# Difference between Internal and External Hydration of Hardened Cement Paste under Microwave Curing

Authors: Guoshun Yan; Jiazheng Li; Yuqiang Lin; Xia Chen
Journal: Advances in Materials Science and Engineering
 (2021)

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## Abstract

In order to investigate the difference between internal and external hydration of hardened cement paste under microwave curing, a comparative study on the hydration products, hydration degree, fracture morphology, and pore structure between the inner part and outer part of hardened cement paste (Φ120 mm × 120 mm) under microwave curing was carried out by XRD-Rietveld refinement, TG-DSC, SEM, and MIP methods. The results show that the total hydration degree of the inner part is lower at early ages, but with the hydration, there is little difference in the hydration degree between inner and outer parts at later ages. Apart from granular AFt crystal formed in the inner part of hardened cement paste, there is little difference in the fracture morphology between internal and external hydration. The total porosity of the outer part is lower than that of the inner part.


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## Body

## 1. Introduction

Microwave can accelerate the hydration of cement [1, 2] and supplementary cementitious materials [3] to improve the early age compressive strength [4, 5] with little damage to later-age strength and durability [6–8]. Therefore, much attention has been paid to the application of microwave curing for the precast concrete production [9–11] or rapid repair of cement-based materials [12]. Application of microwave curing also shows a broad perspective on the alkali activated materials [13, 14]. With the development of society, economic and ecological needs have to be considered. Microwave process can shorten mold removal time significantly but also reduce the carbon emission by replacement of fossil fuels. Therefore, microwave curing is thought to be a promising method for rapid curing.The principle of microwave heating for concrete is different from that of traditional steam heating. The concrete is dielectric composite material. Except the aggregate, all of the other components can absorb the microwave energy [4], which can be transferred to be heat energy according to equation (1). The heating performance is affected by the dielectric constants.(1)ΔTΔt=PρCP=2πfε0εr″E2ρCP,where ΔT is the temperature difference of specimens; Δt is the time for microwave radiation; P is the microwave energy; ρ is the density of the dielectric material; Cp is the specific heat capacity; ƒ is the frequency of the electric field; ε0 is the permittivity of vacuum; εr″ represents the imaginary part of complex permittivity; and E is the electric field intensity.However, the dielectric constants also affect the penetration depth of microwave through concrete according to the following equation:(2)Dp=12πf/vPεr′1+tanδ2−1/2,where Dp is the penetration depth; vp is the propagation rate of microwave in dielectric materials; εr′ is the real part of complex permittivity; and tan δ is the loss tangent.When the microwave penetrates through concrete, volumetric heating will be formed, leading to a synchrony heating in the inner part and outer part of the concrete. Therefore, when compared with conventional heating, microwave curing shortens the heat conduction time. However, according to the research of Rattanadecho et al. [15], the penetration depth increases with the increase in temperature. For example, the penetration depth of microwave at 2.45 GHz for type I Portland cement paste with water to cement ratio of 0.4 increases from 31 mm to 38 mm when the temperature increases from 30 to 90°C. This is close to the thickness of reinforcement protective layer. In general, the thickness of plain concrete is also larger than 4 cm in practical application. Therefore, in order to investigate the difference between internal and external hydration of hardened cement paste under microwave curing, in this paper, a comparative study on the hydration products, hydration degree, fracture morphology, and pore structure between the inner part and outer part of hardened cement paste (Φ120 mm × 120 mm) under microwave curing was carried out by XRD-Rietveld refinement, TG-DSC, SEM, and MIP methods.


## 2. Materials and Methods

### 2.1. Materials

Portland cement with 28 days compressive strength of 42.5 MPa (China United Cement Corp., Beijing, China) was utilized for the current study, and deionized water was used for the mixing water. The density and specific area of the cement are 3150 kg/m3 and 350 m2/kg, respectively. Table 1 summarizes the chemical properties and mineral components of cement clinkers. Table 2 shows the chemical properties of cement. A cylindrical specimen (Φ120 mm × 120 mm) is prepared for the study with the water/cement ratio of 0.50.Table 1
Chemical and mineral components of cement clinkers (wt.%).
SiO2Al2O3Fe2O3CaOMgOSO3Na2Of-CaOC3SC2SC3AC4AF21.904.813.4165.151.950.510.650.9556.3220.296.9810.37Table 2
Chemical components of cement (wt.%).
SiO2Al2O3Fe2O3CaOMgOSO3Na2Of-CaOCL-LOI20.814.923.4162.652.382.650.670.810.0122.01


