WO1998018739A1

WO1998018739A1 - Method for production of a set-accelerating composition for cement

Info

Publication number: WO1998018739A1
Application number: PCT/NO1996/000258
Authority: WO
Inventors: Harald Justnes
Original assignee: Nyfotek As
Priority date: 1995-05-26
Filing date: 1996-10-30
Publication date: 1998-05-07
Also published as: NO300588B1; NO952096D0; NO952096L

Abstract

Method for manufacturing of a set-accelerating and corrosion inhibiting composition for cement, comprising ammonium containing technical calcium nitrate, where one provides a reaction composition comprising (i) ammonium containing technical calcium nitrate dissolved in water and preferentially in the presence of an alkaline compound, (ii) an epoxy compound as represented by general formula (I), where R1, R2, R3 and R4 independently is hydrogen, substituted or unsubstituted, saturated or unsaturated alkyl; aralkyl; phenyl; or a heteroatomic group. The resulting reaction composition is maintained at a pressure and a temperature which maintains the reaction composition in a liquid phase and in a period of time sufficient to convert at least parts of the ammonia dissolved in the reaction composition to amine containing products, especially alkanol-containing compounds.

Description

METHOD FOR PRODUCTION OF A SET-ACCELERATING COMPOSITION FOR CEMENT.

The invention concerns a method for the manufacture of a set accelerating and corrosion inhibiting composition for cement, as described in the introductory part of claim 1.

Background

The increasing problem of chloride induced corrosion of reinforcement steel in concrete has led to development of several chloride free cement compositions. Special attention has been directed towards the manufacturing of chloride free set-accelerating compositions in order to substitute the commonly used calcium chloride accelerator. Calcium nitrate (Ca(NO₃)₂) was suggested as a basis component in a set-accelerating composition in combination with triethanol amine, see e.g. US patent No. 4.337.094. According to this publication an additive composition comprising an aqueous solution of calcium nitrate and polyalkanol amine is used in an attempt to avoid using the unfavourable chloride component. The polyalkanol component, comprised mainly of diethanolamine and triethanolamine, is also known to have an accelerating effect on the hardening of the cement, and is added according to this publication directly to an aqueous calcium nitrate solution. The primary purpose of this patent seems to be the achievement of the combination of the accelerating effect on the hardening together with use of a cheap raw material in the form of an alkanol distillate composition. Rettvin and Masdal ("The use of Calcium Nitrate Solutions as a Set-Accelerating Admixture in Slipforming of High Strength Concrete", Proc. ERMCO'95, [1995] Istanbul, Turkey) showed that addition of a 50% calcium nitrate solution (ammonium free) to cement gave a hardening-acceleration that was proportional to the added amount up until 0.5% of the weight of the cement. They have also described utilisation of calcium nitrate to obtain the necessary slip forming rate under construction of the shafts for the Troll platform in the North Sea (height 369 metres).

In order to investigate the possibility of using the cheap and easily available raw material calcium nitrate, which is primarily used as a fertiliser, a thorough study was started in 1992. The study was with regard to the utilisation of technical calcium nitrate, which is a complex nitrate salt based on calcium and ammonium with some crystal water, as an admixture to concrete. A typical composition for technical calcium nitrate is described by the formula xNH₄NO₃ . yCa(NO3)₂ . zH₂O, where x= 0.092, y =0.500 and z= 0.826. Ammonium may however also exist in the form of other salts. The effect of adding technical calcium nitrate on the setting characteristics of the cement, the compressive strength of the concrete, chloride induced corrosion of steel and freezing- and melting resistance and so forth is available from the following reports:

