Abstract:
The invention has as object a cementitious composition for forming mortars or concretes having reduced tendency to react with alkali, characterized in that it comprises as additive at least a compound of general formula:
 
[R—N—(CH 2 —COOH) n ] x   (I)
 
R being an aliphatic or aromatic hydrocarbon chain, n=2 or 3 and x=1 or 2.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a national phase of PCT application No. PCT/EP2012/076326, filed Dec. 20, 2012, which claims priority to IT patent application No. MI2011A002327, filed Dec. 20, 2011, all of which are incorporated herein by reference thereto. 
     FIELD OF THE INVENTION 
     The present invention relates to a cementitious composition for forming mortars or concretes having reduced tendency to react with alkali. 
     PRIOR ART 
     In the concretes and mortars field, the alkali-aggregate reaction (AAR) is an end product degradation phenomenon associated with a chemical reaction between reactive silica contained in certain types of aggregate and the cement matrix. It is generally accepted that the alkali-aggregate reaction can take place when the following series of conditions occur simultaneously:
         i. Presence of sufficient moisture in the concrete (permanently or temporarily)   ii. Presence in the aggregates of a sufficient content of species reactive to alkalis (primarily reactive silica)   iii. Presence of a sufficient content of alkali to the cement paste placed in contact with the aggregates.       

     The AAR phenomenon is generally difficult to control since the physicochemical mechanisms that govern the evolution thereof have very slow kinetics. The negative effect on the works can also be seen many years after the execution thereof, through the development of a network of cracks in concrete. The mechanical characteristics of the concrete can consequently be degraded and/or the functionality of the work may be lost. 
     The mechanism of action of the AAR comprises an initial step of increasing the alkalinity of the solution following the dissolution of sodium, potassium and calcium ions from different sources. In a subsequent step, the Si—O bonds in the silica exposed on the surface of the aggregate are hydrated to form a gel containing H 2 SiO 4   2− , H 3 SiO 4   −  ions and cations such as K+, Na+, Ca 2+  in varying proportions. This gel coating the surface of the aggregate exposed to attack tends to absorb water molecules and to expand, locally generating tractive forces in the cement matrix, which can fracture where sufficient resistance to traction has not been developed. 
     More in particular, the hydroxyl ions generated from hydration of the alkali act as primer of the chemical reaction, thus the establishment of a high pH can generally be considered a favourable condition for the reaction itself. 
     The occurrence of AAR can lead to undesired phenomena in the works, such as:
         widespread cracking   discolouration around the cracks   exudation of gel from the cracks   misalignment of adjacent sections   joint closures   localized explosion phenomena       

     To limit the occurrence of the AAR phenomenon, one can intervene on the basic mechanisms of the phenomenon by attempting to exclude at least one of the conditions (i, ii, iii) above. More in particular, point (i) being strongly dependent on the conditions of product exposure and point (ii) being inherent to the origin of the aggregates, modifying the effects thereof is often difficult or impractical. As concerns point (iii), certain possible strategies for reducing the risk of AAR occurrence are:
         I. use of components for concrete having a limited alkali content   II. use of materials with latent hydraulic or pozzolanic activity mixed with the concrete (pozzolanas, fly ash, dross, microsilica, metakaolin, etc.).   III. use of inhibitor additives, which is the case of the present invention.       

     SUMMARY OF THE INVENTION 
     According to the present invention, a cementitious composition is proposed, that is for forming mortars or concretes having reduced tendency to react with alkali, characterized in that it comprises as an additive at least a compound of general formula:
 
[R—N—(CH 2 —COOH) n ] x   (I)
 
     R being an aliphatic or aromatic hydrocarbon chain, n=2 or 3 and x=1 or 2. The following are preferred compounds of general formula (I):
 
CH 3 —N—(CH 2 —COOH) 2  
 
(CH 2 ) m —(N—(CH 2 —COOH) 2 ) 2 , con m≧2
 
C 6 H 5 —N—(CH 2 —COOH) 2  
 
C 6 H 10 —(N—(CH 2 —COOH) 2 ) 2  
 
     One overall preferred compound of general formula (I) is ethylene diamine tetraacetic acid, or EDTA, of formula:
 
