Sea-dredged concretes of an artificial island, which had disintegrated at the coastal revetment due to the Great Kanto Earthquake in 1923 two years after its completion and subsequently sunk into the sea due to waves, were examined petrographically. Steel-reinforced concrete caisson (wall and infill) of a breakwater built in the 1910s and non-reinforced concrete of a warehouse (wall) contained cement particles showing characteristics of production by rotary kiln. Belite predominated forming a large cluster composed of small crystals often having pale colored cross-lamellae, occasionally associated with slender alkali-aluminate. RC caisson also contained cement from bottle kiln with coarsely developed alite and aluminate, which was used in mix with cement from rotary kiln. The gravel used in the reinforced caisson was similar in size to that in modern concretes, but it was much coarser in the non-reinforced members, suggestive of a size specification of the aggregate used. Chert and sandstone pebbles in the warehouse concrete produced microscopic cracks with surrounding ASR gel. Characteristics of the cement hydrates and the behavior of chloride ions in the concrete were discussed reviewing previous data. These observations revealed the early stage of the cement industry 100 years ago in Japan in which the rotary kiln was gradually replacing the bottle kiln.
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