H. Justnes and 0 Vennesland, "CNas Concrete Admixture " , Report STF70 F92096, (1992), SINTEF; H. Justnes, "CNas Set Accelerator for French Cements ", Report STF70 F93013, (1993), SINTEF; and H. Justnes, "The influence of Technical Calcium Nitrate (CN) on the Setting Time, Heat Evolution, Strength Development and Durability of Cement, Mortar and Concretes ", Report STF70 F93138 (1993), SINTEF; H. Justnes and E.C. Nygaard: "Technical Nitrate as Set Accelerator for Cement", Nordic Concrete Research, No. 13, p 70-87 (1993); H. Justnes and E.C. Nygaard: "The Influence of Technical Calcium Nitrate Additions on the Chloride Binding Capacity of Cement and the Rate of Chloride Induced Corrosion of Steel Embedded in Mortars ", Proceeding of the Int. Conf. on Corr. and Corr. Protection of Steel in Concrete, vol 1, p. 491-502 (25-28 July 1994) Sheffield, UK; H. Justnes and E.C. Nygaard "Technical Nitrate as Set Accelerator for Cement at Low

Temperatures ", 4th CANMET/ ACI Int. Conf. on Superplasticizers and other Chemical Admixtures in Montreal, Canada, supplementary papers p. 71-80 (11-13 October 1994); H. Justnes and E.C. Nygaard: "Technical calcium nitrate as set accelerator for cement pastes at low temperatures ", Advances in Cement Research, 8, No. 31, 101-109 (July 1996), and H. Justnes: "Technical calcium nitrate as set accelerator for cement at low temperatures ", Cement and Concrete Research, Vol 25, No. 8, pp 7966-1774, Pergamon Press, NY (August 1995).

Technical calcium nitrate has accordingly proven to be a useful set-accelerator for cement with the primary objective of avoiding chloride induced corrosion of the concrete. A substantial disadvantage with calcium nitrate, however, is that the ammonia component of the nitrate, which in a solution will exist as ammonium ions (NH₄ ⁺), will be liberated as gaseous ammonia when untreated technical calcium nitrate is used directly as the accelerator. This disadvantage limits the use of technical calcium nitrate to low concentrations and to open areas, like for bridge constructions, due to the sickly smell and the health hazard of ammonia. To overcome this problem, it has been attempted to convert ammonia in technical calcium nitrate to hexamethylene tetramine by adding formaldehyde, see DE- Al-35 43 874. Formaldehyde is, however, not well suited for use in such a connection, firstly with respect to the handling of the manufacturing of the accelerator composition itself, secondly with respect to emission of any remaining, unconverted formaldehyde from the concrete under setting.

US patent No. 4.897.120 is a further development of the concept in the German publication mentioned above, and involves use of tri- or tetra-methyl glycolurile (TMGU) for immobilising ammonium under alkaline conditions. The process described in this US publication is, however, complex and time consuming and represents a costly production on a commercial scale. In addition the manufacturing of the set-accelerator will yield some undissolved hexamine and glycolurile, which will cause a depositing problem. It should further be mentioned that this product has not been put into any commercial use.

The Objective

The main objective with the present invention is to obtain a method for the manufacturing of a set-accelerator for cement, which will render it possible to use a technical calcium nitrate as a set-accelerator. An underlying objective is to provide a substance which also can exercise a corrosion inhibiting effect on the reinforcement steel in concrete and promote the setting process further.

The invention

These objectives are achieved with a method according to the characterising part of claim 1. Additional favourable features are disclosed in the dependant claims. The invention concerns a method for the manufacture of a set-accelerating and corrosion inhibiting composition for use in cement containing technical calcium nitrate. According to the invention one or more epoxy compounds with the formula

R ,_^ /°\ /^R3 R₂ ^/ R₄

are added to an aqueous solution of technical calcium nitrate. R,, R₂, R₃ and R₄ may independently from each other, be hydrogen, substituted or unsubstituted, saturated or unsaturated alkyl; aralkyl, phenyl; or a heteroatomic group, preferentially under alkaline conditions, whereby the epoxy compound reacts with ammonia absorbed in the solution of the technical calcium nitrate, thereby forming on or more amine containing compounds.

