**4. Parameters affecting water sorption of hcps**

Changes in the pore structure by altering w/c or adding pozzolanic materials as well as incorporating hydrophobic agents are the main parameters affecting the water sorption of hcps. In this part, the effect of each parameter will be illustrated by presenting the results from experimental


In order to reduce undesirable macro-air pores, an air-detraining admixture based on modified polysiloxanes with 0.5% dry matter was used in all of the mixes. In addition, a plasticizer based on ligno-sulfonates with 40% dry matter and a super plasticizer (SP) based on modified acrylic polymers with 30% dry matter were used in the cement paste mixes. The composition of the mixes is listed in **Table 1**. The water to binder ratio (w/b) is also given for the SF series in this table since pozzolanic materials work as a part of binder in cementitious composites. The samples were cured in water for 12 weeks, and then dried in ventilated oven for 2 weeks at 50°C. Parameters obtained from capillary suction test are given in **Table 2**. Capillary suction porosity (εcsuc), capillary submersion porosity (εcsub), and pressure saturated porosity (εps) are calculated from the mass gain after capillary suction, submersion in water at 1 (atm) and submersion in water at 50 (atm), respectively. Furthermore, **Figures 2**–**4** show the pore distribution of different mixes with the same w/c. Note that this test is not an accurate test for porosimetry and is intended for characterizing material behavior under water sorption. The results show that the total capillary porosities (εcsuc + εcsub) are decreased for the Oil samples due to reduction in water absorption by using hydrophobic agents. Unlike impregnating agent emulsion that was used by Haugan [4], alkyl alkoxysilane had a minor effect on internal hydrophobic treatment for all the three selected mixes with different w/c, showing that not all the hydrophobic agents developed for surface treatment are also effective in reducing water

Water Sorption of Hardened Cement Pastes http://dx.doi.org/10.5772/intechopen.76378 67

Furthermore, using silica fume has decreased εcsuc and increased εcsub and it is more obvious in lower w/c. In fact, there is a minor reduction in the total capillary porosity of SF samples, but the reason and amount of this reduction is different from the Oil samples. Moreover, considering Ref and SF samples with w/b in the same range, we can see that although the total porosity has slightly increased for SF samples, εcsuc has decreased, indicating more resistance to water transport in SF samples due to reduction in pore size and connectivity between the pores.

The oil shows the best effect on reducing absorption, with a large amount of pore space that can only be accessed by high water pressure. The performance of oil is probably due to some water repellency effect, whereas the performance of silica fume is due to a reduction in pore size and connectivity between the pores. In fact, changes in the pore structure by altering w/c have only changed the total porosity of the material and did not have a significant effect on reducing the capillary suction of hcps. Adding silica fume had a minor effect on reducing the total capillary water absorption (εcsuc + εcsub) but decreased the capillary suction by creating a denser pore structure. However, using a low amount of proper hydrophobic agent as admix-

In addition to the results shown in **Table 2**, the degree of hydration was calculated from w/c and εtot shown in this table to calculate the dry sample density (ρd) according to Power's model [11] which agreed very well with measured ρd (mainly less than 2% difference). Thus, these

"Pore protection factor" (PF) is a criterion for assessing frost resistance of concrete in Finnish Standard SFS 4475 [12]. It is defined as the air content as a percentage of the total porosity (PF = εair/εtot). It is worth noting that εps is usually considered as εair for normal concrete, but the abovementioned hcps contain a low amount of air pores as judged from **Table 2** for the

sorption when used as admixture.

ture can reduce the water sorption of hcps significantly.

hcps behave as they should in terms of Powers model.

**Table 1.** Mix proportions for reference (Ref) samples and the sample containing rapeseed oil (Oil), alkyl alkoxysilane (Si), and silica fume (SF).

investigations [5, 8]. **Table 1** presents mix proportions for four mix series of hcps. The effect of changes in w/c is investigated by considering three different ratios of 0.58, 0.44, and 0.36 in each series.

Portland cement (CEM I 45.5 R) with a specific area of 550 (m<sup>2</sup> /kg) and a specific density of 3120 (kg/m3 ) were used. The series include reference (Ref) which is plain hcp, two series containing hydrophobic agents, and one series containing silica fume (SF) as pozzolanic material.

