'sacrete' is not the proper mix. I believe the premade mortar mix is 3 to 1 sand. - fine for mortar but not for fire mix. I tried it when building my first fireplace and it cracked out almost immediately - leaving me a nice tuckpoint repair job. second time around I paid closer attention to the fire mix and abused the fireplace for 6 years before moving - cracked bricks yes but the 'mortar' stayed in place.