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Hydration and microstructure of cement-based materials under microwave curing
Construction and Building Materials 114 (2016) 831–838
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Hydration and microstructure of cement-based materials under microwave curing Yaning Kong a, Peiming Wang a,⇑, Shuhua Liu b, Zhiyang Gao c a
School of Materials Science and Engineering, Tongji University, Shanghai 201804, China State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, China c Changjiang River Scientific Research Institute, Wuhan 430012, China b
h i g h l i g h t s Microwave curing improves the early strength when compared to steam curing. Microwave curing reduces the pores in the range of >100 nm. Microwave curing forms short-rod AFt and diminishes the particle size of CH. Microwave curing increases the absorption of S, Mg and K by C-S-H gel.
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Article history: Received 29 September 2015 Received in revised form 26 March 2016 Accepted 29 March 2016
Keywords: Microwave curing Cement-based materials Hydration Microstructure
a b s t r a c t By reducing the curing time, microwave curing can enhance the productivity, save the capital and decrease the plant areas for precast concrete when compared with the steam curing. Based on the results of 6-h and 24-h compressive strength tests the optimum curing regime was selected. The sample microwave cured using the selected curing regime was then compared against samples cured using (a) normal curing (b) steam curing at 40 °C for 10 h and (c) steam curing at 80 °C for 4 h by performing the compressive strength, XRD, TG-DSC, SEM-EDS and MIP. The results indicate that, compared with the steam curing at 80 °C, microwave curing improves the compressive strength of mortar before the age of 28 days, increases the porosity of mortar slightly, while reduces the pores in the range of >100 nm greatly, forms short-rod AFt and smaller particle size of calcium hydroxide, and increases the adsorption of K, S and Mg by C-S-H gel. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction There are many advantages when the curing period is reduced, such as productivity improvement, capital saving, and the reduction of workshop area, et al. Thermal curing technique has been used in the precast concrete for a long time. However, as a poor heat conductor, it takes >10 h to complete one curing cycle before demoulding [1]. In addition, concrete strength after 28 days is lower than that under standard curing because the fast early hydration of cement under such a high temperature, which results in the formation of large amounts of very fine C-S-H gel surrounding the unhydrates, and hence causing hindrance of diffusion and further development of strength [2]. High-frequency electromagnetic heating, such as microwave enhanced heating, is able to reduce such heterogeneity due to its ⇑ Corresponding author. E-mail address: [email protected] (P. Wang). http://dx.doi.org/10.1016/j.conbuildmat.2016.03.202 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
superior penetration depth. Microwave energy is attenuated by the vibration of polar molecules and dipoles and the resulting friction between the molecules generates heats rapidly throughout the concrete. Research by Watson [3] showed that 28-day compressive strength of microwave cured specimens displayed only half the strength compared to normally cured specimens. However, by optimizing the microwave curing parameters, strength can be improved effectively [4–6], making microwave curing a potential alternative method for accelerating cements hydration. Research by Xuequan et al. [2] found an increase in early strength under microwave curing, without any detrimental effect at later ages. Results of the permeability revealed that the specimens under microwave curing were denser than the reference specimens, inferring a reduction in porosity and the action of plastic shrinkage. Studied by Leung et al. showed that type III Portland cement concrete with microwave curing can develop early-age strength (at 4.5 h) and later-age strength (7 day) that compare very favorably with commercially available rapid hardening concrete as
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well as concrete containing accelerating admixtures [7]. Makul et al. [8] recently reported that the use of a Cober Electronics industrial microwave generator (2.45 GHz, 6.0 kW) into a multimode applicator, resulted in specimens that exhibited increased strength over those cured under autoclaved and lime saturated conditions, particularly at early ages. In general, the performances of the concrete are influenced by the hydration and microstructure. Microstructural and compositional analysis did reveal differences in the cement specimens based on the applied curing regime [8]. Ettringite (AFt) was found in the microwaved and autoclaved specimens, but not in specimens cured under lime saturated water, where plate-like calcium hydroxide or possibly monosulfate (AFm) was identified by authors. Microwave curing promoted disorder within the C-S-H phase, which provides cement with its long-term mechanical strength. Microwave curing made the C-SH crystals smaller, fractured and more randomly orientated. And the Xenotile (Ca6(SiO3)6(H2O)) was poorly resolved, suggesting an uncertain and complicated shape, despite being crystalline. Intrinsic properties of materials affect the way they interact with the electric and magnetic fields of the microwaves. In order to discuss the heating kinetic, in general, non-magnetic material was assumed [8,9]. Therefore, the complex (electric) permittivity comprised with real and imaginary parts can be defined as the relationship expressed in Eq. (1). Where, e0r and e00r are the real ! pffiffiffiffiffiffiffi and imaginary parts, and j ¼ 1.