(CH 2 —COOH) 2 —N—(CH 2 ) 2 —N—(CH 2 —COOH) 2  
 
     The present invention also has as object a composition comprising at least one hydraulic binder, water and, optionally, one or more aggregates, and/or one or more mineral additions, and/or fibres for cements, and/or one or more additives, preferably comprising at least one of the compounds of general formula (I) in an amount by weight ranging from 0.01% and 1% by weight with respect to the binder, as inhibitor of AAR. 
     In one embodiment, this composition comprises EDTA in an amount by weight ranging from 0.2% to 0.4% by weight with respect to the binder. 
     Preferably, it comprises EDTA in amount by weight equal to 0.28% by weight. 
     The present invention also has as object an addition for a cementitious composition for forming mortars or concretes having reduced tendency to react with alkali, characterized in that it comprises at least one of said compounds of general formula:
 
[R—N—(CH 2 —COOH) n ] x   (I)
 
     R being an aliphatic or aromatic hydrocarbon chain, n=2 or 3 and x=1 or 2. The present invention also has as object the use of at least one compound of general formula:
 
[R—N—(CH 2 —COOH) n ] x   (I)
 
     R being an aliphatic or aromatic hydrocarbon chain, n=2 or 3 and x=1 or 2. 
     as an additive for a cementitious mixture for forming mortars or concretes in order to reduce the tendency to react with alkali of mortar or concrete. 
     The term hydraulic means a material in powder form, in dry state, which when mixed with water, provides plastic mixtures that are able to solidify and harden. Cements means in particular those included in European EN 197-1 standard. 
     The cementitious compositions in question are divided into pastes, i.e. compositions free of inert aggregates, and conglomerates, i.e. compositions containing at least one inert aggregate. The conglomerates are in turn divided in mortars, containing fine aggregates such as for example sand, and concretes, containing both fine aggregates and coarse aggregates such as gravel, pebbles and crushed aggregate selected for example from those classified according to the European EN 12620 standard. 
     The present invention is directed at mortars and concretes in particular. 
     Mineral addition means any type of finely subdivided inorganic material that can be added to the concrete to impart improved mechanical resistance and durability characteristics. The additions can be inert, pozzolanic or can have latent hydraulic activity, for example selected from those permitted by European EN 206-1 standard. 
     For example, a concrete compliant with European EN 206-1 standard, having an addition in excess of 10 kg/m 3  is object of the invention. 
     According to the present invention, at least one compound of formula (I), for example EDTA, is introduced as an additive to the cementitious mixture, for example to form concrete, directly into the mixer or preventively dissolved in the mixing water or on the addition thereof. 
     The amount of EDTA added to the cementitious mixture is preferably between 0.01% and 1% by weight of hydraulic binder. More preferably, the dose of EDTA is between 0.2% and 0.4% by weight of binder. Even more preferably, the dose of EDTA is equal to 0.28% by weight of binder. 
     In the present invention binder means the sum of cement and addition. 
     A cement of the present invention is in particular selected according to European EN 197-1 standard. An addition is in particular selected according to European EN 206-1 standard. 
     A cementitious mixture according to the present invention may comprise additions with latent hydraulic or pozzolanic activity, such as fly ash, microsilica, finely ground granulated blast furnace slag. As hydraulic binders said cements according to European EN 197-1 standard are preferred. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Characteristics and advantages of the present invention are described in grater detail in the following examples, provided by way of a non-limiting example of the present invention. 
     EXAMPLES 
     In the described examples, EDTA was used for the preparation of mortar mixtures according to the invention, dissolving it in the mixing water in the mixer. 
     Aggregate containing reactive silica and that is therefore susceptible to AAR was used; the reactive species content was 25% on average. NaOH was introduced to the mixtures as an alkali source, dissolved in the mixing water in the content of 1% by weight expressed as Na 2 O referring to the binder. 
     Mortar specimens were prepared having 4 cm×4 cm×16 cm dimensions were prepared. The determination of performance was performed by measuring the deformation of the specimens, 24 hours from casting, under the following conditions:
         in a 1N of NaOH solution at 80° C., onerous both due to the high temperature and due to the continuous supply of alkali during exposure.   in water at 60° C., onerous on account of the acceleration of the speed of the AAR due to the high temperature exposure.       

     Example 1 
     The effect of EDTA in modifying AAR was evaluated in mortar mixtures containing fly ash, as shown in Table 1, using strongly accelerating conditions of exposure (NaOH 1N at 80° C.). or performance of the tests, the following proportions of mixture were adopted using a cement CEM II/A-LL 42.5 R: water-binder ratio equal to 0.55—binder weight—NaOH dissolved in the mixing water in a proportion of 1% by weight expressed as weight of Na 2 O relating to the binder, understood as the sum of the cement and pozzolanic addition. EDTA was added to one of the two mixtures in a proportion of 0.28% by weight of binder, equal to a value of 0.07% by weight referring to the mortar. A second mixture wherein EDTA was not added is shown as reference. It can be observed that using EDTA, the expansions were significantly reduced, the desired technical effect thereby being achieved. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Deformation 
                   
                   
               
               
                   
                 (expansion) 
                 Maturation in NaOH 1N at 80° C. 
               
