SAND LIME BRICK
NURDEEN .M .OUSM
1.GENERAL CONSIDERATION OF THE SANDLIME BRICK INDUSTRY
Sand-lime brick or the "calcium silicate brick" , may be defined as a mass of sand grains cemented together by hydrated calcium silicate. If the lime used, in the manufacture is dolomitic there will be in the bond hydrated magnesium silicate as well as calcium silicate, but experience has shown that the best product is obtained when the percentage of magnesium oxide in the lime is low. The reaction between lime and silica resulting in the formation of hydrated lime silicates takes place slowly at ordinary temperatures in moist atmospheres, but in practice it is hastened by the action of steam. Sand-lime brick differs from sandstone, mortar, and silica brick primarily in the nature of the bond.
Sandstone is formed from beds of sand which have been laid down and covered by later strata. The sand is first closely packed by the pressure of the strata above. The cementation, or formation of bond, then begins, and is effected by the deposition, in the interstices between the sand grains, of various chemical substances that may be carried in solution by percolating water. Such substances are calcium, magnesium, and iron carbonates; calcium, barium and strontium sulphates; etc. The bond in the sandstone is purely a mechanical one, and the stone possesses its strength, its power of resistance to frost, heat, etc., by virtue of its compactness.
In the case of mortar, mortar brick, etc., the sand and one constituent of the bonding material are incorporated at the time of preparation of the mixture. In order that the mass may harden, it is necessary that another constituent, viz: carbonic acid, be added to change the soluble calcium hydroxide into the insoluble carbonate. This is obtained from the atmosphere. The bond in this case is almost entirely mechanical, as it is in the case of natural sandstone. Hydraulic mortars differ from ordinary lime mortars in that they are capable of hardening without the addition of carbonic acid. The bond in this mortar consists of silicates and aluminates of calcium and magnesium, but there is no appreciable chemical reaction taking place between the silica of the sand and any of the bonding constituents. The bond in this case is mechanical, as before.
The material of silica bricks resembles that of sand-lime bricks more closely, perhaps, than does any other material. Silica bricks are made quite extensively for use as refractory linings for furnaces. They consist of sand grains cemented together by true calcium silicate. They are prepared by mixing lime and silica in the proportion of about 3-4 per cent of the former to 96-97 per cent of the latter, subjecting to great pressure in molding, and burning at a rather high temperature. The lime reacts with the silica of the sand forming a kind of glass which cements the sand grains together. The difference between a silica brick and the common sand-lime brick is that in the former little lime is used, and the hardening is done by firing in a kiln ; while sand lime bricks contain much more lime and they are hardened by the action of saturated steam under pressure. In silica bricks the bond consists of a glass containing no water; while in the case of sand-lime bricks, the bond is not a glass but a hydrated silicate, very much akin, no doubt, to some of the constituents of set Portland cement. Exclusive of the impurities always found in the raw materials of sandlime bricks, their primary constituents are silica (SiO2 ), lime (CaO), and water (H2O).
2. THE RAW MATERIALS OF THE SAND-LIME BRICK INDUSTRY.
The materials used in the manufacture of sand-lime bricks are clean quartz sand., or other material rich in silica, and lime, with the preference in favor of calcium lime rather than the magnesian variety. Bricks apparently good at the time of manufacture can be made from impure sands, but such bricks will not withstand the action of the weather sowell as those in which purer sands are used.
Sand is one of the products resulting from the disintegration of rocks. It may, therefore, be composed of many different kinds of minerals and consequently vary widely in chemical composition. With reference to the size of its particles, sand lies between the clays, on the one hand, and the gravels, on the other. In some sands there is gradation of sizes, while in others the grains are all of nearly the same size. Furthermore, sand may vary widely in the character of the grains, some of them being round, while others are sharp.
Most of our soils, sands, sedimentary rocks, etc., were formed originally by the disintegration of igneous rocks. Where the molten material or lava cooled rapidly it produced obsidians or glasses of uniform composition, but not corresponding to any definite chemical formula. Where, on the other hand, the mass of material cooled slowly crystallization took place, several constituents separated as minerals, and comparatively coarse-grained rocks of which granite is a type resulted. In granite are distinct grains of quartz, mica, hornblende, feldspar, and several other minerals. When rocks of this kind are exposed to 'the action of the weather, the softer, ,and less resistant constituents crumble away first. Quartz, which is the most resistant of all the constituents, is left behind, and collects in stream beds as sand.
Feldspar is also found in some sands, soils, and clays, but being less resistant than quartz, it is less abundant than this mineral in the residues left by weathering. Feldspar sands-, limestone sands, chalks, marls, etc., are entirely unsuited to take, the place of ordinary sand in the manufacture of sand-lime bricks. In its generally accepted sense, the term "sand" signifies quartz sand, and it is quite essential that sand should be of this character when it is to be used in the manufacture of sand-lime bricks, although, for some other purposes, such as for mortar, concrete, etc., feldspathic sands or sands of any other character will do quite well.
The Kind of sand used in the manufacture of sand-lime bricks in most countries of the world are river, lake, sea, crushed stone, weathered stone and ore tailings sand. In terms of impurities which found in the sand are, clay substance is an almost ever present impurity in sands, as is also ferric oxide, one of the decomposition products of iron bearing minerals which are nearly always present in igneous rocks.