hydration was tested by X-ray diffraction analysis (XRD) and thermo gravimetric-differential scanning calorimetry (TG-DSC), the morphology of mortars was measured by scanning electron microscopy-energy dispersive X-ray Spectrum (SEM-EDS). The pore structure of mortars was tested by mercury intrusion porosimetry (MIP). 2. Material and methods 2.1. Material The chemical composition of P.I 42.5 cement is listed in Table 1. The ISO standard sand conforming to Chinese National Standard GB/T 17671-2005 is used. Specimens with a diameter of 4 cm and the height of 8 cm were prepared by cylindrical molds processed by Nylon with a wall thickness of 4 mm. The water to cement ratio of mortar is 0.45. The sand to cement ratio is 3.00. The water to cement ratio of paste is 0.25. 2.2. Curing regimes Four curing regimes i.e. normal curing (20 °C, RH 95%), steam curing at 40 °C for 10 h, steam curing at 80 °C for 4 h, and microwave curing were studied. 2-h delay period prior to steam curing was needed. The rates of temperature increase and drop of steam curing were 15 °C/h. The microwave radiation was fixed in the microwave oven with a chamber of 23 L. Eight specimens with molds on a turntable was heated together to radiate homogeneously. The nomenclature of specimens with specific microwave curing methods was listed in Table 2. After the curing in Table 2, all of the specimens were cured in water at room temperature after demoulding. 2.3. Test methods
e ¼e r
0 r
! j e00r
ð1Þ
However, the influences of microwave curing on the hydration and microstructure of cement-based materials cannot solely be attributed to excessive material shrinkage and acceleration of hydration resulting from rapid dewatering due to high temperature rises under microwave treatment. In addition, in order to further understand the relationships between the microstructure and the superior performances of cement-based materials cured under microwave, the optimum microwave curing regime was selected. The compressive strength of mortar under the optimum microwave curing regime was compared with that of mortar under normal curing, 40 °C steam curing and 80 °C steam curing. The
According to the curing regime of M5, the upper-surface temperature of the specimens in mold were measured before the microwave turning on or after the microwave turning off immediately (in 10 s) using an infrared thermal scanner with its emissivity set to 0.95, which is the optimum to detect concrete temperatures [10]. The temperature history of the mortar under microwave curing is shown in Fig. 1. The compressive strength was tested by a hydraulic compression testing machine with a loading rate of 2.4 kN/s. For later use in SEM, EDS and MIP, the cores of broken mortar at 1 day and 28 days were collected and put into the ampere bottles filled up with absolute ethyl alcohol to terminate hydration. The pastes were vacuum-oven-dried at 40 °C until constant mass and then ground to powder for XRD and TG-DSC tests. XRD analysis was carried out by using a Rigaku D/max 2550 X-ray diffractometer at a continuous scanning rate of 10°/min with Graphite-monochromatized Cu Ka radiation generated at 40 kV and 200 mA. The TG-DSC experiments were mea-
Table 1 The chemical composition of P.I 42.5 cement (mass%). SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
Na2O
f-CaO
Loss on ignition
20.81
4.92
3.41
62.65
2.38
2.65
0.67
0.81
2.01
Table 2 The sample numbers and microwave curing regimes. Number
Curing regimes
N S40 S80 M1 M2 M3 M4 M5 M6 M7 M8 M9 M10
Temperature 20 ± 2 °C; Relative humidity >95% Steam curing at 40 °C Steam curing at 80 °C D60 + M5 + I30 + M5 + I30 + M5 D60 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML30 D60 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML30 D60 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML10 D60 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML20 D60 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML15 D60 + L10 + I30 + L10 + I30 + L10 + I30 + L10 + I30 + L10 + I45 + ML15 D120 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML20 D30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML20 D30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML20 + I40 + ML15
Note: D represents delay time from the moment adding water into the cement to the first microwave radiation; I represents interval time between two times microwave radiation; M, ML and L represent output power of 440w, 260w and 140w, respectively. For example, D60 + M5 + I30 + M20 means that, at the condition of output power of 440w, 60 min after adding water is needed before the first microwave radiation for 5 min, then shut down the radiation for 30 min, then radiates for 20 min at last.
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Fig. 1. The temperature history of the mortar under microwave curing. sured on the SDT Q600 analyzer of TA company in the temperature range from 20 °C to 1000 °C with about 20 mg of sample under the dynamic N2 atmosphere. An AutoPore IV 9500 V1.09 Mercury Intrusion Pore Apparatus was used to calculate the pore sizes with penetration pressure from 413.685 MPa to 1.6 kPa (3– 798,339 nm). FEI Quanta 650 FEG was used for SEM-EDS tests.
3. Results and discussion 3.1. Compressive strength Except for the workability, the early-age compressive strength is also very important for precast concrete to enhance the productivity, save the capital and decrease the plant areas et al. Therefore, in order to optimize the microwave curing regimes, the results of 6-h and 24-h compressive strength of mortars under ten microwave curing regimes (M1–M10) using different power levels and duration of microwave radiation were investigated. The results are shown in Fig. 2. According to the compressive strength of M5, M8 and M9 we can see that the delay time has insignificant effect on the early strength. The duration time or output power of the microwave radiation is important. Long time and high output power of microwave radiation cause water evaporation and air expansion, leading to fracture of mortars, especially for the previous times microwave radiation. Shortage of microwave radiation time (M1, M2) cannot form satisfactory compressive strength. A short-final radiation (M4, M6) is not enough for the improvement of compressive strength quickly, but over curing of the last microwave radiation causes the excessive loss of water (M3). Here, M5 with the final radiation of 20 min is the best. The lower output power is better
Fig. 2. The compressive strength of mortar under different microwave curing regimes.
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Fig. 3. The compressive strength of mortars under four curing regimes at different ages.
for the compressive strength at 1 day (M7), but the total curing period is longer. Therefore, M5 is considered as the optimum curing regime in our study and is used for the SEM-EDS and MIP tests. The compressive strength of M5 at other ages is present in Fig. 3. As shown in Fig. 3, it can be seen that microwave curing can improve the early strength. The compressive strength cured under microwave is the highest in the four curing regimes at 1, 3, 7 and 28 days. At the age of 28 days, the compressive strength of mortar cured under microwave is 1.8, 1.7 and 1.5 times higher than that with normal curing regime, 40 °C steam curing and 80 °C steam curing, respectively, while the strength at 6 h is about 120%, 70% and 50% of that cured under normal curing, 40 °C steam curing, and 80 °C steam curing respectively. The microwave can reduce the water to cement ratio dramatically, and according to our test, the water to cement ratio of M5 at the age of 6 h after microwave curing is about 0.3. Microwave accelerates the hydration of cement and reduces the water to cement ratio, leading to the plastic shrinkage in the early age. Therefore, the structure of mortar should be denser than that under other curing regimes. In addition, microwave is volumetric heating without thermal gradient, which is good for compressive strength.
3.2. hydration products As seen from the Figs. 4 and 5, except for unhydrated clinkers, CH is the mainly crystalline phase. At the age of 1 day, the content
Fig. 4. The XRD patterns of samples cured under different regimes at early ages.
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Fig. 5. The XRD patterns of samples cured under different regimes at the age of 28 days.
of CH under microwave curing is more dominant than that under 40°C steam curing and normal curing. CH content at 6 h is more than that under normal curing at 1 day. In addition, the intensity of clinkers cured under microwave is weaker than that of normal curing and steam curing obviously. Just from the observation of the XRD, the content of CH under microwave curing is a little lower than that under 80 °C. This can explained by the existence of the poor crystallization and preferred orientation of CH. In order to further understand the content of CH, we have calculated the content of CH by DSC in the 3.4. The results show that the content of CH of the sample under microwave at 1d is higher than that under 80 °C. In addition, the reduction of the water to cement ratio also contributed to the increase of the compressive strength as shown in Fig. 3. The Intensity of CH cured under steam at 40 °C is stronger than that under microwave curing and 80 °C steam curing at 28 days, which is in consistency with the results in Fig. 8. This can be attributed to the high temperature, which accelerates the hydration at early age, forming large amounts of hydration products around the cement particles hindering the diffusion of ions.
