Setting times mixing and placing

Cottin (C50) earlier studied pastes of Ciment Fondu, largely by the TEM replica method. The microstructures of pastes of low w/c ratios hydrated for 7 days at 12°C were too compact to show more than occasional detail, but with pastes of w/c = 0,7 much detail could be observed in cavities exposed by fracture, which were lined with small crystals of CAH,o and sometimes contained plates of C,AHg. The crystals of CAH[q, which were hexagonal prisms up to 1 pm long and 0.2 pm wide, were present in rounded aggregates. Hydrous alumina was seen in a paste of w/c = 1.0 hydrated at 30 C it occurred in the lining of a cavity, mainly as rounded masses of randomly oriented platelets. 

Halse and Pratt (H57) reported SEM observations on pastes hydrated at various temperatures. In those hydrated at 8°C or 23 C, the main feature was fibrous material that was considered to be hydrous alumina, but which could also have been partly dehydrated CAH,q. The hydrating grains of cement were surrounded by shells of hydration products, from w hich they tended to become separated in a manner similar to that observed with Portland cement pastes (Section 7.4.2) though the authors recognized that this could have been partly due to dehydration. Two-day-old pastes hydrated at 40 C showed spheroidal particles of CjAH and thin, flaky plates of gibbsite. In pastes mixed with sea water, hydration took place more slowly, but no other effects on microstructural development were observed. 

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