One percent of cement mass (mc) of a silane-based product (100% alkyl alkoxysilane) which has a small molecular size of 5–10 Å was used in the "Si" series. This material is developed for surface hydrophobation but it was used here as an admixture to study its effect for internal hydrophobation. In addition, food quality rapeseed oil with 8% saturated, 62% monounsaturated, and 30% polyunsaturated fatty acids was used as the other hydrophobic admixture in the "Oil" series with a dosage of 1% mc. The oil was selected as an environmental friendly substitute for existing chemical hydrophobic agents [9, 10]. Today, there are different hydrophobic agents in the market as concrete admixture where the producer claims effective hydrophobicity and no negative effect on mechanical properties; the selected hydrophobic agents here are to show how these agents may affect the water sorption of hcps.

The "SF" series contain densified silica fume with a specific density of 2200 (kg/m<sup>3</sup> ) and a dosage of 20% mc. This high dosage was considered to see the effect of finer pore structure in water suction of hcps.

In order to reduce undesirable macro-air pores, an air-detraining admixture based on modified polysiloxanes with 0.5% dry matter was used in all of the mixes. In addition, a plasticizer based on ligno-sulfonates with 40% dry matter and a super plasticizer (SP) based on modified acrylic polymers with 30% dry matter were used in the cement paste mixes. The composition of the mixes is listed in **Table 1**. The water to binder ratio (w/b) is also given for the SF series in this table since pozzolanic materials work as a part of binder in cementitious composites.

The samples were cured in water for 12 weeks, and then dried in ventilated oven for 2 weeks at 50°C. Parameters obtained from capillary suction test are given in **Table 2**. Capillary suction porosity (εcsuc), capillary submersion porosity (εcsub), and pressure saturated porosity (εps) are calculated from the mass gain after capillary suction, submersion in water at 1 (atm) and submersion in water at 50 (atm), respectively. Furthermore, **Figures 2**–**4** show the pore distribution of different mixes with the same w/c. Note that this test is not an accurate test for porosimetry and is intended for characterizing material behavior under water sorption. The results show that the total capillary porosities (εcsuc + εcsub) are decreased for the Oil samples due to reduction in water absorption by using hydrophobic agents. Unlike impregnating agent emulsion that was used by Haugan [4], alkyl alkoxysilane had a minor effect on internal hydrophobic treatment for all the three selected mixes with different w/c, showing that not all the hydrophobic agents developed for surface treatment are also effective in reducing water sorption when used as admixture.

Furthermore, using silica fume has decreased εcsuc and increased εcsub and it is more obvious in lower w/c. In fact, there is a minor reduction in the total capillary porosity of SF samples, but the reason and amount of this reduction is different from the Oil samples. Moreover, considering Ref and SF samples with w/b in the same range, we can see that although the total porosity has slightly increased for SF samples, εcsuc has decreased, indicating more resistance to water transport in SF samples due to reduction in pore size and connectivity between the pores.

investigations [5, 8]. **Table 1** presents mix proportions for four mix series of hcps. The effect of changes in w/c is investigated by considering three different ratios of 0.58, 0.44, and 0.36 in

**Table 1.** Mix proportions for reference (Ref) samples and the sample containing rapeseed oil (Oil), alkyl alkoxysilane

**Rapeseed oil (% mc )**

0.44 0 0 0 0.6 8 0.36 0 0 0 0.6 8

0.44 1 0 0 0.6 8 0.36 1 0 0 0.6 8

0.44 0 1 0 0.6 8 0.36 0 1 0 0.6 8

Ref 0.58 0 0 0 0.6 8

Si 0.58 1 0 0 0.6 8

Oil 0.58 0 1 0 0.6 8

SF w/b = 0.48 0.58 0 0 20 0.72 8 w/b = 0.40 0.44 0 0 10 0.66 8 w/b = 0.37 0.44 0 0 20 0.72 8 w/b = 0.34 0.44 0 0 30 0.78 *(SP)* 8 w/b = 0.30 0.36 0 0 20 1.44 *(SP)* 8

