Manufactured sand, called crushed rock fines in some places, made in aggregate quarries is the best alternative to natural sand. When it is naturally produced after primary, secondary and tertiary crushing of stones at quarries, it may have gap graded particle size distribution and sometimes the clay content is also higher depending on the sorting process. When there is rain making the clay stick to the stones before dumping into the primary crusher, the clay content contaminated in crushed rock fines will also depend on the efficiency of the clay screen at the crushing line. There're also special additives (called clay blocker in some places) to alleviate the effect of high water demand of the clay in concrete. In some places, crushed rock fines are further processed by tailor made cone crusher to make the particle shape rounder and then sieved to different particle size ranges and re-mix again to come up with a smooth overall grading distribution before use. However, this will increase the cost and induce some waste materials.

Research has shown that some materials (saw dust) can be used to replace sand but getting the quantity required may be the challenge

Following the answers

Let me also add granulated plastic as another artificial material that can be used to replace sand.

I believe Bart is looking for a material that can substitute natural sand in concrete with massive use. In this regard, supplementary waste materials (e.g. saw dust, granulated plastic, etc.)in concrete mainly for environmental protection are not feasible solutions on this issue.

I concur with Sai's response -- I do not think sawdust will bind properly in concrete nor will most plastics.

This is high time to think seriously about saving natural sand, lot of work is on to replace natural sand, but no material found suitable till date for replacement of sand completely in concrete,all new materials are having disadvantages as compared with natural sand, so it is point of concern