as what we used is from sanding dust and mixing with resin becomes very hard like rock but gives also weight so that is not good. why not silica?

Silica is good for increasing the density of resin, but it doesn't give it strength like rovings do. It doesn't form a matrix, there is no way of transferring load. So really it is just thick resin behind the gelcoat, which is better than a void as voids can expand and blister the gelcoat with heat etc. But still, the gelcoat will chip easier than it would with rovings. 

so best you suggest chopped frp?

Yep. If you can get it, loose strand is better than mat. Just simpler and easier to use. But of course you can just pull apart mat and do the same job. 

Here you go - http://www.cfsnet.co.uk/acatalog/CFS_Catalogue__Chopped_Strands_399.html

Make a paste and apply into corners with mixing stick.

Chopped ends http://www.ecfibreglasssupplies.co.uk/p-770-chopped-strands-6-mm.aspx