**Silica fume (% mc )** **Plasticizer (% mc )**

**Air-detraining admixture (g/l)**

taining hydrophobic agents, and one series containing silica fume (SF) as pozzolanic material. One percent of cement mass (mc) of a silane-based product (100% alkyl alkoxysilane) which has a small molecular size of 5–10 Å was used in the "Si" series. This material is developed for surface hydrophobation but it was used here as an admixture to study its effect for internal hydrophobation. In addition, food quality rapeseed oil with 8% saturated, 62% monounsaturated, and 30% polyunsaturated fatty acids was used as the other hydrophobic admixture in the "Oil" series with a dosage of 1% mc. The oil was selected as an environmental friendly substitute for existing chemical hydrophobic agents [9, 10]. Today, there are different hydrophobic agents in the market as concrete admixture where the producer claims effective hydrophobicity and no negative effect on mechanical properties; the selected hydrophobic agents here are to show how these agents may affect the water sorption of hcps.

The "SF" series contain densified silica fume with a specific density of 2200 (kg/m<sup>3</sup>

dosage of 20% mc. This high dosage was considered to see the effect of finer pore structure in

) were used. The series include reference (Ref) which is plain hcp, two series con-

/kg) and a specific density of

) and a

Portland cement (CEM I 45.5 R) with a specific area of 550 (m<sup>2</sup>

**Mix type w/c Alkyl alkoxysilane** 

66 Cement Based Materials

**(% mc )**

each series.

(Si), and silica fume (SF).

3120 (kg/m3

water suction of hcps.

The oil shows the best effect on reducing absorption, with a large amount of pore space that can only be accessed by high water pressure. The performance of oil is probably due to some water repellency effect, whereas the performance of silica fume is due to a reduction in pore size and connectivity between the pores. In fact, changes in the pore structure by altering w/c have only changed the total porosity of the material and did not have a significant effect on reducing the capillary suction of hcps. Adding silica fume had a minor effect on reducing the total capillary water absorption (εcsuc + εcsub) but decreased the capillary suction by creating a denser pore structure. However, using a low amount of proper hydrophobic agent as admixture can reduce the water sorption of hcps significantly.

In addition to the results shown in **Table 2**, the degree of hydration was calculated from w/c and εtot shown in this table to calculate the dry sample density (ρd) according to Power's model [11] which agreed very well with measured ρd (mainly less than 2% difference). Thus, these hcps behave as they should in terms of Powers model.

"Pore protection factor" (PF) is a criterion for assessing frost resistance of concrete in Finnish Standard SFS 4475 [12]. It is defined as the air content as a percentage of the total porosity (PF = εair/εtot). It is worth noting that εps is usually considered as εair for normal concrete, but the abovementioned hcps contain a low amount of air pores as judged from **Table 2** for the


**Table 2.** Capillary suction porosity (εcsuc), capillary submersion porosity (εcsub), pressure saturated porosity (εps), total porosity (εtot), average density of solids (ρs), and dry sample density (ρd), derived from capillary suction test.

reference materials. On the other hand, in case of internal hydrophobation of these samples, a

**Figure 3.** Pore distribution of different mixes with the w/c of 0.44 for reference (Ref) sample and the samples containing

**Figure 2.** Pore distribution of different mixes with the w/c of 0.36 for reference (Ref) sample and the samples containing

1% rapeseed oil (Oil), 1% alkyl alkoxysilane (Si), and 20% silica fume (SF).

1% rapeseed oil (Oil), 1% alkyl alkoxysilane (Si), and 20% silica fume (SF).

considerable amount of the capillary pores are not filled after 3 days of submersion in water at 1 atm but they fill at 50 atm. The apparent air voids may thus be a part of εps. **Figure 5** shows PF values for different mixes by considering εps as εair. According to this figure, it is concluded that although silica fume gives a denser pore structure, it has a minor effect on blocking the pores from the suction of water at atmospheric pressure. On the contrary, oil has been effec

tive in increasing the PF value, indicating water repellency effect in the pores. This effect has

Eq. (1), La Places or Washburn's equation, shows the pressure that forces water with surface

*σ*) into a pore of radius (

been increased by a reduction in w/c where the overall pore size becomes smaller.

tension between air and water (


*r*). Since the contact angle for the

Water Sorption of Hardened Cement Pastes http://dx.doi.org/10.5772/intechopen.76378 69

**Figure 2.** Pore distribution of different mixes with the w/c of 0.36 for reference (Ref) sample and the samples containing 1% rapeseed oil (Oil), 1% alkyl alkoxysilane (Si), and 20% silica fume (SF).