Fig. 7. (a) TG-DSC curves of pastes cured under different regimes at early ages. (b) TG-DSC curves of pastes cured under different regimes at 28 days.
3.3. Chemically bonded water Though quantitative research on hydration of binder is difficult, the calculation of chemically bonded water is relatively accurate. The content of chemically bonded water mainly depends on the quantity of hydration products, which relates to the extent of hydration. On the basis of reference [11], the evaporable water
Fig. 8. The content of CH of samples under different curing regimes.
was removed above 105 °C and the samples were heated to 1000 °C. Therefore, the formula (2) is used to calculate the chemically bonded water.
H¼
Fig. 6. The content of chemically bonded water.
w105 C w1050 C c w1050 C
ð2Þ
where w is the mass of corresponding temperature; c is the loss on ignition. As can be seen in Fig. 6, the content of chemically bonded water of sample under microwave curing is 9.73% at 6 h, which is higher than that under 80 °C steam curing and normal curing at the age of 1 day. At the same age of 1 day, the chemically bonded water of
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sample under microwave curing is 12.07%, which is higher than that under all other curing regimes. This can be attributed to the acceleration effects of microwave. However, the content of chemically bonded water of sample cured under microwave is lower than that of samples with normal curing and steam curing at 40 °C, and is higher than that of samples under steam curing at 80 °C at the age of 28 days. Although high temperature curing will promote the hydration at early age, fine and dense C-S-H gels will be produced around the cement particles, which will impede the later hydration process. The results indicate that microwave heating is better than steam curing at 80 °C. Obviously, the steam curing at 40 °C is the best as seen from the chemically bonded water simply, however, it requires longer curing period.
3.4. Ca(OH)2 content The TG-DSC curves of pastes under different curing regimes are shown in Fig. 7. The endothermic peaks around 180 °C is formed by decomposition of ettringite [12]. From the Fig. 7a, more AFt is formed with microwave curing, leading to a higher compressive strength at early age. Compared with XRD, because of part of CH is poor crystallization and preferred orientation, the content of CH calculated by DSC is more accurate. In general, the temperature of decomposition of CH occurs around 430 °C, and the endothermic peak at about 700 °C can be attributed to the decomposition of calcium carbonate [12], which can be neglected in this study. In this study, the enthalpy for thermal decomposition of CH is shown in Fig. 8. According to the calculation, the content of CH cured under microwave at the age of 6 h is the same with that cured under normal condition at the age of 1 day, and is higher than that cured under 40 °C and 80 °C steam at 1 day. Microwave radiation increases the water molecular polarity, accelerating the dissolution of cement particles, which is beneficial to the hydration. The liquid membrane around the cement particles can be disturbed by the microwave, making it much thinner, which can also attenuate the resistance of diffusion process and accelerate the cement hydration process. At the same age of 1 day, CH content of samples under microwave curing is the highest. At the age of 28 days, the CH content cured under microwave is lower than that with normal curing, however, which is higher than under steam curing at 80 °C. This is consistent with the results of the chemically bonded water (Fig. 6). As well as steam curing, microwave curing can also accelerate the hydration of cement due to its high temperature, leading to fine C-S-H around cement particles, impeding the further hydration at the later age. However, the compressive strength of mortar under microwave curing is the highest, illustrating that due to improved microstructure, microwave significantly impacts the
Fig. 9. The differential curves of pore size distribution at the age of 28 days.
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compressive strength development, such as the plastic shrinkage at early ages caused by water reduction.
3.5. Pore structure The distribution of pore diameter at the age of 28 days is shown in Fig. 9. The pore diameters at 80 nm and 100 lm with highest peak are the most probable pore sizes. Obviously, contrary to steam curing, the microwave increases the most probable pore size at 80 nm. However, the most probable pore size at 100 lm is reduced sharply under microwave curing when compared with steam curing, though the most probable pore size under microwave curing is a bit higher than that under normal curing. The total porosity of the mortar under different curing regimes at the age of 28 days is shown in the Fig. 10. From the Fig. 10 we can see that the microwave curing increases the total porosity of mortar slightly when compared with normal curing and team curing at 80 °C. In order to gain more insight into the pore size distribution of mortar under different curing regimes, the measured pore distributions are divided into four size ranges [13] as shown in Fig. 10: gel micropores (<4.5 nm), mesopores (4.5–50 nm), middle capillary pores (50–100 nm), and large capillary pores (>100 nm). The large capillary pores of mortar under microwave curing are similar to that of mortar under normal curing, and are lower than that under steam curing. The middle capillary pores of the specimen under normal curing are similar to that under steam curing. But the total middle capillary pores are increased under microwave curing. Compared with normal curing, the mesopores under microwave curing is reduced, and is increased when compared with steam curing. This illustrates that the decrease of large capillary pores and the improvement of pore distribution contributes to the high compressive strength of mortar under microwave curing when compared with steam curing at the age of 28 days. Three reasons contribute to the phenomenon. Firstly, the shrink of capillary pores produced by plastic shrinkage at early time [2] mainly affects the pores >100 nm. On the other hands, the acceleration effects of microwave curing as discussed in the CH content make a difference to it. The final one may be attributed to the microwave effects. It has been reported that microwave radiation enhancements the rates of activated processes in solids involving material transport [14], which are generally considered to be microwave effects because a reduction in activation energy is required. Though a universally agreed scientific basis for microwave effects is not available, a model based on thermodynamic stability of pores in a matrix of grains can be used to provide a rea-
Fig. 10. The porosity of mortar under different curing regimes.
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sonable explanation. The dielectric inhomogeneity of the mortar results in very high electric field enhancements at the solid phase-pore and particle-particle interfaces [15]. The local field enhancements improve the material flux in the convex surfaces of the pores [15] and results in non-isotropic pore closure, thus influencing the local driving force for densification of the microstructure. Seen from Fig. 10, if the microwave effects exist, the mesopores and the middle capillary pores are influenced most remarkably. In addition, during the microwave curing, the temperature increase leads to the air expansion in the pores. Because of the hardening of the matrix, the expansion pores cannot cause shrinkage, which also contributes to the increase of the middle capillary pores (50–100 nm).
