Pore Structure Development in Portland/Fly Ash Blends
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PORE STRUCTURE DEVELOPMENT IN PORTLAND/FLY ASH BLENDS
DAVID J. COOK*, HUU T. CAO* AND EVERETT P. COAN** * National Building Technology Center, Chatswood, NSW 2067, Australia. ** University of New South Wales, Kensington, NSW 2033, Australia.
ABSTRACT Pore structure development in portland/fly ash blends was investigated using mercury porosimetry and methanol exchange techniques. The progress of hydration was monitored using compressive strength tests. The specimens were made using four water-cement ratios and were hydrated over a one-year period in lime-saturated water. Mercury porosimetry results indicated that the blended cement pastes generally had higher total porosity than plain cement pastes. The major contribution to this increase in porosity was in the form of smaller pore sizes. With reactive fly ash at 20% replacement, the pore structure of mature paste consists mainly of pores nominally smaller than 0.05 pm in diameter. Diffusion parameters obtained from the methanol exchange results were found to be inversely related to the volume of large pores (nominally >0.05 pm) and also to the volume of small pores (nominally 0.05.m
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Total Porosity
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----I 100
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Total Porosity Vol. Pores < 0.05um
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oreity
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Pore structure development at water-solids ratio 0.42 for (a) Figure 5. plain C#l paste, (b) plain C#2 paste, (c) C#1/20% F#l blend, (d) C#1/20% F#2 blend, (e) C#1/40% F#l blend, and (f) C#2/20% F#l blend. changes in slope of the exchange curve, the parameter T' was introduced and The is defined as the duration from the half-exchange point to saturation. results of the methanol exchange experiments are tabulated in Table II. From these results, the following observations can be made: 1. 2. 3. 4.
TI/2 increased with age of hydration. T1/2 decreased with higher water-cement and fly ash replacement ratios. Fly ash F#l blends showed higher values of T1 / 2 than blends using F#2. Cement C#l blends showed higher T 1 / 2 values than C#2 blends for the same type of fly ash.
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U.IO
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0.10%
0% (1
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0.1 1 Pore ODiaeter
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0.01
0.1 1 10 Pore Diameter
•0I 1o0 Uft
Figure 6. Effect of fly ash replacement percentage at 7 days hydration and water-solids ratio 0.42 for (a) F#1/C#l blends and (b) F#2/C#l blends.
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Figure 7. Cement C#2 blends at 7 days hydration at water-solids ratio 0.42 for (a) F#l blends and (b) F#2 blends.
a 0. 1 . 2
'I pW..
0.01
0.1 1 Pore DViamter
10
100 uM
0.1 1 Pore Diameter
Figure 8. Effect of fly ash type in cement C#1/20% fly ash replacement blends, water-solids ratio 0.42 at (a) 28 days hydration and (b) 100 days hydration.
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