# The Influence of Ground Fly Ash on Cement Hydration and Mechanical Property of Mortar

Authors: Jingjing Feng; Jianwei Sun; Peiyu Yan
Journal: Advances in Civil Engineering
 (2018)

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## Abstract

In this study, the ground fly ash is made of ordinary grade I fly ash by grinding. Compared with grade I fly ash, the influence of ground fly ash on cement hydration and mechanical property of mortar was investigated. The results show that ground fly ash can improve the hydration of cement at all the ages compared with grade I fly ash, and not only does its pozzolanic reaction start earlier, but the reaction degree is higher and the speed is quicker. Before 3 days, the contribution of ground fly ash to the strength is mainly due to physical filling and microaggregate effect. After that, the contribution of pozzolanic effect to the strength becomes obvious and can significantly increase the compressive strength after 60 days and the flexural strength after 28 days. The ground fly ash is better than grade I fly ash to optimize the pore structure of hardened pastes. It can significantly reduce the number of harmful pores (>20 nm) and increase the number of harmless pores (<20 nm), which refines the pore structure and makes the structure denser.


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## Body

## 1. Introduction

Fly ash is a by-product from the residue of coal combustion, and it is a kind of pozzolana. Fly ash tends to be spherical and micrometers to millimeters particles and has a relatively high reactivity depending on the chemical formation of its individual particle [1]. These made the demand of fly ash grow steadily over the past 30 years [2, 3], especially in concrete and construction industries [3, 4]. Therefore, fly ash has become the most widely used mineral admixtures in all kinds of concrete [5, 6]. The main chemical components of fly ash are Al2O3, SiO2, and Fe2O3 [7]. And fly ash consists of a major amorphous phase and a small amount of crystalline phases [8, 9]. At early ages of hydration, fly ash particles often act as microaggregates to fill the pores of binders and nuclei for the hydration products [10]. While at later ages, the chemical effect of fly ash works. Fly ash can react with Ca(OH)2, which is a product of cement hydration, to form C–S–H gel [11, 12]. This is the so-called pozzolanic reaction. The pozzolanic reaction of fly ash is much slower than the hydration of cement [13].The utilization of fly ash not only saves energy but also improves the properties of concrete. With the fly ash addition, the heat of hydration of high-volume concrete is reduced [14–16]. Liu et al. [17] proved that fly ash could improve the fluidity of concrete. Fly ash also can decrease the drying shrinkage of concrete to some degree [18–21]. Furthermore, an appropriate addition of fly ash could enhance the resistance of concrete to sulfate attack and chloride penetration [22–24], thus reducing the risk of steel corrosion [25]. Other properties such as strength [6], acid resistance [26], wear resistance [27], and leaching properties [28] could be improved by replacing part of cement with fly ash.The specific surface area of the fly ash used in current practical engineering is between 300 and 400 m2/kg. Fly ash with a specific surface area of 600 to 700 m2/kg can be obtained from grinding. A large part of the glass phase can be ground up to pieces. And the particle size of fly ash is mainly distributed below 10 μm by grinding. The activity of fly ash depends mainly on the structures and properties of the amorphous composition. And the filling effect of fly ash depends mainly on the particle size. Therefore, theoretically it is practicable to make fly ash play a greater role in concrete by grinding.In this research, pure cement and ordinary fly ash were used as a reference. With a variety of experimental test methods, the effects of ground fly ash and grade I fly ash on cement hydration, pore structures of hardened pastes, and strength of mortar were comparatively studied. More importantly, the mechanism of grinding fly ash in hydration and hardening of cement-based composite cementitious materials was discussed.


