Construction Research & Technology GmbH (Trostberg, DE) has patented an improved freeze-thaw durability cementitious composition is provided that uses at least partially degradable polymeric particles that are blended directly into the cementitious composition mixture. The degradable polymeric particles provide void spaces in the cementitious material matrix, and such void spaces act to increase freeze-thaw durability of the cementitious material. The formulation includes nano-silica according to U.S. Patent 7,648,575 issued on January 19th, 2010.
According to inventors Michael L. Kerns and Bruce J. Christensen, the cementitious composition's freeze-thaw resistance is provided by the incorporation of predictable "air" contents by use of small degradable polymeric particles having selected dimensions and optionally an added chemical or blend of chemicals that generate gas when dispersed into the cementitious mixture.
While not intending to be limited by theory, the polymeric particles are unstable in highly alkaline environments (as would be found in cementitious compositions) and degrade via base catalyzed hydrolysis over a period of hours, days, or weeks to leave a void. The degradable polymeric particles are also susceptible to degradation by enzymes and bacteria, as well as by catalysis from transition metals that are present in the cementitious composition. The void spaces are therefore created during the hydration process, during set, and further after the cementitious composition has hardened.
Traditional air entrainment techniques are variable in their efficacy and polycarboxylates are known in the art for higher-than-desirable air contents. The disclosed degradable particles allow for heavy use of defoaming agents to eliminate any adventitious air that might be brought about through variability in other raw materials in the mix design.
The use of degradable polymeric particles eliminates most of the problems in the current art. Degradable polymeric particles include fully degradable particles and at least partially degradable particles. By partially degradable, it is meant that a portion of the particle structure may not be susceptible to hydrolytic degradation, thus leaving a partially filled cavity. It also makes possible the use of materials, i.e., low grade, high-carbon fly ash which are currently landfilled as they are not usable in air-entrained cementitious compositions without further treatment. This results in cement savings, and therefore economic savings. In one embodiment the voids "created" by this approach are about 6 volume percent or less.
Traditional air entrainment techniques are variable in their efficacy and polycarboxylates are known in the art for higher-than-desirable air contents. The disclosed degradable particles allow for heavy use of defoaming agents to eliminate any adventitious air that might be brought about through variability in other raw materials in the mix design.
The use of degradable polymeric particles eliminates most of the problems in the current art. Degradable polymeric particles include fully degradable particles and at least partially degradable particles. By partially degradable, it is meant that a portion of the particle structure may not be susceptible to hydrolytic degradation, thus leaving a partially filled cavity. It also makes possible the use of materials, i.e., low grade, high-carbon fly ash which are currently landfilled as they are not usable in air-entrained cementitious compositions without further treatment. This results in cement savings, and therefore economic savings. In one embodiment the voids "created" by this approach are about 6 volume percent or less.
