quartz doesn’t absorb water and Silica sand too if has been washed and dried, because only little amount of clay minerals, quite normally found about filler part on river sand can absorb something.
About UHPC fillers aren’t a problem and water demand is managed and reduced at minimum possible using policarboxylates superplasticizers.
The key point is to mix properly to obtain the best hydration with consequently the highest strength development. Also quality and amount of fibers is important to maintain stability.