3.6. Morphology As seen from Fig. 11, at the early age (1 day), the average particle size of CH produced under normal curing and steam curing is large. However, the CH in mortar cured under microwave is comparatively small. The length-diameter ratio of ettringite cured under normal and steam conditions is larger than that of ettringite cured under microwave. The ettringite formed in the mortar under microwave curing is short-rod shape. These also contribute to the high early strength of mortar under microwave curing. The influence of microwave on the interfacial transition zone (ITZ) is more significant for mortar because the coarse aggregate can constrain the expansion and local boiling, which leads to
Fig. 11. Morphology of specimens at 1 day.
Y. Kong et al. / Construction and Building Materials 114 (2016) 831–838
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Fig. 11 (continued)
Fig. 12. Microstructure of mortar at the age of 28 days.
cracks and holes [7]. Another reason is that microwave may attributes to the reduction of water to cement ratio, which ameliorates the ITZ and leads to the increase of the strength. Compared with normal curing and steam curing, though a more loose and porous structure is formed under microwave curing, the microwave lowers the water to cement ratio, improves the interfacial transition zone between the paste and aggregates, which will contribute to the strength and durability performance.
The microstructure of mortar at 28 days under different curing regimes is shown in Fig. 12 and the positions of numbers on the pictures are tested by energy dispersive X-ray spectroscopy (EDS) for the elements distribution of hydration products. The C-S-H gel present in Portland cement has a composition of 1.5–1.9 CaOSiO2nH2O. The number n of water molecules depends on relative humidity and temperature [16]. The average chain length increases as the Ca/Si ratio decreases. The formation of C-S-H with
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Table 3 EDX analysis for hydration products under different curing regimes at the age of 28 days/Atomic%.
Normal curing 40 °C steam curing 80 °C steam curing Microwave curing
O
Al
Si
Ca
Mg
S
K
Fe
Al/Si
Ca/Si
Ca/(Si + Al)
60.42 71.47 71.88 70.44
1.05 2.64 2.97 3.41
12.23 10.89 8.88 3.08
26.31 12.45 13.00 9.77
/ 0.36 1.44 3.34
/ 1.18 1.17 4.81
/ 0.56 0.66 3.84
/ 0.44 / 1.31
0.09 0.24 0.33 1.11
2.15 1.14 1.46 3.17
1.98 0.92 1.10 1.51
a low Ca/Si leads to an increased uptake of aluminum in the C-S-H: C-A-S-H. Aluminums enters C-S-H mainly at the bridging sites in the silicate chains [17]. As C-S-H with a lower Ca/Si ratio and higher alumina content leads to an increase in alkali uptake by C-A-S-H: C-A-K-S-H [18], which is probably caused by the negative charges due to the ionization of silanol groups, the partial substitution of Si(IV) by Al(III) and the lower concentrations of calcium. That is why the content of K and Mg increase in our study. However, microwave reduces the Al/Ca, and disperses the Si/Ca, though the Al/Si increases in Table 3. This can be attributes to that microwave diminishes the particle size of CH, which is mixed in the C-AK-S-H, leading to the increase of the Ca/Si ratio and the Ca/(Si + Al). This may also explain the theory that the microwave impeding the K transfer to the glass fiber when compared with the normal curing [19]. Table 3 EDX analysis for hydration products under different curing regimes at the age of 28 days (Atomic %). 4. Conclusions The compressive strength of mortar under microwave curing at 6 h is 1.2, 0.7 and 0.5 times higher than that under normal curing, 40 °C steam curing and 80 °C steam curing at 1 day, respectively. The large capillary pores proportion of mortar under microwave curing is similar to that under normal curing, and is lower than that under steam curing. Compared with normal curing and steam curing, microwave curing increases the total middle capillary pores. Mesopores content under microwave curing is reduced when compared with normal curing, and is increased when compared with steam curing. Compared with the normal curing and steam curing at 80 °C, though microwave curing improves the porosity slightly, the compressive strength is improved. Microwave curing can accelerate the hydration of cement, but as well as the steam curing, cements microwave cured generate fine products wrapped around the cement particles, hindering the further hydration at later ages. Microwave curing produces large amounts of short-rod AFt, diminishes the particle size of CH, and increases the absorption of S, K and Mg by C-S-H gel. Acknowledgements The authors would like to acknowledge the Twelfth Five-year National Science-technology Support Plan (2012BA20B02) and
the Fifth Grant Program of the Administrative Office of Laboratory and Equipment of Tongji University (0002015036) for supporting this research. References [1] G.J. Verbeck, R.H. Helmuth, Structures and physical properties of cement paste, in: Proceedings of the 5th International Symposium on the Chemistry of Cement, 1968, pp. 1–32. [2] W. Xuequan, D. Jianbgo, T. Mingshu, Microwave curing technique in concrete manufacture, Cem. Concr. Res. 17 (2) (1987) 205–210. [3] A. Watson, Curing of concrete, Microwave Power Eng. 2 (1968) 108–118. [4] C.K. Leung, T. Pheeraphan, Determination of optimal process for microwave curing of concrete, Cem. Concr. Res. 27 (3) (1997) 463–472. [5] D. Sohn, D.L. Johnson, Microwave curing effects on the 28-day strength of cementitious materials, Cem. Concr. Res. 29 (2) (1999) 241–247. [6] I.B. Topcu, M.U. Toprak, D. Akdag, Determination of optimal microwave curing cycle for fly ash mortars, Can. J. Civ. Eng. 35 (4) (2008) 349–357. [7] C.K. Leung, T. Pheeraphan, Microwave curing of Portland cement concrete: experimental results and feasibility for practical applications, Constr. Build. Mater. 9 (2) (1995) 67–73. [8] N. Makul, P. Keangin, P. Rattanadecho, B. Chatveera, D.K. Agrawal, Microwaveassisted heating of cementitious materials: relative dielectric properties, mechanical property, and experimental and numerical heat transfer characteristics, Int. Commun. Heat Mass Transfer 37 (8) (2010) 1096–1105. [9] P. Ratanadecho, K. Aoki, M. Akahori, A numerical and experimental investigation of the modeling of microwave heating for liquid layers using a rectangular wave guide (effects of natural convection and dielectric properties), Appl. Math. Model. 26 (3) (2002) 449–472. [10] J. Somaratna, D. Ravikumar, N. Neithalath, Response of alkali activated fly ash mortars to microwave curing, Cem. Concr. Res. 40 (12) (2010) 1688–1696. [11] S. Liu, Y. Kong, L. Wang, A comparison of hydration properties of cement–low quality fly ash binder and cement–limestone powder binder, J. Therm. Anal. Calorim. 