CS252104B1 - The agent extends the pot life of inorganic building materials - Google Patents

Abstract

The use of a new, previously unknown effect of urea for extending the workability time of inorganic building materials without affecting the kinetics of the hydration reaction based on the effect of urea, in the presence of which the phenomenon called false solidification does not occur in inorganic building materials mixed with water, the origin of which is the rapid formation of bulky amorphous insoluble calcium compounds, but rather the slow formation of crystalline products, which occurs late after the actual solidification of the inorganic building materials. This phenomenon is very striking in the presence of additives containing a sulfo- or carboxy group in the molecule. The use of urea allows for better use of the good properties of plasticizers or flow-through agents, especially in transport concretes, self-leveling screeds, the creation of perfect surfaces in exposed concretes and the machine application of interior and exterior plasters.

Description

The invention relates to the use of a new, previously unknown property of urea, which can be used primarily to extend the workability time of inorganic building materials.

Nowadays, the use of additives influencing the initial or final mechanical and physical properties of inorganic building materials is increasingly common in the construction industry. These include additives influencing the kinetics of the hydration reaction, i.e. accelerators or setting retarders, additives influencing the permanent properties of building materials during processing, e.g. plasticity, fluidity, thixotropy, water retention, resistance to subzero temperatures during processing and during the hydration reaction, or additives and combined effects.

The current prevailing types of plasticizers, flow agents and setting retarders contain a sulfo group in the molecule, which reacts rapidly with calcium ions of building materials to form insoluble salts of macromolecules of the auxiliary additives. This results in a phenomenon that precedes the actual setting of the building material and is referred to by some authors as the so-called false setting, which significantly shortens the workability period of inorganic building materials, especially the fluidity of liquefied building materials. A similar, but less pronounced phenomenon also occurs with inorganic building materials not modified in the above-mentioned way. Accelerators (hardening accelerators) and frost-resistant additives based on carbonates and calcium chloride behave similarly, i.e. they significantly shorten the workability period.

The plethifying, or liquefying, effect of urea is well known, patented at the beginning of the century.

Recently, several other uses of urea as an additive to inorganic building materials have been registered, e.g. AO 167115 by Bruthanae, which uses the reduction of the freezing point of its aqueous solution to prepare building materials that set and harden at a reduced temperature, at concentrations above 5% by weight. The flow-through effect of urea allows a reduction in the water coefficient and thus partially paralyzes its retarding effect at these concentrations.

The retarding effect of urea is also mentioned by P. Tatard in British Pat. No. 2,001,620 A, p. 1, lines 25-26, while the simultaneous use of nitrates and sulfates for retarding CaSO^ is very debatable. In the described compositions, the retarding effect is primarily due to the addition of phosphogypsum, the added urea, during hardening when the compositions are heated above 105 °C in the pH range of 2-5, condenses with the melemlin or urea formaldehyde resole and thus increases the useful properties of the product, which is mainly related to lines 41-45, p. 2. In British Pat. No. apise 622 755 (also class C 04 B 13/26) the same author together with β M. Laquerbem use urea-formaldehyde resin in a dose of 1 to 20% by weight, which in these concentrations improves the workability by retarding the solidification of building materials not based on clay and cement, in addition to over-precipitating the composition. The accelerating effect of urea in small doses (up to 0.3%) on the rate of crystallization of building materials is also known, used e.g. to suppress the retarding effect of sulfonated phenol-formaldehyde polycondensate - Czechoslovak selective plasticizer for cement building materials UHAPÓIW SF or universal GDR plasticizer VISKOUENT based on melamine-formaldehyde sulfonated polycondensate.

The known effects of urea on the kinetics of hydration and crystallization of building materials can therefore be summarized as follows:

1. In doses up to 0.3% by weight of the construction binder, a slight to pronounced acceleration effect on the kinetics of solidification is manifested, depending on the type of binder.

2. In doses above 2% by weight, urea exhibits a retarding effect depending on the type of binder and urea concentration (see Fig. 2 and 3).

3. Based on the known state of the art, when the use of urea for retardation of building materials was considered, it was not possible to expect that urea would also be able to be used to extend the processing time of inorganic building materials without affecting the kinetics of the hydration reaction.

