GB2170495A - Cement accelerator - Google Patents

Abstract

A cement composition contains calcium nitrite in an amount of 0.1 to 3.0 wt% based on cement and a thiocyanate in an amount of 10 to 200 wt% based on said calcium nitrite, and optionally an amino alcohol in an amount of 0.005 to 0.06 wt% based on cement. Owing to these additives, the cement composition sets and hardens rapidly and has improved non-corrosive performance.

Description

SPECIFICATION Cement composition Field of the invention The present invention relates to a cement composition which sets and hardenes rapidly and does not corrode reinforcing steel.

Description of the prior art Concrete is prepared by mixing cement, aggregates, water, and admixtures according to a variety of mix proportions. And concrete is incorporated with a certain substance that promotes the hydration of cement in order to utilize a limited number of forms efficiently or in order to speed up the setting and hardening rate of concrete at normal or low temperatures. Such a substance is called a cement accelerator which has been developed in so many kinds of substance such as chlorides, sulfates, thiosulfates, and carbonates of alkaline earth metal e.g., calcium and alkali metal e.g., sodium. They are mostly of no practical use, except calcium chloride. Calcium chloride has long been used because of its low price and good accelerating performance.It increases the initial strength of concrete by 30 to 40% when it is added to cement in an amount of 1 wt % based on cement. However, it has a disadvantage of corroding reinforcing steel in concrete. Thus, there has been a demand for a new accelerator that replaces calcium chloride.

In Japanese Patent Publication No. 22957/1967, there was proposed calcium nitrite as a non-corrosive accelerator. Since then, calcium chloride was partly replaced by this new comer. Being inferior to calcium chloride in accelerating performance, calcium nitrite is not so practical where rapid setting and hardening are required. In Japanese Patent Laid-open No. 67325/1976, there was proposed the combination of calcium nitrate and calcium formate which would improve the accelerating performance of calcium nitrite while keeping the non-corrosive properties of calcium nitrite. The above mentioned accelerator composition, however, cannot be made into a solution of practically high concentration because the solubility of calcium formate toward water is only 10% or so. In addition, the composition is not so excellent in accelerating performance and not so economical as calcium chloride.

U.S. Patent No. 3,782,991 discloses an amino alcohol or a composition of amino alcohol and sodium silicate as an accelerator. They are not satisfactory in accelerating performance. West Germany Patent No. 2,611,419 discloses an accelerator composed of calcium formate as a major component and calcium thiocyanate as a minor component. This composition is of no practical use because calcium thiocyanate is corrosive and calcium formate is not highly soluble in water and not so good in accelerating performance.

Although it is known that an accelerator of practical value should have both the accelerating performance of calcium chloride and the non-corrosive properties of calcium nitrite, such a product has not been available so far.

Summary of the invention It is an object of the present invention to provide a cement composition incorporated with an admixture which has both the non-corrosive properties of calcium nitrite and the accelerating performance of calcium chloride.

The cement composition of this invention contains 0.1 to 3.0 wt% of calcium nitrite (based on cement) and 10 to 200 wt% of thiocyanate (based on calcium nitrite). The cement composition may optionally contain 0.005 to 0.06 wt% of amino alcohol (based on cement).

Detailed description of the invention The cement used in this invention is hydraulic cement that sets and hardenes through the hydration reaction. It includes, for example, normal Portland cement, high-early-strength Portland cement, moderate heat Portland cement, sulfate-resistant cement, fly ash cement, and Portland blast-furnace cement.

The calcium nitrite used in this invention may be a commerical product which does not adversely affect the setting and hardening of cement and does not corrode reinforcing steel. It may be in the form of either powder or aqueous solution, the latter being convenient for use. The thiocyanate used in this invention is a compound represented by the formula M(SCN) (where M denotes a metal atom and n is an integer corresponding to the valence of M). This compound is also called rhodanate, rhodanide, or thiocyanide. Examples of M include sodium, potassium, magnesium, and calcium. Preferred examples of thiocyanate used in this invention include sodium thiocyanate and calcium thiocyanate. They may be used in combination with each other.The amino alcohol used in this invention is a water-soluble organic compound of such a structure that the hydrogen atom of an amine is replaced by an alkanol group. It includes, for example, monoethanolamine, diethanolamine, triethanolamine, N,N-dimethylethanolamine, N,N-diethylethanolamine., N-(ss-aminoethyl)ethanolamine, N-methylethanolamine, and N-methyldiethanolamine. Preferable among them are triethanolamine, diethanolamine, and monoethanolamine. They may be used in combination with one another.

