**4. Fostering photocatalysis through cement microstructure**

In the previous section, we have exposed the most significant effects that nanoparticles of titanium dioxide, as filler, cause on the microstructure of the hydrated cement. Extensive research has been reported to show these clear modifications in the microstructure: decrease of porosity, increase of density in the C-S-H gel and increase of hydration products in the cement matrix. However, there exist not many works dedicated to the study of the cement microstructure as a valid support to favor the processes of photocatalysis. In this section, we try to explore the most relevant microstructural characteristics of hydrated cement for the promotion of the photocatalytic activity of nano-TiO<sup>2</sup> .

The first view of the problem indicates that in order to obtain reasonable rates of photocatalytic activity, the substrate must be able to adsorb the pollutant particles and trap them in some way to expose them to the photocatalytic process. The adsorption of contaminants on the surface of the photocatalyst is, therefore, an important factor in achieving high rates of degradation. To promote adsorption, composite structures comprising inert domains coexisting with photocatalytic domains can be used. In this way, the pollutants are adsorbed first in the inert sites and then diffuse to the photocatalytic domains. This is the so-called "Adsorb and Shuttle" (A&S) effect [10]. In this sense, there are important benefits derived from the use of the microstructure of the cement matrix as a suitable substrate for the nano-TiO<sup>2</sup> photocatalyst. The TiO<sup>2</sup> nanoparticles are located in the porous microstructure of the cement, where surface and volume irregularities occur. This defects population also promotes the adsorption of inorganic species that are trapped on the surface. On the other hand, since the water molecules participate in the photocatalytic process both as adsorbents, as OH sources and as reaction products, it is obvious that the water content that naturally possesses the cementitious materials can have a significant effect on photocatalytic efficiency.

It is evident then that the porosity and the presence of water of hydration in the microstructure of the cement are influential factors in the photocatalytic activity. With the aim to deeply analyze these factors, we propose a framework that helps identify the conditions for potential photocatalysis substrates in the cement material. Porosity and nanostructure of C-S-H gel are the two key elements in the microstructure of this material that affect the promotion of TiO<sup>2</sup> photocatalysis and will be studied in the following subsections.

### **4.1. Pore system of cement microstructure**

**4. Fostering photocatalysis through cement microstructure**

promotion of the photocatalytic activity of nano-TiO<sup>2</sup>

**Figure 3.** (A) Detail of SEM micrograph of TiO<sup>2</sup>

inter-hydrate space. (B) Same TiO<sup>2</sup>

54 Cement Based Materials

of amorphous inner C-S-H gel formed around the TiO<sup>2</sup>

catalyst. The TiO<sup>2</sup>

In the previous section, we have exposed the most significant effects that nanoparticles of titanium dioxide, as filler, cause on the microstructure of the hydrated cement. Extensive research has been reported to show these clear modifications in the microstructure: decrease of porosity, increase of density in the C-S-H gel and increase of hydration products in the cement matrix. However, there exist not many works dedicated to the study of the cement microstructure as a valid support to favor the processes of photocatalysis. In this section, we try to explore the most relevant microstructural characteristics of hydrated cement for the


The first view of the problem indicates that in order to obtain reasonable rates of photocatalytic activity, the substrate must be able to adsorb the pollutant particles and trap them in some way to expose them to the photocatalytic process. The adsorption of contaminants on the surface of the photocatalyst is, therefore, an important factor in achieving high rates of degradation. To promote adsorption, composite structures comprising inert domains coexisting with photocatalytic domains can be used. In this way, the pollutants are adsorbed first in the inert sites and then diffuse to the photocatalytic domains. This is the so-called "Adsorb and Shuttle" (A&S) effect [10]. In this sense, there are important benefits derived from the use

of the microstructure of the cement matrix as a suitable substrate for the nano-TiO<sup>2</sup>

tious materials can have a significant effect on photocatalytic efficiency.

surface and volume irregularities occur. This defects population also promotes the adsorption of inorganic species that are trapped on the surface. On the other hand, since the water molecules participate in the photocatalytic process both as adsorbents, as OH sources and as reaction products, it is obvious that the water content that naturally possesses the cementi-

.


