Last month a client in Nigeria called. His new machine was producing bricks that snapped when bent, and the site rejected the whole batch. We reviewed it remotely for a long time and found the fault was not the machine. It was the raw material ratio. Cement sat at only 8 percent, and the aggregate carried high mud content, so strength never built up.

We see this often. Buyers study machine parameters for two weeks before purchase, then guess at the ratio, scooping cement and shoveling stone powder by feel. When the bricks fail, the first thought is that the machine is defective.

Mixing is not guesswork

The ratio is a balance of three elements. The binder, mainly cement and sometimes fly ash, holds the aggregate together. The aggregate, stone powder, crushed stone waste, or slag, gives shape and lowers cost. Water lets the mix blend evenly. Once this balance breaks, bricks turn either brittle or loose.

How to mix solid brick

For MU10 grade solid brick, a stable ratio runs as follows.

● Cement 10 to 12 percent, about 300 kg per cubic meter of blocks

● Stone powder or crushed stone waste 82 to 85 percent, about 1.4 tons per cubic meter

● Fly ash 0 to 6 percent, mainly to improve workability, not the more the better

● Water-cement ratio 0.28 to 0.32; too wet and the brick collapses, too dry and it will not compact

How to mix hollow brick

Hollow brick needs a different ratio. Thin walls make it more sensitive to aggregate size, so cement goes up slightly to hold strength.

● Cement 9 to 11 percent

● Fly ash or slag 15 to 25 percent; this cuts weight and saves cement

● Stone powder as the remainder; grain size must pass a 5 mm sieve

● Water-cement ratio about 0.30, slightly higher than solid brick

Three common pitfalls

● Too much fly ash. Some clients hear fly ash is cheap and replaces cement, then add it up to 30 percent. The 7-day strength comes out at half the target. Fly ash contributes almost no early strength; cement must set the brick first.

● Untested aggregate mud content. If stone powder mud exceeds 5 percent, a mud film separates the cement bond and the brick layers when bent. Ask the supplier for a mud-content test before feeding material.

● Water-cement ratio by feel. On rainy days the aggregate moisture changes, yet the same water is added and the brick collapses. Fit a simple moisture meter, or adjust water daily by weather.

Three questions to ask your material supplier

If the ratio is unclear, ask the supplier these three questions first.

● What is the mud content of the stone powder, and is there a test report?

● What grade is the fly ash? Grade 1 has high activity; grade 3 is basically a filler.

● What is the aggregate grain distribution, and can it pass a 5 mm sieve?

Every Dongyue machine ships with a recommended ratio table, but that is a general version. For your local materials, have a technician cast a sample board and test its strength before mass production. On ratio, the machine gives no answer. The brick does.