Morphogenesis and microstructure of concrete-derived calthemites

Abstract

Distributions of calcareous calthemite deposits have been widely documented, but depositional processes and the architecture of their internal fabrics are not well understood. These concrete degradation products from a case study area in western Canada have external morphologies comparable to calcite speleothems formed in natural limestone caves, but the internal microstructural architecture and mineralogy are markedly different. The mineralogy consists of mostly calcite with secondary halite and minor percentages of trona and portlandite. A novel morphogenetic model explains depositional processes resulting in calcareous crusts that follow fractures of an overlying concrete surface, and how sufficient structural integrity provided by the internal architecture supports attachment areas of tubular soda straws. Interiors of these multi-cm crusts consist of curvilinear calcite laminae arrayed as sub-parallel walls, compartmentalizing water–gas interfaces along variously interconnected conduits and basin-form chambers. Overall porosities of 40–60% or more are prevalent, in contrast to < 1% associated with externally similar drapery-form crusts deposited within natural limestone caves. Several calcite fabrics new to calthemite deposits are described, including dendritic shrubs that coalesce into concentric growth rings along central canals of soda straws.