Civil Engineering Reference
In-Depth Information
70
Control
P1 + 600 ml water
P1 + 800 ml water
P1 + 1000 ml water
60
50
40
30
20
10
0
0.1
1
10
Time after Casting (Hours)
100
1000
Figure 2.24
Percentage hydration versus time for mortar specimens microwave cured
at different powers. Because of the lack of power metres, water loads were
used to measure the microwave power delivered at each power setting. The
relative value of the power levels can be considered as roughly proportional
to the amount of water. (From Hutchison, R.G., Chang, J.T., et al., Thermal
acceleration of Portland-cement mortars with microwave-energy,
Cement
and Concrete Research
, 1991,
21
(5):795-799. With permisson.)
The increased hydration rate in microwave-cured mortar has also been
confirmed by x-ray diffraction (XRD). XRD characterisation has indicated
that the characteristic peaks of unhydrated C
3
S and C
2
S in microwave-
cured mortar are lower than conventionally cured mortar specimens. The
lower amounts of unhydrated C
3
S and C
2
S in the microwave-cured mortar
corroborate observation of increased hydration rate caused by microwave
curing. The collapse of capillary pores and densification of concrete because
of the removal of water from the fresh mix are also considered to contrib-
ute to the increased early strength development rate achievable through
microwave curing [2]. It has been shown that microwave curing can con-
siderably reduce the permeability of concrete/mortar, leading to a denser
microstructure in the microwave-cured mortar vis-à-vis the standard cured
mortar, which can result in a stronger and more durable concrete/mortar
(Figure 2.25) [2]. The densification of concrete mortar/concrete during
microwave curing may be attributed to the plastic shrinkage and the result-
ing reduction in the porosity of mortar through removal of some of the free
water at early ages.
As discussed, based on the results of a series of basic experimental
studies, early investigations on microwave curing of concrete concluded
that microwave heating generally increases both the early and long-term
strengths of mortar/concrete. However, further studies indicated that the
strength development trend of microwave-cured cementitious materials is
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