116 (2) (2014) 937–943. [12] W. Sha, E.A. O’Neill, Z. Guo, Differential scanning calorimetry study of ordinary Portland cement, Cem. Concr. Res. 29 (9) (1999) 1487–1489. [13] P.K. Mehta, P.J. Monteiro, Concrete: Microstructure, Properties, and Materials, McGraw-Hill, London, 2006. [14] G. Link, S. Miksch, S. Takayama, M. Thumm, Direct experimental demonstration of non-thermal microwave effects during sintering of ceramics, Plasma Science, 2006. ICOPS 2006, in: IEEE Conference RecordAbstracts. The 33rd IEEE International Conference on, IEEE, 2006, p. 354. [15] M. Willert-Porada, A microstructural approach to the origin of ‘‘microwave effects” in sintering of ceramics and composites, in: Microwaves: Theory and Application in Materials Processing IV, 1997, pp. 153–163. [16] B. Lothenbach, K. Scrivener, R.D. Hooton, Supplementary cementitious materials, Cem. Concr. Res. 41 (12) (2011) 1244–1256. [17] I.G. Richardson, G.W. Groves, The incorporation of minor and trace elements into calcium silicate hydrate (C-S-H) gel in hardened cement pastes, Cem. Concr. Res. 23 (1) (1993) 131–138. [18] S.-Y. Hong, F.P. Glasser, Alkali sorption by C-S-H and C-A-S-H gels: Part II. Role of alumina, Cem. Concr. Res. 32 (7) (2002) 1101–1111. [19] J. Pera, J. Ambroise, M. Oriol, Microwave processing of glass-fiber reinforced composites—modification of the microstructure, Adv. Cem. Based Mater. 6 (3) (1997) 116–122.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Hydration and microstructure of cement-based materials under microwave curing Yaning Kong a, Peiming Wang a,⇑, Shuhua Liu b, Zhiyang Gao c a
School of Materials Science and Engineering, Tongji University, Shanghai 201804, China State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, China c Changjiang River Scientific Research Institute, Wuhan 430012, China b
h i g h l i g h t s Microwave curing improves the early strength when compared to steam curing. Microwave curing reduces the pores in the range of >100 nm. Microwave curing forms short-rod AFt and diminishes the particle size of CH. Microwave curing increases the absorption of S, Mg and K by C-S-H gel.
a r t i c l e
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Article history: Received 29 September 2015 Received in revised form 26 March 2016 Accepted 29 March 2016
Keywords: Microwave curing Cement-based materials Hydration Microstructure
a b s t r a c t By reducing the curing time, microwave curing can enhance the productivity, save the capital and decrease the plant areas for precast concrete when compared with the steam curing. Based on the results of 6-h and 24-h compressive strength tests the optimum curing regime was selected. The sample microwave cured using the selected curing regime was then compared against samples cured using (a) normal curing (b) steam curing at 40 °C for 10 h and (c) steam curing at 80 °C for 4 h by performing the compressive strength, XRD, TG-DSC, SEM-EDS and MIP. The results indicate that, compared with the steam curing at 80 °C, microwave curing improves the compressive strength of mortar before the age of 28 days, increases the porosity of mortar slightly, while reduces the pores in the range of >100 nm greatly, forms short-rod AFt and smaller particle size of calcium hydroxide, and increases the adsorption of K, S and Mg by C-S-H gel. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction There are many advantages when the curing period is reduced, such as productivity improvement, capital saving, and the reduction of workshop area, et al. Thermal curing technique has been used in the precast concrete for a long time. However, as a poor heat conductor, it takes >10 h to complete one curing cycle before demoulding [1]. In addition, concrete strength after 28 days is lower than that under standard curing because the fast early hydration of cement under such a high temperature, which results in the formation of large amounts of very fine C-S-H gel surrounding the unhydrates, and hence causing hindrance of diffusion and further development of strength [2]. High-frequency electromagnetic heating, such as microwave enhanced heating, is able to reduce such heterogeneity due to its ⇑ Corresponding author. E-mail address: [email protected] (P. Wang). http://dx.doi.org/10.1016/j.conbuildmat.2016.03.202 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
superior penetration depth. Microwave energy is attenuated by the vibration of polar molecules and dipoles and the resulting friction between the molecules generates heats rapidly throughout the concrete. Research by Watson [3] showed that 28-day compressive strength of microwave cured specimens displayed only half the strength compared to normally cured specimens. However, by optimizing the microwave curing parameters, strength can be improved effectively [4–6], making microwave curing a potential alternative method for accelerating cements hydration. Research by Xuequan et al. [2] found an increase in early strength under microwave curing, without any detrimental effect at later ages. Results of the permeability revealed that the specimens under microwave curing were denser than the reference specimens, inferring a reduction in porosity and the action of plastic shrinkage. Studied by Leung et al. showed that type III Portland cement concrete with microwave curing can develop early-age strength (at 4.5 h) and later-age strength (7 day) that compare very favorably with commercially available rapid hardening concrete as
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well as concrete containing accelerating admixtures [7]. Makul et al. [8] recently reported that the use of a Cober Electronics industrial microwave generator (2.45 GHz, 6.0 kW) into a multimode applicator, resulted in specimens that exhibited increased strength over those cured under autoclaved and lime saturated conditions, particularly at early ages. In general, the performances of the concrete are influenced by the hydration and microstructure. Microstructural and compositional analysis did reveal differences in the cement specimens based on the applied curing regime [8]. Ettringite (AFt) was found in the microwaved and autoclaved specimens, but not in specimens cured under lime saturated water, where plate-like calcium hydroxide or possibly monosulfate (AFm) was identified by authors. Microwave curing promoted disorder within the C-S-H phase, which provides cement with its long-term mechanical strength. Microwave curing made the C-SH crystals smaller, fractured and more randomly orientated. And the Xenotile (Ca6(SiO3)6(H2O)) was poorly resolved, suggesting an uncertain and complicated shape, despite being crystalline. Intrinsic properties of materials affect the way they interact with the electric and magnetic fields of the microwaves. In order to discuss the heating kinetic, in general, non-magnetic material was assumed [8,9]. Therefore, the complex (electric) permittivity comprised with real and imaginary parts can be defined as the relationship expressed in Eq. (1). Where, e0r and e00r are the real ! pffiffiffiffiffiffiffi and imaginary parts, and j ¼ 1.
