Title of Invention

AERATED CONCRETE AND A PROCESS FOR THE MANUFACTURE THEREOF

Abstract

A process for the manufacture of aerated concrete comprises the preparation of an aqueous slurry from a mix containing 25-90 % by wt. of pozzolan material; 1-45% by wt. of lime; up to 45% by wt. of OPC; 2-15% of sulphate bearing compound; 0.05-3% of an aeration agent; and water at ambient or above ambient temperature. The slurry is poured into a mould and allowed to swell under aeration before curing the same for attaining hardness.

Full Text

This invention relates to aerated concrete and a process for manufacturing thereof. The said process does not involve the use of autoclave or foaming agents, but gives a cellular/aerated concrete product with impressive pores and pore refinement, at lesser plant cost and utility costs. Such foaming agents usually contain organic proteins, and are commercially available as branded products. The term "aerated concrete" covers the materials variously described as gas, cellular, or foamed concrete. It is a material known for its lightweight duly obtained by the entrainment of a large volume of air or gas. In 1914 the Swedes discovered a mixture of cement, lime, water and sand that expands by adding aluminium powder, resulting in a material like wood but without the disadvantages of combustibility, decay and termite damage. This material is further optimized and developed to what we know as autoclaved aerated concrete (AAC) which otherwise is called autoclaved cellular concrete. The greater part of autoclaved aerated concrete production has been in the form of building bbck, which because of the lightness in weight, can be very much larger in size than the clay brick, thus effecting economies in handling and in construction. The normal range of technical properties of the product is as follows: Wet Strength (Kg/cm2) 21-60 Volume Stability (% of length) ... 0.05-0.01 Density (Kg/m^) 400-750 The strengths attainable are more dependent upon density than any other single factor, and, as indicated above, the densities at which these materials are made are usually fixed as low as possible consistent with the strength required. The following data show the manner in which the strength of typical autoclaved aerated concrete varies with density. The low thermal conductivity is an important property of aerated concretes, giving them an advantage for many uses over most of the alternative materials. It is almost entirely a function of density and is independent of the nature of bonding material i.e., cement or lime. But the thermal conductivity is slightly greater with sand as aggregate than other aggregates such as fly ash and burnt shale which are characteristically lower in conductivity. There are several processes for making aerated concrete: In one process a slurry of Portland cement and finely ground siliceous aggregate, such as ground sand, fly ash, ground burnt shale, or ground slag or a combined mixture of them is admixed with aluminum powder. The reaction of aluminum with lime and alkalis released from the cement cause evolution of bubbles of hydrogen which are entrapped in the mass and cause it to swell: In certain processes lime is used instead of cement. After 3-6 hours the set material is cut to the required shape by tensioned wires. Then the product is cured in high-pressure steam for 14-18 hours at 10-14 bar pressure. The aeration in aluminium route was developed in Sweden and became popular throughout the world, more so in Expiree and North America. In some processes practiced in Germany, hydrogen peroxide and bleaching powder are used to initiate reactions to generate oxygen as the media of aeration as follows: in canal nuisance processes, a loaming again is use, pricier lips, lu entrap air in the mix. In one method the agent is added to the cement, aggregate and water and the whole mass is vigorously mixed. In some cases, a stiff foam is prepared and then mixed with the cement-sand or lime-sand slurry. The foam injection process utilizes chemically stabilized foams similar to those used in fire fighting. By use of suitable foam generators, with which the bubble size and the amount of air to a given volume of liquid are closely controlled, and injection of a known volume of foam into a mix any required density of product can be achieved. The method is, however, utilized more for aerated concretes made on the construction site in contrast to the factory-made autoclaved process. One method in which air is whipped directly into a mix containing air-entraining agents cannot incorporate as much gas and cannot therefore achieve such low densities as the other methods mentioned. It has been used for concrete of a density down to about 1300kg/cu.m. The first batch of aerated concretes such as "aero Crete" in UK were made with OPC (Ordinary Portland Cement), sand and aluminium powder. They were allowed to harden at ordinary temperatures and, although of reasonable strength, suffered from excesswglv high driving shrinkage. The Swedish Spore and Ytong, and Danish Doro are some of the successive developments in autoclave route to answer the drying shrinkage. The drying shrinkage of the autoclave-cured aerated concretes tends to be higher than most other materials used in brick or block construction, and manufacturers have to give considerable attention to the factors involved to keep the shrinkage as low as possible. Operation of autoclave is a cost-intensive process step; one on account of high capital cost of the equipment and the other owing to dependence on energy-intensive input like steam. Hence it is desirable to dispense with this process step. In foaming agent route, the strength attainable in autoclave is compensated by the enrichment of cement which gives no cost benefits. The foaming agent in the order of Rs 40,000 to 60,000 per ton is prohibitively costly. The object of this invention is to rationalize the plant cost by avoiding the cost-intensive equipment such as autoclave, as well as the cost of utilities such as foaming agents, in order to make aerated concrete production sustainable in practice and affordable in price. The process for the manufacture of aerated concrete, according to this invention, comprises the preparation of an aqueous slurry from a mix containing 25-90% by weight of puzzling material; 1-45% by weight of lime; 2-15% of sulphate ambient temperature; pouring the slurry into a mould; allowing the slurry to swell under aeration and curing the same for attaining hardness and strength thereafter. The Mix: Puzzling ... 