Restricted Hydration of Mass-Cured Concrete Containing Fly Ash
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RESTRICTED HYDRATION OF MASS-CURED CONCRETE CONTAINING FLY ASH R.H. MILLS* and N. BUENFELD** *Department of Civil Engineering, University of Toronto, 35 St. George St., Toronto, Ontario, Canada M5S 1A4 **Department of Civil Engineering, Imperial College, Imperial College Road, London, SW7 2BU, United Kingdom Received 1 December,
1986; refereed
ABSTRACT Cement paste and concrete specimens containing three different mixtures of portland cement (PC) and high-lime fly ash (FA) were subjected to various curing conditions, and the strengths, non-evaporable water, and porosities were compared with control mixes containing portland cement only. Strength and porosity data indicated that the cementing action of the mixtures was, in all cases, inferior to portland cement. For each mixture the strength loss resulting from imperfect curing, i.e. sealed or exposed to 50% relative humidity, was greater than for portland cement. In the case of concrete drying from one surface, strength differences were found, but these were not as clearly defined as those obtained in the paste specimens. INTRODUCTION Laboratory concrete is invariably cured by immersion in water or exposure to 100% relative humidity. Site concrete receives little water curing except, perhaps, near the surface. The current ACI standard [1] allows the use of sealing materials to prevent evaporation of water from the surface, but does not address the effect of self-desiccation due to chemical shrinkage [2]. Shalon and Ravina [3] reported substantial differences between water cured and sealed concrete even when the water/cement ratio was as high as 0.92.
The ACI
Standard [1] recognises that portland-pozzolan cements require 50% more curing than portland cements. In an earlier paper [4], micrographs of cement pastes sealed against gain or loss of moisture showed that the pastes were deficient in hydration products in comparison with water cured specimens. EXPERIMENTAL Materials Table I shows the composition of the portland cement (PC) and fly ash (FA) used. The fly ash had a pozzolanic activity value of 16.4 MPa at 7 days (d), and a portland cement pozzolanic activity value at 104%, tested according to ASTM C 618-83. Cement Paste Specimens Because an important aspect of the tests concerned the capacity of mixing water to support hydration, it was important to avoid bleeding. This was done by dosing the high water/cement (wo) mixes with inert rock flour (RF) derived from crystalline quartz. All mixes were made with 491 kg/m 3 initial water content. Mix proportions used are given in Table II. Thirty specimens were cast from each mix in plastic containers, 50 mm x 28 mm diameter, each with a tightly fitting lid. After 24 hours (h), 10 specimens were placed in a water bath with the lids removed (W), 10 were left sealed (S) and 10 were exposed to the laboratory atmosphere (D). After 30 and Mat. Res. Soc. Symp. Proc. Vol. 86. c 1987 Materials Research Society
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Table I Composition of Portland Cement and Fly Ash
PC FA
Na2 0
MgO
A120 3
Si03
SO3
K2 0
CaO
Fe 203
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