To prepare the cement composition of this invention, the above-mentioned calcium nitrite is added in an amount of 0.1 to 3.0 wt% based on cement, and the thiocyanate is added in an amount of 10 to 200 wt% based on the calcium nitrite, and, optionally, the amino alcohol may be added to those in an amount of 0.005 to 0.06 wt% based on cement. These components exhibit greatly improved accelerating performance and non-corrosive properties on account of their synergistic effect. Therefore, these components as a whole are comparable to calcium nitrite in non-corrosive properties and to calcium chloride in accelerating performance when compared on the basis of unit weight.

If the amount of calcium nitrite is less than 0.1 wt% based on cement, the resulting cement composition is not satisfactory in non-corrosive properties. Even if the amount of calcium nitrite is increased more than 3.0 wt%, no additional improvement is made in the non-corrosive properties, but the accelerating performance per unit weight of the additive decreases. If the amount of thiocyanate to be added together with calcium nitrite is less than 10 wt% based on calcium nitrite, the synergistic effect on the accelerating performance is not produced. If the amount of thiocyanate exceeds 200 wt%, the non-corrosive properties are adversely affected. The preferred amount is 20 to 100 wt% based on calcium nitrite.

When an amino alcohol is used in combination with the above-mentioned two components, it should be added in an amount of 0.005 to 0.06 wt% based on cement. The amino alcohol will further improve the accelerating performance.

Example 1 One percent aqueous solutions each containing calcium nitrite and sodium thiocyanate or calcium thiocyanate were prepared as shown in Table 1. They were examined for non-corrosive properties by measuring the natural electrode potential which is generated when a sample of reinforcing steel was dipped in them. The change with dipping time of natural electrode potential was measured according to JIS A6205 using a silver-silver chloride electrode and calcium hydroxide-saturated aqueous solution in place of pure water. The results are shown in Table 1.

It is noted from Table 1 that in experiments where the amount of thiocyanate is more than 300 wt % based on calcium nitrite, the potential rise toward less noble, which leads to the corrosion of reinforcing steel. On the other hand, in experiments (Nos. 4, 5, 8, and 9) where the amounts of thiocyanate is less than 100 wt% based on calcium nitrite, the potential increases toward noble as in experiment No. 1 where calcium nitrite is used alone. This indicates that they have the non-corrosive properties.

TABLE 1 Potential with respect to silver chloride electrode (-mV) Additive composition (%) Dipping time (hours) Experiment No. Ca(NO2)2 NaSCN Ca(SCN)2 1 3 6 24 48 72 96 120 144 168 1 100 - - 218 194 183 146 145 144 143 144 144 144 2 - 100 - 603 651 630 561 562 586 570 584 573 573 3 - - 100 490 512 582 550 556 543 551 540 536 541 4 75 25 - 225 219 216 191 183 173 164 158 155 155 5 50 50 - 225 218 215 190 183 174 164 159 155 155 6 25 75 - 236 245 245 423 455 527 541 513 500 500 7 12 88 - 405 477 519 506 488 473 504 519 500 500 8 75 - 25 244 223 201 172 167 163 162 163 161 161 9 50 - 50 213 202 185 166 161 155 154 155 155 155 10 25 - 75 256 270 257 301 372 379 380 384 389 389 11 12 - 88 231 253 318 378 430 445 451 452 454 454 Examples 2 to 7 and Comparative Examples 1 to 5 Different kinds of mortar each containing accelerators as shown in Table 2 were prepared according to JIS R5201. The accelerating performance was evaluated by measuring the Proctor penetration resistance when the proctor needle is penetrated into the mortar in depth of 0.5 inches according to ASTM-C40363T. The results are shown in Table 2.

It is noted from Table 2 that the additive components greatly improve the accelerating performance on account of their synergistic effect.

TABLE 2 Amount of additive on cement (%) Proctor penetration resistance (psi) Example No. Ca(NO2)2 Ca(SCN)2 NaSCN TEA CaCl2 4 hours 5 hours 6 hours 7 hours 2 1.0 - 0.2 - - 170 340 880 2240 3 1.0 - 0.2 0.025 - 135 430 1160 2920 4 0.8 - 0.4 0.030 - 185 460 1360 3600 5 1.0 0.20 - - - 160 320 840 2000 6 1.0 0.20 - 0.025 - 165 360 1280 3360 7 0.8 0.40 - 0.030 - 210 520 1200 2880 1 * - - - - - 65 150 240 520 2 * 1.0 - - - - 130 280 660 1520 3 * - - - - 1.0 200 440 1200 3040 4 * - - 1.0 - - 130 240 620 1680 5 * - 1.0 - - - 160 300 800 2000 * Comparative Examples TEA stands for triethanolamine.