### 2.2. Methods

The curing regimes under microwave heating are listed in Table3. After 1 day, the specimens are cured in water with the ambient temperature of 20 ± 2°C.Table 3
The curing regime and compressive strength.
No.Curing regimeCompressive strength (MPa)6 h1 d120 d1ML15Swelling——2ML10 + I30ML15Breaking——3ML10I30ML5 + I30ML5 + I30ML5 + I30ML2012.4——4ML10 + I30M5 + I30M5 + I30M5 + I30M2510.0—19.65ML5 + I30ML5 + I30ML5 + I30ML5 + I30M2010.1——6ML5 + I30ML5 + I30M5 + I30M5 + I30M2012.7—28.27M5 + I30M5 + I30M5 + I30M5 + I30M1517.718.429.58M5 + I30M5 + I30M5 + I30M2016.321.433.2Note. ML and M represent the output power of 260 w and 440 w, respectively, and I represents the interval time between two radiations, min. The followed Arabic numerals represent the time, min.A hydraulic machine is used to test the compressive strength. The loading rate is 2.4 kN/s. The X-ray diffraction (XRD) analysis is carried out by using a Rigaku D/max 2550 X-ray diffractometer (Rigaku Corp., Japan) with Cu Kα radiation generated at 40 kV and 250 mA. The scanning rate is 0.02°/step, and every step stays for 4 seconds. The specimen is pulverized into powder and then mixed with 20 wt.% α-Al2O3 uniformly to refine by the Rietveld method with TOPAS. FEI QUANTA 200 is used for SEM-EDS tests. An AutoPore IV 9500 V1.09 Mercury Intrusion Pore Apparatus is used to calculate the pore sizes with penetration pressure from 1 to 29948 psi (pore size varies from less than 10 nm to 216800 nm). The TG-DSC experiments are measured on the SDT Q600 analyzer of TA Company with approximately 20 mg of sample under the dynamic N2 atmosphere. The heat for CH decomposition is used for the characterization of the CH content according to the DSC curves. Chemically bonded water (H) of paste is calculated according to the TG test. The calculation method was as follows:(3)H=w105°C−w1000°Cw1000°C−LOI,where w is the weight of samples at corresponding temperature and LOI is the loss on ignition of paste.


## 2.1. Materials

Portland cement with 28 days compressive strength of 42.5 MPa (China United Cement Corp., Beijing, China) was utilized for the current study, and deionized water was used for the mixing water. The density and specific area of the cement are 3150 kg/m3 and 350 m2/kg, respectively. Table 1 summarizes the chemical properties and mineral components of cement clinkers. Table 2 shows the chemical properties of cement. A cylindrical specimen (Φ120 mm × 120 mm) is prepared for the study with the water/cement ratio of 0.50.Table 1
Chemical and mineral components of cement clinkers (wt.%).
SiO2Al2O3Fe2O3CaOMgOSO3Na2Of-CaOC3SC2SC3AC4AF21.904.813.4165.151.950.510.650.9556.3220.296.9810.37Table 2
Chemical components of cement (wt.%).
SiO2Al2O3Fe2O3CaOMgOSO3Na2Of-CaOCL-LOI20.814.923.4162.652.382.650.670.810.0122.01


