specimens at 20°C. For example, at 60°C for a 6-hour thermal treatment and a W/C of 0.50, the 28-day mechanical strength increased from 46 MPa without calcium nitrite to 57 MPa with calcium nitrite, whilst the 28-day mechanical strength of the control specimen at 20°C were 61 MPa.