This reference covers the defects most often seen in an autoclaved aerated concrete (AAC) plant — from casting and pre-curing through cutting and autoclaving — with the root cause of each and the adjustment that corrects it. It is written for plant operators and process engineers who need to diagnose a fault on the floor and act.
The structure is consistent throughout: a symptom, then the likely cause and the fix. Where several causes apply, each is listed on its own. "Cake" means the aerated body before autoclaving; "slurry" is the mixed batter; "dry solids" means lime, cement and fly ash together.
● Casting temperature too high — lower the casting temperature.
● Too much aluminium powder — reduce the aluminium powder dosage.
● Slurry too thin — raise the slurry consistency.
● Too little lime or cement — increase the lime or cement.
● Fly ash too coarse and slurry poorly cohesive — increase the dry-solids content (fly ash powder, lime, cement) and use a saponin-based foam stabiliser.
● Too much lime, high internal cake temperature — reduce the lime.
● Thin slurry with excess water pooling at the mould edges — raise the slurry specific gravity, use a stabiliser and reduce the slurry volume.
● High casting temperature driving excess gas — lower the casting temperature.
● Aluminium particles too coarse — change the aluminium powder grade.
● Slurry too thick — lower the slurry specific gravity.
● Too much lime or cement — reduce the lime or cement dosage.
● Casting temperature too high — lower the casting temperature.
● Total batch weight too low — increase the total mix.
● Too little aluminium powder — increase the aluminium powder dosage.
● Uneven strength, hard inside and soft outside: low ambient temperature — raise the ambient temperature.
● Weak side-corner strength at cutting, cake collapses: cutting too soft — raise the cutting hardness.
● Slow surface set, high water-to-solid ratio, low dry solids (especially lime): (1) increase lime and cement; (2) reduce the slurry volume; (3) raise the slurry consistency.
● Mould frame striking the cake: overhead crane travel not synchronised — (1) remove oil film from the rail surface; (2) fix the crane travel drive; loose flip-spreader arms — fix the flip-spreader.
● Mould frame distorted: (1) correct the frame assembly; (2) correct the frame itself.
● Weak bottom-edge strength at cutting, cake dropped too hard: follow the procedure — the cake must pause when it is 10 cm above the cutting table, then be lowered slowly.
● Low water-to-solid ratio, poor toughness: (1) lower the slurry specific gravity; (2) increase the slurry volume; (3) reduce the lime.
● Fine fly ash, early strength lagging: (1) raise the cutting hardness; (2) raise the fly ash sieve residue.
● High bulk density: (1) control the density; (2) use an additive to raise the cutting strength.
● Wet cake, over-long pre-cure, continuous vertical cracks from top down — high water-to-solid ratio and low dry solids (especially lime): (1) reduce the slurry volume or raise its consistency; (2) increase the lime; (3) reduce the gypsum to speed stiffening.
● Dry cake, too short pre-cure, intermittent breaks at several levels — fly ash too fine: raise the fly ash sieve residue; too much lime: reduce the lime; too little gypsum: raise the gypsum proportion.
The surface throws large, deep blowholes and sinks; the blown area sets into a honeycomb-like hard lump.
● Casting temperature too high — lower the casting temperature.
● Water-to-solid ratio too high — tighten the water-to-solid ratio.
● Too much lime — reduce the lime.
● Too little gypsum — increase the gypsum.
● Lime absorbs water poorly and cement is low — increase both lime and cement.
● Lime calorific value insufficient — raise the casting temperature.
● Water-to-solid ratio too high — reduce the water-to-solid ratio.
● Fly ash too fine — raise the fly ash sieve residue.
● Cutting too soft — raise the cutting hardness.
● The "bread head" (uncut top crust) is too thick — cut it away.
● Blade distorted, worn or scaled — put the blade right.
● Cutting too soft — raise the cutting hardness.
● Long pre-cure, slow hardening: (1) reduce the water-to-solid ratio; (2) add lime and cement; (3) raise the casting temperature; (4) reduce the gypsum.
● Dry cracking, cutting hardness too high — cut a little earlier. If hardening is fast: (1) increase the water-to-solid ratio; (2) reduce lime and cement; (3) lower the casting temperature; (4) add gypsum.
● Mould oil poorly applied or poor quality: (1) oil evenly and completely; (2) switch to an oil that meets specification.
● Surface shrinkage and sinkage near the mould frame, "mouse-nest" shape — low cement, high water-to-solid ratio, slow hardening: (1) add cement; (2) reduce the water-to-solid ratio.
● High internal lime heat, long aeration: (1) reduce the lime; (2) lower the casting temperature; (3) increase the gypsum.
After autoclaving, product on the side plate clings to the plate and will not separate at the original cut seam.
● Cutting too soft — raise the cutting hardness.
● Long pre-cure, slow hardening: (1) reduce the water-to-solid ratio; (2) add lime and cement; (3) raise the casting temperature; (4) reduce the gypsum.
