These are caused either by over-burning or under-burning.
Over-burnt. - In the former case, a hard and heavy substance is produced, burnt almost to a clinker, which slakes with very great difficulty, and after a considerable lapse of time.
This can only be remedied by screening out the hard portions and grinding them to a fine powder; otherwise any larger particles that may be left will slake after the mortar has been laid in the work, and may do great damage to the masonry.
Under-burnt. - When, on the other hand, the stone has been under-burnt, a somewhat similar result occurs, but from a different cause.
The substance produced consists partly of a perfect cement or hydraulic lime, and partly of free quicklime. The latter is prevented, by the setting action of the cement, from slaking at once, but does so eventually, and with the same disastrous consequences as occur with over-burnt lime.
This dangerous action may, however, be got rid of by free exposure of the lime or cement, so as to air-slake the caustic portions, or by frequently reworking the mortar, or by adding a proportion of soluble silicates, which anticipate and prevent the slaking action. The latter is, however, seldom if ever done in practice.
Dead-burnt Lime is lime that has been imperfectly calcined and will not slake with water.
This may be caused by under burning, so that only part of the carbonic acid is expelled, the resulting substance being a compound of quicklime and carbonate of lime, which refuses to slake.
Hydraulic lime may be rendered "dead" by over-burning; the silicates are partially fused and coat the stone, so that the evolution of the carbonic acid is prevented 2 (see p. 234).
Flare-burnt Lime is lime burnt in flare kilns, in which it is kept cleaner than in tunnel kilns owing to its not being in contact with the fuel.
1 Certain varieties of overclayed cements yield a deep bronze-coloured clinker which, as it cools on coming from the kiln, disintegrates spontaneously into a fine flakey greyish powder which produces an inert cement. - M.P.I.C.E., vol. lxii. p. 81.