## 2. Materials and Methods

### 2.1. Raw Materials

An ordinary Portland cement which is used for testing admixtures was applied in our research, and the specific surface area of the cement is 346 m2/kg. Grade I fly ash used was produced by a thermal power plant in China, the specific surface area of the fly ash is 384 m2/kg, and water demand is 88%. The chemical composition of cement and grade I fly ash is shown in Table 1. Standard sand was used as the aggregate for mortar specimens. Polycarboxylate superplasticizer with water reduction rate 19.1% was used to adjust the flowability and workability of fresh mortars and pastes.Table 1
Chemical compositions of cement and fly ash.
Composition
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
Na2O
f-CaO
Cl−
Cement
21.56
4.44
2.78
62.83
2.32
3.14
0.6
0.79
0.01
Fly ash
57.60
21.90
2.70
3.87
1.68
0.41
1.05
—
—The ground fly ash was made of grade I fly ash using the ball milling method in the laboratory. The rotating speed is 250 rad/min, ball to binder ratio is 4 : 1, and the milling time is 5 hours. The specific surface area of the prepared ground fly ash is 651 m2/kg, and its water demand is 97%. The microstructures of ordinary and ground fly ash are shown in Figures 1 and 2, respectively. And the particle size distribution of the three raw materials is shown in Figure 3.Figure 1
Micromorphology of grade I fly ash.Figure 2
Micromorphology of ground fly ash.Figure 3
Particle size distribution of raw materials.


### 2.2. Mix Methods

#### 2.2.1. Mortar Specimens

Three groups of mortar specimens were prepared. Pure cement mortar was the reference group, and the other two groups were mortar with 25% grade I fly ash addition and mortar with 25% ground fly ash addition. The three groups of mortars were marked as Cement, FA-25%, and GFA-25%. Their sand to binder ratios and water to binder ratios were 3 : 1 and 0.4, respectively. The superplasticizer was added to the mortars to achieve a jumping table flow of 200 ± 5 mm. The specimens were cured in standard curing boxes for 28 days and then cured in water and ambient temperature to the testing age.


#### 2.2.2. Hardened Paste Specimens

The hardened paste specimens were casted and cured in the same conditions as mortar specimens. The specimens were immersed in acetone to discontinue the hydration of pastes at the testing age and used for microstructure analysis.


## 2.1. Raw Materials

An ordinary Portland cement which is used for testing admixtures was applied in our research, and the specific surface area of the cement is 346 m2/kg. Grade I fly ash used was produced by a thermal power plant in China, the specific surface area of the fly ash is 384 m2/kg, and water demand is 88%. The chemical composition of cement and grade I fly ash is shown in Table 1. Standard sand was used as the aggregate for mortar specimens. Polycarboxylate superplasticizer with water reduction rate 19.1% was used to adjust the flowability and workability of fresh mortars and pastes.Table 1
Chemical compositions of cement and fly ash.
Composition
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
Na2O
f-CaO
Cl−
Cement
21.56
4.44
2.78
62.83
2.32
3.14
0.6
0.79
0.01
Fly ash
57.60
21.90
2.70
3.87
1.68
0.41
1.05
—
—The ground fly ash was made of grade I fly ash using the ball milling method in the laboratory. The rotating speed is 250 rad/min, ball to binder ratio is 4 : 1, and the milling time is 5 hours. The specific surface area of the prepared ground fly ash is 651 m2/kg, and its water demand is 97%. The microstructures of ordinary and ground fly ash are shown in Figures 1 and 2, respectively. And the particle size distribution of the three raw materials is shown in Figure 3.Figure 1
Micromorphology of grade I fly ash.Figure 2
Micromorphology of ground fly ash.Figure 3
Particle size distribution of raw materials.


## 2.2. Mix Methods

### 2.2.1. Mortar Specimens

Three groups of mortar specimens were prepared. Pure cement mortar was the reference group, and the other two groups were mortar with 25% grade I fly ash addition and mortar with 25% ground fly ash addition. The three groups of mortars were marked as Cement, FA-25%, and GFA-25%. Their sand to binder ratios and water to binder ratios were 3 : 1 and 0.4, respectively. The superplasticizer was added to the mortars to achieve a jumping table flow of 200 ± 5 mm. The specimens were cured in standard curing boxes for 28 days and then cured in water and ambient temperature to the testing age.


