9.4. Hydraulic lime and lime containing binders

Hydraulic lime is the product, produced by the moderate burning of limestone, which contains 6-25% fine-grained clay and sand impurity substances. The silicates and calcium aluminoferrites, which provide lime the hydraulic properties, are formed at the burning of such fine-grained marl limestone along with calcium oxide.

There are weakly hydraulic and strongly hydraulic limes. The content of active calcium oxide and magnesium in the first one are 40-65%, in the second one 5-40%. The main characteristic of the raw materials for hydraulic lime is the hydraulic index (– ratio of the calcium oxide percentage to the total content of alumina, silicon and ferric oxides). It is 9-4.5 for the weakly hydraulic lime, and 4.5-1.7, for strongly hydraulic limes. Weakly hydraulic lime on interaction with water intensively slakes and scatters in the powder, whereas the strongly hydraulic limes slowly slakes or does not slake completely. The initial setting time of hydraulic lime occurs after 0.5-2 hours, the final set takes from 2 to 16 hours. With increasing free calcium oxide content the hardening accelerates.

The ultimate compressive strength for samples of lime-sand mortars of the proportion 1:3 (one part by weight of lime, 3 parts of normal sand) after 28 days for weakly-hydraulic lime must be at least 1.7 MPa, strongly hydraulic - 5 MPa. Hydraulic lime is used to manufacture building mortars for masonry and wall plastering in dry or wet environments. The mortars, based on it, are more durable than on air lime, but less plastic. Hydraulic lime can be used also to produce mixed lime containing binders and concrete of low strength classes.

Hydraulic lime containing binders include the binding materials, produced by grinding or mixing of air-hardening or hydraulic lime with active mineral additives(- natural and artificial substances that when mixed with lime and water gives a paste, which after hardening in the air conditions can continue to harden in water). Examples of natural active mineral additives are tuffs, trasses, diatomites, etc. The blast-furnace and fuel slags, ashes and others can be attributed to artificial additives.

The ability to bind Ca(OH)₂ at normal temperature (activity of mineral additives) is characterized by the amount of absorbed CaO from the lime mortars within 30 days. In binders, which include lime, the content of active calcium and magnesium oxides is 10-30% by the weight.

Hardening of these binders at normal conditions is slow. It is stipulated mainly by the interaction of calcium hydroxide with active silica and formation of hydrosilicates. Compressive 28 days strength of these binders is in the range of 5-20 MPa. The hardening rate at temperature 80-100 °C and at enhanced humidity is substantially accelerated.

A significant deficiency of lime-containing binders, is their low stability in air conditions. Stability may be improved by addition of gypsum additives, calcium or sodium chlorides, and by the replacement of air-hardening lime with hydraulic lime. They are also characterized by comparatively low freeze-thaw resistance. It is expedient to use them for mortars and concretes of low strength in underground or submerged structures and to manufacture products using thermal treatment, especially autoclaving.