Calcium Hydroxide Deposition in the Near Interfacial Zone in Plain Concrete

The near interfacial zone in plain concrete, as seen by back‐scatter electron microscopy, is characterized by calcium I hydroxide (CH) deposition and large pores. CH was deposited around many of the aggregate particles, irrespective of rock type as irregular but conspicuous coatings. The common thickness of the CH coating was about 2–3 μm, but occasionally it thinned or thickened to form lenslike deposits up to 8 μm wide. A greater amount of CH was encountered in zones confined between close‐packed aggregates where the precipitated CH bridged the spaces between aggregates. It is suggested that the formation of the CH coating combined with the presence of large pores makes this region vulnerable to solution ingress and chemical attack.