### 2.2.2. Hardened Paste Specimens

The hardened paste specimens were casted and cured in the same conditions as mortar specimens. The specimens were immersed in acetone to discontinue the hydration of pastes at the testing age and used for microstructure analysis.


## 2.2.1. Mortar Specimens

Three groups of mortar specimens were prepared. Pure cement mortar was the reference group, and the other two groups were mortar with 25% grade I fly ash addition and mortar with 25% ground fly ash addition. The three groups of mortars were marked as Cement, FA-25%, and GFA-25%. Their sand to binder ratios and water to binder ratios were 3 : 1 and 0.4, respectively. The superplasticizer was added to the mortars to achieve a jumping table flow of 200 ± 5 mm. The specimens were cured in standard curing boxes for 28 days and then cured in water and ambient temperature to the testing age.


## 2.2.2. Hardened Paste Specimens

The hardened paste specimens were casted and cured in the same conditions as mortar specimens. The specimens were immersed in acetone to discontinue the hydration of pastes at the testing age and used for microstructure analysis.


## 3. Results and Discussion

### 3.1. Heat of Hydration

The hydration heat of composite cementitious materials was tested by using ToniTechnik7338 Isothermal Differential Calorimeter. The cumulative hydration heat Q and exothermic rate dQ/dt were measured within the initial 72 hours at 25°C. The water to binder ratios kept 0.4 and the total mass of binders was 6 g in each experiment. The cumulative hydration heat and exothermic rate of all the samples are exhibited in Figure4.Figure 4
Hydration heat curves of composite cementitious materials.Figure4 shows that the additions of grade I fly ash and ground fly ash both lowered the hydration heat compared to that of pure cement, because replacing part of cement with fly ash resulted in the reducing content of cement in binders. In comparison with pure cement, the second exothermic peak of binder with grade I fly ash addition appeared later, while with ground fly ash addition, the second exothermic peak appeared earlier. In addition, the total hydration heat and exothermic rate of Sample GFA-25% were both higher than Sample FA-25%.In the process of hydration, the precipitation of C–S–H and Ca(OH)2 required Ca2+ concentration in the solution reaches the saturation. When replacing part of cement with grade I fly ash, some of Ca2+ was adsorbed on the surfaces of fly ash particles and the decrease of the cement content led to the decrease of Ca2+ concentration in the liquid phase, thus delaying the time of Ca2+ to reach saturation. Therefore, adding grade I fly ash has an effect of delaying the early-age hydration and further delay the time of the appearance of the second exothermic peak. However, the particle size of ground fly ash is significantly smaller than that of grade I fly ash. In the early-age hydration of Sample GFA-25%, the fly ash particles act as nucleation of the cement’s hydration products, which to some extent can promote the hydration and thus slightly advance the second exothermic peak.