hydration was tested by X-ray diffraction analysis (XRD) and thermo gravimetric-differential scanning calorimetry (TG-DSC), the morphology of mortars was measured by scanning electron microscopy-energy dispersive X-ray Spectrum (SEM-EDS). The pore structure of mortars was tested by mercury intrusion porosimetry (MIP). 2. Material and methods 2.1. Material The chemical composition of P.I 42.5 cement is listed in Table 1. The ISO standard sand conforming to Chinese National Standard GB/T 17671-2005 is used. Specimens with a diameter of 4 cm and the height of 8 cm were prepared by cylindrical molds processed by Nylon with a wall thickness of 4 mm. The water to cement ratio of mortar is 0.45. The sand to cement ratio is 3.00. The water to cement ratio of paste is 0.25. 2.2. Curing regimes Four curing regimes i.e. normal curing (20 °C, RH 95%), steam curing at 40 °C for 10 h, steam curing at 80 °C for 4 h, and microwave curing were studied. 2-h delay period prior to steam curing was needed. The rates of temperature increase and drop of steam curing were 15 °C/h. The microwave radiation was fixed in the microwave oven with a chamber of 23 L. Eight specimens with molds on a turntable was heated together to radiate homogeneously. The nomenclature of specimens with specific microwave curing methods was listed in Table 2. After the curing in Table 2, all of the specimens were cured in water at room temperature after demoulding. 2.3. Test methods
e ¼e r
0 r
! j e00r
ð1Þ
However, the influences of microwave curing on the hydration and microstructure of cement-based materials cannot solely be attributed to excessive material shrinkage and acceleration of hydration resulting from rapid dewatering due to high temperature rises under microwave treatment. In addition, in order to further understand the relationships between the microstructure and the superior performances of cement-based materials cured under microwave, the optimum microwave curing regime was selected. The compressive strength of mortar under the optimum microwave curing regime was compared with that of mortar under normal curing, 40 °C steam curing and 80 °C steam curing. The
According to the curing regime of M5, the upper-surface temperature of the specimens in mold were measured before the microwave turning on or after the microwave turning off immediately (in 10 s) using an infrared thermal scanner with its emissivity set to 0.95, which is the optimum to detect concrete temperatures [10]. The temperature history of the mortar under microwave curing is shown in Fig. 1. The compressive strength was tested by a hydraulic compression testing machine with a loading rate of 2.4 kN/s. For later use in SEM, EDS and MIP, the cores of broken mortar at 1 day and 28 days were collected and put into the ampere bottles filled up with absolute ethyl alcohol to terminate hydration. The pastes were vacuum-oven-dried at 40 °C until constant mass and then ground to powder for XRD and TG-DSC tests. XRD analysis was carried out by using a Rigaku D/max 2550 X-ray diffractometer at a continuous scanning rate of 10°/min with Graphite-monochromatized Cu Ka radiation generated at 40 kV and 200 mA. The TG-DSC experiments were mea-
Table 1 The chemical composition of P.I 42.5 cement (mass%). SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
Na2O
f-CaO
Loss on ignition
20.81
4.92
3.41
62.65
2.38
2.65
0.67
0.81
2.01
Table 2 The sample numbers and microwave curing regimes. Number
Curing regimes
N S40 S80 M1 M2 M3 M4 M5 M6 M7 M8 M9 M10
Temperature 20 ± 2 °C; Relative humidity >95% Steam curing at 40 °C Steam curing at 80 °C D60 + M5 + I30 + M5 + I30 + M5 D60 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML30 D60 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML30 D60 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML10 D60 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML20 D60 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML15 D60 + L10 + I30 + L10 + I30 + L10 + I30 + L10 + I30 + L10 + I45 + ML15 D120 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML20 D30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML20 D30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML5 + I30 + ML20 + I40 + ML15
Note: D represents delay time from the moment adding water into the cement to the first microwave radiation; I represents interval time between two times microwave radiation; M, ML and L represent output power of 440w, 260w and 140w, respectively. For example, D60 + M5 + I30 + M20 means that, at the condition of output power of 440w, 60 min after adding water is needed before the first microwave radiation for 5 min, then shut down the radiation for 30 min, then radiates for 20 min at last.
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Fig. 1. The temperature history of the mortar under microwave curing. sured on the SDT Q600 analyzer of TA company in the temperature range from 20 °C to 1000 °C with about 20 mg of sample under the dynamic N2 atmosphere. An AutoPore IV 9500 V1.09 Mercury Intrusion Pore Apparatus was used to calculate the pore sizes with penetration pressure from 413.685 MPa to 1.6 kPa (3– 798,339 nm). FEI Quanta 650 FEG was used for SEM-EDS tests.
3. Results and discussion 3.1. Compressive strength Except for the workability, the early-age compressive strength is also very important for precast concrete to enhance the productivity, save the capital and decrease the plant areas et al. Therefore, in order to optimize the microwave curing regimes, the results of 6-h and 24-h compressive strength of mortars under ten microwave curing regimes (M1–M10) using different power levels and duration of microwave radiation were investigated. The results are shown in Fig. 2. According to the compressive strength of M5, M8 and M9 we can see that the delay time has insignificant effect on the early strength. The duration time or output power of the microwave radiation is important. Long time and high output power of microwave radiation cause water evaporation and air expansion, leading to fracture of mortars, especially for the previous times microwave radiation. Shortage of microwave radiation time (M1, M2) cannot form satisfactory compressive strength. A short-final radiation (M4, M6) is not enough for the improvement of compressive strength quickly, but over curing of the last microwave radiation causes the excessive loss of water (M3). Here, M5 with the final radiation of 20 min is the best. The lower output power is better
Fig. 2. The compressive strength of mortar under different microwave curing regimes.
