Dehydration-hydration reactivity of laumontite: analyses and tests for easy detection

Abstract

AbstractHydration reactions are known to affect rock or aggregate stability in construction; laumontite is not usually considered to be a ‘problem-mineral’ though drill cores from the very low-grade metamorphic altered andesites and volcanoclastic rocks from Central Chile showed detachments of shotcrete in a tunnel exposed to periodic water flow, with expandable clay phases presumed to be responsible for the observed failure. Abundant laumontite detected in the cores may also be responsible for the detachment, however, resulting from the structural expansion and contraction in response to hydration and drying. Clay reactivity in construction projects is often tested on site by 30 days of ethylene glycol exposure, but adequate monitoring options for laumontite are not deployed. Options for laumontite characterization involving a combination of water immersion and slaking and modified oedometer-based expansibility tests were used here to observe the response to laumontite expansion pressure. All tests were formulated considering minimal implementation efforts for building sites or the easy availability of analytical and testing facilities.Laumontite was identified by optical microscopy, semiquantitative X-ray diffraction, and automated mineralogical analysis. A combination of the latter two methods provided reliable information about the presence of sub-microscopic laumontite and a visual impression of the textural arrangement of the zeolite in the rock.A slaking test based on four cycles of immersion followed by drying and final application of weight (simulated overburden) is best suited to indirect detection and for demonstrating rock reactivity due to the presence of laumontite. Rocks with laumontite show expansion when crushed, recompacted and fitted into an oedometer, but mineralogical information is required for adequate interpretation of the results.