certain
proportions is burned until it is partly fused (some natural marl is
already of about the right composition), and the clinker so produced is
ground to powder, the product is called cement. When this material is
moistened it sets to a hard stone-like mass which retains its hardness
even when exposed to the continued action of water. It can be used for
under-water work, such as bridge piers, where mortar would quickly
soften. Several varieties of cement are made, the best known of which is
Portland cement.

~Growing importance of cement.~ Cement is rapidly coming into use for a
great variety of purposes. It is often used in place of mortar in the
construction of brick buildings. Mixed with crushed stone and sand it
forms concrete which is used in foundation work. It is also used in
making artificial stone, terra-cotta trimmings for buildings, artificial
stone walks and floors, and the like. It is being used more and more for
making many articles which were formerly made of wood or stone, and the
entire walls of buildings are sometimes made of cement blocks or of
concrete.

~Calcium carbonate~ (CaCO_{3}). This substance is found in a great many
natural forms to which various names have been given. They may be
classified under three heads:

1. _Amorphous carbonate._ This includes those forms which are not
markedly crystalline. Limestone is the most familiar of these and is a
grayish rock usually found in hard stratified masses. Whole mountain
ranges are sometimes made up of this material. It is always impure,
usually containing magnesium carbonate, clay, silica, iron and aluminium
compounds, and frequently fossil remains. Marl is a mixture of limestone
and clay. Pearls, chalk, coral, and shells are largely calcium
carbonate.

2. _Hexagonal carbonate._ Calcium carbonate crystallizes in the form of
rhomb-shaped crystals which belong to the hexagonal system. When very
pure and transparent the substance is called Iceland spar. Calcite is a
similar form, but somewhat opaque or clouded. Mexican onyx is a massive
variety, streaked or banded with colors due to impurities. Marble when
pure is made up of minute calcite crystals. Stalactites and stalagmites
are icicle-like forms sometimes found in caves.

3. _Rhombic carbonate._ Calcium carbonate sometimes crystallizes in
needle-shaped crystals belonging to the rhombic system. This is the
unstable form and tends to go over into the other variety. Aragonite is
the most