WO2007022852A2

WO2007022852A2 - Accelerator for portland cement

Info

Publication number: WO2007022852A2
Application number: PCT/EP2006/007569
Authority: WO
Inventors: Cristiano Maltese; Carlo Pistolesi; Marco Squinzi
Original assignee: Mapei S.P.A.
Priority date: 2005-08-23
Filing date: 2006-07-31
Publication date: 2007-03-01
Also published as: WO2007022852A3; ITMI20051589A1

Abstract

This invention discloses an accelerating admixture for Portland cement favouring high compressive strength development, in particular at early curing ages, and comprising: a) aluminium hydroxide or alumina and/or sodium aluminate; b) aluminium sulphate; c) hydrofluoric acid; and/or d) inorganic or organic phosphorus based acid or salts; and/or e) boric acid or a boron derivative; and/or f) ascorbic acid; and/or g) urea.

Description

LOW COST ACCELERATOR FOR PORTLAND CEMENT

The present invention relates to liquid accelerators for Portland cement favouring fast early compressive strength development. They are also able to reduce effectively the final setting time of the sprayed concrete and are characterised by low production costs. BACKGROUND OF THE INVENTION

The excavation of tunnels requires large amount of cementitious materials (mortars and /or concrete) which are used to prepare temporary or definitive protective shells. The cement mixtures are directly sprayed on the rocky surface, by means of high pressure nozzles, without the necessity of moulds. In order to speed up the tunnelling work and for safety reasons, the cement materials should satisfy the following requirements: 1) to adhere permanently on the rock walls; 2) to have low rebound (a phenomenon arising mainly by the high spraying pressure) which causes large concrete losses; 3) to harden very quickly; 4) to develop high early mechanical strengths. Only if the aforementioned conditions occur, a structural consolidation of the tunnel can be obtained allowing fast excavation rates and safe working conditions. Accelerating admixtures are usually utilised in order to reach this target. These products have to guarantee an excellent adhesion of the sprayed material to the rock wall and a rapid compressive strength development. The first condition can be evaluated by measuring the capability of the accelerator to reduce the setting time of a cement paste. The second requirement is determined by measuring the mechanical strength development during the first hours of hydration of a cementitious mortar or concrete.

In the past, several alkaline accelerators were used, such as soda, potash, silicates or aluminates of alkali metal. Nevertheless, these accelerators are known to negatively affect the long term mechanical strengths. Moreover, due to their alkaline nature, they are irritating to the skin and particular protective devices are requested for the workers' s safety. Furthermore, alkali metal substances reacting with aggregates, could favour alkali silica reaction (ASR) which negatively affects the concrete properties. Finally, they release alkaline substances that, by increasing the pH of ground waters, could be dangerous polluting agents.

These problems favoured the development "low in alkali" or "alkali- free" accelerators. According to European regulations (Osterreichischer Betonverein, Sprayed Concrete Guideline, Wien, March 1999 and pr EN 934-5 "Admixtures for Sprayed Concrete-Definitions, Requirements, Conformity, Marking and Labelling"), an accelerating admixture is classified as "alkali-free" when the concentration of sodium and potassium, expressed as equivalents of Na₂O, is lower than 1%. Lithium is also an alkali metal, however the scientific literature shows that it does not negatively affect the concrete and therefore it is not considered in the calculation of equivalents of Na₂O.

Among these new "alkali-free" admixtures, EP 1167317 describes an accelerator consisting of fluoro aluminates and aluminium sulfate. These accelerators can be in the form of water solutions (with good long term stability) which cause a quick concrete setting, thereby allowing good adhesion to the rock walls. Nevertheless, they retard, in particular during the first hours of hydration, an effective mechanical strength development of the sprayed cementitious material.