The lime intended for use in the manufacture of sand-lime bricks should be of a high grade. It should, therefore, be burned from good materials in the best manner for producing good lime. The raw materials from which limes are burned are limestone of various degrees of purity, and marble. The impurities found in limestone and. consequently, in the burned lime, are ferric oxide, alumina, silica, and occasionally traces of other substances. When present in the limestone, these materials usually combine with the lime during burning, and so render a part of it inactive.
Magnesia is usually present in commercial limes to, some extent. This oxide is quite similar to the oxide of calcium and when the lime is intended for use in mortars, plaster, etc., is not considered particularly objectionable. It is much less basic, however, than calcium oxide; and consequently it reacts with silica much less readily than pure lime, when used in the manufacture of sand-lime bricks. Limes have been variously classified as white and gray; hot and cool; fat and lean; quick and slow. These terms are intended to distinguish between limes relatively pure on the one hand, and those containing considerable clay or magnesia on the other.
In burning lime it is necessary in some way to heat the stone to a temperature of 900 or 1000 and maintain this temperature for a sufficient length of time to allow the chemical reaction, which is started by the heat, to run to completion. The processes used in burning lime are numerous. . The numerous types of kilns are used, which are constructed so as to make use of coal, wood, gas, etc., as fuel.
Lime is produced from limestone through these steps:
·Limestone (calcium carbonate) is heated in a kiln or kiln pit until burnt, giving off carbon dioxide and leaving calcium oxide (quicklime).
·Quicklime is mixed with water; this produces heat. If only a little water is added, the result is a dry powder called hydrated lime or lime hydrate. If a lot of water is added, this 'slaking' process forms calcium hydroxide, usually called 'slaked lime' or 'lime putty'. This is then left to mature for several weeks. This process is called 'hydration.
Non-hydraulic lime (slaked lime) hardens by a slow process of drying and carbonation, reacting with atmospheric carbon dioxide to form calcium carbonate. This takes a period of some weeks. Hydraulic lime, on the other hand, sets rapidly by reacting with water in a matter of hours. A non-hydraulic lime can be made to set much more rapidly by the addition of an hydraulic or 'pozzolanic' additive. This practice is known as 'gauging'. Typical additives are finely crushed brick powder or cement. These contain highly reactive silica and/or alumina, which give a rapid chemical set by reaction with water. Of these, cement is by far the most widely used in the UK, and the cheapest. Typical proportions, commonly used, are 16 (cement:lime:sand) and 19. Use sharp sand with lime.
3. TECHNIQUE OF THE MANUFACTURE OF SAND-LIME BRICKS
The first step in the manufacture of sand-lime bricks is the mixing of the sand and lime, and it is in this that the various processes differ. The subsequent steps the pressing or molding and hardening of the mixture are practically the same in all plants, except as to kind of machinery used.
In the past the the manufacture of sand-lime bricks, According to the Keramisches Jahrbuch, 1909, there were in use in Germany in 1908 five processes for preparing the mixture used in the sand-lime brick industry. These were known as:
a- The pure hydrate methods, burned lime is slaked (in any way) without the addition of sand, pulverized, and finally mixed with sand and pressed.
b-Mixed methods, burned lime is slaked with a part of the sand, then mixed with the remainder and pressed.
c-burned lime is partly pulverized and mixed with a part of the sand, and then placed in the silo for a while, after which it is mixed with the remainder of the sand and pressed.
d-Caustic lime methods, the entire quantity of sand is mixed with the pulverized caustic lime in a mixing machine, and then is cured in a silo and pressed.
e-pulverized caustic lime is mixed with all the sand in a slaking machine until a good mixture is obtained, when it is pressed.
But, In the United States, sand lime brick has been produced since the early 1900s and the process were almost same and as shown.
a-Lime is slaked before being mixed with any part of the sand.
b- Caustic lime is mixed with a part of the sand and slaked. It is then mixed with the remainder of the sand and pressed.
c- Caustic lime is finely pulverized and mixed with a part of the sand and sufficient water to completely hydrate it. It is then stored for a time in a silo, after which it is mixed with the remainder of the sand and pressed.
d-Caustic lime is finely pulverized and mixed with all the sand and sufficient water to insure complete hydration. It is then stored in a silo for a time and pressed.
e-Caustic lime is finely pulverized and mixed with all the sand. The mixture is then passed through a machine in which the lime is completely hydrated, after which it is pressed.
f-The caustic lime and a part of the dried sand are ground together. This mixture is then mixed with the remainder of the sand and sufficient water added to insure complete hydration of the lime. The mixture is then siloed for a day and pressed.
Nowadays , technology has evolved in the manufacture of sand lime bricks , but the general method is in all cases the same .
Sand lime brick, is made with sand or other siliceous material. The following is an analysis of a typical sand-lime brick: silica (SiO), 84%; lime (CaO), 5 to 10%; alumina and oxide of iron, 2%; water, magnesia and alkalis, 7%. and hydrated lime, then stream in high pressure autoclave at 400°F for up to 8 hours. In the autoclave, the lime reacts chemically with the silica to form hydrated silicate , a strong and durable cementing agent that binds the sand particles together.
The units have a natural near-white color with a slight yellow, gray, or pink tint, depending on the of sand used. With the addition of natural or synthetic pigments, dark earth tones, reds, blacks, and light pastel colors can be or produced, including blues and green. Tow colors can be blended for a swirled mixture, or units can dipped in acid after hardening to intensify their color. Unit surfaces are smooth and uniform, the finer the sand particles, the smooth surfaces. Texture is is produced by sandblasting, mechanical brushing, or adding flint aggregate to the mix. Splitting hardening units produces a natural rockface finish.