Influence of Curing at Different Relative Humidities on the Hydration and Porosity of a Portland/Flyash Cement Paste


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Title: Influence of Curing at Different Relative Humidities on the Hydration and Porosity of a Portland/Flyash Cement Paste

Author(s): D. C. Killoh, L. J. Parrott, and R. G. PatelI

Publication: Special Publication

Volume: 114


Appears on pages(s): 157-174

Keywords: binders (materials); blended cements; curing; durability; environments; exposure; fly ash; humidity; hydration; porosity; pozzolans; portland cements; tests; water; Materials Research

Date: 5/1/1989

Curing of concrete is impaired by exposure to drying at early ages. Removing water from the surface layers restricts the binder reactions and pore structure development. High porosity in the surface region will allow the ingress of deleterious agents, which can lead to durability problems. Present work reports results obtained by hydrating a flyash blended cement under drying conditions. Comparisons are made with similar results from a portland cement. Small samples of OPC/pfa (70/30) paste with a water binder ratio of 0.59, initially cured under saturated conditions for 7 days, were exposed at 20 C in a CO2-free environment, to various preselected relative humidities. After 28 and 91 days, the extent of reaction and the porosities of the samples were measured by thermogravimetry and methanol adsorption, respectively. Results show the extent of hydration falls when changing from saturated to 70 percent relative humidity (rh) conditions; below this rh, it is virtually constant. From the shape of the TGA curve, it seems that there is little change in the nature of the gel phase. The pozzolanic reaction appeared to cease below 80 percent rh. Total porosity only fell very slightly with increasing relative humidity even after 91 days exposure. Under drying conditions (70 percent rh) the large-diameter porosity was three times greater than large-diameter porosity obtained under saturated conditions. From these tests it is clear that to promote reaction and to effect a decrease in the volume of large pores, the relative humidity must be greater than 95 percent, at least during early-age curing.