US 6302954 and EP 1114004 disclose accelerators based on aluminium carboxylates and aluminium sulfate. These accelerators are in the form of water solution and, compared to those based on fluoro aluminates, are characterised by a faster development of early mechanical strength with a lower capability to reduce the setting time and, therefore, they can negatively affect the adhesion of the fresh sprayed material on the rock walls, determining a large rebound.

WO 2005040059 discloses accelerators composed of fluoro-aluminium carboxylates, sodium aluminate and manganese sulphate able to reduce efficaciously final setting time and favouring a proper compressive strength development.

WO 03/029163 describes liquid accelerators based on amorphous aluminium hydroxide, formic acid, phosphoric acid and aluminium sulphate.

WO 2004/106258 discloses setting accelerators containing amorphous aluminium hydroxide, organic dibasic acid anhydride, aluminium sulphate, magnesium sulphate and eventually stabilized with phosphoric acid.

All the accelerators favouring high compressive strength development (US 6302954; EP 1114004; WO 2005040059; WO 03/029163; WO 2004/106258) require the use of amorphous aluminium hydroxide for its high solubility in soft acids. Nevertheless, its high price increases the cost of these accelerators. Furthermore, due to their low capability to reduce the setting, in presence of water coming from the rock walls, dangerous concrete collapses can occur. In these cases, high accelerator dosages are necessary.

DESCRIPTION OF THE INVENTION The object of the present invention is to provide accelerating admixtures for Portland cement capable to develop high mechanical strength while maintaining an efficient setting time reduction. They are also characterised by low production costs as they contain a low amount of amorphous aluminium hydroxide. These accelerators show better performances than those described in the prior art, as they are able to reduce setting and favour, at the same time, an excellent mechanical strength development without the need of high concentration of amorphous aluminium hydroxide. The "alkali-free" accelerating admixtures according to the invention comprise: a) the reaction products at temperatures higher than 50⁰C of an aluminium hydroxide or alumina and/or sodium aluminate with; b) aluminium sulphate; c) hydrofluoric acid; and/or d) an inorganic or organic phosphorus based acid or salt or a mixture thereof; and/or e) a boric acid or a boron derivative; and/or f) ascorbic acid; and/or g) urea; with the proviso that: molar ratio 0.5<Al/F<95; 0.33<Al/P<161 (if an inorganic or organic phosphorus based acid or salts is present); 0.7<Al/S<15; 1<A1/B<46 (if a boric acid or a boron derivative is present); 0<Al/Na<140; alkali content (expressed as % of Na₂O)<l.

The product performances are further improved by the addition of amines or alkanolamines, in particular diethanolamine.

The accelerator of the present invention may be in the form of a clear or turbid solution and, after water evaporation, the resulting powder keeps the same performances as the original solution.

The accelerator of the present invention can be prepared, as water solution, by reacting in water (at temperature higher than 50⁰C) crystalline aluminium hydroxide or alumina and/or sodium aluminate with hydrohalogenic acid and/or an inorganic or organic phosphorus based acid and/or boric acid and/or ascorbic acid and/or urea. A clear or slight turbid solution confirms the formation of soluble aluminium complexes. During the second step, keeping the temperature at approx. 6O⁰C, aluminium sulfate or aluminium basic sulfate is added, stirring the mixture to obtain a clear solution. Afterwards, amorphous aluminium hydroxide is added and stirred to obtain a clear or slightly turbid solution. Any impurities can be filtered off. The accelerator efficiency may be further improved by addition of an alkanolamine or amine. The characteristics and the advantages related to the use of the accelerator of the present invention are described in more details in the following examples.

All the components of the examples are expressed as per cent by weight. Example 1 Cement pastes were prepared by mixing a portland cement (Cement type I 42,5 R), water (water to cement ratio, W/C = 0.26) and 1% of polycarboxylate superplasticizer (Dynamon SX by Mapei). Several dosage of an accelerator of the invention (Formula 1) and the final setting time of the resulting mixtures was measured according to Vicat's method. The accelerator dosage necessary to obtain a final set lower than two minutes was evaluated. This value is roughly connected to the dosage that can be employed in job site in order to avoid concrete collapses. A commercial accelerator (rich in amorphous aluminium hydroxide) was also evaluated.

