The book in question is a space navy-/space opera-type book. The human ships in the book(s) used primarily lasers as their weapons. As such they have countermeasures that I think were known as "sand bags;" they released said countermeasures which used sand and particulates to weaken laser beams before the the ship takes a hit.
At some stage they run into aliens whom are hostile and more powerful than the humans. None of the lasers can breach the alien hulls/shields. I remember one of the characters thought to use the "sand bags" as a weapon by laying them in the path of the Aliens whom are moving at considerable speed and when they hit the sand it overpowers their shields.
At first I thought it was from the Lost Fleet books, but I have recently re-read them and the "sand bags" were not used.