AAC Block Plant

Overview

An AAC block plant makes autoclaved aerated concrete, a lightweight building block full of tiny air cells that give it low weight and good insulation while still carrying load. The plant works in two stages. First it pours a thin slurry that foams up like a cake as aluminium powder reacts with lime to release hydrogen gas. Once that cake has stiffened it is cut into blocks with wires. Then the cut blocks are cured under high-pressure steam in an autoclave, where the lime and the fine ground sand react to form the calcium silicate that gives the finished block its strength. The result is a block about a fifth the weight of dense concrete that can be sawn and drilled with hand tools yet still build load-bearing walls.

The plant is built from a Raw Material Batching section that doses the raw materials, a Slurry Mixing and Pouring that blends and pours the slurry, a Mould and Rising Chamber and rising chamber where the cake forms, a Cutting Machine machine, an Autoclave System system that cures the blocks, a Cake and Product Handling section that moves cakes and packs product, a Steam and Boiler Plant plant that raises the steam, and a Control System system that ties it all together.

Batching the raw materials

The Raw Material Batching section stores each material in its own Material Silo and doses it by weight. A Screw Feeder draws powder into a Weigh Hopper that rests on a Load Cell, so the controller can hit the recipe weight for each batch. The silica sand has to be ground fine before it will react, so it passes through the Sand Ball Mill, a rotating drum where a Grinding Ball Charge charge crushes the sand to a wet slurry inside a Mill Liner. The small but critical aluminium dose is metered by the Aluminium Dispenser, since too little leaves the cake dense and too much makes it rise too far and collapse.

Mixing, pouring, and rising

All the weighed materials drop into the Slurry Mixing and Pouring, where a high-speed Mixer Impeller driven by the Mixer Motor blends them in under a minute. Even dispersion of the aluminium matters, because the powder reacts with the lime to release hydrogen and the gas has to form evenly through the mix for the cake to rise level. The Pouring Spout fills each Mould Box only about half full, leaving room for the slurry to roughly double in height. The poured moulds rest in the Rising Chamber, a warm room near 40 degrees C, where the cake finishes rising and stiffens over a few hours into a green solid firm enough to cut but soft enough that a wire passes through it. A Tilting Crane then opens the Side Form walls and tips the cake onto a cutting pallet.

Cutting the green cake

The Cutting Machine machine slices the soft cake into blocks before any hard cure. A Cutting Frame carries three wire harps and feeds them through the cake on Ball Screw drives that run on a Linear Guide Rail pair for a straight, even cut. The Vertical Wire Harp harp sets the block width, the Horizontal Wire Harp harp sets the height, and the Cross-cut Wire Harp harp cuts the length and trims the rough top and bottom skin. Each wire is kept straight by a Wire Tensioner, because a slack wire wanders and the blocks come out off-square. The cut cake travels on the Cutting Trolley toward the autoclave still in one piece, with the cuts only lightly bonded.

Autoclave curing

The Autoclave System system gives the blocks their strength. Trolleys of cut cake roll on the Rail Track into a long Autoclave Vessel that seals behind a Vessel Door. A Steam Valve follows a programmed curve that raises the vessel to about 12 bar and 190 degrees C and holds it for several hours, while a Pressure Sensor tracks the pressure and a Safety Relief Valve guards the limit. Under that heat and pressure the lime and ground sand react to tobermorite, the mineral that carries the load in the finished block. A Condensate Recovery system recovers heat by passing spent steam from a finishing vessel to one starting up, which cuts the energy bill on a plant that runs several vessels around the clock. The steam itself comes from the Steam and Boiler Plant plant, where a Feedwater Pump forces treated water into the Steam Boiler.

Handling and use

After curing, the Cake and Product Handling section takes over. A Product Tilting Crane lifts the cured cake off its pallet, the Block Separator breaks the lightly bonded cut joints into individual blocks, and the Packing Line stacks, wraps, and straps them onto transport pallets. The finished blocks build internal and external walls in housing and commercial work, where their light weight speeds laying, their air cells cut heating and cooling loads, and the same material gives both structure and insulation in one course. Output is set by the number of autoclave vessels and the steam the Steam and Boiler Plant can raise, since curing is the slowest step in the line.