## 2.2. Methods

The curing regimes under microwave heating are listed in Table3. After 1 day, the specimens are cured in water with the ambient temperature of 20 ± 2°C.Table 3
The curing regime and compressive strength.
No.Curing regimeCompressive strength (MPa)6 h1 d120 d1ML15Swelling——2ML10 + I30ML15Breaking——3ML10I30ML5 + I30ML5 + I30ML5 + I30ML2012.4——4ML10 + I30M5 + I30M5 + I30M5 + I30M2510.0—19.65ML5 + I30ML5 + I30ML5 + I30ML5 + I30M2010.1——6ML5 + I30ML5 + I30M5 + I30M5 + I30M2012.7—28.27M5 + I30M5 + I30M5 + I30M5 + I30M1517.718.429.58M5 + I30M5 + I30M5 + I30M2016.321.433.2Note. ML and M represent the output power of 260 w and 440 w, respectively, and I represents the interval time between two radiations, min. The followed Arabic numerals represent the time, min.A hydraulic machine is used to test the compressive strength. The loading rate is 2.4 kN/s. The X-ray diffraction (XRD) analysis is carried out by using a Rigaku D/max 2550 X-ray diffractometer (Rigaku Corp., Japan) with Cu Kα radiation generated at 40 kV and 250 mA. The scanning rate is 0.02°/step, and every step stays for 4 seconds. The specimen is pulverized into powder and then mixed with 20 wt.% α-Al2O3 uniformly to refine by the Rietveld method with TOPAS. FEI QUANTA 200 is used for SEM-EDS tests. An AutoPore IV 9500 V1.09 Mercury Intrusion Pore Apparatus is used to calculate the pore sizes with penetration pressure from 1 to 29948 psi (pore size varies from less than 10 nm to 216800 nm). The TG-DSC experiments are measured on the SDT Q600 analyzer of TA Company with approximately 20 mg of sample under the dynamic N2 atmosphere. The heat for CH decomposition is used for the characterization of the CH content according to the DSC curves. Chemically bonded water (H) of paste is calculated according to the TG test. The calculation method was as follows:(3)H=w105°C−w1000°Cw1000°C−LOI,where w is the weight of samples at corresponding temperature and LOI is the loss on ignition of paste.


## 3. Results and Discussion

### 3.1. Compressive Strength

The specimen ML15 cured under microwave of 260 w for 15 minutes is swelled as shown in Figure1(a). The water evaporation and air expansion happen quickly because of the rapid temperature rising, leading to the expansion failure. The specimen cured with No. 2 curing regime is shown in Figure 1(b). Although the first radiation for 10 minutes is good for hardening, the expansion stress by the followed 15 min radiation is higher than the tensile strength of hardened cement paste. Therefore, the duration of microwave radiation cannot be too long at the early age. The specimens under the other curing regimes are shown in Figure 1(c). From Figure 1(c), it is seen that there are some air holes on the surface of the specimens, which may be attributed to insufficient vibration.Figure 1
The specimens under microwave curing.The compressive strength of hardened cement paste is listed in Table3. Although the output energy is very high, the compressive strength of hardened cement paste cured by regimes of No. 3 and 4 is still relatively lower. This may be because of the excessive water loss caused by the continuous radiation, which is not conducive to the cement hydration. According to the compressive strength, the specimens cured by the regime of No. 7 were selected for the hydration and microstructure analysis. The sampling positions of inner part A and outer part B are as shown in Figure 2.Figure 2
The sampling position for hydration and microstructure analysis.


### 3.2. Hydration Products

The crystal structures of mineral phases are listed in Table4. The XRD patterns and results of Rietveld refinement are as shown in Figure 3 and Table 5.Table 4
The crystal structure of phases used in Rietveld refinement.
PhaseSpace groupa (Å)b (Å)c (Å)α (°)β (°)γ (°)ReferenceC3SC1m133.10787.035518.52119094.13790[16]C2SP121/n15.486.769.289085.4590[17]C4AFIbm25.58414.65.374909090[18]C3APa-315.26315.26315.263909090[19]CHP-3m13.5893.5894.9119090120[20]KatoiteIa-3d12.569512.569512.5695909090[21]Al2O3R-3c4.76064.760612.9949090120[22]Figure 3
The XRD-Rietveld refinement patterns: (a) inner part; (b) outer part.
(a)(b)Table 5
The results of XRD-Rietveld refinement.
SamplePhase (wt.%)Rp (%)Rwp (%)C3SC2SC3AC4AFKatoiteCHAmorphousAl2O3Inner part21.312.63.51.62.67.830.620.03.284.58Outer part20.215.83.41.72.19.127.720.03.204.46From Figure3, we can see that the crystal phases of hydration products are calcium and katoite. As seen from Table 5, the hydration of C3S in the inner part is a little lower than that in the outer part, but the trend is reversed for the hydration of C2S. The Ca/Si ratio of C-S-H on average is about 1.7 [23, 24], that means, according to equations (4) and (5) [25], 1 mol C3S produces 1.5 mol CH, but 1 mol C2S only produces 1 mol CH. Therefore, more CH is produced in the outer part. Katoite is usually formed in the cement paste under elevated temperature curing. However, in this study, only slight difference in amount of katoite is observed between inner and outer part; therefore, it is hard to identify the effect of microwave curing on the formation of katoite.(4)3CaO⋅SiO2+nH2O⟶xCaO×SiO2⋅yH2O+3−xCaOH2,(5)2CaO⋅SiO2+mH2O⟶xCaO×SiO2⋅yH2O+2−xCaOH2.


