I have been learning about tabby!
Tabby is a type of concrete which was widely used in the pre-Civil war American Southeast. The ingredients are sand, water, shells and lime (calcium oxide), with the lime produced by crushing and burning more shells (which are calcium carbonate). The resulting concrete-like structures are phenomenally durable, especially compared to wood in the subtropical climate. It is in the ruined and unmaintained buildings (as depicted) you can appreciate the shell concrete. The nonruined ones are painted and in use and look like any other building if perhaps more solid than more recent brick or woodframe buildings.
Here is a quote from Thomas Spaulding, a Georgia plantation owner and great advocate of tabby buildings.
The Original Progressive Farmer: The Agricultural
Legacy of Thomas Spalding of Sapelo
Spalding’s affinity for tabby arose from this perceived permanence.
Growing up in Frederica, Spalding observed the ruins of the fort and
town and noted that he had “seen time destroy everything but them.”...
If properly cared for, Spalding believed, buildings like his South End
House could last many lifetimes, enduring the forces of man and
nature. Indeed, many tabby structures remain standing—and in some
cases are still being utilized––two centuries and many violent storms
In 1830, Spalding wrote an article for the Southern
Agriculturist, entitled “On the mode of Constructing Tabby Buildings
and the propriety of improving our plantations in a permanent manner.”
Spalding began his article by arguing that “no man who cultivates his
own land, should erect upon it wooden or temporary buildings.”
Plantation buildings, whether homes or buildings for agricultural
purposes, should be built to withstand the tests of time. Temporary
structures required constant maintenance and improvement, and suffered
inevitable decay. Durable, permanent buildings were therefore more
economically beneficial, as they saved planters much time and energy
long term. Tabby, according to Spalding, was the most economical
material that could withstand the tests of time. Furthermore, tabby
was convenient and affordable when the proper materials were
Tabby as far as I can tell was used where there were large deposits of oyster and other shells - usually taken from "shell middens" centuries of shell accumulation in native shell middens.
But could there be giant shell middens in non-coastal areas? Could a rainforest have shell middens suitable for turning into tabby concrete?
Depicted: a colossal freshwater mussel midden from Indiana. Anyplace there are people and water that harbors shellfish, people will harvest and eat the shellfish and throw the shells in a heap, where over the years they pile up into huge troves.
It is not great stretch to propose that shell middens could exist in the rainforest (though if they do I don't know about them. Links welcome!) Along with wood to burn the shells into lime, you now have 3 of the 4 needed ingredients. The trickiest component might be sand. Fortunately sand tends to collect itself and your people will know where to find it. It is easier to haul sand a distance than it is cut stone!