Autoclaved Asbestos-Cement Products

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Title: Autoclaved Asbestos-Cement Products

Author(s): Julie C. Yang

Publication: Special Publication

Volume: 32

Issue:

Appears on pages(s): 117-140

Keywords: asbestos-cement products; autoclaving; binders (materials); calcium silicate hydrates; chemical analysis; compressive strength; curing; differential thermal analysis; flexural strength;

Date: 7/1/1972

Abstract:
Curing of asbestos-cement products by high-pressure autoclave, including saturated steam and superheated steam conditions is dis-cussed and compared. The binder produced under the various conditions were characterized by x-ray diffraction, DTA, microscopic point-count estimation of composition, pore spectra, uncombined Ca(OH)2 analysis, density, and strength determinations. The desired high-strength binder was essenti-ally tobermorite and some CSHn gel. Al-substituted tobermorite and CSHII) may be present in small amounts, but C2SH, Al2O3, Fe2O3, and S03-bearing hydrous phases were not detected. Superheated steam conditions should be avoided at all time, and saturated steam autoclaving can be carried out usually at pressures around 100-l 10 psig (7.0 - 7.7 kgf/cm2) for times of about 16 hr Curing in saturated steam, utilizing a water spray technique, is recommended because it permits the desired binders to be formed over a broad pressure range and also reduces significantly the autoclaving time. An optimum autoclaving cycle is recommended that uses stepwise curing at two pressure levels in the presence of water spray: 5.5 hr at 115 psig (8.1 kgf/cm2 followed by a 3.0 hr at about 130 psig (9.1 kgf/cm2).