Cement companies can manufacture AAC blocks and can compete with stand alone AAC units in the country.
The raw materials required for AAC production are readily available in any cement manufacturing plant. The process does not require installation of a steam boiler or a power plant and can utilise the waste-heat from the clinker cooler exhaust gases for steam curing of aerated concrete. The method also earns carbon credits not only for the green product being made, but also for waste-heat utilisation. Although, there are more than 35 standalone AAC manufacturing units in India, very limited attempts have been made to manufacture AAC by the cement plants. One reason behind this is the lack of awareness about the new technologies that were developed recently in this field. DS Venkatesh elaborates on the technology offered by Cemeng for AAC production in a cement plant.

What is AAC
AAC is lightweight autoclaved aerated concrete, which is completely cured, inert and stable form of calcium silicate hydrate. It is a structural material, approximately one quarter in weight of the conventional concrete. It is composed of minute cells/air pockets, which give the material its lightweight and high thermal insulation characteristics. It is available as blocks and as pre-cast reinforced units for building floors, roofs, walls and lintels.

Raw material
Raw materials for AAC vary with the manufacturer and the location. The kinds of materials that could be used are detailed in the ASTM C1386 specifications. They include some, or all, of the following: fine silica sand; Class F fly ash; hydraulic cements; calcined lime; gypsum; expansive agents such as finely ground aluminium powder or paste; and water.

AAC is produced by mixing quartz sand and/or pulverised fly ash (PFA), lime, cement, gypsum/anhydrite, water and aluminium and is hardened by steam-curing in autoclaves. The silica is obtained from silica sand, fly ash (PFA), crushed silica rock and/or stone. It is possible to obtain silica as a by-product coming from other processes such as foundry sand or burgee from glass grinding; although, it can be used only if the levels of alkali or other impurities are not too high. The calcium is obtained from quick lime, hydrated lime and cement. Gypsum acts as a catalyst and enhances the properties of AAC. Careful regulation of the amount of aluminium powder gives accurate control over the density of the final product.

Cement with the least per cent of clinker would be the cheapest and suitable, e.g., Portland limestone cement. If milling of siliceous material is required, Cemeng suggests grinding of a composite mix of siliceous materials together with cement clinker, lime and gypsum/anhydrous. The ground material can be stored in a single bin. It also eliminates the need for multiple handling of individual constituents and weigh batchers. Cemeng employs a PSRG mill function in open circuit to produce the desired fineness of the composite mix.

Process flow
Cemeng has simplified the process flow to minimise the number of equipment and material handling requirements in mini AAC plants. The process gets rid of ?wet cutting? the green cake as it is possible only if the plant is involved in exclusive production of smaller blocks. Other AAC products with or without reinforcement certainly require ?dry milling? of cured cakes for profiling. Cemeng moulds for AAC are wheel-mounted units with a base plate and sliding sidewalls. There is no need for rotation or dismantling and re-assembling of side plates. Loaded moulds are transferred directly into the autoclaves for steam curing. Cemeng autoclaves generate the required steam in the autoclave itself. Separate boiler is not required. For mini AAC plants, Cemeng suggests after-cutting/milling of cured blocks for economic benefits.

The important unit operations involved in AAC production are gravimetric proportioning and mixing of constituents with water to form the slurry. This is followed with secondary mixing with expansive agents, pouring the slurry into casting moulds and then allowing sufficient time for initial hydration. Once the material is hydrated it gains enough strength to support its own weight and can undergo de-moulding/cutting. The cakes are then transferred into autoclave for high pressure steam curing. Once cooled, the autoclaved blocks are ready for after-cutting/milling as per the required profile. The AAC cost depends mainly on the cost of mineral binders and the expansive agents used. The cost of silica can vary from location to location.

Cement plant and Cemeng mini AAC production line
Cemeng mini AAC production line can be installed in an existing cement plant. Cement plants are already processing and handling both siliceous and mineral binder constituents, except for lime and sand. Also, ground raw meal, preheater ESP dust, pre-calcined meal from bottom most stage of preheater can partially or wholly replace lime. Sand may be replaced by ground slag and cinder. Clinker dust from cooler ESP and gypsum can replace cement. Besides, clinker cooler exhaust air may be effectively utilised for production of steam required for autoclaving, thus eliminating the need for a separate boiler set up.

AAC production capacity, on a thumb rule basis, can be considered as twenty cubic meter per day for every 100 tpd production capacity of the clinkerisation unit. This is based on steam production using gases only from the from clinker cooler exhaust.

Manufacturing process
To make AAC, sand is ground to the required fineness in a ball mill and is stored along with other raw materials. The raw materials are then batched by weight and are delivered to the mixer. Measured amounts of water and expansive agent is added to the mixer to prepare a cementitious slurry.

Preparation of slurry
Slurry preparation is a batch process. When AAC is being made from dry constituents, Cemeng employs separate weighbin augur dosers for each constituent the Cemeng weighbin augur doser, which uses a combination of weight and volumetric filling. A vertical auger looks like a corkscrew. The auger rotates in the hopper filled with lose powder. As it turns, it drives the powder through the bottom of the hopper into a narrow tube, where the powder is drawn down by a turning screw. The auger runs through the narrow tube, creating a tight fit. As the screw turns, it pulls the prescribed amount of powder down the tube. The agitator keeps the feed flowing to the centre of the auger. You can control the amount of powder delivered by setting the number of revolution made by the auger.

