Ask the Builder: The secret to ancient concrete steps

I knew limestone was hard and durable as a geology student, but I wasn’t aware that dense, fine-grained limestone can be heated to create the lime that’s then used as brick mortar, plaster and a super-adhesive additive in regular concrete. When you heat up limestone, its chemical composition is CaCO3, you drive off the CO2 and are left with CaO.

Once you grind up this CaO into a powder and then add water, you end up re-creating the limestone. This is why plaster is so hard. This is why true whitewash made from lime and then painted on brick buildings, fences or other walls lasts for decades. You’re basically painting on or troweling on a thin layer of limestone, for goodness sake!

You can now see why it’s a good idea to add hydrated lime to concrete, can’t you? The Portland cement is strong, but why not give it a little boost. Keep in mind the normal minimum standard for exterior concrete exposed to cold temperatures is a six-bag mix. This means six 94-pound bags of Portland cement are in each cubic yard of concrete. It’s probably a better idea to add seven or eight bags to your exterior concrete.

You can bet the old masons building the steps added extra cement, and they most likely put in a shovelful or two of hydrated lime. The railroad bridge engineers did the same. This is why you see concrete railroad bridge abutments in your city or town in such great shape after 100 or more years.