Examples 8 to 13 and Comparative Examples 6 to 10 The additives were added to cement in varied ratios as shown in Table 3. One part of the cement was mixed with 2 parts of standard sand, with the water-cement ratio being 65%, according to JIS R5201. The resuiting mortar was formed into a specimen measuring 4 x 4 x 16 cm. The compression strength of the specimen was measured 1 day, 7 days, and 28 days after curing in water at 20 C, whereby the accelerating performance was evaluated. The results are shown in Table 3.

In Comparative Example 7 where 1 wt% (based on cement) of calcium nitrite was added, the compression strength of the mortar specimen after curing for 1 day was about 80% of that of the mortar specimen containing no additives (Comparative Example 6). Even after 7 days of curing, it increased to only about 110%. This indicates that calcium nitrite alone accelerates setting and hardening only a little. In Example 10 where 0.8 wt% of calcium nitrite, 0.4 wt% of calcium thiocyanate, and 0.030 wt% of triethanolamine were added to cement, the compressive strength of the mortar specimen was 148% of that of control after 1 day of curing and 132% of that of control after 7 days of curing. This suggests a remarkable accelerating effect.

TABLE 3 Amount of additive based on cement lO/ol Compressive strength (kglcm2) Example No. Ca(NO2)2 Ca(SCN)2 NaSCN TEA CaCI2 1 day 7 days 28 days 8 1.0 - 0.20 - - 67.2 297 416 9 1.0 - 0.20 0.025 - 71.7 326 436 10 0.8 - 0.40 0.030 - 78.4 304 420 11 1.0 0.20 - - - 68.9 271 411 12 1.0 0.20 - 0.025 - 75.6 278 417 13 0.8 0.40 - 0.030 - 82.9 312 422 6 * - - - - - 56.0 236 407 7* 1.0 - - - - 45.9 260 410 8* - - - - 1.0 74.5 322 401 9* - - 1.0 - - 68.9 271 409 10* - 1.0 - - - 67.2 257 405 * Comparative Examples TEA stands for triethanolamine.

Effect of the invention: It is said that calcium nitrite produces the corrosion preventive effect because it prevents the passive state film on reinforcing steel in alkaline concrete from being destroyed. It is a surprise to find that the cement composition of this invention exhibits almost the same non-corrosive properties as the one which contains calcium nitrite alone, although it contains 100 wt% (based on calcium nitrite) of thiocya nate that causes corrosion.

It is an unexpected effect that a very small amount of amino alcohol greatly improves the accelerating properties of cement composition containing calcium nitrite and thiocyanate.

The cement composition of this invention can be made into cement paste, mortar, and concrete ac cording to the usual mix proportions. In addition, it can be incorporated with a water-reducing agent such as sulfonated melamine-formaldehyde condensate, sulfonated naphthalene-formaldehyde conden sate, sodium ligninsulfonate, hydroxycarboxylate, and polyol; air-entraining agent; and other ordinary cement admixtures such as fly ash and slug powder. Thus the cement composition of this invention is of great practical use and is effective in improving the properties of mortar and concrete.

Claims (7)

1. A cement composition which comprises containing therein calcium nitrite in an amount of 0.1 to

3.0 wt% based on cement and a thiocyanate in an amount of 10 to 200 wt% based on said calcium nitrite.

2. A cement composition as claimed in Claim 1, wherein the thiocyanate is sodium thiocyanate, po tassium thiocyanate, calcium thiocyanate, or magnesium thiocyanate, or a mixture thereof.

3. A cement composition which comprises containing therein calcium nitrite in an amount of 0.1 to

3.0 wt% based on cement, an amino alcohol in an amount of 0.005 to 0.06 wt% based on cement, and a thiocyanate in an amount of 10 to 200 wt% based on said calcium nitrite.

4. A cement composition as claimed in Claim 3, wherein the amino alcohol is triethanolamine, die thanolamine, or monoethanolamine, or a mixture thereof.

5. A cement composition as claimed in Claim 3 or Claim 4, wherein the thiocyanate is sodium thiocy anate, potassium thiocyanate, calcium thiocyanate, or magnesium thiocyanate, or a mixture thereof.

6. A cement composition as claimed in claim 1 and substantially as herein described in any one of the specific examples hereinbefore set forth.

7. Each and every novel embodiment herein set forth taken either separately or in combination.