### 3.2. Thermal Analysis

The content of Ca(OH)2 in hardened pastes was measured by thermogravimetry (TG). In order to prevent carbonization of hardened pastes during the heating process, N2 was used as the protective atmosphere. The heating rate was 10°C/min and the maximum temperature was 900°C in TG tests. The TG-DTG results of the three kinds of pastes at the age of 3, 28, 90, and 360 days are presented in Figure 5, and the corresponding Ca(OH)2 contents are shown in Table 2. Δ1 and Δ2 in Table 2, respectively, represented the difference values in Ca(OH)2 contents of FA-25% and GFA-25% compared to 75% Ca(OH)2 content of Cement.Figure 5
TG-DTG curves of composite cementitious materials.Table 2
The content of Ca(OH)2 in the hardened cement paste at different ages (%).
Age
Cement
FA-25%
GFA-25%
Δ1
Δ2
3 d
16.60
13.30
13.14
0.86
0.69
28 d
18.36
15.73
13.03
1.96
−0.74
90 d
21.20
15.52
14.76
−0.38
−1.14
360 d
21.48
15.48
13.36
−0.63
−2.75The continuous hydration of cement leads to the constant increase of Ca(OH)2 content, but the pozzolanic reaction of fly ash will consume some Ca(OH)2. A large number of studies [29, 30] have shown that the reaction degree of fly ash at the age of 3 days is generally less than 5% and at the age of 7 days less than 10%. It can be referred from Table 2 that Ca(OH)2 content of FA-25% and GFA-25% increased first and then decreased with the increase of ages, which was due to the relative values between forming rate and consuming rate of Ca(OH)2. However, Ca(OH)2 content of GFA-25% was less than that of FA-25% at all ages. Furthermore, Ca(OH)2 content of GFA-25% reached its maximum at the age of 90 days while Ca(OH)2 content of FA-25% reached its maximum at the age of 28 days. This indicates that the pozzolanic activity of ground fly ash is higher than that of grade I fly ash and that ground fly ash has more significant effect on promoting the hydration of cement.It is known from hydration heat analysis that grade I fly ash retards the early-age hydration of cement, but Δ1 kept positive from 3 days to 28 days and increased with ages, indicating that fly ash promotes the hydration of cement. The higher slope of hydration heat curve of FA-25% after 24 hours compared to that of Cement also proves this point. The main reason is that addition of grade I fly ash increased the effective water to cement ratio so that the retarding effect of grade I fly ash on hydration appeared mainly in the initial 24 hours. Δ1 reached maximum at 28 days, then generally changed to negative, and grew negatively, indicating that the pozzolanic reaction of grade I fly ash presented very low degree within initial 28 days, but the reaction degree was obviously promoted after 28 days and the hydration rate exceeded that of Cement, resulting in more Ca(OH)2 consumption than its formation and negative Δ1; the value of Δ1 expressed little difference at 90 days and 360 days, indicating that the pozzolanic reaction of grade I fly ash was relatively fast from 28 days to 90 days and then became slow.Δ2 of GFA-25% was positive at 3 days, meaning that ground fly ash always promoted the hydration of cement. After 3 days, Δ2 significantly grew negatively with the increase of ages, indicating that after 3 days the pozzolanic activity of ground fly ash began to increase greatly and the hydration rate of GFA-25% was higher than that of Cement, even at the ages of up to 360 days. It can be seen from the data in Table 2 that the pozzolanic reaction degree of ground fly ash is slightly higher than that of grade I fly ash at initial 3 days, while after 3 days, the pozzolanic activity of ground fly ash began to increase significantly and is greatly higher than that of grade I fly ash. With the increase of ages, the gap of pozzolanic activity between ground fly ash and grade I fly ash became significant.


### 3.3. Compressive Strength and Flexural Strength

The changes of compressive strength and flexural strength of three kinds of mortars with ages are shown in Figures6 and 7, respectively.Figure 6
The compressive strength versus ages.Figure 7
The flexural strength versus ages.Figure6 shows that, in the initial 28 days, the compressive strength of Cement was the biggest and grew fast, but grew slowly after 28 days, whereas the compressive strengths of FA-25% and GFA-25% basically maintained a line growing trend until 360 days and the compressive strength of GFA-25% was higher and grew faster than that of FA-25%. At the age of 360 days, the compressive strength of GFA-25% was 1.25 times that of Cement while the compressive strength of FA-25% was still less than that of Cement, indicating that the contribution of ground fly ash to pozzolanic activity was higher than that of grade I fly ash, and the pozzolanic activity of ground fly ash worked earlier than that of grade I fly ash. After 3 days, the pozzolanic activity of ground fly ash began to work and continuously contributed to compressive strength. These results are consistent with TG-DTG results.Figure7 shows that the growing trend of flexural strength was similar to that of compressive strength. The difference was that, at 1 day, the flexural strength of GFA-25% was obviously higher than that of the other two mortars. Because the hydration degree at 1 day was very low so that the filling effect and microaggregate effect of ground fly ash worked. At the age of 14 days, the flexural strength of GFA-25% was almost the same as that of Cement. For comparative study, mortar with 50% ground fly ash was casted with the same w/b. Its flexural strength was higher than that of the other three mortars at 28 days and 1.06 times that of Cement, but its compressive strength did not exceed that of Cement until 180 days. Therefore, adding ground fly ash is more beneficial to improve flexural strength than compressive strength of mortar. The more the replacement ratio is, the higher the later-age (after 28 days) flexural strength will be. This may be mainly related to the fineness of ground fly ash and the morphology of the particles after mechanical grinding. The fine fly ash particles can fill the pores between the hydration products, and it makes the microstructure denser, thus improving the flexural strength of mortar.