### 3.3. Hydration Degree

The TG-DSC curves of specimens are shown in Figure4. According to the calculation, the variation of chemically bonded water and calcium hydroxide is listed in Table 6. Because of the existence of amorphous CH [26–28], the quantitative analysis of CH is more accurate by DSC than XRD. Therefore, the CH content calculated by DSC results is used for the analysis of hydration degree. As seen from Table 6, the hydration degree of outer part is higher than that of inner part because of higher content of CH observed at 6 hours. From references [1, 2], microwave curing can reduce the surface tension of pore solution and increase the nucleation by increasing collision of molecules caused by the movement of molecules along with the electromagnetic field, accelerating the hydration of cement. As the penetration depth of microwave is smaller than 4 cm, the microwave only affects the outer part of specimens; therefore, the hydration degree of outer part is higher. However, with the deepening of hydration of cement, the hydration degree of inner part and outer part grows close to each other. This may be attributed to the curing in water, which is conducive to the hydration of the anhydrous at later ages.Figure 4
The TG-DSC curves of specimens: (a) inner part at 6 hours; (b) outer part at 6 hours; (c) inner part at 120 days; (d) outer part at 120 days.
(a)(b)(c)(d)Table 6
The content of chemically bonded water and calcium hydroxide.
Inner partOuter part6 hours120 days6 hours120 daysChemically bonded water (wt.%)9.019.610.719.7Heat for CH decomposition (J/g)92.5129.8108.2128.6


### 3.4. Fracture Morphology

The fracture morphology of hardened cement paste at the age of 6 hours is as shown in Figure5. As seen from Figures 5(a) and 5(b), there are capillary pores both in the inner and outer parts. The products (IP) of hydration of cement particles both in the inner and outer parts of hardened cement paste present a flocculent structure as shown in Figures 5(c) and 5(d). However, there is a lot of granular crystal in the inner part as demonstrated in Figure 5(c). This may be nano-sized ettringite (AFt). In addition, there is flaked AFm crystal as shown in Figures 5(e) and 5(f) and bunchy C-S-H as shown in Figures 5(g) and 5(h) formed both in the inner part and outer part.Figure 5
The fracture morphology of inner part and outer part of specimens: (a) inner part; (b) outer part; (c) inner part; (d) outer part; (e) inner part; (f) outer part; (g) inner part; (h) outer part.
(a)(b)(c)(d)(e)(f)(g)(h)


### 3.5. Pore Structure

The pore size distribution of hardened cement paste is shown in Figure6. From Figure 6(a), the most probable pore diameter of pastes in inner part at the age of 6 hours has two peaks around 0.03 and 0.07 μm, respectively, and the most probable pore diameter in outer part has a peak around 0.05 μm. With the deepening of hydration, the most probable pore diameter is decreased. At the age of 120 days, the most probable pore of both the inner part and outer part has peaks around 0.2, 0.06, and 0.01 μm, and an another peak around 0.02 μm is observed for the most probable pore of the inner part of hardened cement paste.Figure 6
The pore size distribution curves of hardened cement paste: (a) 6 hours; (b) 120 days.
(a)(b)According to the research of Mehta [29], the pores in the hardened cement paste are divided into three groups: meso pores (<50 nm), middle capillary pores (50–100 nm), and large capillary pores (>100 nm). The pore diameter of pastes is grouped as shown in Figure 7. As seen from Figure 7, at the age of 6 hours, the total porosity in the outer part is lower than that in the inner part; the porosity of large capillary pores and meso pores is lower in the outer part, but the porosity of middle capillary pores is higher. With the deepening of hydration, the total porosity of inner and outer parts is gradually decreased. At the age of 120 days, the porosity in the outer part is still lower than that in the inner part. This is primarily attributed to the decrease in the porosity of capillary pores.Figure 7
The porosity of hardened cement paste.