Knowledge about the effect of urea was obtained during work on the liquefaction of inorganic building materials containing calcium such as CaO, Ca(OH) ₂ , CaSO^ and CaCl ₂ . The problem of false setting, which was not so significant in unplasticized and even more so in unliquefied building materials, begins to manifest itself significantly with the use of additives containing sulfo-, oxalate- or carboxy groups in the molecule. The addition of a liquefaction agent or plasticizer containing one of these groups in the molecule immediately improves their workability and increases their mobility compared to building materials without these additives at the same water coefficient. However, their workability and mobility deteriorate rapidly and their consistency increases due to the preferential reaction of the sulfo group of the additive with the calcium ion of the binder with the formation of an amorphous insoluble precipitate with a large surface area. Most foreign manufacturers of plasticizers therefore recommend adding the plasticizer just before applying the building material, e.g. for ready-mixed concrete, not to the mixer, but to the automix upon arrival at the application site.

The mechanism of action of urea can be easily demonstrated by a simple in vitro experiment.

A small amount of a flow-through fluid (Umaforo SM, SF, SU, Melment, Mighty, Viskomin, Silfix, Plaetlfikátor S, Ligoplast, Ralentol, etc.) is added to water containing calcium ions (lime water, gypsum water, CaClg solution, etc.). After a short time, approximately 1 to 5 minutes (depending on the type of fluid), an opalescent turbidity develops, the increasing intensity of which can be precisely determined nephelometrically. After approximately 15 to 25 minutes, the active ingredient is practically quantitatively bound as its amorphous calcium salt. In the presence of urea, turbidity does not develop. After 250 to 800 minutes, shiny needle-like crystals of calcium and the additive begin to precipitate on the walls of the test tube. The crystallization of this salt is quantitatively completed only after several days. A similar, albeit slower, phenomenon can be observed in the effect of atmospheric CO2 on a saturated solution of Ca(OH) ₂ - lime water, and therefore analogously also on ready-mixed concrete transported over longer distances by automixer with slow mixing.

False setting and its suppression are most pronounced in building materials based on calcium sulfate as a binder. Table 1 and Fig. 1 derived from it show the effect of urea on the fluidity measured as the cake size (STSEV 826-77) of the liquefied self-leveling material based on anhydrite (KONHYDRIT), prepared according to example 1.

Table "1"

Laboratory fluidity measurement on a plasticity meter

time since mixing (min) 0 5 10 20 40 80 120 160 average 0 22.5 21.5 19.5 16.3 12.6 9.0 7.5 7.5 cakes % urea (cm) 1 23.5 23.0 23.0 22.4 21.6 20.5 19.3 7.5

Measurement of the onset of setting, setting time and setting interval of the same mass using an automatic Vicat apparatus depending on the addition of urea is shown in Table 2 and Fig. 2 and 3.

Table 2

Vicat hardening:

Addition of urea Start of solidification Setting time Solidification interval % wt (minutes) (minutes) (minutes) 0 210 335 125 0.5 245 330 85 1.0 295 340 45 1.5 305 345 40 2.0 310 360 50 3.0 363 470 107 4.0 425 810 385

In Fig. 1 is a graph of fluidity measured by a plasticity meter for 1 to 0% urea, 2 to 1% urea

Fig. 2 shows the beginning, time and interval of solidification

Fig. 3 shows the course of the solidification curves on the Vicat apparatus A - false solidification region B - retarded solidification region

The disadvantages of the so-called false solidification of inorganic building materials are eliminated by the addition of urea according to the invention, which consists in the fact that in its presence there is no rapid formation of amorphous precipitates of insoluble salts, but rather a slowed formation of crystalline products, which practically quantitatively suppresses the so-called false solidification of inorganic building materials and thereby prolongs their workability until the onset of actual solidification.

It is advantageous to use urea with inorganic shrinkage compounds bound by one of the following substances: anhydrite, cement, gypsum or lime or mixtures thereof.

It is advantageous if the necessary amount of urea is contained in the plasticizer or flow additive.

It is also advantageous if the necessary amounts of urea contain an additive that slows down or accelerates the solidification of the inorganic building material.