**Figure 3.** Pore distribution of different mixes with the w/c of 0.44 for reference (Ref) sample and the samples containing 1% rapeseed oil (Oil), 1% alkyl alkoxysilane (Si), and 20% silica fume (SF).

reference materials. On the other hand, in case of internal hydrophobation of these samples, a considerable amount of the capillary pores are not filled after 3 days of submersion in water at 1 atm but they fill at 50 atm. The apparent air voids may thus be a part of εps. **Figure 5** shows PF values for different mixes by considering εps as εair. According to this figure, it is concluded that although silica fume gives a denser pore structure, it has a minor effect on blocking the pores from the suction of water at atmospheric pressure. On the contrary, oil has been effective in increasing the PF value, indicating water repellency effect in the pores. This effect has been increased by a reduction in w/c where the overall pore size becomes smaller.

Eq. (1), La Places or Washburn's equation, shows the pressure that forces water with surface tension between air and water (*σ*) into a pore of radius (*r*). Since the contact angle for the

**Mix type**

Ref

**w/c** 0.58 0.44 0.36 0.58 0.44 0.36 0.58 0.44 0.36

> Silica fume

w/b = 0.48

w/b = 0.40

w/b = 0.37

w/b = 0.34

w/b = 0.30

> **Table 2.**

sample density (ρd), derived from capillary suction test.

0.36

25.2 ± 0.8

7.9 ± 0.7

2.5 ± 0.3 Capillary suction porosity (εcsuc), capillary submersion porosity (εcsub), pressure saturated porosity (εps), total porosity (εtot), average density of solids (ρs), and dry

35.6 ± 1.2

2719 ± 31

1751 ± 12

0.44

26.0 ± 2.0

10.7 ± 1.7

2.8 ± 0.5

39.5 ± 0.2

2699 ± 4

1632 ± 4

0.44

33.1 ± 2.4

4.8 ± 2.2

2.9 ± 0.1

40.8 ± 0.1

2716 ± 6

1607 ± 1

0.44

37.4 ± 0.5

1.9 ± 0.2

2.2 ± 0.4

41.5 ± 0.1

2704 ± 11

1582 ± 10

0.58

43.1 ± 0.8

2.3 ± 0.9

2.0 ± 0.3

47.4 ± 0.1

2687 ± 2

1413 ± 2

19.6 ± 0.6

3.8 ± 1.6

13.3 ± 1.1

36.7 ± 0.1

2679 ± 2

1695 ± 4

22.7 ± 1.8

6.5 ± 1.5

12.2 ± 1.0

41.1 ± 0.4

2642 ± 6

1548 ± 14

29.8 ± 1.9

12.0 ± 1.4

7.4 ± 0.8

49.2 ± 0.1

2631 ± 5

1335 ± 2

Oil

33.4 ± 3.3

4.0 ± 3.2

1.0 ± 0.1

38.4 ± 0.1

2724 ± 3

1677 ± 2

39.9 ± 0.4

1.7 ± 0.6

1.3 ± 0.5

43.0 ± 0.3

2675 ± 7

1526 ± 14

46.6 ± 0.7

2.7 ± 0.5

1.7 ± 0.3

51.1 ± 0.1

2686 ± 2

1315 ± 3

Silane

35.8 ± 1.4

1.5 ± 1.4

0.6 ± 0.1

38.0 ± 1.4

2737 ± 10

1698 ± 6

42.3 ± 0.1

0.4 ± 0.0

0.4 ± 0.0

43.1 ± 0.1

2712 ± 3

1543 ± 3

68 Cement Based Materials

49.2 ± 0.5

1.1 ± 0.6

0.4 ± 0.1

50.7 ± 0.2

2696 ± 2

1329 ± 4

**εcsuc (vol%)**

**εcsub (vol%)**

**εps (vol%)**

**εtot (vol%)**

**ρs (kg/m3**

**)**

**ρd (kg/m3**

**)**

**Figure 4.** Pore distribution of different mixes with the w/c of 0.58 for reference (Ref) samples and the samples containing 1% rapeseed oil (Oil), 1% alkyl alkoxysilane (Si), and 20% silica fume (SF).

