The thousand pillar temple near Warangal is being renovated by the ASI. They are using the ancient technique of building walls using river sand, kadukkai (terminalia chebula) and jaggery. It has always surprised me that the Golconda fort has walls that look quite new and are still standing, whereas, modern day houses last maybe 3 or 4 decades at best and some of the recent flats look as if they will crumble in the second monsoon they face.
There is an article about how the Roman forts were built, and why they are still standing in some places. They used volcanic ash and lime, and treated the bricks by immersing them in seawater. Two different but very effective technologies.
Many houses built in the 19th century, have walls that shine, sort of glowing walls. This was done by crushing shells from the riverbed and using that powder as plaster for the walls....mother of pearl walls so to speak.
These technologies may not be useful now since we cannot afford the river sand to build so many houses, but surely there might have been some technologies that we have lost, that could be of use to us now.