25-90% by wt (selected from at least one of the following Substances, namely, fly ash, ground burnt clay, Burnt shale, natural puzzling) Lime ... 1-45% by wt OPC (as commensurate Replacement to lime) ... 0-44% by wt Sulphate bearing compound Such as gypsum, potassium sulphate. Sodium sulphate ... 2-15% by wt Aeration agent such as Aluminium powder, hydrogen peroxide, Sodium bicarbonate ...0.05 to 3.0%by wt Superpozzolan (optional) such as Silica illume, rice husk ash or met kaolin ... 0-15% by wt Filler material (optional) ... Sand/stone dust or any Inert dust of siliceous/ calcareous nature Process water ... At ambient or hot water to be decided based on raw material characteristics. N.B: If OPC (the use of which is optional) is contained in the mix, there must be lime at least by one per cent by weight of the mix, and 44% by weight may be constituted by OPC or by lime or by both. Example 750 gm of fly ash is mocked with 220 gm of OPC and 30 gm of lime. 38 gm of gypsum is also added to the mix. The raw materials, added with 467 gm of water, are stirred with 2 gm of aeration chemical for five minutes. The said slurry is then poured into moulds and allowed for complete aeration for 4 hours. After aeration the product is subjected for accelerated curing Stage I at elevated temperature of 80°C and saturated water vapor for 6 hours for pre-hardening so that the same could be cut into necessary size of blocks with suitable device such as pensioned wire. The cut product is put into insulated chambers (to avoid heat loss) for accelerated curing Stage II at elevated temperature of 80°C and saturated water vapor, for strength gain. The cured product is taken out of the curing chambers after 48 hours, allowed for conditioning for about 24 hours. The said product had a strength of 41 kg/cm^ and a density of 720 kg/m^. The following other mix designs as prepared in the above process and strengths derived there from are illustrative of the scope of the invention. In the above approach, wherever the selected fly ash is of poor quality, the chemistry can be augmented by using hot water of 70-90 "C for mix preparation. OPC is used to make available lime, released out of its hydration, to the pozzolanic reactions and also to render early strengths to the matrix. The studies on water absorption indicate that the product has pores but without interconnectivity to facilitate lightweight but at the same time lesser water absorption. The following data substantiate this observation: Density kg/cu.m Water absorption Sand lime AAC 740 38% Subject patent 720 30% This is owing to the superfine particle size of fly ash, which cannot be attained by ground sand with any amount of grinding that facilitates floating of particles during aeration to allow development of small pores. Also the reactivity of fly ash, in contrast to sand, facilitates localized chemistry, leading to pore refinement. WE CLAIM : 1. A process for the manufacture of aerated concrete, comprising the preparation of an aqueous slurry from a mix containing 25-90% by wt. of pozzolan material; 1-45% by wt. of lime; 2-15% of sulphate bearing compound; 0.05-3% of an aeration agent and water at ambient or above ambient temperature; pouring the slurry into a mould; allowing the slurry to swell under aeration and curing the same for attaining hardness and strength thereafter. 2. A process as claimed in claim I wherein the curing is carried out at two stages, each stage at elevated temperatures upto 100 oC in the presence of saturated water vapour and over a period such as here described; the elevated temperatures being achieved by hot air. low pressure steam or hot water. 3. A process as claimed in claim 1 or claim 2 wherein the mix contains upto 15% super pozzolan by weight. 4. A process as claimed in any one of the preceding claims wherein the mix contains filler materials. 5. A process as claimed in any one of the preceding claims wherein the mix contains OPC, such that there is lime at least by 1% by weight of the mix, and up to 44% by weight is consthuted by OPC or by lime or by both. 6. A process as claimed in any one of the preceding claims wherein the pozzolan material is selected from at least one of the following substances, namely, fly ash, ground burnt clay, burnt shale, natural pozzolan. 7. A process as claimed in any one of the preceding claims wherein the sulphate bearing compound comprises at least one of the following substances namely calcium sulphate (gypsum), potassium sulphate, sodium sulphate. 8. A process as claimed in any one of the preceding claims wherein the aeration agent comprises a substance which releases gas on chemical reaction, such as aluminium powder, hydrogen peroxide, sodium bicarbonate. 9. A process as claimed in any one of the preceding claims wherein super pozzolan comprises up to 15% by weight of at least one of the following substances namely silica fiime, rice husk ash, metakaoHn. 10. A process as claimed in any of the preceding claims wherein the filler materials comprise sand/stone dust or any inert dust of siliceous/calcareous nature. 11. A process for the manufacture of aerated concrete substantially as herein described. 12. Aerated concrete whenever manufactured by a process as claimed in any one of the preceding claims.

Documents:

0976-mas-2000 abstract-duplicate.pdf

0976-mas-2000 abstract.pdf

0976-mas-2000 claims-duplicate.pdf

0976-mas-2000 claims.pdf

0976-mas-2000 correspondence-others.pdf

0976-mas-2000 correspondence-po.pdf

0976-mas-2000 description (complete)-duplicate.pdf

0976-mas-2000 description (complete).pdf

0976-mas-2000 form-1.pdf

0976-mas-2000 form-13.pdf

0976-mas-2000 form-19.pdf

0976-mas-2000 form-26.pdf