I've just been building a couple of retaining walls in the garden. The mortar I've used is a 4:1 ratio mix that I know works well. However, even after 48 hours, the mortar has not gone properly hard and is still very soft and crumbly. The cement has been stored in plastic bags, on a pallet in my garage since I bought it 5 months ago. It has a "use by" date of December 2012.....but doesn't appear to have "gone off" - it's still dry and powdery/ no lumps. Having said that - I've used cement well after date in the past and not experienced this .... it's a new one on me and a complete mystery! Anybody got any ideas why my mortar won't go hard?