I don't think adding calcium (2+) to sand would increase the net positive charge on sand, because it is unlikely that calcium will attach to sand. Even there is some attachment, it will not be sufficient to increase net positive charge on sand.

If the aim is to increase net positive charge on quartz, you may consider coating iron oxide (positively charged mineral at neutral pH) on sand. There are many methods to coat iron oxide on sand. You can find those here:

http://www.sciencedirect.com/science/article/pii/S0043135496001613

http://pubs.acs.org/doi/abs/10.1021/es050159h

Briefly, to coat iron oxide on sand, sand is soaked in iron solution (iron chloride or nitrate). The iron oxide is precipitated on surface of sands by either slowly increasing the pH (addition of NaOH) or heating the solution until it is completely evaporated.