What veg and pellets are you feeding? Guinea pigs excrete excess calcium through their urine so whitish compound is normal.
If it is gritty however, you may be feeding too much calcium. Try reducing their calcium intake and see if that helps
I found that my guinea pig pee is leaving this white compound , I am observing it from 4-5 days and today it's little gritty too , what could it be? Help?