### 3.4. Pore Structures

Autopore II 9220 mercury intrusion porosimeter was applied to determine the porosity of hardened pastes. The maximum pressure of the tests was 400 MPa, and pores between 3 × 105 nm and 3.14 × 105 nm could be examined. The pores were classified as follows: harmless pores (<20 nm), little harmful pores (20–100 nm), harmful pores (100–200 nm), and much harmful pores (>200 nm). The pore structures of different kinds of hardened pastes are shown in Figure 8 according to this classification method.Figure 8
Cumulative pore volume histogram.It can be learned from Figure8 that, at the age of 3 days, the cumulative pore volume and much harmful pores of Cement were the least, so that its compressive strength and flexural strength were the highest (as shown in Figures 6 and 7); the cumulative pore volume of FA-25% and GFA-25% was almost the same, but the number of much harmful pores in FA-25% was twice that in GFA-25%, leading to the lowest compressive strength and flexural strength of FA-25%.With the increase of ages, the hydration degree of cement continuously increased and the pozzolanic activity of fly ash gradually appeared. The newly formed hydration products were constantly filling the pores of the hardened pores, so the total pore volume of the three mixes significantly decreased. Nevertheless, at the age of 360 days, the number of harmless pores in FA-25% and GFA-25% had a significant increase, indicating that the C–S–H and/or C–A–H gel formed from the pozzolanic reaction between fly ash and Ca(OH)2 had a good refining effect on pore structures. Furthermore, the number of harmless pores in GFA-25% was the least, indicating that the refining effect of ground fly ash on pore structures was the greatest.


## 3.1. Heat of Hydration

The hydration heat of composite cementitious materials was tested by using ToniTechnik7338 Isothermal Differential Calorimeter. The cumulative hydration heat Q and exothermic rate dQ/dt were measured within the initial 72 hours at 25°C. The water to binder ratios kept 0.4 and the total mass of binders was 6 g in each experiment. The cumulative hydration heat and exothermic rate of all the samples are exhibited in Figure4.Figure 4
Hydration heat curves of composite cementitious materials.Figure4 shows that the additions of grade I fly ash and ground fly ash both lowered the hydration heat compared to that of pure cement, because replacing part of cement with fly ash resulted in the reducing content of cement in binders. In comparison with pure cement, the second exothermic peak of binder with grade I fly ash addition appeared later, while with ground fly ash addition, the second exothermic peak appeared earlier. In addition, the total hydration heat and exothermic rate of Sample GFA-25% were both higher than Sample FA-25%.In the process of hydration, the precipitation of C–S–H and Ca(OH)2 required Ca2+ concentration in the solution reaches the saturation. When replacing part of cement with grade I fly ash, some of Ca2+ was adsorbed on the surfaces of fly ash particles and the decrease of the cement content led to the decrease of Ca2+ concentration in the liquid phase, thus delaying the time of Ca2+ to reach saturation. Therefore, adding grade I fly ash has an effect of delaying the early-age hydration and further delay the time of the appearance of the second exothermic peak. However, the particle size of ground fly ash is significantly smaller than that of grade I fly ash. In the early-age hydration of Sample GFA-25%, the fly ash particles act as nucleation of the cement’s hydration products, which to some extent can promote the hydration and thus slightly advance the second exothermic peak.


