I worked pouring residential foundations for about a year and a half straight in my late 20's. In so doing, I got to know way more than I ever wanted to about the stuff. One problematic fact, very likely the same problem that the Romans ran into in the declining years of the Western Empire, is that hydraulic cement (aka Portland cement) is extremely energy intensive to produce. Water, gravel and sand are everywhere but the precise formula for cement is not easily reproduced outside of a few geographically fortunate (or unfortunate, depending on your point of view) areas. If my memory serves, the western Untied States for example, was without a local source for cement until sometime in the late 19th century. Clearly we will want to know how to make decent mortar in the coming years, although everything from mud to cow dung has been successfully used in various places. So, I agree that having knowledge of the surprisingly complex process of making cement widely known and practiced would probably be a good thing for the coming low-tech millennia.

From a general ecological point of view, I don't care for the centralized aspect of concrete batch plants, nor the massive CO2 emissions produced by the process and industry.

From a personal aesthetic point of view, I don't like working with the stuff nor do I care for the way that concrete products look, especially as they age. For now, it's a relatively cheap and fast way to build things, but like most of our culture's cheap and fast practices, it is extremely wasteful and frequently results in hideously ugly structures. As long as I can get a little mortar for grouting tile, I'm good with no more concrete structures!

I live in the mountains by lime kilns, old and new. California. Cement and lumber from here rebuilt san francisco after the great earthquake.

The modern lime kiln, the cement plant only ceased operating in the last 10 years. I believe we still have plenty of of feedstock on this mountain. I think It was shut down due to the normal reasons of predatory buyout. It was bought out by a large international conglomerate that ran it breifly, a few years, then shut it down and sold off all the land. There was an extremely large amount of land, good forest, that was the property associated with that cement plant.

making lime cement is easy, not hard to reproduce. You get some limestone or marble or shells and heat up VERY hot to make lime. Powder the lime. Slake it with water. Add sand and gravel.

I have done this small scale when I homeschooled my daughter, along with field trip to hike out to some of the historical kilns and the cement plant

She did a mosaic decoration on the outside of a clay flowerpot using the mix and a few decorative things

lime concrete is great for building with in general, it takes longer to cure and I believe it does not set up under water. But for house foundations, we could very easily have lime concrete.

I have often thought about the spread of the use of lime in parallel with the cultivation of maize. The corn seems to have spread first, followed by the use of lye and slaked lime to nixtamalize the corn, followed (at least as far north as the Anasazi culture) by whitewashing of mud plastered adobe or stone walls. Adobe bricks, mortared with mud (interestingly, everyone that works with mortar or concrete STILL calls it 'mud') and then thickly whitewashed, will withstand rain and wind erosion indefinitely as long as it's regularly renewed. It also is a beguiling canvas; who could resist a huge white wall? AND, just as with the frescos of Michelangelo, adding colors to semi-cured plaster is a way of producing a durable and long lasting artwork. Some of the streets in old colonial towns of Mexico still maintain a version of this (see the photo). It's no wonder that mural art is so developed in that country; centuries of practice!