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Fig. 3. The compressive strength of mortars under four curing regimes at different ages.
for the compressive strength at 1 day (M7), but the total curing period is longer. Therefore, M5 is considered as the optimum curing regime in our study and is used for the SEM-EDS and MIP tests. The compressive strength of M5 at other ages is present in Fig. 3. As shown in Fig. 3, it can be seen that microwave curing can improve the early strength. The compressive strength cured under microwave is the highest in the four curing regimes at 1, 3, 7 and 28 days. At the age of 28 days, the compressive strength of mortar cured under microwave is 1.8, 1.7 and 1.5 times higher than that with normal curing regime, 40 °C steam curing and 80 °C steam curing, respectively, while the strength at 6 h is about 120%, 70% and 50% of that cured under normal curing, 40 °C steam curing, and 80 °C steam curing respectively. The microwave can reduce the water to cement ratio dramatically, and according to our test, the water to cement ratio of M5 at the age of 6 h after microwave curing is about 0.3. Microwave accelerates the hydration of cement and reduces the water to cement ratio, leading to the plastic shrinkage in the early age. Therefore, the structure of mortar should be denser than that under other curing regimes. In addition, microwave is volumetric heating without thermal gradient, which is good for compressive strength.
3.2. hydration products As seen from the Figs. 4 and 5, except for unhydrated clinkers, CH is the mainly crystalline phase. At the age of 1 day, the content
Fig. 4. The XRD patterns of samples cured under different regimes at early ages.
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Fig. 5. The XRD patterns of samples cured under different regimes at the age of 28 days.
of CH under microwave curing is more dominant than that under 40°C steam curing and normal curing. CH content at 6 h is more than that under normal curing at 1 day. In addition, the intensity of clinkers cured under microwave is weaker than that of normal curing and steam curing obviously. Just from the observation of the XRD, the content of CH under microwave curing is a little lower than that under 80 °C. This can explained by the existence of the poor crystallization and preferred orientation of CH. In order to further understand the content of CH, we have calculated the content of CH by DSC in the 3.4. The results show that the content of CH of the sample under microwave at 1d is higher than that under 80 °C. In addition, the reduction of the water to cement ratio also contributed to the increase of the compressive strength as shown in Fig. 3. The Intensity of CH cured under steam at 40 °C is stronger than that under microwave curing and 80 °C steam curing at 28 days, which is in consistency with the results in Fig. 8. This can be attributed to the high temperature, which accelerates the hydration at early age, forming large amounts of hydration products around the cement particles hindering the diffusion of ions.
Fig. 7. (a) TG-DSC curves of pastes cured under different regimes at early ages. (b) TG-DSC curves of pastes cured under different regimes at 28 days.
3.3. Chemically bonded water Though quantitative research on hydration of binder is difficult, the calculation of chemically bonded water is relatively accurate. The content of chemically bonded water mainly depends on the quantity of hydration products, which relates to the extent of hydration. On the basis of reference [11], the evaporable water
Fig. 8. The content of CH of samples under different curing regimes.
was removed above 105 °C and the samples were heated to 1000 °C. Therefore, the formula (2) is used to calculate the chemically bonded water.
H¼
Fig. 6. The content of chemically bonded water.
w105 C w1050 C c w1050 C
ð2Þ
where w is the mass of corresponding temperature; c is the loss on ignition. As can be seen in Fig. 6, the content of chemically bonded water of sample under microwave curing is 9.73% at 6 h, which is higher than that under 80 °C steam curing and normal curing at the age of 1 day. At the same age of 1 day, the chemically bonded water of
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sample under microwave curing is 12.07%, which is higher than that under all other curing regimes. This can be attributed to the acceleration effects of microwave. However, the content of chemically bonded water of sample cured under microwave is lower than that of samples with normal curing and steam curing at 40 °C, and is higher than that of samples under steam curing at 80 °C at the age of 28 days. Although high temperature curing will promote the hydration at early age, fine and dense C-S-H gels will be produced around the cement particles, which will impede the later hydration process. The results indicate that microwave heating is better than steam curing at 80 °C. Obviously, the steam curing at 40 °C is the best as seen from the chemically bonded water simply, however, it requires longer curing period.
3.4. Ca(OH)2 content The TG-DSC curves of pastes under different curing regimes are shown in Fig. 7. The endothermic peaks around 180 °C is formed by decomposition of ettringite [12]. From the Fig. 7a, more AFt is formed with microwave curing, leading to a higher compressive strength at early age. Compared with XRD, because of part of CH is poor crystallization and preferred orientation, the content of CH calculated by DSC is more accurate. In general, the temperature of decomposition of CH occurs around 430 °C, and the endothermic peak at about 700 °C can be attributed to the decomposition of calcium carbonate [12], which can be neglected in this study. In this study, the enthalpy for thermal decomposition of CH is shown in Fig. 8. According to the calculation, the content of CH cured under microwave at the age of 6 h is the same with that cured under normal condition at the age of 1 day, and is higher than that cured under 40 °C and 80 °C steam at 1 day. Microwave radiation increases the water molecular polarity, accelerating the dissolution of cement particles, which is beneficial to the hydration. The liquid membrane around the cement particles can be disturbed by the microwave, making it much thinner, which can also attenuate the resistance of diffusion process and accelerate the cement hydration process. At the same age of 1 day, CH content of samples under microwave curing is the highest. At the age of 28 days, the CH content cured under microwave is lower than that with normal curing, however, which is higher than under steam curing at 80 °C. This is consistent with the results of the chemically bonded water (Fig. 6). As well as steam curing, microwave curing can also accelerate the hydration of cement due to its high temperature, leading to fine C-S-H around cement particles, impeding the further hydration at the later age. However, the compressive strength of mortar under microwave curing is the highest, illustrating that due to improved microstructure, microwave significantly impacts the
Fig. 9. The differential curves of pore size distribution at the age of 28 days.
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compressive strength development, such as the plastic shrinkage at early ages caused by water reduction.