## 3.2. Thermal Analysis

The content of Ca(OH)2 in hardened pastes was measured by thermogravimetry (TG). In order to prevent carbonization of hardened pastes during the heating process, N2 was used as the protective atmosphere. The heating rate was 10°C/min and the maximum temperature was 900°C in TG tests. The TG-DTG results of the three kinds of pastes at the age of 3, 28, 90, and 360 days are presented in Figure 5, and the corresponding Ca(OH)2 contents are shown in Table 2. Δ1 and Δ2 in Table 2, respectively, represented the difference values in Ca(OH)2 contents of FA-25% and GFA-25% compared to 75% Ca(OH)2 content of Cement.Figure 5
TG-DTG curves of composite cementitious materials.Table 2
The content of Ca(OH)2 in the hardened cement paste at different ages (%).
Age
Cement
FA-25%
GFA-25%
Δ1
Δ2
3 d
16.60
13.30
13.14
0.86
0.69
28 d
18.36
15.73
13.03
1.96
−0.74
90 d
21.20
15.52
14.76
−0.38
−1.14
360 d
21.48
15.48
13.36
−0.63
−2.75The continuous hydration of cement leads to the constant increase of Ca(OH)2 content, but the pozzolanic reaction of fly ash will consume some Ca(OH)2. A large number of studies [29, 30] have shown that the reaction degree of fly ash at the age of 3 days is generally less than 5% and at the age of 7 days less than 10%. It can be referred from Table 2 that Ca(OH)2 content of FA-25% and GFA-25% increased first and then decreased with the increase of ages, which was due to the relative values between forming rate and consuming rate of Ca(OH)2. However, Ca(OH)2 content of GFA-25% was less than that of FA-25% at all ages. Furthermore, Ca(OH)2 content of GFA-25% reached its maximum at the age of 90 days while Ca(OH)2 content of FA-25% reached its maximum at the age of 28 days. This indicates that the pozzolanic activity of ground fly ash is higher than that of grade I fly ash and that ground fly ash has more significant effect on promoting the hydration of cement.It is known from hydration heat analysis that grade I fly ash retards the early-age hydration of cement, but Δ1 kept positive from 3 days to 28 days and increased with ages, indicating that fly ash promotes the hydration of cement. The higher slope of hydration heat curve of FA-25% after 24 hours compared to that of Cement also proves this point. The main reason is that addition of grade I fly ash increased the effective water to cement ratio so that the retarding effect of grade I fly ash on hydration appeared mainly in the initial 24 hours. Δ1 reached maximum at 28 days, then generally changed to negative, and grew negatively, indicating that the pozzolanic reaction of grade I fly ash presented very low degree within initial 28 days, but the reaction degree was obviously promoted after 28 days and the hydration rate exceeded that of Cement, resulting in more Ca(OH)2 consumption than its formation and negative Δ1; the value of Δ1 expressed little difference at 90 days and 360 days, indicating that the pozzolanic reaction of grade I fly ash was relatively fast from 28 days to 90 days and then became slow.Δ2 of GFA-25% was positive at 3 days, meaning that ground fly ash always promoted the hydration of cement. After 3 days, Δ2 significantly grew negatively with the increase of ages, indicating that after 3 days the pozzolanic activity of ground fly ash began to increase greatly and the hydration rate of GFA-25% was higher than that of Cement, even at the ages of up to 360 days. It can be seen from the data in Table 2 that the pozzolanic reaction degree of ground fly ash is slightly higher than that of grade I fly ash at initial 3 days, while after 3 days, the pozzolanic activity of ground fly ash began to increase significantly and is greatly higher than that of grade I fly ash. With the increase of ages, the gap of pozzolanic activity between ground fly ash and grade I fly ash became significant.


