Modern architectural companies involved in historical restorations and ambitious modern construction projects, like Wolverhampton-based Telling, largely utilising neoteric hydraulic lime and lime mortar solutions, sets tough requirements towards cement quality. It should offer good adhesion after mixing with water and sand, but only after a certain period of time (45 to 60 minutes), so it can be prepared and used diligently. According to the theory of academician Baikov, cement setting takes place in three stages. First, there is the hydration of the mixture to form particles of calcium hydroxide, which is excreted in the amorphous state, glues cement grains. The second stage is setting of the cement. Then begins the third step – crystallisation or solidification. Calcium hydroxide particles become larger, turning into a needle-like crystals that seem to be riddling the amorphous mass of calcium silicate and sealing it. It should be noted that hardening of cement is the most effective when the temperature is not too low. Therefore, in winter the builders are taking measures to heat the buildings that are under construction.
A higher quality than the silicate has aluminous cement, produced with the use of clays of a low silica content. The main components of this cement are calcium aluminates. It is more expensive than silica, but it offers more advantages. So, aluminous cement better resists the action of the sea water, it hardens quickly, and moreover, the addition of water to calcium aluminate provides exothermic reaction. This is very important because it is possible to conduct the work in winter with no extra expenses on heating designs. Within hours the solidified calcium aluminate cement has a strength that some lime types achieve only in a month. Not accidently, it is called ‘stone glue’. With this cement at your disposal you can prepare the concrete, which is not afraid of water, does not burn in the fire and doesn’t lose its properties for long decades to come.
However, at it melts at excessively high temperature; and if we have no other choice but to start building houses of steel, the walls of the buildings would have needed to be 40 times thicker than concrete ones, as metals readily give off heat. When people weigh the pros and cons, it turns out that there is no better concrete building material than the mixture of cement, gravel, sand and water. This filler is enveloped with cement mortar, laid in the appropriate form, carefully compacted. The hardened mass forms a solid monolith.
Concretes are classified by the strength, bulk density and use. The most important characteristic of the resulting material is the volume weight. For example, heavy concrete – the one that is used most often – has a volume weight of about two tons per cubic meter. Lightweight and ultra-light concrete weigh one and a half tons to three hundred kilograms per cubic meter. Concrete structures are reinforced by introducing filler slag, pumice, tuff gravel and/or expanded clay (expanded clay). Particularly lightweight concrete grade is obtained by foaming the mass in different ways.