Civil Engineering Reference
In-Depth Information
slower at low temperatures. During setting, moisture escapes, which needs to be
ventilated.
Gypsum
Calcined gypsum is a widely-used binder. It is usual to grind the calcined
substance with larger or smaller parts of lime or dolomite, which act as cata-
lysts for setting. The calcined gypsum can even be used as plaster of Paris as
it is.
In Germany, a plaster cement which can compete functionally with Portland
cement is developed. This is a hydraulic product.
Additives in cement
Cement is often complemented with additives, either while dry or during mix-
ing when water and other mineral constituents are added. The first additives
were used as early as 1920, but only in small amounts. During the 1960s and
1970s the amounts grew. In Denmark there are now additives in 60-70 per cent
of all concrete (Strunge, 1990). The actual amounts vary, but the additives seldom
form more than 1 per cent of the weight of the cement. Amongst the most impor-
tant additives are:
•
Airing agents
, used to increase the workability, reduce the need for water,
etc. These additives are benzene-compounds and phenolaldehyde conden-
sates.
•
Water reducing agents
up to 5-10 per cent by weight which reduce the surface
tension of water. Examples are waterglass, sodium and soda.
•
Accelerators
, which increase the rate of setting. Calcium chloride at 1.5 per cent
by weight. Different amounts of sodium, potassium, lithium or ammonia salts
can also be used. Triethanolamine, waterglass, soda and aluminium com-
pounds can be used.
•
Retarders
, which delay setting during transport. These contain sugar, petrol,
etc.
•
Water-repellents
, which make the substance more waterproof. Metal salts from
stearic acid such as zinc stearate and silicone are used.
•
Adhesive agents
, which increase the tensile strength and ability of the cement to
adhere to other materials, such as polyvinyl acetate and polyvinyl proprion-
ate.
Cement products and pollution
To produce Portland cement in rotary kilns requires the use of energy sources
such as coal, heavy oil or gas. Effluent from combustion, therefore, is the same as
