From Fly Ash to Finished Blocks: A Practical Walkthrough of an AAC Plant
— By Maruti Hydraulics Limited
A step-by-step practical guide walking through every stage of an AAC block manufacturing plant operation.
An AAC block plant is a sequential chemical-mechanical production system in which every stage is interdependent. A problem at Stage 2 (slurry preparation) will not manifest as a visible defect until Stage 7 (autoclave curing) or even Stage 8 (finished block quality testing) — by which time several tonnes of raw material and 14 hours of processing time have been invested in that batch. Understanding the complete production sequence — including the quality checkpoints and failure modes at each stage — is essential for anyone investing in or operating an AAC block plant in India.
This walkthrough describes a standard 300 CBM/day fly ash-based AAC block plant in full operational detail, from raw material intake to finished block dispatch. The plant configuration is typical of Maruti Hydraulics-supplied plants in Maharashtra, Rajasthan, and Karnataka.
Plant Layout Overview
A 300 CBM/day AAC block plant occupies approximately 8–12 acres (3.2–4.9 hectares). The site layout is organised into distinct functional zones: raw material storage and handling (silos and weigh bridges), production hall (batching, mixing, mould filling, pre-curing), cutting area (tilting, wire cutting), autoclave bay (autoclaves, steam and condensate piping), finished goods yard (covered stacking area), and utilities (boiler house, compressor room, electrical substation). The production hall is typically a PEB (Pre-Engineered Building) structure with 30–40 m clear span to accommodate the production line without interior columns.
Stage 1: Raw Material Intake and Storage
Raw materials for a fly ash-based AAC plant are: fly ash (from thermal power stations), ordinary Portland cement (IS 269 or IS 8112, 43 or 53 grade), quicklime (calcium oxide, minimum 85% CaO), gypsum (calcium sulphate dihydrate, minimum 80% CaSO₄·2H₂O purity), and aluminum paste (65–70% aluminum metal content in mineral oil carrier).
Fly ash arrives by tanker truck or covered tipper and is pneumatically conveyed to a dedicated fly ash silo with 200–500 tonne capacity. Cement arrives by cement tanker and is conveyed to a separate cement silo (50–100 tonne capacity). Quicklime arrives in covered trucks and is conveyed to a lime silo (50–100 tonne). Gypsum typically arrives in bags or bulk and is stored in a covered shed. Aluminum paste arrives in sealed 20 kg drums and is stored in a shaded, ventilated drum store (aluminum paste is flammable and must not be stored near open flames or heat sources).
Each bulk silo is equipped with: load cells for real-time inventory monitoring, aeration pads on the cone section to prevent material bridging, a bag filter vent on the top to capture displaced dust during tanker filling, and a level indicator with high-level alarm. The control room monitors all silo levels from the SCADA system, and automatic reorder alerts prevent production stoppages from material shortages.
Intake quality check: Every fly ash consignment should be tested for specific gravity (target: 2.0–2.2 g/cm³), fineness (passing 45-micron sieve: minimum 70%), and SiO₂ content (minimum 40%). Lime should be tested for available CaO (minimum 80%) and slake temperature (minimum 60°C at 3 minutes). Reject consignments that fail these intake specifications — using substandard raw materials through a production batch is always more expensive than the inconvenience of rejection.
Stage 2: Fly Ash Slurry Preparation
Dry fly ash from the silo is mixed with water in a high-shear slurry mixer (ball mill or paddle mixer) to produce a uniform, lump-free slurry at 60–65% solids by weight. The slurry preparation system typically runs continuously to maintain a buffer of prepared slurry in an agitated storage tank (capacity: 30–60 m³), which feeds the batching system on demand.
Slurry quality parameters: Specific gravity target 1.58–1.65 kg/litre (checked by the operator every 2 hours using a calibrated density cup). Temperature: the slurry enters the mixer at approximately 30–35°C. If ambient temperature is low (winter night shifts in Rajasthan can be 8–12°C), preheated water may be required to maintain target slurry temperature. The slurry storage tank agitator runs continuously — fly ash settles rapidly when agitation stops, creating a dense sediment layer that is difficult to re-suspend.
