Fly Ash and Recycled Aggregate for Blocks in Syria: Grading, Testing and Mix Design

Fly ash and recycled aggregate for blocks in Syria turn two waste streams into raw material, but only after the debris is crushed, screened and cleaned, and only after the fly ash is treated as a cement replacement rather than a filler. Rebar, plastic and gypsum board have to leave the stockpile before the crusher, because each one costs strength differently. Fines under 10 mm taken off as soil, mud on rubble and water absorbed by old mortar all push cement and water demand up. The moulding host then has to compact a lighter, rougher mix, a question of vibration force and pressing pressure rather than block size.

Which tests decide whether rubble is fit for block aggregate?

Four tests settle the question before a single bag of cement is committed: gradation, mud content, moisture content and loose bulk density. Gradation shows whether the blend fills the mould evenly. Mud content shows how much cement is spent coating dirt. Moisture content shows how much mixing water is already inside the pile. Loose bulk density shows what a metered cubic metre of the pile weighs. Test every new source and every stockpile change.

Sampling discipline is the part that fails in practice. Take a sample from the top, middle and bottom of a delivered load, mix them into one composite sample of a few kilograms, and record the date, the source and the numbers in one book kept at the batching station. Repeating the same four tests on a fixed interval is what turns a pile of debris into a raw material you can price.

Crushing, screening and picking out what ruins a block

Debris becomes aggregate in a fixed sequence, and each step removes something that would have cost strength later. A grate feeder takes the fine soil off before crushing. Air separation and manual sorting pull out the light and the wrong material, and iron removal stations catch rebar at several points in the circuit, because one bar that reaches the mould can crack it. The output is recycled aggregate and recycled sand in several grades, which is what a batching machine needs to hold a mix.

• Rebar and wire: removed at every iron removal stage before the material reaches the crusher.
• Plastic, wood and paper: lifted out by air separation, then picked by hand off the belt.
• Gypsum board and plaster: kept out of the stockpile, because sulphate bearing material forces a softer binder system.
• Fines under 10 mm: taken off as soil by the grate feeder instead of staying in the aggregate.

Why fly ash and slag change curing, not the mould

Fly ash and slag replace part of the cement, so the mix hardens on a slower curve and reaches its later strength further out. The block has to stay moist and undisturbed for longer, and stacking it early freezes a low value no press can repair. On a line with layered curing, the pallet lifter and the high level finger car carry the stack into a 3D curing kiln and hold each layer in place, which extends curing without a bigger yard.

Recycled aggregate pulls the other way. Old mortar on the surface of crushed concrete is porous, so a particle absorbs water while the mixer is running and takes it out of the paste. A blend with a high share of crushed rubble behaves like a drier, rougher batch than the gradation curve suggests, and needs more mixing water and cement to reach the same density. Keep a measured moisture figure for every stock and correct with batching water, never with a guess.

What grading does the mould actually need?

The mould does not care where the particles came from, only how they pack. A blend short of mid size particles leaves voids, and voids are filled with cement paste, the most expensive component in the mix. Blend crushed rubble with natural sand, add recycled sand from the same crushing line where the grading stays coarse, then re-test. If one aggregate cannot hold the curve, two stockpiles metered separately by a batching machine will.

Top size is set by the mixer, not by preference. Our twin shaft mixers accept up to 80 mm for pebbles and 60 mm for crushed stone, and the batching machine weighs each fraction on a three point load cell against a pre-set mix ratio, with a vibration device that keeps damp recycled sand flowing. That is what makes a two stock blend repeatable on a wet afternoon.

Why a waste blend needs more pressing capacity, not less

Rubble particles are angular, and with fly ash in the binder the mix is leaner in paste, so a block reaches the same density only with more compaction energy. The numbers: a QST8-15 presses with 95 kN of vibration force, a QST1500 with 220 kN and a QST1800 with 280 kN, all at a rated pressure of 31.5 MPa, and vibration frequency sits between 3,800 and 4,500 times per minute across the QST range. A dry, lean, angular mix is the case that uses it.

Cycle time is where extra energy turns into output. The QST series runs a moulding cycle of 15 to 25 s, and the pallet free QSMT series runs 13 to 18 s at a rated pressure of 21 MPa and a vibration frequency of 3,200 to 4,600 times per minute. After a switch to a recycled blend, plan for the slower end of the band until mix and moisture control are stable, then take the speed back.

Does a recycled mix change the machine you should buy?

It changes the specification, not the machine class. A moulding host matched to a target of 30,000 to 250,000 pieces per 8-hour shift, using vibration plus press moulding, keeps force in reserve for a lean angular blend. That is also why a fully automatic line with layered curing suits a plant running fly ash and rubble, and why the same line is staffed at 3 to 6 persons per shift once handling and cubing are automated.

Ask one question of any machine before buying it for a waste blend: does it hold rated pressure and vibration frequency on the leanest mix you intend to run, at the pallet size you intend to use. A 1,400 x 1,400 x 30 mm pallet and a 1,150 x 1,100 mm moulding area cover most hollow and solid block work. Installed capacity of 104 kW on the largest host in the range is an output decision, not a materials decision.

Send five items and we will size the line

Five items set the configuration: the block sizes you will sell, the aggregate and fly ash sources you can actually get, the test results you already hold for gradation, mud content, moisture and bulk density, the pallet size you plan to run, and the voltage and frequency at your site. With those we return the mould list and the machine class that carries your mix, and we say plainly where a recycled blend will cost you speed.

The crushing and screening package, the batching and mixing group and the curing system are configured around the material you have, not a catalogue page. Send your stockpile photographs and the four test numbers, and we will tell you whether the blend in front of you can carry the block you want to sell.