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 IA4 ** Department of Civil Engineering, Imperial College, Imperial College Road, London, SW7 2BU, United Kingdom
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 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-dessication 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] recognizes 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 them to be deficient in hydration products in comparison with water cured specimens.
EXPERIMENTAL Materials Table I shows the composition of the portland cement and fly ash used. The fly ash had a pozzolanic activity value of 16.4 MPa at 7 days, and a portland cement pozzolanic activity value of 104%, tested according to ASTM C618-83.
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 mixes with inert rock flour (RF) derived from crystalline quartz. All mixes were made with 491 kg/m3 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, 10
Mat. Res. Soc. Symp. Proc. Vol. 85. 11987 Materials Research Society
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Table I Composition of portland cement and fly ash
PC FA
Na 2 0
MgO
A12 0 3
Si0 3
SO3
K2 0
CaO
Fe 2 03
0.4 7.6
2.9 3.9
4.7 19.2
21.3 35.0
4.0 4.9
0.4 0.8
62.9 20.4
2.1 6.0
Table II 8
Cement paste mixes
3
Mix
Reference
wob
PC
Mass (kg/m ) FA Water
RF
454 680
100 100 100
30 45 60
0.30 0.45 0.60
1638 1091 818
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