Stage 3: SCADA Batching and Mixing
The SCADA batching system is the quality control heart of the AAC plant. For each production batch, the PLC control system executes the following sequence automatically, with all weights logged to the batch database:
- Fly ash slurry dosed to the mixer from the slurry storage tank (gravimetric dosing via flow meter and weigh hopper — typically 1,600–1,800 kg per batch)
- Ordinary Portland cement dosed by screw conveyor from cement silo to weigh hopper (typically 90–130 kg per batch)
- Quicklime dosed by screw conveyor to weigh hopper (typically 80–120 kg per batch)
- Gypsum dosed by screw conveyor (typically 15–25 kg per batch)
- Warm water added (40–45°C, approximately 200–400 litres) to adjust final mix water-to-binder ratio
- Aluminum paste pre-diluted in a separate heated vessel (150–200 litres of warm water at 40°C, stirred for 2 minutes) then added to the mixer as the final ingredient
The mixer runs for 3–4 minutes after all materials are added. Discharge is triggered by the SCADA system when the mixer torque profile indicates complete homogenisation. Total batch mass: 2.8–3.2 tonnes for a standard 6.2 m × 1.5 m × 0.65 m mould. Mixer discharge to mould filling takes less than 90 seconds to prevent premature gas generation before the mould is full.
The SCADA system logs: batch number, date and time, recipe name, actual weight of each material (with deviation from target), slurry temperature at mixing, mixer torque profile, and total mixing time. Any batch where a material weight deviates more than ±2% from target triggers an automatic alarm and requires supervisor sign-off before the mould is filled. This quality gate prevents out-of-recipe batches from entering the production stream.
Stage 4: Mould Preparation, Filling and Pre-Curing
Steel moulds (6.2 m × 1.5 m × 0.65 m, fabricated from 8–10 mm structural steel plate) are prepared before each use by cleaning residual cake from the previous batch, applying a light coat of release oil (typically mineral oil or a dedicated mould release compound) to all interior surfaces, and verifying that all side panels are correctly seated and the bolts are tightened.
The filled mixer discharges the slurry through a bottom valve directly into the prepared mould positioned beneath the mixer on a mould trolley. The operator uses a vibrating table (30–60 second vibration cycle) to eliminate trapped air pockets and ensure the slurry settles uniformly across the mould base.
Within 5–10 minutes of filling, the aluminum reaction begins producing visible gas. The cake rises from the initial fill level (approximately 60–65% of mould depth) to overflow level over 30–60 minutes. During this rise phase, the mould must not be moved or vibrated — the fragile gas bubble structure is disrupted by movement. The temperature in the pre-curing area must be maintained at 40–50°C using underfloor heating pipes or overhead radiant heaters — warm temperature accelerates lime-aluminum reaction and green cake strength development.
The pre-curing duration depends on ambient temperature and lime reactivity — typically 2–4 hours. The test for readiness is the penetrometer hardness test: the operator presses a standard steel cone into the green cake surface and reads the penetration depth. A reading of 1.5–2.5 kg/cm² indicates sufficient green strength for cutting. Too soft (<1.5 kg/cm²): cutting wires drag the cake — blocks deform and edges crumble. Too hard (>2.5 kg/cm²): cutting resistance increases, wire breakage risk rises. Getting this judgement right is one of the key operator skills that only develops with experience and good process documentation.
Stage 5: Tilting and Demoulding
Once the green cake passes the penetrometer test, the mould and cake assembly is moved by the plant's overhead crane to the tilting station. The tilting machine is a hydraulic fixture that grips the long edges of the mould frame, lifts the entire assembly, and rotates it 90° — from lying flat (horizontal cake) to standing upright (vertical cake). During this rotation, the mould side panels slide off the cake on a roller bearing guide system, leaving the green cake standing on its bottom cutting plate (a steel or hardwood platform that travels through the cutting machine with the cake).
The tilting operation must be smooth and controlled — shock or vibration during tilting can crack the green cake, creating internal fractures that become visible only after autoclave curing as structural blocks with reduced strength. The tilting speed is programmable on modern hydraulic tilting machines — the optimal tilt profile varies with green cake hardness and ambient temperature.
Stage 6: Wire Cutting
The green cake on its cutting plate is moved to the wire cutting machine on a transfer trolley. Modern wire cutting machines use a stationary cutting frame with tensioned wires arranged in a fixed grid, while the green cake is pushed through the frame on its trolley at controlled speed (typically 200–400 mm/minute, depending on cake hardness).
The cutting frame carries two sets of wires:
- Horizontal wires (parallel to the cutting plate): cut the cake into layers, defining block height (typically 200 mm height = 3 horizontal wires for a 650 mm deep cake, allowing for top skin trim)
- Vertical cross-wires (perpendicular to direction of travel): cut each layer into individual blocks, defining block width (100 mm, 150 mm, 200 mm, 225 mm, or 250 mm)
A 6.2 m × 1.5 m × 0.65 m cake cut to 600 × 200 × 100 mm blocks yields approximately 600 blocks per batch, minus top and edge trimmings (about 8–10% waste by volume). The waste trimmings (top skin — the dense surface layer — plus edge columns) drop into a scrap slurry collection trough below the cutting frame. This green waste is re-slurried with water and added back to the fly ash slurry preparation circuit, eliminating raw material waste. Wire cutting machines in modern AAC plants can accommodate field-changeable wire spacing to switch between block sizes without major adjustment time — a flexibility requirement for plants serving mixed-size markets.
