CS235584B1 - Cement blend with increased strength increase, for hardening temperatures below 0 ° C - Google Patents

Abstract

The invention concerns a cement mixture with increased strength, especially for hardening temperatures below 0 °C. Its essence is that, in addition to Portland clinker-based cement and optionally water and/or filler, it contains 0.3 to 15% by weight of the cement mixture of a soluble salt of formic acid, for example calcium formate, with the ratio of formate to nitrate being from 5:100 to 100:5 parts by weight. The cement mixture according to the invention can also be produced in the form of a dry prefabricated mixture containing all components except mixing water. However, it can also be produced by the usual procedure for the production of mortars and concretes, in which the formate and nitrate are added in the form of an aqueous solution or in powder form to the other components of the mixture.

Description

The invention relates to a cement mixture with increased strength, especially for hardening temperatures below 0 °C.

In construction practice, it is often required to accelerate the increase in the strength of cement mixtures, especially when hardening in cold weather. As a rule, the most effective accelerators are chlorides, such as calcium or sodium chloride, which, however, cause corrosion of the steel reinforcement of concrete and therefore cannot be used in the production of reinforced concrete. Therefore, mixtures of calcium chloride with a corrosion inhibitor of steel reinforcement, such as sodium nitrite NaNOz, or nitrite — calcium nitrate Ca(NO2)2. Ca(NOj]2 (Guidelines for the application of chemical additives in concrete, Strojizdat Moscow 1980), while the recommended doses of antifreeze additives are as follows, depending on the type of additive and temperature:

hardening temperature concrete dosage range °C additives % weight up to — 5 3 to 6 — 6 to —10 5 to 9 —11 to —15 7 to 10

The anti-corrosion effect of sodium or calcium nitrite is, however, only temporary; the gradual oxidation of nitrite to nitrate and carbonation of concrete creates the risk of chloride corrosion of the steel reinforcement of concrete. However, the practical use of nitrites as anti-freeze additives in construction industry poses very difficult problems, because all nitrites are poisons. The urea additive [urea, or carboxyldiamide CO(NH2)2] has also been known for a long time, which is used either by itself as an anti-freeze additive or in combination with various inorganic salts, for example nitrate or calcium nitrite (Ratinov, V. B., Rozenberg, T. I.: Additives in concrete, Strojizdat Moskva 1973). However, urea has little effect on lowering the freezing point of mixing water and slows down the hydration of cement. Urea also causes carbonation of concrete and severe corrosion of concrete (Moskvin, V. M., Chernov, A. V.: Mechanism of concrete corrosion in carbamide solutions, Collection of scientific workers of the NIIŽB GOS-STROJA SSSR "Corrosion and durability of reinforced concrete in aggressive environments", Strojizdat Moscow 1980, pp. 15 to 21). Therefore, it is advisable to reduce the urea content in the composition of antifreeze additives as much as possible, or not to add urea to antifreeze additives at all.

The use of formates, such as calcium, as a hardening accelerator for concrete mixtures is also known. However, the addition of calcium formate does not ensure a sufficient increase in the strength of cement mixtures at negative temperatures.

The disadvantages of cement mixtures with the above additives are eliminated in the cement mixture according to the invention. Its essence is that, in addition to Portland clinker-based cement and optionally filler and/or water, it contains 0.3 to 15% by weight of the cement of a mixture of a soluble salt of formic acid, for example calcium or sodium formate, and a salt of nitric acid, for example calcium, sodium or potassium nitrate, the ratio of formate to nitrate being from 5:100 to 100:5 parts by weight.

As a cement based on Portland clinker, for example, slag Portland or Portland cement can be used in the cement mixture according to the invention, but at temperatures below 0°C, pure Portland cements are best suited.

During the tests, it was surprisingly found that the cement mixture according to the invention does not cause corrosion of the steel reinforcement of the concrete, has an accelerated increase in strength even at normal hardening temperatures (e.g. 20 °C) and has a good increase in strength even at hardening temperatures of -10 °C to -15 °C. The advantages of the cement mixture according to the invention are shown by the following examples of embodiments:

Example 1

The following mortars were produced from gray Portland cement of class PC 400 from the Mokrá site according to the procedure according to ČSN 72 2117: — comparative — with the addition of 8% calcium chloride CaCl2 — with the addition of 4% calcium formate

Ca(HCOO)2 and 4'% potassium nitrate KNOs. . .

Unlike ČSN 72 2117, however, the constant water coefficient of 0.5 was not observed, but the constant workability (plasticity) according to ČSN 73 2441 was observed. ........... ./

Test samples of mortars measuring 40 X 40 X 160 mm were hardened at the following hardening regimes:

a) test samples that were stored immediately after manufacture in a climate-controlled cabinet with a temperature of -10 C.