The accelerator of the invention (Formula 1) was prepared according to the above mentioned procedure: 0.8 g of crystalline aluminium hydroxide were added to 44.2 g of water at 7O⁰C in a glass vessel; 0.5 g of hydrofluoric acid (40% water solution) and 2.5 g of phosphoric acid (75% water solution) were then added to the resulting mixture that was stirred in a water bath at 70°C, till a clear or slightly turbid solution was obtained (about 30 minutes); Afterwards aluminium sulfate (A1₂(SO₄)₃14H₂O - 40 g) was added and stirred for 30 minutes to have a clear or slight turbid solution; therefore amorphous aluminium hydroxide (52% Al₂O₃ - 6 g) was added to the mixture, stirring for 30 minutes; 5 g of diethanolamine (85% water solution) were then added to the solution, stirring to obtain a clear o slightly turbid pale yellow solution.

Table 1. Composition of accelerating admixtures, final setting time and dry solid content

Formula 1

Water (44.2g)

Cristalline aluminium hydroxide 59% Al₂O₃ (0.8g)

Hydrofluoric acid 40% water solution (0.5g)

Phosphoric acid 75% water solution (2.5g)

Aluminium sulphate 17.2% Al₂O₃ (40.Og)

Amorphous aluminium hydroxide 54% Al₂O₃(O. Og)

DEA 85% water solution (6.Og)

Dry solid content

Accelerator Dosage ( %) Final setting time

(% - EN 480/8)

7 I¹ 10"

Formula 1 5 I¹ 30" 40. 3

4 V 50"

Commercial 10 I¹ 50" 2

Product 9 T 40" The results of Table 1 show that only 4% by cement mass of Formula 1 are necessary to obtain a final setting lower than two minutes. On the contrary, 9% by cement mass of the commercial accelerator are necessary to get a final setting lower than 2 minutes. These results clearly point out that Formula 1 is more effective than the commercial accelerator. Example 2

An accelerator according to the invention was prepared as previously described in Example 1 following the composition reported in Tab. 2 (Formula 2) with the difference that sodium aluminate replaced crystalline aluminium hydroxide and, after acid additions, the mixture was stirred for 10 minutes.

Cement pastes were prepared by mixing a portland cement (Cement type II B-S 42,5 N), water (water to cement ratio, W/C = 0.26) and 1% of polycarboxylate superplasticizer (Dynamon SX by Mapei). The minimum accelerator dosage of accelerator to be added on these fresh made cement paste samples in order to obtain a final setting time lower than 2 minutes was determined.

The results are shown in Table 2.

Table 2. Composition of accelerating admixtures, final setting time and dry solid content

The results of Table 2 show that 8% by cement mass of Formula 2 are necessary to obtain a final setting lower than two minutes. On the contrary, to reach the same result, 10% by cement mass of the commercial accelerator are necessary. These data clearly indicate that Formula 2 is more effective than the commercial accelerator.

Example 3

In this example, the compressive strength development of mortars containing an accelerating admixture of the present invention (Formula 3) was measured. A commercial accelerator (rich in amorphous aluminium hydroxide) was also evaluated. The tests were carried out on specimens having the following composition:

450 g Cement;

1350 g Standardized sand;

0.4 g Antifoaming agent; 4.5 g Superplasticizer (Dynamon SX by Mapei);

202.5 g Water;

31.5 g Accelerator

The mortars were prepared according to EN 196/1. The accelerating admixture was added at the end of the mixing cycle and further mixed for 10". At early curing ages the mechanical strength was measured by a digital force gauge (Osterreichischer Betonverein, Sprayed Concrete Guideline, Wien, March 1999) and it was expressed in N. At later curing ages, when the specimens (40x40x160mm) become harder, the mechanical strength was determined according to the EN 196/1 and the values were expressed in MPa. Table 3. Composition and compressive strength development

