I am not the right oerson to answer that although i am a material controller i am not directly involve on purchase.
The idea is build a table, bom, that list all the materials you will need to complete a certain job. Inckude the estnated quantity. Say on your sample, how many bags or kg of cement do you need. Then when you puchase the material, you will know hiw many to allocate fir this job, +5% allowance for loss/damage/shrinkage.