Carbonation Behavior of Mortars and Concretes Made With Fly Ash


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Title: Carbonation Behavior of Mortars and Concretes Made With Fly Ash

Author(s): P. Schubert

Publication: Special Publication

Volume: 100


Appears on pages(s): 1945-1962

Keywords: blast furnace slag; carbonation; compressive strength; concretes; concrete durability; curing; fly ash; mortars (material); permeability; Materials Research

Date: 4/1/1987

Fly ashes react with Ca(OH)2 and water to form calcium-silicate-hydrates. This consumption of Ca(OH)2 may increase the carbonation rate of fly ash concretes and mortars. On the other hand, the pore size distribution will be changed toward smaller pores due to the long term pozzolanic reaction of the fly ash so that the diffusion of CO2 will be diminished. Investigations have been carried out with mortars made of different types of cements and different fly ashes (different granulometry) from bituminous coal by variation of water-(cement + fly ash) ratio and the time of curing in water. The depth of carbonation dc, the carbonation rate vc, and the compressive strength fc have been determined during storage in the laboratory (20 C and 65 percent relative humidity) over several years. The test results have been compared with results of other investigations. There is an approximately linear relationship between vc and 1 / û fc28 (fc28 = compressive strength at 28 days). Related to the same compressive strength, there is no significant difference between concretes and mortars with and without fly ash as long as cements with normal CaO content are used, the cement content c is not too low ( > about 250 kg/m3) and the fly ash content f is not too high (f/c > about 0.3). In the case of concretes and mortars with low CaO contents, an increasing fly ash content leads to an increase in carbonation rate. Fly ashes with different granulometry have not influenced the carbonation behavior marked. However, the time of moist curing had a very strong influence.