**Figure 5.** Pore protection factor (PF) for reference (Ref) sample and the samples containing rapeseed oil (Oil), alkyl alkoxysilane (Si), and silica fume (SF).

hydrophobed surface is more than 90°, the pressure sign will be positive, thus the smaller the pore radius, the larger the repellency effect if these small pores are hydrophobed. The pore structure of hcp is more complicated than a capillary tube with connections between the pores. In addition, the hydrophobic agents may not cover all the pore surface areas. One may generally assume that the pore structure of hcps with lower w/c is finer than the higher w/c, simply by comparing the volumetric fraction of gel pores. Consequently, it is expected that water-repellant admixtures will be more effective in lower w/c if these smaller pores are the main part of the pore system that is impregnated. This effect is observed in the current example as an increase in εps by using hydrophobic agents (**Figure 6**).

$$P = \frac{(-2\sigma\cos\theta)}{r} \tag{1}$$

Note that the behavior of hcps to water sorption may have some differences with concrete specimens. Hardened cement paste gets some micro-cracks during drying period. Visual observations of micro-cracks on these pastes indicate that they cause faster water suction when placed on the water surface. These micro-cracks will, however, close after water absorption and are not expected to highly change the total porosity, but they may open some of the pores that were not accessible to capillary water and reduce pressure-saturated porosity. Since the effect of cracking can be more in higher w/c due to a higher amount of capillary water, the PF value may be less for higher w/c compared to lower w/c for Oil samples. On the other hand, the interfacial transition zone (ITZ) between the cement paste and aggregates in concrete that is the weak part of the concrete matrix can be the reason for different behavior of hydrophobic agents in hcp and concrete. In addition, pore blocking by oil droplets and denser pore structure are the other possible reasons that have been mentioned by [3] for less PF values in some concrete samples. The effect of denser pore structure can be seen in **Figure 5** by comparing the PF value for Ref samples with w/c = 0.36 and SF samples with w/c = 0.44 which have a w/b of 0.37. The PF values for SF samples are higher than Ref samples due to denser and more discontinuous pore structure,

but this effect is not comparable to the water repellency effect of rapeseed oil.

storage is usually registered in the following order during capillary suction test:

In order to measure the resistance to capillary suction, Smeplass and Skjølsvold [6] have sug-

front reaches the top surface of the specimen with the height "*h*" (see **Figure 7**). The mass

*cap*, and the corresponding absorption value, *Qcap*, that the water

Water Sorption of Hardened Cement Pastes http://dx.doi.org/10.5772/intechopen.76378 71

**5. Effective moisture transport factor**

**Figure 6.** Hydrophobic and non-hydrophobic surfaces [13].

gested calculating the time, *t*

• 1, 2, 3, 4 and alternatively 5 days.

• 10 and 30 min,

• 1, 2, 3, 4, and 6 h,

**Figure 6.** Hydrophobic and non-hydrophobic surfaces [13].

Note that the behavior of hcps to water sorption may have some differences with concrete specimens. Hardened cement paste gets some micro-cracks during drying period. Visual observations of micro-cracks on these pastes indicate that they cause faster water suction when placed on the water surface. These micro-cracks will, however, close after water absorption and are not expected to highly change the total porosity, but they may open some of the pores that were not accessible to capillary water and reduce pressure-saturated porosity. Since the effect of cracking can be more in higher w/c due to a higher amount of capillary water, the PF value may be less for higher w/c compared to lower w/c for Oil samples. On the other hand, the interfacial transition zone (ITZ) between the cement paste and aggregates in concrete that is the weak part of the concrete matrix can be the reason for different behavior of hydrophobic agents in hcp and concrete.

In addition, pore blocking by oil droplets and denser pore structure are the other possible reasons that have been mentioned by [3] for less PF values in some concrete samples. The effect of denser pore structure can be seen in **Figure 5** by comparing the PF value for Ref samples with w/c = 0.36 and SF samples with w/c = 0.44 which have a w/b of 0.37. The PF values for SF samples are higher than Ref samples due to denser and more discontinuous pore structure, but this effect is not comparable to the water repellency effect of rapeseed oil.