3.5. Pore structure The distribution of pore diameter at the age of 28 days is shown in Fig. 9. The pore diameters at 80 nm and 100 lm with highest peak are the most probable pore sizes. Obviously, contrary to steam curing, the microwave increases the most probable pore size at 80 nm. However, the most probable pore size at 100 lm is reduced sharply under microwave curing when compared with steam curing, though the most probable pore size under microwave curing is a bit higher than that under normal curing. The total porosity of the mortar under different curing regimes at the age of 28 days is shown in the Fig. 10. From the Fig. 10 we can see that the microwave curing increases the total porosity of mortar slightly when compared with normal curing and team curing at 80 °C. In order to gain more insight into the pore size distribution of mortar under different curing regimes, the measured pore distributions are divided into four size ranges [13] as shown in Fig. 10: gel micropores (<4.5 nm), mesopores (4.5–50 nm), middle capillary pores (50–100 nm), and large capillary pores (>100 nm). The large capillary pores of mortar under microwave curing are similar to that of mortar under normal curing, and are lower than that under steam curing. The middle capillary pores of the specimen under normal curing are similar to that under steam curing. But the total middle capillary pores are increased under microwave curing. Compared with normal curing, the mesopores under microwave curing is reduced, and is increased when compared with steam curing. This illustrates that the decrease of large capillary pores and the improvement of pore distribution contributes to the high compressive strength of mortar under microwave curing when compared with steam curing at the age of 28 days. Three reasons contribute to the phenomenon. Firstly, the shrink of capillary pores produced by plastic shrinkage at early time [2] mainly affects the pores >100 nm. On the other hands, the acceleration effects of microwave curing as discussed in the CH content make a difference to it. The final one may be attributed to the microwave effects. It has been reported that microwave radiation enhancements the rates of activated processes in solids involving material transport [14], which are generally considered to be microwave effects because a reduction in activation energy is required. Though a universally agreed scientific basis for microwave effects is not available, a model based on thermodynamic stability of pores in a matrix of grains can be used to provide a rea-
Fig. 10. The porosity of mortar under different curing regimes.
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sonable explanation. The dielectric inhomogeneity of the mortar results in very high electric field enhancements at the solid phase-pore and particle-particle interfaces [15]. The local field enhancements improve the material flux in the convex surfaces of the pores [15] and results in non-isotropic pore closure, thus influencing the local driving force for densification of the microstructure. Seen from Fig. 10, if the microwave effects exist, the mesopores and the middle capillary pores are influenced most remarkably. In addition, during the microwave curing, the temperature increase leads to the air expansion in the pores. Because of the hardening of the matrix, the expansion pores cannot cause shrinkage, which also contributes to the increase of the middle capillary pores (50–100 nm).
3.6. Morphology As seen from Fig. 11, at the early age (1 day), the average particle size of CH produced under normal curing and steam curing is large. However, the CH in mortar cured under microwave is comparatively small. The length-diameter ratio of ettringite cured under normal and steam conditions is larger than that of ettringite cured under microwave. The ettringite formed in the mortar under microwave curing is short-rod shape. These also contribute to the high early strength of mortar under microwave curing. The influence of microwave on the interfacial transition zone (ITZ) is more significant for mortar because the coarse aggregate can constrain the expansion and local boiling, which leads to
Fig. 11. Morphology of specimens at 1 day.
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Fig. 11 (continued)
Fig. 12. Microstructure of mortar at the age of 28 days.
cracks and holes [7]. Another reason is that microwave may attributes to the reduction of water to cement ratio, which ameliorates the ITZ and leads to the increase of the strength. Compared with normal curing and steam curing, though a more loose and porous structure is formed under microwave curing, the microwave lowers the water to cement ratio, improves the interfacial transition zone between the paste and aggregates, which will contribute to the strength and durability performance.
The microstructure of mortar at 28 days under different curing regimes is shown in Fig. 12 and the positions of numbers on the pictures are tested by energy dispersive X-ray spectroscopy (EDS) for the elements distribution of hydration products. The C-S-H gel present in Portland cement has a composition of 1.5–1.9 CaOSiO2nH2O. The number n of water molecules depends on relative humidity and temperature [16]. The average chain length increases as the Ca/Si ratio decreases. The formation of C-S-H with
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Table 3 EDX analysis for hydration products under different curing regimes at the age of 28 days/Atomic%.
Normal curing 40 °C steam curing 80 °C steam curing Microwave curing
O
Al
Si
Ca
Mg
S
K
Fe
Al/Si
Ca/Si
Ca/(Si + Al)
60.42 71.47 71.88 70.44
1.05 2.64 2.97 3.41
12.23 10.89 8.88 3.08
26.31 12.45 13.00 9.77
/ 0.36 1.44 3.34
/ 1.18 1.17 4.81
/ 0.56 0.66 3.84
/ 0.44 / 1.31
0.09 0.24 0.33 1.11
2.15 1.14 1.46 3.17
1.98 0.92 1.10 1.51
a low Ca/Si leads to an increased uptake of aluminum in the C-S-H: C-A-S-H. Aluminums enters C-S-H mainly at the bridging sites in the silicate chains [17]. As C-S-H with a lower Ca/Si ratio and higher alumina content leads to an increase in alkali uptake by C-A-S-H: C-A-K-S-H [18], which is probably caused by the negative charges due to the ionization of silanol groups, the partial substitution of Si(IV) by Al(III) and the lower concentrations of calcium. That is why the content of K and Mg increase in our study. However, microwave reduces the Al/Ca, and disperses the Si/Ca, though the Al/Si increases in Table 3. This can be attributes to that microwave diminishes the particle size of CH, which is mixed in the C-AK-S-H, leading to the increase of the Ca/Si ratio and the Ca/(Si + Al). This may also explain the theory that the microwave impeding the K transfer to the glass fiber when compared with the normal curing [19]. Table 3 EDX analysis for hydration products under different curing regimes at the age of 28 days (Atomic %). 4. Conclusions The compressive strength of mortar under microwave curing at 6 h is 1.2, 0.7 and 0.5 times higher than that under normal curing, 40 °C steam curing and 80 °C steam curing at 1 day, respectively. The large capillary pores proportion of mortar under microwave curing is similar to that under normal curing, and is lower than that under steam curing. Compared with normal curing and steam curing, microwave curing increases the total middle capillary pores. Mesopores content under microwave curing is reduced when compared with normal curing, and is increased when compared with steam curing. Compared with the normal curing and steam curing at 80 °C, though microwave curing improves the porosity slightly, the compressive strength is improved. Microwave curing can accelerate the hydration of cement, but as well as the steam curing, cements microwave cured generate fine products wrapped around the cement particles, hindering the further hydration at later ages. Microwave curing produces large amounts of short-rod AFt, diminishes the particle size of CH, and increases the absorption of S, K and Mg by C-S-H gel. Acknowledgements The authors would like to acknowledge the Twelfth Five-year National Science-technology Support Plan (2012BA20B02) and
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