Sand prices have increased, and frequent disturbance of river beds to mine it, causes ecological issues. Use of quarry dust, which is a waste from crushing rocks, reduces demand for sand, and may in many cases be cheaper, and better than river sand. Rock dust can be used as a sand substitute for hollow blocks. Mixed aggregates from rock metal production, with a range of sizes, is used.
A study done in India has shown that upto 50% of the sand can be replaced with rock dust, without affecting compressive strength. This study, recommends use of aggregates from 12.5mm and below, along with sand and quarry dust, for best performance. Test results indicated effective usage of stone dust with same compressive strength, comparable tensile strength and modulus of rupture. 2% Superplasticizer made workability equivalent to conventional concrete.
A study done by Nnamdi Azikiwe University, Awka , has found that adding quarry dust to make concrete, makes workability difficult, and requires more water. But after curing for about 2 months, the strength was comparable to regular concrete. About 15 to 25% substitution of the total aggregate mixture, by weight, with rock dust can be done, without affecting strength. Quarry dust has rough, sharp and angular particles and as such causes a gain in strength due to better interlocking.
In India, there is a wide use of quarry dust with mixed sizes upto 5mm, has been used as a substantial replacement for sand. If proper ratio of cement is not maintained, then the hollow blocks may not be cured properly, and will crumble or lose strength in a few years. Since structural use of hollow blocks is minimal, this is not an issue. But a proper ratio, will allow the hollow blocks for walls taking structural loads.
Another study done in Nigeria, found that the ratio of sand to quarry dust, is about 15% for optimal strength.
There are many studies available online, with proper ratios, and curing, we can create hollow blocks, that are comparable in strength.
Rock dust is used as an additive to improve the properties of some clay rich minerals . This is moulded into bricks and fired.