             
          
           
               
                   
                 [μm/m] 
                 7 days 
                 14 days 
                 28 days 
                 90 days 
               
               
                   
                   
               
             
          
           
               
                   
                 Reference 
                 581 
                 931 
                 1313 
                 2144 
               
               
                   
                 EDTA 0.07% 
                 331 
                 463 
                 731 
                 1200 
               
               
                   
                 (present invention) 
               
               
                   
                   
               
             
          
         
       
     
     Example 2 
     The positive effect of EDTA in reducing AAR was verified from tests on mortar mixtures containing fly ash, as shown in Table 2, using accelerating conditions of exposure (water at 60° C.). 
     The following mixture proportions, using CEM II/A-LL 42.5 R, were adopted for performance of the tests:
         water/binder ratio equal to 0.55   aggregate/binder ratio equal to 2.25   fly ash in a proportion of 20% by weight of binder   NaOH dissolved in the mixing water in a proportion of 1% by weight expressed as weight of Na 2 O referring to the binder.       

     EDTA was introduced into one of the two mixtures in a proportion of 0.28% by weight of binder, equal to 0.07% by weight referring to the mortar. A second mixture wherein EDTA was not added is shown as reference. 
     It can be observed the expansions were significantly reduced using EDTA. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Deformation 
                   
                   
               
               
                   
                 (expansion) 
                 Maturation in water at 60° C. 
               
             
          
           
               
                   
                 [μm/m] 
                 7 days 
                 14 days 
                 28 days 
                 90 days 
               
               
                   
                   
               
             
          
           
               
                   
                 Reference 
                 210 
                 150 
                 191 
                 263 
               
               
                   
                 EDTA 0.07% 
                 110 
                 63 
                 47 
                 113 
               
               
                   
                 (present invention) 
               
               
                   
                   
               
             
          
         
       
     
     Example 3 
     The positive effect of EDTA in reducing AAR was verified by tests on mortar mixtures containing powdered glass as addition. The physicochemical characteristics of the powdered glass in question are shown in Table 3 and Table 4. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
             
               
                   
                 reactive SiO 2  [%] 
                 52.88 
               
               
                   
                 SiO 2  [%] 
                 69.0 
               
               
                   
                 Al 2 O 3  [%] 
                 2.70 
               
               
                   
                 Fe 2 O 3  [%] 
                 0.36 
               
               
                   
                 CaO [%] 
                 8.84 
               
               
                   
                 MgO [%] 
                 1.44 
               
               
                   
                 Na 2 O [%] 
                 15.6 
               
               
                   
                 K 2 O [%] 
                 0.84 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
             
             
               
                   
                 BET 
                 m 2 /g 
                 0.59 
               
               
                   
                 Density - ρ 
                 g/cm 3   
                 2.540 
               
               
                   
                 Laser - Sv (specific surface) 
                 m 2 /cm 3   
                 0.99 
               
               
                   
                 Laser - xp (average diameter) 
                 μm 
                 16.7 
               
               
                   
                 Laser - n (amplitude) 
                 — 
                 1.23 
               
               
                   
                   
               
             
          
         
       
     
     The following mixture proportions were adopted for performance of the tests:
         fly ash in a proportion of 20% by weight of binder   aggregate/binder ratio equal to 1.88   water/binder ratio equal to 0.49   NaOH dissolved in the mixing water in a proportion of 1% by weight expressed as weight of Na 2 O referring to the binder.       

     Binder means the sum of cement and powdered glass. 
     EDTA is added to one of the two mixtures in a proportion of 0.28% by weight of binder, equal to 0.1% by weight of mortar. A second mixture wherein EDTA was not added is shown as reference. 
     Table 5 shows the results of tests of the expansion tests in mortar under strongly accelerating conditions of exposure of the AAR (NaOH 1N at 80° C.). It can be observed that by using EDTA the expansions were significantly reduced as follows. 
     
       
         
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Deformation 
                   
               
               
                 (expansion) 
                 Maturation in NaOH 1N at 80° C. 
               
