Expanding cement can be used to make prestressed concrete building components.
Expanding cement, also known as expansive cement, is a relative of Portland cement that contains materials that increase in volume as they set. It's usually used in situations where the shrinkage of conventional cement is undesirable or where the cement needs to create pressure on another part of a structure. Expanding cement works well to prevent leaks and is a common construction material on deep water oil rigs.
Materials
This type of cement is made by using a Portland cement base composed of kilned limestone, clay and gypsum. The limestone and clay are heated together to a temperature of around 2,600 degrees Fahrenheit, which transforms the material into dry pieces of cement. This material is ground with sulfoaluminate clinkers. These clinkers are made by kilning limestone, calcium sulphate and bauxite together at a temperature of about 2,300 degrees Fahrenheit. When exposed to water, sulfoaluminate expands in volume.
History
According to the Encyclopedia Britannica, expanding cements were first invented in France during the mid-1940s. This cement included not just sulfoaluminate and Portland cement but also blast furnace slag, added as a stabilizing agent. It was the first successful expanding cement that worked reliably and remained stable over a long period. Another type of expanding cement, developed in Russia around the same time, used Portland cement, gypsum and alumina cement. Expansive cement ingredients have remained roughly the same since its development, though manufacturers have since improved its predictability, strength and working time.
Uses
Most Portland cements shrink as they dry since the water used to activate them increases their volume. In some applications, this shrinkage can reduce the strength of the cement's bond to nearby objects and structures or create leaks. Expansive cements allow contractors to create large, continuous floor slabs without joints, work well to fill holes in foundations and to create self-stressed concrete that is stronger than conventional Portland cement concrete. Prestressed concrete components for bridges and buildings are made using this material.
Misconceptions
Expanding cement is sometimes referred to as hydraulic cement. However, any cement that reacts with water is technically a hydraulic cement, including Portland cement. All hydraulic cements, including expanding types, will cure under water, but some shrink considerably during the process. Choose cement listed as "expanding," "sulfoaluminate" or "nonshrinking" to avoid this problem.
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