It is also advantageous if the required amount of urea is contained in the additive enabling the inorganic building material to solidify and harden at low and sub-zero temperatures.

It is also advantageous to use the necessary urea content in the catalysts for the solidification and hardening of anhydrite building materials.

The use of urea according to the invention in inorganic building materials is illustrated in the following examples: percentages and parts are by weight.

Example 1

The dry prefabricated anhydrite floor compound KDNHYDRIT, prepared according to AO 204 076 with a composition of 100 parts of fluoranhydrite, 0.5 parts of expanded perlite and 1.5 parts of KgSO^ as a hardening catalyst, was mixed in a continuous mixing and pumping system and made liquid with water and a flowable additive based on the sodium salt of the sulfonated melamine formaldehyde resin UMAPOBJí SK in an amount of 0.35 parts. With a water coefficient V/A = 0.32 and a dosage of 0.75% urea, the flowability time was extended from 20 minutes to 120 minutes, which allowed the pouring of a liquid self-leveling underlay with a thickness of 1.5 cm in a hall with an area of 1,470 m ² without interruption. The flatness control with a two-meter bar did not show deviations greater than + 0.2 aa, the levelness throughout the entire length of the hall within 2 mm.

Example 2

Pumpable transport concrete B 250, retarded and liquefied with the polyphenolic plasticizer and retarding additive RALENTOL, has a workability of 12 to 15 cm according to Abramee after mixing. After a 45-minute journey to the application site by automix, the workability of B 250 is below 7 cm according to Abramee. Difficult emptying of the automix and impaired workability lead to frequent dilution with water on the construction site to achieve the required workability, which results in a decrease in the strength of the concrete even below the control strength limit. The deterioration of workability is blocked by adding 0.5 to 0.8% by weight of urea to the mixer of the central concrete plant, so that at the application site after the same transport time the workability ranges from 10 to 12 cm according to Abramee. This application of urea made it possible to reduce the water coefficient and, with an initial workability of around 10 cm according to Abremee, achieve a control strength one class higher.

Example 3

For winter concreting of exposed concrete, a liquefied concrete mixture was prepared in the central concrete plant with the composition of 330 parts of PC-D 400 dispersion cement, 950 parts of Gravel 0/8, 380 parts of crushed aggregate 8/16, 470 parts of crushed aggregate 16/32, 150 parts of water, in which 3.5 parts of sodium salt of sulfonated melamine and urea copolycondensate with formaldehyde, 18.5 parts of potassium as an antifreeze additive and 3.6 parts of urea were previously dissolved. This concrete mixture had a workability of 23 cm according to Abrams. The half-hour transport to the application site did not change the workability and therefore the AMS was not diluted. Without the urea additive, the concrete would have to be prepared on site in an ambulatory mixer, because the increase in consistency after half an hour of transport would have made it impossible to empty the automix. The application took place in the temperature range of -1 to -7 °C. After removal of the formwork, the strength of the concrete was measured by a class higher.

Example 4

Prefabricated gypsum plaster composed of gypsum, lime, limestone, fine sand, retention and retarding additives and plasticizing additives based on sulfonated urea-formaldehyde resin had the beginning of setting, i.e. the workability time of 40 minutes from mixing with water and the end of setting, i.e. the setting time of approximately 95 minutes. By adding 0.6% urea in relation to the weight of gypsum binder to the mixing water, the workability time was extended to 65 minutes and the setting time was shortened to 85 minutes.

The advantage of the new effect of urea as a means of extending the workability time of inorganic building materials according to the invention is its wide use and application in the construction industry, and its advantage is its use for transport concrete, cast, flowable or self-leveling screeds and underlays.

Due to its very good solubility in water, urea can be used as a solution or a solid in combination with flow-through agents and plasticizers in one solution, in a mixture with solid additives accelerating or retarding solidification or hardening, additives increasing the resistance of the hydrating building material to frost or with additives having combined effects, as well as 8 catalysts for the solidification of anhydrite building materials.

Claims (1)

Use of urea to extend the pot life of inorganic building materials other than anhydrite, cement, gypsum, lime or mixtures thereof.