Building Design and Construction Handbook - Merritt - Ventech!

4.5.1 Hydraulic Limes
BUILDING MATERIALS 4.7
These are made by calcining a limestone containing silica and alumina to a temperature
short of incipient fusion so as to form sufficient free lime to permit hydration
and at the same time leave unhydrated sufficient calcium silicates to give
the dry powder its hydraulic properties (see ‘‘Specification for Hydraulic Hydrated
Lime for Structural Purposes,’’ ASTM C141).
Because of the low silicate and high lime contents, hydraulic limes are relatively
weak. They find their principal use in masonry mortars. A hydraulic lime with more
than 10% silica will set under water.
4.5.2 Quicklimes
When limestone is heated to a temperature in excess of 1700�F, the carbon dioxide
content is driven off and the remaining solid product is quicklime. It consists essentially
of calcium and magnesium oxides plus impurities such as silica, iron, and
aluminum oxides. The impurities are usually limited to less than 5%. If they exceed
10%, the product may be a hydraulic lime.
Two classes of quicklime are recognized, high-calcium and dolomitic. A highcalcium
quicklime usually contains less than 5% magnesium oxide. A dolomitic
quicklime usually contains from 35 to 40% magnesium oxide. A few quicklimes
are found that contain 5 to 35% magnesium oxide and are called magnesian limes.
The outstanding characteristic of quicklime is its ability to slake with water.
When quicklime is mixed with from two to three times its weight of water, a
chemical reaction takes place. The calcium oxide combines with water to form
calcium hydroxide, and sufficient heat is evolved to bring the entire mass to a boil.
The resulting product is a suspension of finely divided calcium hydroxide (and
magnesium hydroxide or oxide if dolomitic lime is used) in water. On cooling, the
semifluid mass stiffens to a putty of such consistency that it may be shoveled or
carried in a hod. This slaked quicklime putty, when cooled and preferably screened,
is the material used in construction. Quicklime should always be thoroughly slaked.
The yield of putty will vary, depending on the type of quicklime, its degree of
burning, and slaking conditions, and will usually be from 70 to 100 ft 3 of putty per
ton of quicklime. The principal use of the putty is in masonry mortars, where it is
particularly valuable because of the high degree of plasticity or workability it imparts
to the mortar. It is used at times as an admixture in concrete to improve
workability. It also is used in some localities as finish-coat plaster where full advantage
may be taken of its high plasticity.
4.5.3 Mason’s Hydrated Lime
Hydrated limes are prepared from quicklimes by addition of a limited amount of
water. After hydration ceases to evolve heat, the resulting product is a fine, dry
powder. It is then classified by air-classification methods to remove undesirable
oversize particles and packaged in 50-lb sacks. It is always a factory-made product,
whereas quicklime putty is almost always a job-slaked product.
Mason’s hydrated limes are those hydrates suitable for use in mortars, base-coat
plasters, and concrete. They necessarily follow the composition of the quicklime.
High-calcium hydrates are composed primarily of calcium hydroxide. Normal dolomitic
hydrates are composed of calcium hydroxide plus magnesium oxide.