Oh dear now I am in unknown territory Cement over the Pond is different? I wonder if you are thinking of a bag that has Sand and Cement that are mixed to make Mortar? Portland Cement (just that) over here is all very fine almost like Talcum Powder as dust like you say. I wonder if you have an older bag where some dampness has got in to make part of the cement harden? Whatever Dusty Portland would be the 1 part of the suggested Mix.
JonD