Which type of sand should I use?

Morning all! I'm planning on sowing some nantes carrots this month -weather/conditions allowing - but our soil is slightly heavy so I'm going to add a bit of sand in with the soil where I plant them. The question is... do I have to buy special horticultural gritty sand from the garden centre or can I buy 'normal' sand from my local builder's merchants?  :?:

ordinary concreating sand from b& q sometimes called sharp sand.
wash well and mixwith growbags or ordinary soil depending on how big your bed is and how many carrots you want.
dave.