Cake surface shows a crazed pattern before cutting.
● Over-burnt lime, poor early water absorption and high later calorific: (1) add cement; (2) reduce lime; (3) lower the casting temperature; (4) add gypsum.
● Undissolved lime nodules after set: lime charged too early and stuck to the mixer wall — charge the lime only after the slurry is complete; uneven mixing — (1) extend mixing time; (2) replace the mixer impeller.
● Over-burnt lime that will not slake: do not use over-burnt lime; if the lime dosage is high, reduce it; if the lime is too coarse, lower the sieve residue.
● Cake has large water bubbles (honeycomb) or is locally over-hard — eliminate the honeycomb defect; if over-hard, cut a little earlier.
● Wire damaged — replace the wire.
● Foreign object inside the cake — inspect the system and remove the source.
● Wire damaged — replace the wire.
● Foreign object in the cake — clear it.
● Cutting too hard — cut a little earlier.
● Lower limit set too low — raise the transverse lower-limit position.
● Too long a dwell at the lower limit — shorten the dwell time.
During longitudinal cutting the back of the cake is missed in places.
● Cake offset on the cutting table — re-align the cake position.
● Debris on the cutting table — clear it.
During longitudinal cutting the top wire does not enter the cake.
● Debris on the cutting table — clear it.
● Top wire not held by the pressure plate — place the top wire under the pressure plate.
● Top wire too loose — replace the top wire.
● Wire carries the cake top up on its return — loose wire: replace the wire.
● Dry cake: (1) cut a little earlier; (2) adjust the mix so the cake hardens more slowly.
The lowest course of product is not fully cut through.
● Dry cake: (1) cut softer; (2) adjust the mix so the cake hardens slightly slower.
● Cake too hard — cut a little earlier.
● Wire too loose or made too long — fit the correct wire.
● Spring plate fatigued — replace the spring plate.
After longitudinal cutting the front corner is taken away by the blade.
● Cut too thick: (1) cut away the bread head; (2) trim the edge properly.
● Cutting too soft — raise the cutting hardness.
On the return stroke the transverse wire does not follow the original path and re-cuts the cake.
● Loose wire — replace the wire.
● Cutting too soft — raise the cutting hardness.
● Cutter not firmly fixed — fix the loose transverse-cutter brake.
Product dimensions fall out of tolerance.
● Wires hung in the wrong position — re-hang the wires.
● Blade position wrong — adjust the blade position.
● Upright column shifted — adjust the column position.
● Transverse ruler wrongly fitted — adjust the ruler position.
● Spring plate position wrong — adjust the spring plate.
● Wire too loose — replace the wire.
● Spring plate fatigued — replace the spring plate.
● Cake mis-positioned on the cutting table: (1) clear debris from the table; (2) set the cake square.
● Each block shows a longitudinal crack — high impact during marshalling onto the autoclave car: (1) operate the winch in steps; (2) the car does not run smoothly — clear debris from the track.
● Each block shows a transverse crack — the cake with its side plate strikes the cart upright, causing impact and skew: (1) align the side plate with the cart; (2) pack under the cart.
● Earthworm-like internal cracks, product sometimes still intact — too much lime, stiffening faster than aeration: reduce the lime; water-to-solid ratio too small, trapped gas expands: increase the water-to-solid ratio; fly ash with high carbon content: use low-carbon fly ash.
● Fine external line cracks, product generally broken — fly ash too fine, low surface strength: raise the fly ash sieve residue; thin slurry, much water, long pre-cure: reduce the water-to-solid ratio; ambient temperature too low: raise it.
● Low bulk density — thin or short slurry: raise the slurry specific gravity; low dry solids: increase the dry-solids content.
● Not fully autoclaved, non-standard curing: (1) apply vacuum; (2) drain condensate promptly; (3) hold the curing temperature and time; fly ash too fine: raise the sieve residue; unreasonable mix: (4) hold a proper calcium-to-silicon ratio; (5) hold a proper water-to-solid ratio; (6) hold a proper gypsum proportion.
● No binder framework, fly ash too fine: (1) raise the fly ash sieve residue; (2) use an additive to adjust.
● Many cracks — resolve the crack issues above.
● Excess water loss, cake parked too long before autoclaving — send to autoclave promptly.
Every mould in the same autoclave shows a frame-shaped crack around its edge.
● Heating rate too fast — reduce the heating rate.
● Surface turns blue-black rather than the normal blue-green — fly ash with high carbon content: blend in low-carbon fly ash; parked too long before autoclaving, moisture lost: shorten the pre-autoclave parking time.
● Surface stays black long after rain — product rained on, fly ash high in carbon: (1) keep under cover; (2) lower the fly ash carbon content.
Most AAC defects trace back to three variables: the water-to-solid ratio, the lime-to-cement balance, and the casting temperature. Get those right and most cutting and autoclaving faults largely take care of themselves. When a fault appears, change one variable at a time and record the result — that is how a plant builds its own process window.