Stage 7: Autoclave Loading and Curing
Cut blocks on their steel trolleys are transferred to the autoclave bay. Each trolley holds one full mould-worth of cut blocks — approximately 600 × 100 mm blocks (8–10 trolleys per autoclave batch). The autoclave trolleys run on rails set into the concrete floor, aligned precisely with the autoclave rail inside the vessel. At a 300 CBM/day plant, the standard autoclave is 2.68–3.0 m internal diameter and 36–40 m long, accommodating 8–10 loaded trolleys per charge.
Once the autoclave is fully loaded, the end door is sealed (typically a swing door with a quick-acting locking ring mechanism) and the steam admission process begins per the programmed cycle:
- Pre-heating / ramp-up phase: 1.5–2 hours. Steam pressure rises gradually from atmospheric to target operating pressure. Slow ramp-up prevents thermal shock cracking in the green blocks. Steam is admitted through a header running the full length of the autoclave for uniform temperature distribution.
- Isothermal hold phase: 8–12 hours at 10–12 bar (temperature approximately 180–185°C). This is the tobermorite synthesis phase. The block chemistry is completing — calcium silicate hydrate crystals are growing. This phase must never be shortened regardless of production pressure.
- Controlled depressurisation: 1–1.5 hours. Pressure is released in stages, not suddenly. Rapid depressurisation causes moisture flash evaporation within the block pore structure, creating microcracking and dimensional instability. Condensate from the autoclave is collected in a condensate tank for boiler feedwater recovery.
Total autoclave cycle time: 11–15 hours. A 300 CBM/day plant requires 2 autoclaves to maintain continuous production — while one autoclave is being unloaded and reloaded, the other is mid-cycle.
Stage 8: Block Separation, Palletising and Dispatch
Cured blocks emerge from the autoclave at elevated temperature (60–80°C) on their trolleys. The blocks are moved to the block separation area where a hydraulic separator arm pushes individual block columns apart laterally, breaking the thin mortar bridges between adjacent blocks that formed during autoclave curing. Proper separation without corner chipping requires the separator arm to be calibrated for the specific block size and the exact autoclave cycle being used.
Separated blocks are collected on a conveyor and fed to the palletising station, where they are stacked on wooden pallets in a standard pack configuration. A typical pallet holds 1.0–1.2 m³ of 100 mm blocks (approximately 80–90 blocks per pallet). Pallets are strapped with polypropylene banding, wrapped with stretch film for weather protection, and labelled with: manufacturer name and BIS certification mark, block grade and density class per IS 2185, nominal dimensions, batch number, production date, and plant location. The labelling requirement is part of IS 2185 product certification compliance.
Finished block pallets are stored in the covered dispatch yard. IS 2185 requires a minimum 24-hour storage period after autoclave unloading before dispatch — this allows residual steam pressure within the block pore structure to equilibrate with atmospheric pressure. Dispatching blocks too soon after autoclaving can cause stack deformation (blocks are still warm and slightly more pliable) and moisture reading errors on site.
Utilities and Support Systems
The production process described above depends on reliable operation of several utility systems:
- Industrial boiler: Coal, biomass, or gas-fired, rated at 6–12 tonnes of steam per hour at 12–14 bar. The boiler is the utility most critical to production continuity — a boiler failure stops all autoclave operation within 30 minutes. Maintaining a minimum 30-day coal stockpile is standard practice at well-run Indian AAC plants.
- Compressed air system: Oil-free compressed air at 6–8 bar for pneumatic actuators, SCADA instrumentation, and sand blasting equipment. A dedicated screw compressor with air dryer and 500-litre receiver is standard.
- Electrical supply: A 300 CBM/day plant requires approximately 600–900 kW of connected load. A dedicated power factor correction capacitor bank is advisable to avoid reactive power penalties from the DISCOM.
- Water supply: Process water consumption is approximately 2–3 m³ per CBM of block production, or 600–900 m³/day for a 300 CBM/day plant. A borewell plus overhead storage tank (day's supply minimum) is standard. Effluent from the production area (wash water, scrap slurry overflow) is collected and recycled — no liquid effluent discharge is permitted under MPCB norms.
Maruti Hydraulics has designed and commissioned over 50 AAC block plants across India and internationally. To see this process in action at one of our reference plants, or to discuss a new plant investment, contact our engineering team. View our complete AAC plant equipment range including autoclaves, SCADA batching systems, and wire cutting machines.