Formula 3

Water (41.5 g)

Sodium aluminate 18% Al₂O₃ / 21% Na₂O (2.5g)

Hydrofluoric acid 40% water solution (3.5g)

Phosphoric acid 75% water solution (0.5g)

Aluminium sulphate 17.2% Al₂O₃ (40.Og)

Amorphous aluminium hydroxide 54% Al₂O₃ (6.Og)

DEA 85% water solution (6.Og)

Compressive Strength

Accelerator Dry solid content

(% - EN 480/8)

1 hour 2 hours 3 hours 4 hours 5 hours

Formula 3 130 N 261 N 400 N 1.2 MPa 1.4 MPa 40.4

Commercial 2O N 55 N 150 N 199 N 0.9 MPa 58.2 product

The results clearly indicate that the admixture of the present invention (Formula 3) favours significantly higher compressive strength development than an ordinary rich in amorphous aluminium hydroxide commercial product.

Example 4

An accelerator according to the invention was prepared as previously described in Example 1 following the composition reported in Tab. 4 (Formula 4) with the difference that: 1) sodium aluminate replaced crystalline aluminium hydroxide; 2) Amino tris (methylene phosphonic) acid replaced phosphoric acid.

Cement pastes were prepared by mixing a portland cement (Cement type II A-LL 42,5 R), water (water to cement ratio, W/C = 0.27) and 1% of polycarboxylate superplasticizer (Dynamon SX by Mapei). It was evaluated the minimum dosage of accelerator (both Formula 4 and a commercial one) to be added on these fresh made cement paste samples in order to obtain a final setting time lower than 2 minutes.

The results are shown in Table 4.

Table 4. Composition of accelerating admixtures, final setting time and dry solid content

Formula 4

Water (41.5g)

Sodium aluminate 18% Al₂O₃ / 21% Na₂O (2.5g)

Hydrofluoric acid 40% water solution (3.5g)

Amino tris methylene phosphonic acid 50% water solution (0.5g)

Aluminium sulphate 17.2% Al₂O₃ (40.Og)

Amorphous aluminium hydroxide 54% Al₂O₃ (6.Og)

DEA 85% water solution (6.Og)

Dry solid content

Accelerator Dosage (%) Final setting time (% - EN 480/8)

T 15"

Formula 4 39.4

1' 55"

5' 30'

Commercial Product 2' 30' 47.1

11 1' 58'

The results of Table 4 show that 9% by cement mass of Formula 4 are necessary to obtain a final setting lower than two minutes. On the contrary, 11% by cement mass of the commercial accelerator (rich in amorphous aluminium hydroxide) are necessary to get a final setting lower than 2 minutes. These results clearly point out that Formula 4 is more effective than the commercial accelerator. Example 5

An accelerator according to the invention was prepared as previously described in Example 1 following the composition reported in Tab. 5 (Formula 5) with the difference that: 1) sodium aluminate replaced crystalline aluminium hydroxide; 2) boric acid replacing phosphoric acid.

Cement pastes were prepared by mixing a portland cement (Cement type II A-LL 42,5 R), water (water to cement ratio, W/C = 0.27) and 1% of polycarboxylate superplasticizer (Dynamon SX by Mapei). It was evaluated the minimum dosage of accelerator (both Formula 5 and a commercial one) to be added on these fresh made cement paste samples in order to obtain a final setting time lower than 2 minutes.

The results are shown in Table 5.