— 14 days at—10 °C — 14 days at —10 °C and 14 days at 20 °C in humid air (above 90% relative humidity).

bj 28 days in humid air

Since frozen concrete has higher strengths than thawed concrete (Mironov, S. A.: Theory and methods of winter concreting, Strojizdat Moscow 1975), and this phenomenon naturally occurs even when antifreeze additives are used (Rosenberg, T. I. et al.: Investigation of hardening processes of concrete with complex additives at temperatures below 0 C, Proceedings of the II. International Symposium RILEM on Winter Concreting in 1975 in Moscow, Part I, pp. 69 to 81, Strojizdat Moscow 1975), the strengths of test specimens stored at negative temperatures were tested immediately after thawing, which was within 1 hour after removing the test specimens from the area with negative temperatures and placing them in air with normal laboratory temperature (approx. 20 °C). The results are given in Table 1.

Table 1

Cement mixture In Strength in MPa c 14 days - -10°C 14 days -10°C 28 days humid· 14 days -j-20 °C (+20°C] Pull behind Pressure Pull for Push Pull for Push bending bending . bending .

Comparative

without additives with 8% chloride 0.50 0.1 1.6 4.7 16.5 7.9 40.5 calcium CaCl2 with 4% formate Ca, 0.53 0.5 2.9 3.3 12.5 3.7 20.2 4 ^% potassium nitrate 0.49 1.17 4.2 'θ' ¹ ., λ 20.8 ' . 7.1 ' ³⁶ - ⁶ Results achieved, which are listed in first 16 hours — 5 °C Table 1 shows that cement mixture another 8 hours and 5 days —15°C

According to the invention, it gained strength even at a temperature of -10 °C, and had the best strength after being transferred from freezing to an environment with positive temperatures.

Example 2

White cement from the Rohožník site was used for the tests. Test samples of mortars were produced using a similar procedure as in Example 1; immediately after production, the test samples were placed in an air-conditioned room with the following temperature regime:

next 4 days. -18 °C

Then they were taken out of the air-conditioned space. Some of the test samples were tested for compressive and tensile strength immediately after thawing, while others were stored for another 7 days in humid air (approx. 20 °C, above 90% relative humidity). The results are shown in the following table 2.

Table 2

Additive in Strength in MPa immediately after thawing after another 7 days at 20 °C

Pull for Pressure Pull for Bending Bending Pressure

Comparative with 4 °/o calcium formate Ca,(HCOO)2 with 8% calcium formate 0.5 CafHCOOjz with 8% sodium nitrate 0.47 NaNCb 0.48 with an ingredient of the stage of the invention — 2% calcium formate a 0.46 2% sodium nitrate 0.47

0.10 0.8 4.15 20.2 0.20 1.1 4.7 19.9 0.54 2.4 4.14 19.9 1.50 6.9 4.66 17.7 1.84 9.2 5.0 21.3

From the strengths of the mortars listed in Table 2, it can be seen that the combination of calcium formate and sodium nitrate according to the invention in a total amount of 4% of additives by weight of cement exceeded the effects of 4% and 8% calcium formate and 8% sodium nitrate, as well as the additive effect of 4% calcium formate and 8% sodium nitrate during hardening at negative temperatures, and did not have a negative effect on the further increase in strength after moving the test samples to an environment with positive temperatures.

Tests have shown that the cement mixture according to the invention does not cause corrosion of steel reinforcement in concrete.

The cement mixture according to the invention can be used with advantage, especially during work in winter, such as monolithic construction of assembled reinforced concrete structures.

The cement mixture according to the invention can be easily produced by one of the following processes:

a) a solution of nitrate and formate is prepared in mixing water, which is then added to the cement mixture in the usual way during production

b) a mixture of formate and nitrate is prepared in their crystalline or powder form, after which it is added as a powder additive to the cement mixture during production.

c) a mixture of cement and crystalline or powdered formate and nitrate is prepared, and this mixture is used in the same way as cement in the production of a cement mixture

d) a mixture of additives, cement and filler, for example silica sand or limestone wood, is prepared, to which only mixing water is added before use.

The above example does not limit the use of the cement mixture according to the invention, which can also be used at all hardening temperatures of cement mortars and concretes that are common in conventional technologies for their production.

Claims (1)

2 3 5 5 8 Ί 7 The cementitious composition according to the invention can be used particularly advantageously in the works, such as monolithic-mounted reinforced concrete structures. The cementitious composition of the present invention can be easily prepared by one of the following processes: a) a solution of nitrate and freeze-dried water is prepared, which is then customarily added to a mixture of b) to form a mixture of formate and nitrogen in the crystalline or powder form, and then added as a powder additive to the cementitious composition. c) preparing a mixture of cement and crystalline or powdered formate, and the mixture being used in the same manner as cement in the manufacture of the cementitious composition; d) preparing a mixture of additives, cement and filler, for example, silica sand, or crushed stone; to which only added water is added before use. The example does not limit the use of the cementitious composition according to the invention, which can also be used at all temperatures of curing of cementitious mortars and concretes which are common in conventional production techniques. OBJECT Cement blend with increased growth, not exceeding curing temperatures below 0 ° C, consisting of cement with Portland clinker and possibly filler and / or water, characterized in that it contains 0.3 to 15% of the weight of the cement mixture of the soluble salt of formic acid, for example, calcium or sodium formate, and the salt of nitric acid, for example, calcium, sodium, or potassium nitrate, wherein the ratio of formate to nitrate is from 5: 100 to 100: 5 parts by weight tional. Severografia, n. P., Závod 7, Most Price 2,40 Kcs