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Solarpunk Building Materials
As we've said elsewhere, solarpunk buildings should use materials appropriate to their surroundings. Climate, weather (including rainy seasons, severe heat, long winters, etc), threats to the structure (such as insects and mold), and available materials should all be a factor, as should the needs of the community - perhaps importing an advanced material from further away will allow a structure to last and cause less harm than rebuilding it regularly.
The following is a far-from-exhaustive list of materials and building styles you may want to include depending on your setting:
Salvage
Few solarpunk stories seem to take place in 'clean slate' settings like space colonies (though some great ones absolutely do!), and if anything, a solarpunk society on earth would probably strive to preserve intact habitats and do less greenfield construction than our present day one. So it's likely existing buildings would see a fair bit of upkeep and modification, if only because they already exist and the resources to build them have already been spent.
But there are plenty of circumstances where maintaining a building or bringing it up to modern standards isn't worthwhile, and deconstruction makes more sense. Deconstructed (and even demolished!) buildings can yield all sorts of useful building materials, depending on the structure's condition at the time, and the care with which it's taken down. Generally this is a less cost-efficient source of materials than extraction from raw sources but part of that is because our current society has had a lot of time to iterate and improve on logging, mining, and other extractive fields - a solarpunk society might get equally good at deconstruction. And there are a few advantages: deconstruction sites/sources of salvaged material are almost guaranteed to be much closer to the places they'll be reused, and these materials are generally retrieved in a much more ready-to-use state. For example, dimensional lumber pulled from a stick-frame house might have some extra nails stuck in it, but it's already cut to size and will be easier to work down than a green tree, which must be cut down, transported, milled to rough dimensions, dried, possibly treated with preservatives, milled to final dimensions, and transported again.
Ideally, a deconstructed structure should provide every building material used in its construction, but realistically there are going to be limitations. Shingles (asphalt roofing shingles or wooden siding shingles) are going to be basically impossible to remove intact. Some forms of insulation (especially old stuff like newspaper, sawdust, or asbestos) won't be worth reusing and may even necessitate hazardous materials disposal. Horsehair plaster can be extremely fragile and probably wouldn't be worth reusing even if it survived removal, transportation and storage. Even modern sheetrock will be a pain to salvage. Some wood will be rotten or infested with ants, termites, or other insects. Wiring and plumbing will take a lot of inspection and some careful documentation of original use before it's considered safe to reuse, and many folks will be (not unreasonably,) reluctant to use reuse it even then. Even brick and concrete can be damaged by the elements.
That said, a tremendous amount of construction supplies, from doors and windows, to lumber and plywood can be obtained, saving both the materials and avoiding transportation and wasted space in a landfill. To get an idea of the sheer scope of materials, hardware, and stuff which might be salvaged, here's a few real life businesses which wholesale recovered materials:
- https://junkyardsnearme.net/building-material-salvage-yards-near-me/ - this one has some good photos of salvage yards
- https://www.seconduse.com/inventory/ - this appears to be an inventory of thousands of items from bowling alley parts to cabinet doors to light switch covers
Even concrete slabs can be cut up to produce new building materials or reused in creative ways! This is huge as concrete rubble accounts for 25-30% of solid waste in landfills.
Concrete rubble can also be reused in other ways, such as in landscaping. There's also a rich history of cobblestone buildings where concrete rubble may be a suitable replacement for stone.
Geopolymers
If you're looking for a drop-in replacement for Portland Cement (which is used to produce the vast majority of our built-up concrete environment) then you probably want Geopolymers. The concrete industry is a huge portion of human CO2 production today (around 8% total), due both to the release of CO2 from the chemical process of baking the limestone, and from burning fuel to produce the tremendous amounts of heat necessary for that reaction.
Geopolymer is almost carbon-neutral, repurposes industrial waste, covers all the uses of concrete, and is much better performing overall. The only reason it hasn't replaced Portland cement already is the broad hegemony of the established industry –which won't matter as industry localizes. Ideally, we want a carbon-negative alternative to allow our built habitat to become a carbon sink, but that's going to take a while. In the meantime, geopolymers are probably the best we can do. So where concrete construction can't be replaced, that seems more likely to become the cement alternative of choice.
https://www.youtube.com/watch?v=_b5dgyB6K6w
Metakaolin. It seems like in an ideal solarpunk world, Blast Furnace Slag and Fly Ash wouldn't be produced, at least not in the kind of quantities they are now or that geopolymers might need, though landfill mining could provide a supply for awhile. So it seems like our longer-term options (as I understand them) are Metakaolin (a natural resource that'll need to be obtained and processed) or palm oil fuel ash left over from the burning of palm shells and husks and fibers in the production of palm oil (the industry has issues, but it seems like it could be done sustainably).
Quartz mining tailings, clay, silt, sewage sludge, all sorts of sources of calcium-aluminates can also be used and would be likely sources once coal is fully obsolete. https://www.youtube.com/watch?v=1rcCSDVIUtg . Mineral wool waste is yet another possible source, which compliments urban farming as mineral wool is a waste product of hydroponics, as well as used for housing insulation. The base materials are very common on Earth and the number of possible sources are remarkable, with more or less levels of energy in processing.
From a visual standpoint, there's probably not much to see with geopolymer production because of the simplicity. It's basically just a measured mixing process. And that work would be done in most any conventional industrial building or workshop. As you anticipated the more visible use of direct solar energy might be in the thermal preparation of metakaolin or the other source materials, which is likely to be done near those sources. (because it's more efficient to transport refined materials than crude materials) The unusual thing about using geopolymer compared to concrete is that it hardens very rapidly and so would often be mixed for use on-demand at the work location even at a relatively large volume. So you would have a pre-mix of the dry ingredients then an on-demand liquid mix of water, alkaline 'activator' (sodium silicate waterglass –usually pre-diluted with water as it's physically heavy and can be very viscous, but also shipped as a powder), and 'hardener' (sodium hydroxide). (I've seen all the fluids premixed together, other times added individually, the water content adjusted according the latent moisture in the aggregates and the work site (it can be poured and cast underwater), though the shelf-life of this mixture is unclear to me) We're familiar with concrete being mixed in a big plant within a couple hours driving distance from a work site and delivered by caravans of traditional cement trucks.