## 3.3. Compressive Strength and Flexural Strength

The changes of compressive strength and flexural strength of three kinds of mortars with ages are shown in Figures6 and 7, respectively.Figure 6
The compressive strength versus ages.Figure 7
The flexural strength versus ages.Figure6 shows that, in the initial 28 days, the compressive strength of Cement was the biggest and grew fast, but grew slowly after 28 days, whereas the compressive strengths of FA-25% and GFA-25% basically maintained a line growing trend until 360 days and the compressive strength of GFA-25% was higher and grew faster than that of FA-25%. At the age of 360 days, the compressive strength of GFA-25% was 1.25 times that of Cement while the compressive strength of FA-25% was still less than that of Cement, indicating that the contribution of ground fly ash to pozzolanic activity was higher than that of grade I fly ash, and the pozzolanic activity of ground fly ash worked earlier than that of grade I fly ash. After 3 days, the pozzolanic activity of ground fly ash began to work and continuously contributed to compressive strength. These results are consistent with TG-DTG results.Figure7 shows that the growing trend of flexural strength was similar to that of compressive strength. The difference was that, at 1 day, the flexural strength of GFA-25% was obviously higher than that of the other two mortars. Because the hydration degree at 1 day was very low so that the filling effect and microaggregate effect of ground fly ash worked. At the age of 14 days, the flexural strength of GFA-25% was almost the same as that of Cement. For comparative study, mortar with 50% ground fly ash was casted with the same w/b. Its flexural strength was higher than that of the other three mortars at 28 days and 1.06 times that of Cement, but its compressive strength did not exceed that of Cement until 180 days. Therefore, adding ground fly ash is more beneficial to improve flexural strength than compressive strength of mortar. The more the replacement ratio is, the higher the later-age (after 28 days) flexural strength will be. This may be mainly related to the fineness of ground fly ash and the morphology of the particles after mechanical grinding. The fine fly ash particles can fill the pores between the hydration products, and it makes the microstructure denser, thus improving the flexural strength of mortar.


## 3.4. Pore Structures

Autopore II 9220 mercury intrusion porosimeter was applied to determine the porosity of hardened pastes. The maximum pressure of the tests was 400 MPa, and pores between 3 × 105 nm and 3.14 × 105 nm could be examined. The pores were classified as follows: harmless pores (<20 nm), little harmful pores (20–100 nm), harmful pores (100–200 nm), and much harmful pores (>200 nm). The pore structures of different kinds of hardened pastes are shown in Figure 8 according to this classification method.Figure 8
Cumulative pore volume histogram.It can be learned from Figure8 that, at the age of 3 days, the cumulative pore volume and much harmful pores of Cement were the least, so that its compressive strength and flexural strength were the highest (as shown in Figures 6 and 7); the cumulative pore volume of FA-25% and GFA-25% was almost the same, but the number of much harmful pores in FA-25% was twice that in GFA-25%, leading to the lowest compressive strength and flexural strength of FA-25%.With the increase of ages, the hydration degree of cement continuously increased and the pozzolanic activity of fly ash gradually appeared. The newly formed hydration products were constantly filling the pores of the hardened pores, so the total pore volume of the three mixes significantly decreased. Nevertheless, at the age of 360 days, the number of harmless pores in FA-25% and GFA-25% had a significant increase, indicating that the C–S–H and/or C–A–H gel formed from the pozzolanic reaction between fly ash and Ca(OH)2 had a good refining effect on pore structures. Furthermore, the number of harmless pores in GFA-25% was the least, indicating that the refining effect of ground fly ash on pore structures was the greatest.


## 4. Conclusions

(1)
At early ages, ground fly ash can promote the hydration of cement and making the second exothermic peak appear slightly earlier. The pozzolanic activity of ground fly ash after 3 d is obviously increased and the hydration rate is greater than that of cement. Compared with grade I fly ash, not only the beginning of pozzolanic reaction of ground fly ash is earlier, but also the degree of reaction and the overall reaction rate of ground fly ash is higher.
(2)
The addition of ground fly ash could avoid the decrease of strength which occurs in the addition of ordinary fly ash, and it can improve the strength at middle and late age (after 60 days). The contribution of ground fly ash to the strength before 3 days is mainly due to filling effect and microaggregate effect. Ground fly ash has a significant contribution to strength after 3 days due to the significant increase in pozzolanic activity. Compared to compressive strength, the addition of ground fly ash is more beneficial to improve the flexural strength.
(3)
The positive effect of ground fly ash on pore structures is more significant than that of grade I fly ash, which makes the number of harmful pores (>20 nm) decrease significantly and the number of harmless holes (<20 nm) increase significantly, and thus the microstructure is denser.



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