## 3.1. Compressive Strength

The specimen ML15 cured under microwave of 260 w for 15 minutes is swelled as shown in Figure1(a). The water evaporation and air expansion happen quickly because of the rapid temperature rising, leading to the expansion failure. The specimen cured with No. 2 curing regime is shown in Figure 1(b). Although the first radiation for 10 minutes is good for hardening, the expansion stress by the followed 15 min radiation is higher than the tensile strength of hardened cement paste. Therefore, the duration of microwave radiation cannot be too long at the early age. The specimens under the other curing regimes are shown in Figure 1(c). From Figure 1(c), it is seen that there are some air holes on the surface of the specimens, which may be attributed to insufficient vibration.Figure 1
The specimens under microwave curing.The compressive strength of hardened cement paste is listed in Table3. Although the output energy is very high, the compressive strength of hardened cement paste cured by regimes of No. 3 and 4 is still relatively lower. This may be because of the excessive water loss caused by the continuous radiation, which is not conducive to the cement hydration. According to the compressive strength, the specimens cured by the regime of No. 7 were selected for the hydration and microstructure analysis. The sampling positions of inner part A and outer part B are as shown in Figure 2.Figure 2
The sampling position for hydration and microstructure analysis.


## 3.2. Hydration Products

The crystal structures of mineral phases are listed in Table4. The XRD patterns and results of Rietveld refinement are as shown in Figure 3 and Table 5.Table 4
The crystal structure of phases used in Rietveld refinement.
PhaseSpace groupa (Å)b (Å)c (Å)α (°)β (°)γ (°)ReferenceC3SC1m133.10787.035518.52119094.13790[16]C2SP121/n15.486.769.289085.4590[17]C4AFIbm25.58414.65.374909090[18]C3APa-315.26315.26315.263909090[19]CHP-3m13.5893.5894.9119090120[20]KatoiteIa-3d12.569512.569512.5695909090[21]Al2O3R-3c4.76064.760612.9949090120[22]Figure 3
The XRD-Rietveld refinement patterns: (a) inner part; (b) outer part.
(a)(b)Table 5
The results of XRD-Rietveld refinement.
SamplePhase (wt.%)Rp (%)Rwp (%)C3SC2SC3AC4AFKatoiteCHAmorphousAl2O3Inner part21.312.63.51.62.67.830.620.03.284.58Outer part20.215.83.41.72.19.127.720.03.204.46From Figure3, we can see that the crystal phases of hydration products are calcium and katoite. As seen from Table 5, the hydration of C3S in the inner part is a little lower than that in the outer part, but the trend is reversed for the hydration of C2S. The Ca/Si ratio of C-S-H on average is about 1.7 [23, 24], that means, according to equations (4) and (5) [25], 1 mol C3S produces 1.5 mol CH, but 1 mol C2S only produces 1 mol CH. Therefore, more CH is produced in the outer part. Katoite is usually formed in the cement paste under elevated temperature curing. However, in this study, only slight difference in amount of katoite is observed between inner and outer part; therefore, it is hard to identify the effect of microwave curing on the formation of katoite.(4)3CaO⋅SiO2+nH2O⟶xCaO×SiO2⋅yH2O+3−xCaOH2,(5)2CaO⋅SiO2+mH2O⟶xCaO×SiO2⋅yH2O+2−xCaOH2.


