First of all, holy sand is considered as an energy substance, which has the function of exorcising evil spirits. Traditionally, people often use magical sand to ward off evil spirits in feng shui layout.
However, the use of sacred sand in graves may bring some adverse effects. According to the principle of geomantic omen, the grave is the place where the dead are buried, belonging to the Yin house, which is different from the Yang house (where people live). More Yin Qi is accumulated in the tomb, which conflicts with the evil energy represented by sand. Because the sacred sand is considered to have the function of exorcism, it may activate the negative energy in the tomb, further enhance Yin Qi's influence, and adversely affect the feng shui fortune of the living.
Therefore, I personally suggest not to use holy sand in graves to avoid unnecessary conflicts and negative effects of geomantic omen. For the burial and sacrifice of the tomb, you can consult the relevant Feng Shui master or religious experts to get more accurate and beneficial suggestions.