Cars Q&A

I park my Hyundai-Santro in my flats' open space. Some residents in the upper floors have done some post-rain, maintenance jobs in their apartments, with WHITE CEMENT and perhaps, poured some power/liquid of WHITE CEMENT over my car's rear-wind shield. On a routine water wash, they are stuck still and an ENAMEL like soot/dust is layered on my car. How do i remove it? What type of a WASH / TREATMENT should i resort to? Help please! Thanks