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as about 30 minutes for heating at 150 W and 15 minutes for heating at a
microwave power of 300 W, resulting in about a 39%-47% increase in the
3-day compressive strength of mortar.
Microwave curing may also improve the strength of the concrete at an
earlier age than those shown in Figures 2.21 and 2.22 [14]. Figure 2.23
shows that a typical mortar with a w/c ratio of 0.5 can achieve compressive
strengths as high as 15 MPa in only 4.5 hours when microwave cured at an
optimal microwave power (400 W in this case). The increased compressive
strength development rate of the microwave-cured cement paste/concrete
at early ages is mainly attributed to the increased rate of hydration of the
microwave-cured cement. Figure 2.24 shows that microwave curing accel-
erates the hydration process during the first 24 hours [12]. The induction
period of the specimens treated with microwaves is shorter than for conven-
tionally cured cement paste, and the temperature in the second exotherm
varies in proportion to the amount of energy absorbed by mortar. As seen
in Figure 2.24, there is virtually no induction period in the mortar cured at
the highest microwave power.
40
30
20
10
0
300 (0.475)
350 (0.471) 400 (0.468) 450 (0.464) 500 (0.457) 600 (0.426)
Microwave Power in Watts (water/cement ratio after heating)
4.5-hour microwave cured
7-day microwave cured
7-day normal cured
Figure 2.23
The 4.5-hour and 7-day strength of mortar cured at different microwave
powers. All specimens were made using type III Portland cement. (From
Leung, C.K.Y. and Pheeraphan, T., Very high early strength of micro-
wave cured concrete.
Cement and Concrete Research
, 1995,
25
(1):136-146.
With permission.)
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