Concrete is practically fireproof in that it can not be consumed by fire,
but unless properly made and of the right materials it will disintegrate,
at least on the surface.
To resist fire concrete should be mixed fairly rich, say, 1:1-1/2:3, or
1:2:4 and special care should be taken to grade the sand and gravel to
secure a dense mixture.
The aggregates should be selected with a view to their fire-resisting
properties. The sand should be siliceous and the larger aggregate
should not disintegrate when heated; hence, marble, granite, limestone,
materials containing quartz, and some gravels are unsuitable. Cinders are
specially valuable, due to their non-conductivity, but can not be used
where strength is required. Trap rock will resist destruction by heat
and produce a strong concrete. Blast furnace slag is very good for this
purpose.
Fireplaces and chimneys of dwellings[2] may be constructed of ordinary
concrete but the back, jambs, and inner hearth, which are directly
exposed to the heat of the fire, should be made of specially prepared
concrete as described above or should be lined with firebrick, although
concrete made with broken hard-burned brick or terra cotta has been
used successfully. If suitable large-sized aggregate is not available a
mixture of one part cement and three parts sand may be used.
[2] See Farmers' Bulletin No. 1230, Chimneys and Fireplaces, U. S.
Department of Agriculture.