My house has a raised wood floor with an enclosed basement space underneath. It currently has a horrible wood laminate that I would like to get rid of. I was wondering if it would be worth my while to put in a thin coat cement floor to gather the sun that comes in all my south facing windows. As I understand it, you can put a thin coat cement floor over wood like this but will there be enough mass to give me any heat collection?
That is entirely dependent on the strength (size of joists, current load, and existing support structure)
but....... it is entirely feasible and gives many options, beside thermal mass it can be poured (usually 2" deep or less) and in floor radiant, or hydronic, heating installed at that time.
If your considering going with less than an inch depth the thermal mass effect will be marginal, even at 2" it will be a significant amount of time before it pays itself back into your coffers.
If weight is a concern aircrete can be used but there is a significant insulating value with its adoption, (not really a desirable feature in thermal mass).
This is a piss poor place to cut corners though, Dry concrete is (very roughly ) 150 lbs per cu ft, and several times that in wet weight. (FWIW 1 cu ft at 2" = 6 sq ft area) so dry weight is rarely a problem but often it needs to be poured in sections and allowed to cure, to avoid dumping massive tonnage on the structure
Hire an engineer for a structural analysis,if your structure is marginal in any way, or the area you wish to cover is large. and possibly a soil analysis if your current footings are on piers, or if your basement/foundation is pre 70's / or built without rebar reinforcement.
Other options to consider are water walls, while they're not pretty they can give you an idea of the impact of thermal mass on your budget / climate for very little investment.
I have a section in my entryway that is laid up with slate and mortared with concrete. In my case it is heated via radiant heat, but it retains heat quite well, BUT that is constant heat. A floor will not get that so much with solar gain as the sun moves about the floor in its daily arc through the sky.
For the rest of my great room, built before radiant heat was common, I never installed radiant tubes in the concrete. But to make life easier for me, and to heat my home still via 100% radiant heat, I used sand over my concrete floor, but under my wooden floor. This is where my radiant floor tubing rests. It even shocked me how well it works, BUT I have 6 inches of concrete mass it is resting on. In your case, you would get quite a bit of gain if you placed reflective material under the floor to help reflect the heat upwards into the room, which they sell and is cheap. Think rolls of tin foil here. It would not replace mass per se, but would be well worth the cost.