This way the ammonia component in the technical calcium nitrate solution is converted practically completely to a non-volatile substance. The epoxy- or epoxide group is a very reactive group, and all epoxide-functional compounds may in principle be used, alone or in combinations, for the purpose of the present invention. The reaction with ammonia runs very easily in aqueous solutions and the reaction is generally exothermic. The reactivity of the epoxy compound decreases however with increasing substitution of the epoxy ring, and thus the reactivity will decrease in the order: propylene oxide > isobutylene oxide > trimethyl-ethylene oxide > tetramethyl-ethylene oxide. While, for instance, ethylene oxide and propylene oxide reacts at room temperatures, a temperature of about 100 °C is required to achieve a satisfactory reaction rate with tetramethyl-ethylene oxide. Among the countless epoxy compounds the following can be mentioned as examples of other epoxide-functional compounds: 2,3-epoxy-propane-l-ol, epichlorohydrin, 2,3 epoxypropyl- isopropylether, 3-hydroxy-pyrrolidine, phenyl-oxacyclopropane, 1,2 diphenyl- oxacyclopropane, b-chlor-ethyl-oxacyclopropane. The temperature for the reaction composition will consequently vary according to the epoxy compound's structure. The pressure in the reaction composition will, with the exception of ethylene oxide as described below, generally be atmospheric.

The primary reaction product from the reaction between ammonia and the epoxy compounds are mono-, di- and tri-alkanolamines, but different by-products are also conceivable. "Alkan" in this context consequently means not only linear or branched alkanes with epoxy-functional groups, but also substituted variants and aromatic groups, alkenyl groups and heteroatomic groups. One of the central aspects with the present invention is that the method according to the invention gives an immobilisation of ammonium in technical calcium nitrate under formation of alkanolamine compounds, which are generally known to have a corrosion inhibiting effect on the reinforcement steel in concrete, as described in more detail below. In order to convert the ammonium ions in the calcium nitrate solution to ammonia, the reaction is carried out under alkaline conditions. This may be obtained e.g. with an addition of calcium hydroxide or magnesium hydroxide, which after completed reaction will be present as nitrate salts, depending upon what form the ammonium component in the technical calcium nitrate has. One might also use alkali- metal hydroxides or -carbonates as a pH-adjusting additive, but these compounds should be avoided in order to minimise unwanted alkali aggregate reactions in the concrete at later use. One should therefore underdose alkaline compounds to e.g. 80% of the ammonium content. The pH-level of the reaction mixture should be in the basic range, but should on the other hand not be too high as one in such a case will risk formation of alcohols in a reaction with water, whereby ammonia or ammonium remains unreacted in the set-accelerator admixture. The pH-level of the reaction composition will however vary in dependence with the ammonium concentration in the composition and the kind of epoxy compound that is used. It is, however, preferred to use ethylene oxide (CH₂CH₂O) which on reaction with ammonia forms a mixture of monoethanolamine (MEA), diethanolamine (DEA) and tetraethanolamine (TEA). The later two ethanolamines DEA and TEA, are especially known to promote the setting reaction of cement. The first mentioned reaction product, MEA, which does not give any significant set-accelerating effect, is on the other hand known to be an effective corrosion inhibitor for steel; this later property applies for alkanolamines in general, see e.g. H. Maeder, "A New Class of Corrosion Inhibitors ", Proceedings of the International Conference on Corrosion and Corrosion Protection of Steel in Concrete, 25-28 July 1994, vol II, page 851, Sheffield, UK. In addition to the ethanolamines mentioned above, one might also get minor amounts of the by-product hydroxyethylether (R₁R₂N-(CH₂CH₂O)n-CH₂CH₂-OH), but this by-product is less pronounced at reduced pH. The ethanolamines are moreover preferred due to their low toxicity. Since ethylene oxide has a low boiling point (13,5°C at atmospheric pressure), it is preferred to conduct the reaction under a moderately raised pressure, preferentially with the aid of an inert atmosphere, e.g. nitrogen, in order to maintain the reaction mixture in a liquid phase at room temperature. Alternatively the reaction composition can be maintained at a temperature below the boiling point of ethylene oxide, but this will slow down the reaction rate. Epoxy compounds are generally toxic, and for that reason one should avoid using epoxy compounds in excess with respect to the concentration of ammonia in the calcium nitrate solution. It is therefore preferred to use stoichiometric or less than stoichiometric amounts of epoxide in comparison to ammonia, and with other words the ratio between the epoxy compounds and ammonia in the reaction composition should be less than or equal to 3. If on the other hand the epoxy compound is added in excess, any remaining unreacted epoxy will in any event be rendered harmless when the set-accelerator composition is added to cement, as the pH of the setting concrete will cause the epoxy compounds to react with water under formation of alcohol compounds. The achieved set-accelerator composition can be used as it is, but from considerations of transportability it is preferred to remove the liquid component, e.g. by vapourization in order to get a set- accelerator composition in a solid state. The solid set-accelerator composition may if, one so wishes be given further treatment like granulating, pelletisation or the like.

