I want to put a small fire pit (3ft x 3ft) into backyard. I checked the stone options at home depot & lowes and just the material will cost around $250.
I was thinking about building a mold and pouring concrete into it instead of using stone. Cost would be less than $50 in this case.
Is it ok to use concrete instead stone in terms of high heat and fire? Should I put some sort of heat shield inside of the fire pit to protect the concrete?
After the suggestions made by @BobOClockand, and @Ecnerwal I decided to go with blocks (got from Lowe's and they are fire resistant) to the bottom I put lava rocks. I checked the outside surface of the blocks for temperature after a while and they were relatively cold, so I will cover the outside with natural stone for better looking.
I put lots of wood into fire pit for one last final test kept it burning for 4-5 hours. The heat was very intense and when I checked the next day, almost all the blocks were cracked.
Conclusion: Even fire resistant blocks need fire insulation with firebrick. I will replace the blocks and add firebricks inside.