             
          
           
               
                 [μm/m] 
                 7 days 
                 14 days 
                 28 days 
                 90 days 
                 126 days 
               
               
                   
               
               
                 Reference 
                 3363 
                 4131 
                 5119 
                 7456 
                 8844 
               
               
                 EDTA 0.1% 
                 2494 
                 3394 
                 3638 
                 4638 
                 4644 
               
               
                 (present 
               
               
                 invention) 
               
               
                   
               
             
          
         
       
     
     Example 4 
     The present example shows that, although EDTA is an acid, its use as an additive for concrete according to the present invention has not shown abatement of the mechanical characteristics arising from negative interactions with a strongly basic cementitious matrix. 
     Table 6 records the determinations of dynamic elastic modulus of the same specimens for which the expansions were recorded in the preceding Table 1 and Table 2. 
     It can be derived from Table 6 that it there has been no decrease in elastic modulus in the time, but that there has been, on the contrary, an increase between 30 and 50%, between 1 and 90 days, in all examined cases. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                   
                 Dynamic 
                 Dynamic 
                   
               
               
                 Specimens the 
                 elastic 
                 elastic 
               
               
                 expansions of 
                 modulus 
                 modulus 
               
               
                 which are shown 
                 (1 day) 
                 (90 days) 
               
               
                 in 
                 [MPa] 
                 [MPa] 
                 % increase 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Table 1 
                 Reference 
                 16691 
                 22110 
                 28 
               
               
                   
                 EDTA 
                 17077 
                 26191 
                 42 
               
               
                 Table 2 
                 Reference 
                 15745 
                 28170 
                 57 
               
               
                   
                 EDTA 
                 17417 
                 29545 
                 52 
               
               
                 Table 5 
                 Reference 
                 15269 
                 17528 
                 14 
               
               
                   
                 EDTA 
                 15913 
                 21895 
                 32 
               
               
                   
               
             
          
         
       
     
     Example 5 (Comparative) 
     The effect of the use of a disodium salt of the EDTA instead of the EDTA in a cement mixture is studied in the present invention. Table 7 shows the test expansion data on mortar under strongly accelerating conditions of exposure (NaOH 1N at 80° C.). 
     The following mixture proportions were adopted for performance of the tests:
         aggregate/cement ratio equal to 1.88   water/cement ratio equal to 0.49   NaOH dissolved in the mixing water in a proportion of 1% by weight expressed as weight of Na 2 O referring to the binder.   Na-EDTA added in a proportion of 0.5% and 2% on cement, respectively equal to 0.1% and 0.3% by weight of mortar.       

     The behaviour of a mixture used as reference wherein Na-EDTA has not been used, is also recorded. 
     Table 7 shows only a mild effect reduction effect of the expansions with respect to the reference, probably to be attributed to the two non-complexing functional groups present in the disodium salt molecule of the EDTA. 
     The present example highlights that the use of a salt of EDTA, in this case a disodium salt, does not produce appreciable effects on the reduction of the expansions. In addition, the use of high sodium salt contents has led to undesirable variations of the rheology and of the mechanical characteristics of the mixtures. More in particular, the higher dose of Na-EDTA (0.3%) caused a strong reduction to 1 day of the elastic modulus with respect to the reference. 
     The use of a salt of EDTA must therefore be considered excluded from the scope of the present invention. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                   
                 Deformation 
                   
                   
                   
               
               
                   
                 (expansion) [μm/m] 
                   
                 Dynamic 
                 Dynamic 
               
               
                 Use of 
                 (Maturation in NaOH 
                 Rheology 
                 elastic 
                 elastic 
               
               
                 disodium 
                 1N at 80° C.) 
                 (Spreading) 
                 modulus 
                 modulus 
               
             
          
           
               
                 salt of 
                 7 
                 14 
                 28 
                 [mm] 
                 [MPa] 
                 [MPa] 
               
               
                 the EDTA 
                 days 
                 days 
                 days 
                 UNI 7044 
                 (1 day) 
                 (7 days) 
               
               
                   
               
               
                 Reference 
                 3650 
                 6006 
                 8919 
                 115 
                 18461 
                 19424 
               
               
                 Na-EDTA 
                 3344 
                 5381 
                 8106 
                 131 
                 18420 
                 20215 
               
               
                 (0.1%) 
               
               
                 (outside 
               
               
                 the present 
               
               
                 invention) 
               
               
                 Na-EDTA 
                 4488 
                 5975 
                 8894 
                 180 
                 7809 
                 18315 
               
               
                 (0.3%) 
               
               
                 (outside 
               
               
                 the present 
               
               
                 invention)