Table 5. Composition of accelerating admixtures, final setting time and dry solid content

Formula 5

Water (41.5g)

Sodium aluminate 18% Al₂O₃ / 21% Na₂O (2.5g)

Hydrofluoric acid 40% water solution (3.5g)

Boric Acid (0.5g)

Aluminium sulphate 17.2% Al₂O₃ (40.Og)

Amorphous aluminium hydroxide 54% Al₂O₃ (6.Og)

DEA 85% water solution (6.Og)

Dry solid content

Accelerator Dosage (%) Final setting time

(% - EN 480/8)

7 2' 20"

Formula 5 39. 1

9 1' 58"

7 5¹ 30"

Commercial

9 T 30" 47. 1 Product

11 1' 58" The results of Table 5 show that 9% by cement mass of Formula 5 are necessary to obtain a final setting lower than two minutes. On the contrary,

11% by cement mass of the commercial accelerator are necessary to get the same result. These data clearly point out that Formula 5 is more effective than the commercial accelerator.

Example 6

In this example, the compressive strength development of mortars containing an accelerating admixture of the present invention (Formula 6) or a commercial accelerator was measured. The tests were carried out on samples having the following composition:

450 g Cement;

135O g Standardized sand;

0.4 g Anti foaming agent;

4.5 g Superplasticizer (Dynamon SX by Mapei); 202.5 g Water;

31.5 g Accelerator

The mortars were prepared according to EN 196/1. The accelerating admixture was added at the end of the mixing cycle and further mixed for 10". At early curing ages the mechanical strength was measured by a digital force gauge (Osterreichischer Betonverein, Sprayed Concrete Guideline, Wien, March 1999) and it was expressed in N. At later curing ages, when the specimens (40x40x160mm) become harder, the mechanical strength was determined according to the EN 196/1 and the values were expressed in MPa.

The results clearly indicate that the admixture of the present invention (Formula 6) favours significantly higher compressive strength development than an ordinary rich in amorphous aluminium hydroxide commercial product. Table 6

Formula 6

Water (42.5 g)

Sodium aluminate 18% Al₂O₃ / 21% Na₂O (2.5g)

Hydrofluoric acid 40% water solution (0.5g)

Boric Acid (0.5g)

Aluminium sulphate 17.2% Al₂O₃ (42.Og)

Amorphous aluminium hydroxide 54% Al₂O₃ (6.Og)

DEA 85% water solution (6.Og)

Compressive Strength

Dry solid content

Accelerante

(% - EN 480/8)

1 hour 2 hours 3 hours 4 hours 6 hours

Formula 6 250 N 1 MPa 1.2 MPa 1.4 MPa 1.8 MPa 42.5

Commercial

4O N 8O N HO N 147 N 1 MPa 47.1 product

Claims

1. An accelerating admixture for hydraulic cement comprising: a) the reaction products obtained from 5O⁰C of an aluminium hydroxide or alumina and/or sodium aluminate with; b) aluminium sulphate; c) hydrofluoric acid; and/or d) an inorganic or organic phosphorus based acid or salt; and/or e) a boric acid or a boron derivative; and/or f) ascorbic acid; and/or g) urea; with the proviso that:

0.50<Al/F<95; 0.33<Al/P<161 (if an inorganic or organic phosphorus based acids or salts are present); 1<A1/B<46 (if a boric acid or a boron derivative is present); 0.7<Al/S<15; 0<Al/Na<140.

2. An accelerating admixture according to claim 1 wherein the phosphorus based derivative is phosphoric acid.

3. An accelerating admixture according to any one of claims 1 and 2 wherein aluminium hydroxide is crystalline or amorphous or a mixture of crystalline and amorphous aluminium hydroxide.

4. An accelerating admixture according to any one of claims 1 to 3 containing also a mono or dicarboxylic acid or hydroxy carboxylic with a number of carbon atoms ranging from 1 to 5 or a mixture thereof.

5. An accelerating admixture according to any one of claims 1 to 4 stabilized with an amine or an alkanolamine.

6. An accelerating admixture according to claim 5 stabilized with diethanolamine.

7. Use of the admixtures of claims 1-6 as accelerating agents for Portland cement.

8. Cement compositions comprising the admixtures of claims 1-6.