Masonry Magazine May 1964 Page. 12
A modern rotary kiln consisting of a long tube that rolls on a horizontal axis. This type of kiln are from 6 to 15 feet in diameter and from 160 to 330 feet in length.
Production rates from these kilns were from 5 to 15 tons per week. This would hardly be enough to keep modern bricklayers busy; however, modifications and innovations have been developed to improve its efficiency and production rate. The shaft kiln is still used successfully in many lime plants today.
The modern rotary kiln consists of a long tube that rolls on a horizontal axis. It has a slight slope from the feed end to the discharge end. These kilns are from 6 to 15 feet in diameter and from 160 to 330 feet in length. Fuel (coal, gas, oil, or a combination of these) is injected at the discharge end which is the calcining or hot zone. The calcining zone is the portion of the kiln where the temperature is above that necessary to convert the stone into lime. As the hot gasses from the combustion of the fuel travel the length of the kiln, heat is absorbed by the stone fed at the other end where the hot gasses are exhausted. The temperature of the stone passing through the kiln is raised above 1800 degrees Fahrenheit. This is the temperature that lime begins to form.
Limestone is made up of the metal calcium, combined with carbon and oxygen. It has the chemical formula of CaCO, or the name Calcium Carbonate. When heated above 1800° F., the carbonate will decompose, or break apart, and carbon dioxide (CO₂) is released. This leaves the calcium left with one oxygen atom, and this is called quicklime (CaO), or calcium oxide. Quicklime is a coarse white solid. It withstands great heat, but can be melted in an electric furnace. Lime is strongly alkaline and readily neutralizes acids. It eats into vegetable and animal substances and often used to destroy refuse. It is called quicklime because of its strong action. Two kinds of limestone are used to burn, or calcine, and convert into lime.
Limestone has been deposited in many different geological periods. All limestone originally formed as calcium carbonate, however, either before or after consolidation, dissolved magnesium compounds in the water may partly replace the calcium with magnesium forming dolomite. This type of stone contains 35 to 40 percent magnesium carbonate. Quicklime can be either that of high calcium or of dolomite.
When water is poured on quicklime, the lime swells and changes into a fine powder, forming slaked lime. This process gives off enough heat to boil water. The fine dry powder is called hydrated lime (Ca(OH)2). High calcium
limes hydrate completely with no difficulty. Dolomitic limes must be hydrated at elevated temperatures and pressures to completely hydrate the magnesium part of the lime. These limes are called special finishing limes (Type S). Dolomitic limes hydrated under atmospheric pressure and just above the boiling point of water will have unhydrated oxides of magnesium and are Type N or normal hydrated lime.
Uses of lime
Mortar is only one of the many areas of industry the lime is used. The largest use is in the manufacture of refractory bricks used to line the inside of steam boilers kilns that burn lime and cement, in furnaces for making iron and steel. Lime is used in the manufacture of steel itself as a fluxing agent to remove impurities as sulfur and silicon. Other large uses are the manufacture of alkalies, and in pulp and paper industry. Lime is used in treating industrial and municipal water supplies for softening and clarifying the supply. Many cities treat their sewage with lime before discharging it. Some industrial waste must be neutralized with lime before they can be discharged into streams or rivers. Other areas in industry that require lime in their processing are calcium carbide sugar refining, glass manufacturing, tanning leather, petroleum refining, uranium milling and paints.
Lime is an important ingredient in mortar. The ancient Romans used lime putty and sand for mortar on buildings that still stand today. Lime and volcanic ash were mixed to make a cement by the Romans. Not until 1824 when a British bricklayer, Joseph Aspdin, invented portland cement, was anything but lime used for mortar. At Harvard University, Massachusetts Hall was laid up with only a lime-sand mortar. It was built in 1720, and has never been tuckpointed.
Today, lime for masonry purposes is made to exacting specifications, providing workability, water retention, bone strength, durability and elasticity. Lime is not used by itself now, because its strength development is too slow for modern rapid construction. For the early strength required, portland cement is used with lime.
During the past 30 years, proprietary masonry cement have been developed as substitutes for lime-portland ce