Weed seeds in leaf compost

I have a huge heap of leaf compost from last years leaves. I used composter and it has worked a treat. The trouble is, when I used the previous years leaf mould on the garden, I got thousands of tree seedlings growing plus a few other weed seeds which had invaded my heap! Is there some chemical I can use on the heap to kill the seeds before I use the leaf mould in the garden? Would something like Roundup be suitable? I don’t want to kill the plants in the garden!