## 3.3. Hydration Degree

The TG-DSC curves of specimens are shown in Figure4. According to the calculation, the variation of chemically bonded water and calcium hydroxide is listed in Table 6. Because of the existence of amorphous CH [26–28], the quantitative analysis of CH is more accurate by DSC than XRD. Therefore, the CH content calculated by DSC results is used for the analysis of hydration degree. As seen from Table 6, the hydration degree of outer part is higher than that of inner part because of higher content of CH observed at 6 hours. From references [1, 2], microwave curing can reduce the surface tension of pore solution and increase the nucleation by increasing collision of molecules caused by the movement of molecules along with the electromagnetic field, accelerating the hydration of cement. As the penetration depth of microwave is smaller than 4 cm, the microwave only affects the outer part of specimens; therefore, the hydration degree of outer part is higher. However, with the deepening of hydration of cement, the hydration degree of inner part and outer part grows close to each other. This may be attributed to the curing in water, which is conducive to the hydration of the anhydrous at later ages.Figure 4
The TG-DSC curves of specimens: (a) inner part at 6 hours; (b) outer part at 6 hours; (c) inner part at 120 days; (d) outer part at 120 days.
(a)(b)(c)(d)Table 6
The content of chemically bonded water and calcium hydroxide.
Inner partOuter part6 hours120 days6 hours120 daysChemically bonded water (wt.%)9.019.610.719.7Heat for CH decomposition (J/g)92.5129.8108.2128.6


## 3.4. Fracture Morphology

The fracture morphology of hardened cement paste at the age of 6 hours is as shown in Figure5. As seen from Figures 5(a) and 5(b), there are capillary pores both in the inner and outer parts. The products (IP) of hydration of cement particles both in the inner and outer parts of hardened cement paste present a flocculent structure as shown in Figures 5(c) and 5(d). However, there is a lot of granular crystal in the inner part as demonstrated in Figure 5(c). This may be nano-sized ettringite (AFt). In addition, there is flaked AFm crystal as shown in Figures 5(e) and 5(f) and bunchy C-S-H as shown in Figures 5(g) and 5(h) formed both in the inner part and outer part.Figure 5
The fracture morphology of inner part and outer part of specimens: (a) inner part; (b) outer part; (c) inner part; (d) outer part; (e) inner part; (f) outer part; (g) inner part; (h) outer part.
(a)(b)(c)(d)(e)(f)(g)(h)


## 3.5. Pore Structure

The pore size distribution of hardened cement paste is shown in Figure6. From Figure 6(a), the most probable pore diameter of pastes in inner part at the age of 6 hours has two peaks around 0.03 and 0.07 μm, respectively, and the most probable pore diameter in outer part has a peak around 0.05 μm. With the deepening of hydration, the most probable pore diameter is decreased. At the age of 120 days, the most probable pore of both the inner part and outer part has peaks around 0.2, 0.06, and 0.01 μm, and an another peak around 0.02 μm is observed for the most probable pore of the inner part of hardened cement paste.Figure 6
The pore size distribution curves of hardened cement paste: (a) 6 hours; (b) 120 days.
(a)(b)According to the research of Mehta [29], the pores in the hardened cement paste are divided into three groups: meso pores (<50 nm), middle capillary pores (50–100 nm), and large capillary pores (>100 nm). The pore diameter of pastes is grouped as shown in Figure 7. As seen from Figure 7, at the age of 6 hours, the total porosity in the outer part is lower than that in the inner part; the porosity of large capillary pores and meso pores is lower in the outer part, but the porosity of middle capillary pores is higher. With the deepening of hydration, the total porosity of inner and outer parts is gradually decreased. At the age of 120 days, the porosity in the outer part is still lower than that in the inner part. This is primarily attributed to the decrease in the porosity of capillary pores.Figure 7
The porosity of hardened cement paste.


## 4. Conclusions

(1)
The inner part is characterized by higher hydration degree of C2S and lower hydration degree of C3S than the outer part under microwave curing, yet the overall hydration degree of the inner part is lower at 6 hours. With the deepening of hydration, there is little difference in the hydration degree between inner and outer parts at the age of 120 days.(2)
Apart from some granular AFt crystals formed in the inner part of hardened cement paste, there is little difference in the fracture morphology between internal and external hydration products.(3)
Under microwave curing, the total porosity of the outer part is lower than that of inner part. The porosity of large capillary pores and mesopores is decreased yet that of the middle capillary pores is increased in the outer part of hardened cement paste.(4)
With the deepening of hydration, the total porosity of hardened cement is still slightly lower in the outer part at the age of 120 days. This is attributed to the reduction of capillary pores.



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*Source: 3307325-2021-10-31.xml*