This just might answer your question
http://www.dansand.co.uk/products/block-paving-sand/--------------------------------------------
Need to get rid of those stubborn weeds in the joints on your patio, driveway or paving? Keep them away for a good long time with our light-coloured Dansand® block paving sand for repointing and repairing granite and concrete paving and flagstones.
This naturally alkaline mixture of oven-dried, nutrient-poor minerals creates desert-like conditions to keep weeds under control in a very natural way – without chemicals. Dansand® block paving sand will keep weeds at bay for years, even with continuous heavy rain – as extensive testing has demonstrated.
Our light-coloured Dansand® block paving sand is easy to use and ideal for narrow joints on patios, sidewalks and driveways with light traffic.
Benefits of Dansand® block paving sand:
>For repointing and repair
>Environmentally friendly
>Highly weed-retardant
>Flexible joint material
>Stable paving
>Water-permeable
>Easy to apply
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Obviously it goes without saying you pressure wash the surface and joints to remove all the moss and weeds first and treat/remove any remaining stubborn weeds by hand like you would normally do. Then you apply the sand afterwards on another day when the blocks, joints and sub base are bone dry.