Class C Fly Ash as a Shrinkage Reducer for Cement Paste


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Title: Class C Fly Ash as a Shrinkage Reducer for Cement Paste

Author(s): S. Tangtermsirikul

Publication: Special Publication

Volume: 153


Appears on pages(s): 385-402

Keywords: cement pastes; drying shrinkage; durability; expansion; flexural strength; fly ash; lime fly ash; shrinkage; strength; Materials Research

Date: 6/1/1995

Introduces some experimental results on the application of ASTM Class C (high lime) fly ash from Thailand to reduce shrinkage of cement paste. Measurement of both autogeneous and drying shrinkage were conducted on cement paste samples. Parameters varied in the autogeneous shrinkage test were type of fly ash (two samples of Class C fly ash, from the same source, with different chemical composition, and a sample of Class F fly ash), cement replacement percentage (0 percent, 30 percent, and 50 percent), curing condition (seal and submerged), and curing period. For autogeneous shrinkage specimens, flexural strength, compressive strength, and setting time were tested to compare the mixtures containing Class C fly ash with those containing Class F fly ash and with those without any fly ash, to derive some basic information for judging the suitability of the Class C fly ash as a pozzolan. From the test results, it was found that Class C fly ash was effective for reducing autogeneous shrinkage and improving flexural strength. The effect was due to expansion which occurred in the samples containing Class C fly ash. For the tested range, the higher the replacement percentage, the more effective the fly ash becomes. Class C fly ash which contained higher SO 3 content was more effective than that with the lower SO 3 content for reducing shrinkage. Water curing was more effective than sealed curing since the expansion process required water; specimens with longer water curing periods showed smaller shrinkages. The pozzolanic activity index of the tested Class C fly ashes was higher than that of the tested Class F fly ash; therefore, higher 28-day compressive strength was observed in paste with the Class C fly ash than in paste with the Class F fly ash. Setting times of pastes with the Class C fly ash were generally shorter than that of the paste with the Class F fly ash. The Class C fly ash also proved to be effective for reducing drying shrinkage, as indicated by comparison of specimens made with and without Class C fly ash. Longer curing periods reduced the drying shrinkage of specimens with and without fly ash.