Dynamic diffusion and precipitation processes across calcium silicate membranes.

Hypothesis: Chemical gardens are tubular inorganic structures exhibiting complex morphologies and interesting dynamic properties upon ageing, with coupled diffusion and precipitation processes keeping the systems out of equilibrium for extended periods of time. Calcium-based silica gardens should comprise membranes that mimic the microstructures occurring in ordinary Portland cement and/or silicate gel layers observed around highly reactive siliceous aggregates in concrete.

Experiments: Single macroscopic silica garden tubes were prepared using pellets of calcium chloride and sodium silicate solution. The composition of the mineralized tubes was characterized by means of various ex-situ techniques, while time-dependent monitoring of the solutions enclosed by and surrounding the membrane gives insight into the spatiotemporal distribution of the different ionic species. The latter data reflect transport properties and precipitation reactions in the system, thus allowing its complex dynamic behavior to be resolved.

Findings: The results show that in contrast to the previously studied cases of iron- and cobalt-based silica gardens, the formed calcium silicate membrane is homogeneous and ultimately becomes impermeable to all species except water, hydroxide and sodium ions, resulting in the permanent conservation of considerable concentration gradients across the membrane. The insights gained in this work may help elucidate the nature and mechanisms of ion diffusion in Portland cements and concrete, especially those occurring during initial hydration of calcium silicates and the so-called alkali-silica reaction (ASR), one of the major concrete deterioration mechanisms causing serious problems with respect to the durability of concrete and the restricted use of many potential sources of raw materials.