Hydraulic lime is a moderate calcination product of marly limestone containing from 6 to 20% of clay impurities. Normal calcination temperature ranges from 900 to 1,100°C, depending on the feed composition. The product is comprised not only of free lime, but also from chemical compounds with oxides of clay: silicate 2CaO • Si02, aluminates 2CaO • AOz, ferrites 2CaO • BegOz calcium. Like the air lime this product represents lump hydraulic lime, which is subjected to grinding mills to produce fine ground quicklime. Yet there is another production option: lump is slaked and unslaked particles (silicates, aluminates and ferrites) are subjected to further grinding followed by mixing and slaking the unslaked parts.
When interacting with water silicates and aluminates remain virtually non-hydrated. Only when this extinction of lime in the dough gradually formed silicate and calcium hydro-aluminates, accompanied by swelling of the compounds and their transition into the jelly state. Depending on the content of calcium oxide in the lime with respect to the amount of silica, sesqui oxides of aluminum and iron can be divided into strongly-hydraulic and weakly-hydraulic.
Currently, the use of hydraulic lime is quite limited – for mortars and concretes of low strength, for laying in damp areas (basements, canals) in low-rise buildings, and others. It is not manufactured now the novel cement – hydraulic binder, gets crushed into a fine moderately burnt powder (not before sintering) lime or magnesia in the marl clay content in these impurities to 25-35%. This kind of cement is completely replaced by more advanced portland cements.
A true to the type portland cement product represents fine grinding of clinker, the resulting uniform roasting before sintering of natural raw materials (marl) or artificial homogeneous raw material mixture defined by composition containing limestone and clay. In the process of grinding the clinker gypsum added in an amount up to 3.5% (based on sulfuric anhydrite). It is possible to partially replace the feed mixture with blast furnace slag or nepheline sludge, if the necessary raw material retains the calculated composition.
The natural minerals in the form of lime marl are rare, so massive construction and design experts, like a UK-based Telling Company, massively utilising hydraulic lime and lime mortar solutions, oftentimes have to rely mainly on synthetic raw mixtures. As noted, they are compiled with the help of rocks with a high content of calcium carbonate (limestone net, chalk, lime tuff, marl limestone, etc.) and species belonging to the aluminous feedstock (heavy clay, marl clay, shale, etc.). Based on the chemical composition of the raw mixture and the desired characteristics calculated clinker composition ratio between its components with the greatest possible precision. Type of raw materials is reflected in the choice of equipment necessary for its preparation, roasting and grinding calcine with additives and its processing to the state the smallest homogeneous powder – portland cement.