Example

This example has the intention to illustrate the reaction process for immobilising ammonia in technical calcium nitrate by the use of ethylene oxide as a reactant. Technical calcium nitrate with the composition xNH₄NO₃ . yCa(NO₃)₂ . zH₂O, where x=0.092, y =0.500 and z=0.826, or with other words 19.00% Ca₂+ , 1.57% NH₄ + , 64,68% NO3- and 14.10% H₂O, all percentages given with respect to total weight of technical calcium nitrate, in an amount of 50 weight parts is dissolved in 50 weight parts of water. To the resulting solution is then added 1.31 weight parts of calcium hydroxide in order to convert about 80% ammonium to ammonia. Thereafter 5.0 weight parts of ethylene oxide is added to this weak basic solution, and the reaction composition is set under a pressure of 2 bar with the aid of nitrogen, whereby the reaction of ammonia to MEA, DEA and TEA progresses at room temperature (20-30 °C). As the reaction is exothermic, cooling is usually applied. It was shown that such a reaction progresses quantitatively and that the product did not have any smell of ammonia when made more alkaline by excess of calcium hydroxide. Alternatively, one may in the example above use 12 weight parts of epichlorohydrin instead of 5 weight parts of ethylene oxide. This will, however, require strong stirring and slight addition of an emulsifier to increase the surface between the aqueous phase and undissolved epichlorohydrin. On the other hand it will in this case not be necessary to set excess pressure to the reaction composition which is in contrast to the case with ethylene oxide.

Claims

1. Method for manufacturing a set-accelerating and corrosion inhibiting composition for cement, comprising ammonium containing technical calcium nitrate, characterized by providing a reaction composition comprising (i) ammonium containing technical calcium nitrate dissolved in water and preferentially in the presence of an alkaline compound, (ii) at least one epoxy compound as represented by the general formula:

R ,C\ R, c — C

R₂ R

where R, , R₂, R₃ and R₄ independently is hydrogen, substituted or unsubstituted, saturated or unsaturated alkyl; aralkyl, phenyl; or a heteroatomic group, and maintaining the reaction composition at a pressure and a temperature that maintains the reaction composition in a liquid phase and in a period of time sufficient to convert at least parts of the ammonia dissolved in the reaction composition to amine containing products.

2. Method according to claim 1, characterized by that the epoxy compound being added in a stoichiometric or a less than stoichiometric amount in comparison to the ammonia in the reaction composition.

3 Method according to claim 1 or 2, characterized in that the epoxy compound is ethylene oxide.

4. Method according to claim 1 or 2, characterized in thtat the epoxy compound is epichlorohydrin.

5. Method according to any one of claims 1 to 4, characterized in that the alkaline compound is calcium hydroxide and/ or magnesium hydroxide.