Cement compositions

Details Cement compositions Cement compositions

1994-01-03

1995-08-22

Bell, Mark L.

Compositions: coating or plastic

Coating or plastic compositions

Inorganic settable ingredient containing

106819, 106820, C04B 2200

Patent

active

054436354

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION

1. Technical Field
This invention relates to a new admixture for cement compositions with the expansion action almost independent of temperature and for preparing cement compositions with improved durability to freezing and thawing, and relates to cement compositions using the admixture.
2. Background Art
An expansion agent is added to cement compositions such as a cement milk, mortar and concrete to provide lightening property, adiabatic property, low contraction property and the like.
Further, an expansion agent is added to, e.g., concrete for being placed in a constraint space where a construction joint cannot integrate with old concrete like a back placing concrete working method to compensate volume decrease.
Conventionally, aluminum powder is generally used as such an expansion agent in view of influence on concrete properties and circumstance, and economy. This expansion agent made of aluminum powder is chemically reacted with alkali components in a cement mixture to generate hydrogen gas. The hydrogen gas acts so as to expand surrounding cement mixture.
However, for aluminum powder, the expansion amount is significantly influenced by temperature. Even if an additive amount of aluminum powder is the same, the expansion amount increases with higher temperature and the expansion amount decreases with lower temperature. Thus, control of expansion amounts is difficult. Further, concrete using aluminum powder as an expansion agent has poor resistance to freezing and thawing. Since aluminum powder is liable to be oxidized in the air, care should be taken to handling and storage thereof.
An object of the invention is to provide an admixture for cement compositions with the expansion action almost independent of temperature, for preparing cement compositions with improved resistance to freezing and thawing, and to provide an admixture for cement compositions with stability in the air; and to provide cement compositions with the admixture.


DISCLOSURE OF THE INVENTION

The invention is an admixture for cement compositions characterized by containing tetrahydroborate, or a cement composition containing such tetrahydroborate, cement, aggregate and water.
Illustrative tetrahydroborate used for the invention are sodium borohydride (NaBH.sub.4), lithium borohydride (LiBH.sub.4), calcium borohydride (Ca(BH.sub.4).sub.2) and ammonium borohydride (NH.sub.4 BH.sub.4). Of them, sodium borohydride is preferably used in view of the handling property.
Sodium borohydride is a complex metal hydride in a white powder form with molecular weight of 37.86. For the thermal stability thereof, it is stable in dry air up to 300.degree. C., it is decompounded (under vacuum) at 400.degree. C. and it is rapidly decompounded above 550.degree. C. to generate hydrogen and a slight amount of diborane. Although it is inflammable and hygroscopic, it can be handled in the air. It is gradually hydrated with moisture to hydrolysis.
It appears that expansion action due to such tetrahydroborate is resulted from that the tetrahydroborate generates hydrogen gas in a cement mixture and this hydrogen gas foams to provide cells inside.
This foam action is hardly influenced by temperature. Expansion amounts due to tetrahydroborate are almost determined by additive amounts thereof. Namely, if additive amounts are the same, the same expansion amount can be obtained, for example, at 20.degree. C. and 5.degree. C. Accordingly, since desired expansion amounts are obtained by adjustment of additive amounts, expansion amounts can be easily controlled.
Tetrahydroborate has an advantageous effect for improving the resistance property against freezing and thawing of a hardened body as well. It appears that this effect is caused by fineness of cells generated by the above foam action.
Cement compositions of the invention basically become a rigid hardened body by chemical reaction between cement and water. The main components thereof are cement, water and aggregate. To the main components are added the above tetrahydroborate and, if necessary, the

REFERENCES:
patent: 4142909 (1979-03-01), Gaines
Database WPI, Derwent Publications Ltd., Class H01, AN 92-164731, SU-A-1 654 542, Jun. 7, 1991.

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