Self Healing Concrete: A Biological Approach

Concrete can be considered as a kind of artificial rock with properties more or less similar to certain natural rocks. As it is strong, durable, and relatively cheap, concrete is, since almost two centuries, the most used construction material worldwide, which can easily be recognized as it has changed the physiognomy of rural areas. However, due to the heterogeneity of the composition of its principle components, cement, water, and a variety of aggregates, the properties of the final product can widely vary. The structural designer therefore must previously establish which properties are important for a specific application and must choose the correct composition of the concrete ingredients in order to ensure that the final product applies to the previously set standards. Concrete is typically characterized by a high-compressive strength, but unfortunately also by a rather low-tensile strength. However, through the application of steel or other material reinforcements, the latter can be compensated for as such reinforcements can take over tensile forces. Modern concrete is based on Portland cement, a hydraulic cement patented by Joseph Aspdin in the early 19th century. Already in Roman times hydraulic cements, made from burned limestone and volcanic earth, slowly replaced the widely used non-hydraulic cements, which were based on burned limestone as main ingredient. When limestone is burned (or “calcined”) at a temperature between 800 and 900◦C, a process that drives off bound carbon dioxide (CO2), lime (calcium oxide; CaO) is produced. Lime, when brought into contact with water, reacts to form portlandite (Ca(OH)2) which can further react with CO2, which in turn forms back into calcite (CaCO3), or limestone, the pre-burning starting material. However, a major drawback of this non-hydraulic cement is that it will not set under water and, moreover, its reaction products portlandite and limestone are relatively soluble, and thus will deteriorate rapidly in wet and/or acidic environments. In contrast, portland cement produces, upon reaction with water, a much harder and insoluble material that will also set under water. For portland cement production a source of calcium, silicon, aluminum, and iron is needed and therefore usually limestone, clay, some bauxite, and iron ore are burned in a kiln at temperatures up to 1, 500◦C. The cement clinker produced is mainly composed of the minerals alite (3CaO.SiO2), belite (2CaO.SiO2), aluminate (3CaO.Al2O3), and ferrite (4CaO.Al2O3.Fe2O3), which all yield specific hydration products with different characteristics upon reaction with water.