The augur screw discharges into a tubular disc conveyor for conveying and transferring directly into the AAC mixer. Subsequently, aluminium paste is added, secondary mixing is carried out and the final slurry is discharged into the AAC moulds.

Casting in moulds
Steel moulds are prepared to receive fresh AAC. If reinforced AAC panels are to be produced, steel reinforcing cages are secured within the moulds. After mixing, the slurry is poured into the moulds. The expansive agent creates small, finely dispersed voids in the fresh mixture, which increases the volume by about 50 per cent within three hours. The moulding of AAC in the mould box, holding for initial strength and de-moulding prior to autoclaving is an important step in reducing the material handling. Conventionally, the base of the moulds-box and three sides are welded together with only one side plate of mountable type. This calls for mould rotation to load the green mould on to the mountable side plate.

Cemeng moulds for AAC are trolley-mounted units with a base plate and sliding sidewalls. During casting, the sidewalls are slided inwards to form a box holding the slurry. The sidewalls keep space all around the green cake for the passage of steam. No rotation or dismantling of the side plates and reassembling are required. After curing in autoclaves, the cake is picked up by a grab and is transferred to the trolley.

Cemeng also offers ?FlexiMold? wherein rectangular shaped pre-stitched permeable cloth bags with open top are held at the base of the trolley. The flexibag is filled half with slurry and the top half is left empty to allow for expansion. As the green cake attains strength, it attains the shape of the flexibag. The telescopic brackets are then lowered. The bracket is held in its lowest position when the trolley is moved into the autoclave. The green cake along with FlexiMold is left undisturbed. After curing, the trolley is moved out and the cured cake in the moulding bag is lifted and transferred to storage. FlexiMold serves as a protective cover for cured block and it is also disposable. The size of the green cake can be set as required and several green cakes can be mounted on a single trolley.

Autoclaving
Autoclaving is steam curing at high temperature and pressure. It is required to achieve the desired structural properties and dimensional stability. The chemical reactions that produce the final calcium silicate hydrate structure happen in the autoclave. The process takes about eight to 12 hours under pressure of about 174 psi (12 bar) and a temperature of about 360?F (180?C), depending on the grade of material produced. Preferably, two autoclaves are used with the casting and precuring section in between. The mixing station is located near the discharge end of the autoclave. The thermic fluid reservoir is located at the feed end of the autoclave. This permits the precuring shed to store the cast moulds for the required duration. The waste heat from grate cooler exhaust is utilised for the heating the thermic fluid in a simple heat exchanger. It is estimated that at least 350-400 kg/hr of steam could be generated per 100 tpd production capacity of clinkerisation unit. To initiate the curing cycle, the thermic fluid, heated to 205?C, is passed through the coils in the reservoir at the bottom of each autoclave to generate steam. The hot steam pressure rises up to 1.75Mpa.

After-cutting/milling of cured AAC Blocks
Steam cured AAC blocks can be transported directly to the marketing yards. After-cutting can be carried out by the stockists or at the construction site itself. Existing granite/stone cutting and polishing units at different cities in the marketing zone can be used to saw the AAC blocks to the required size/dimensions. It is always possible to saw cut the large size AAC blocks to the required size at the construction site. Any broken pieces could be used as lightweight filler, thus nothing is wasted.

Conclusion
Every cement plant has to take green initiatives to safeguard sustainability. Using waste-heat for steam generation is highly cost effective and adds to the profits of AAC production. Besides, the plant will also be a captive consumer of cement. Every cement plant can produce AAC at considerably lower cost and can compete with standalone AAC units. AAC products can save time, labour, cement and sand during construction.

References
Eco-Care Building products: www.primeaac.com
Raw material formulae: Dearye Brick machine
Silica, calcium joined in premium products, by Sandy Herod Pit and Quarry Dec 1987 Pg.72 – 74
Brick manufacture in a Cement Plant by DS Venkatesh, Cemtec Engineering, Secunderabad. Indian Cement Review May 1989, Pages ICR-19 to ICR-25 Green Concrete by Yuvraj Patil, Flycrete. Indian Cement Review, May 2014 ?Let us employ PSRG Milling Technology? by DS Venkatesh, Indian Cement Industry Desk Book, March 2014. www.victoryenergy.com

DS Venkatesh,
Freelance Industrial Consultant
Email: dsvenkatesh40@gmail.com
Former CEO and Director of Cemtec Engineering at Secunderabad, DS Venkatesh is currently working as a freelance industrial consultant. He started as a Design Engineer at ACC and later had a long stint at Holtec-India holding several responsible positions. He has been one of the lead consultants to many rotary based mini cement plants and expansions.

DS Venkatesh has provided technical know-how, design and manufacturing drawings for cement production machinery to many Indian machinery manufacturers. Re-engineering and retrofitting of plant/machinery for enhanced productivity is his forte. His work has helped in enhancement of PSRG milling technology applied in media grinding.