VARIOUS BRICK AGGREGATES USED IN BRICKYARDS
When considering mix design, it is important to know what types of aggregates may be encountered. Firstly let us divide the samples by type:
SAND
Sand is the most common substance found in brick-making. Like most material types, sand comes with its benefits & its drawbacks. Firstly, let's outline the types of sands:
BUILDING SAND
This is the best of the sands. It's used in all aspects of construction & is desireable because of its cohesive properties. It responds well to cement to form high-quality concrete products. Consider mixing with crushed stone to create a less smooth surface on the bricks/blocks, which will make plastering grip much better for a longer lasting wall surface. Adding the stone will also increase the brick strength.
RIVER SAND
River sand usually contains much river debris such as crushed shell fragments & larger silicon particulates. Usually, if sourced far from the river mouth, this type of sand is suitable as long as the water is fresh water with no taste of salt. Some customers have been known to wash their river sand to dispel organic content, which can be a concern when using river sand for brick making.
PLASTER SAND
Plaster sand is very good for fine building work. Usually used with 52,5 cement in tiling & tile grouting as well as normal cement for plastering walls. Using this fine sand is not recommended on its own. It is best to blend this sand with crusher dust or fine grit to average out the cost of the mixture & reduce the brick cost.
SEA SAND
Sea sand is NOT suitable! There are a few number of brick makers that have a system of washing seas sand to rid it of all the salt content, but not only is the time consuming & costly - it yields a type of sand aggregate that consists of near-perfectly round sand particles. This makes the cement requirement much higher because a stronger lattice is needed to keep the smooth round particles connected. Generally, this will make for a weaker brick that may be strong to the pressure test but won't do well in the sheer-force test (think: snapping a bread-stick).
Generally, sand is cheaper than stone because less work is required to "refine" the material. If you are in a remote area, then the transport of the sand may make it as expensive as any stone to transport. In that case, if availability is not an issue, always opt for crushed stone.
STONE
Stone is basically layered rock that is dug up & crushed down to certain gradings to be used for various applications. Anything above 10mm (0.4") is not really suitable for paver/brick/block making. After crushing the stone, the aggregate is sieved to allow the grading to slide off & the remainder to fall through for re-crushing to lower grades.
Let's divide stone into the common gradings:
9.5mm (0.37")
This size is quite large & should typically be restricted to use in concrete block units used to create a framework for other load-bearing support structures. Also, with the stone modulus radius being so large, there will be a very porous appearance to the brick. Do not use this size with the intention of having a smooth face-brick wall.
6.7mm (0.26")
This is a good size stone chip for M4 (4.5"), M6 (6") & M9 (9") cavity or hollow blocks & their similar equivalents. It has stone chips of a decent grading for aesthetics (appearance of the brick from outside) if combined with sand - can be left as facebrick; but excellent provision for plaster grip. Adding sand will allow for a super compression ratio during compaction.
4.5mm (0.18")
Optimal size for adding strength to paving & stock bricks. Stone particles provide jagged edges for cement grip. Blend this stone with sand to fill in the gaps & provide good compression.
CRUSHER DUST
The above gradings are in terms of stones that are caught by the sieve. With crusher dust/crusher run/crushed stone - it is everything from a grading downward. Typically there is COARSE CRUSHER (or coarse grit) and FINE CRUSHER (or fine grit). Fine grit is likely to be 4.5mm (0.18") & below - down to fine "dust."
Crushed dust/grit is THE BEST aggregate to use. It has all the jagged stone chips for the cement to make strong bonding connections that will give the bricks strength as well the fine material to fill the gaps between. However, it is likely to be the most costly. It is best to blend this material with sand to offset the cost.
BASALT
Unlike stone, basalt is actually igneous rock which makes it much stronger & dense. This gives it the added weight one will find as well as the increased difficulty in breaking it down into smaller pieces, especially down to fine dust.
Because adding basalt adds much more weight to the block than we desire without a needed increase in strength, it is preferable to avoid this aggregate type.
FLY ASH
Flyash is a by-product of coal boilers. It is collected at the base as coal ash as well as the air particulates filtered before the smoke exits the gas flutes/chimneys/stacks. It is made of extremely fine particles & can be seen under an electron microscope.
The benefit of using some fly-ash is that it adds excellent thermo-regulation to walls that make use of such blocks. The water requirement for curing the cement is also reduced & the concrete is more workable.
The downside is that it can be quite volatile in practice of brick-making. Much more cement is needed as the ratio of ash content increases. Many areas look for ways of disposing fly ash & brick-making is certainly an avenue that reverses the costs & turning the disposal into profit (or cost savings for utilising the bricks.) The advice is to not use fly ash on its own.
MIX RATIOS
Now that you are familiar with the various raw materials used in bricks, blocks & paver making, take a look the mix design article for tips on blends for different applications.