Chalky sandy cravings

I’m almost 31 weeks pregnant with twins and I cannot stop eating tums! They have that perfect chalky texture I know my iron levels are low and I been taking iron pills its Not helping!! I also know about Pica. Which is what I most likely have. Any suggestions on what I can eat that will be safe for the babies, I have limited myself to 7 tums a day but my cravings are wanting something every couple of hours! Please help nothing too sugary.