Tin Hat Ranch in the video proves through multiple shots that it is the fluid quality of the sand that repeatedly self-heals each hole and friction-grabs the bullet and strips off its jacket.
Wetting it with water and/or cement defeats this important quality. With the resulting cement you have a brick wall that will crack and split apart with just a few directed shots.
On the other hand, if you kept your paper-wrapped cement bags dry like ready-made sandbags and when SHTF slipped them inside the plastic sandbags or tough-contractor trash bags(much cheaper) to protect them from getting wet you would have some unnoticeable, discrete, affordable, ready-to-go/standby protection. (Why do I still have a pallet of cement bags, friend? Well, huh, I’m still planning to build that slab for my future toolshed/barbeque grill). I do believe pure sand is better but like small caliber guns v. bigger ones, dry cement bags are better than a wet pile of sand a block or more away.
Downside is the agregate/rocks in the ready-mix: if a bullet hits them they might ricochet out the bag into you and/or your face. Tin Hat Ranch needs to get off his butt and test this!