Influence of Initial Curing on Pore Structure and Porosity of Blended Cement Concretes

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Title: Influence of Initial Curing on Pore Structure and Porosity of Blended Cement Concretes

Author(s): P. S. Mangat and J. M. El-Khatib

Publication: Special Publication

Volume: 132

Issue:

Appears on pages(s): 813-834

Keywords: absorption; blended cements; crazing; curing; fly ash; porosity; silica fume; slags; superplasticizers; tests; water; Materials Research

Date: 5/1/1992

Abstract:
Paper presents the results of an experimental investigation to determine the influence of curing on pore volume, pore structure, and absorption of the surface zone of blended cement pastes and concretes. The cement replacement materials used were fly ash, ground granulated blast furnace slag, and silica fume. Temperature and relative humidity were the main variable parameters of the early age (up to 14 days) curing regimes adopted--the temperatures used were 20 and 45 C and the different relative humidities were 25, 55, and approximately 100 percent (specimens covered with wet burlap). In addition, a curing membrane was used in one set of experiments. Mercury intrusion porosimetry was carried out on ordinary portland cement and blended cement pastes. Capillary water absorption tests and shallow immersion tests were carried out on samples obtained from the surface zone of concrete cubes, and the latter test was also conducted on whole cubes. The cement replacement level and water-cementitious materials (water-total binder) ratio of the pastes and the concrete mixes were the same, the water-cementitious materials ratio being 0.45. Results show that dry curing at early ages results in higher intruded pore volume, coarse pore structure, and higher absorption of the surface zone compared with initial moist-curing. The effect is more pronounced in fly ash and slag-blended mixes than in control and silica fume mixes. Higher temperatures of curing have a detrimental effect on pore volume of ordinary portland cement paste and silica fume-incorporated cement paste, whereas the effect is beneficial for cement pastes blended with fly ash or slag. Cement pastes and concretes blended with slag are prone to surface crazing, which results in greater porosity of the surface zone of specimens. Surface crazing in slag-blended mixes becomes negligible under initial curing at higher temperature.