nanoparticles. Fibrillar outer C-S-H bridges the spheres filling the

nanoparticles are located in the porous microstructure of the cement, where

photo-

When cement particles are dissolved and surrounded by water, they can react to produce solid reaction products (surface products) or spontaneously nucleate in the capillary water to produce crystals (pore products) [11]. The surface product is mainly the C-S-H gel and the major pore product is portlandite (CH). Yet, the surface product (C-S-H) is still porous media at nanoscale, forming gel pores, thus small molecules can diffuse through, and generally, its surfaces are negatively charged. In **Figure 4**, the presence of surface products (C-S-H, labeled as (1)) and pore products (portlandite crystals, labeled as (2)), can be observed in the microstructure, which occupy different spots, leaving spare room in between them. These voids are macropores (pore size larger than 50 μm), which are normally filled by water vapor. The capillary pores, however, are voids with average radius ranging from 5 to 5000 nm, where water persists even after hydration in completed, and that were previously available for pore products' nucleation.

This landscape draws a complex porous structure whose pore size distribution can vary from nanometers to thousands of microns, if macropores due to inadequate compaction are also

**Figure 4.** Scanning electron micrograph of cement paste displaying (1) C-S-H gel and (2) portlandite crystals as surface product and pore product respectively.

present. Moreover, large pores could be connected to gel pores through the fine capillary pores. Cement porosity is therefore directly related to the water present in the mix. The classification of the state of water in cement paste is generally based on the location where it is held and the nature of its bonding with the solid structure. Apart from the water vapor in the pores and the capillary water, it is suggested that water can also exist as adsorbed water (held by hydrogen bonds on the surface of the hydrated particles) and interlayer water [12]. Thus, the inherent availability of water in the vicinity of capillary pores leads to consider these porous structures as possible inert domains for hosting the photocatalytic nanoparticles.

#### **4.2. Nanostructure of C-S-H gel**

Since the C-S-H gel is the main solid reaction product that is formed during the hydration process, its presence must necessarily be compatible with the nanoparticle housing. As we have seen, nanoparticles not only do not interact negatively with the gel, but they are able to promote its development; therefore, the nanostructure of the C-S-H must be a clear reference in the photocatalytic capacity of TiO<sup>2</sup> .

The transfers of trapped electrons and trapped holes should be fast enough to compete against electron-hole recombination, which is the main process that limits the overall photocatalysis

pores, since these locations are surrounded by vapor water and are thermally stable. In this situation, the nanoparticles exhibit high surface specific area and have defects around them that act as traps for the adsorbents. Moreover, the capillary pores are connected to each other by capillary water, which ensures the provision of water molecules essential for the photo-

In this picture, we now add the presence of C-S-H gel nanostructures as nanosphere clusters, in addition to portlandite crystals that prefer to nucleate in the surface pores. We must remember that the density of the C-S-H gel is very high, and it is also surrounded by negative charge. On the other hand portlandite, namely calcium hydroxide, tends to react with oxygen and carbon dioxide to form calcite. These facts ensure the presence of molecular oxygen in the

Owing to the negative charges of the C-S-H gel nanostructure framework, it is expected that the trapped electron feels the Coulomb repulsion force that pushes the electron away favoring its reaction with adsorbents. The overall effect results in providing charge reservoir sites which promote the interfacial charge transfer mechanism. Cement microstructure can there-

photocatalytic performance.

**-cement composites preparation for photocatalytic** 

When preparing photocatalytic cement, it is crucial to take into account some key factors that will determine both the compound photodegradant capacity and its final performance as structural material. On the one hand, the choice of photocatalytic dosage and insertion method

. The situation is illustrated in **Figure 5**.

; therefore, the charge transfer reactions are

in the photocatalysis conversion process.

Cement Microstructure: Fostering Photocatalysis http://dx.doi.org/10.5772/intechopen.74365 57

nanoparticles are naturally placed in the capillary

performance. All this occurs on the surface of TiO<sup>2</sup>

In the cement microstructure, the TiO<sup>2</sup>

catalytic process to take place.

fore effectively mediate in TiO<sup>2</sup>

vicinity of nano-TiO<sup>2</sup>

**5. Nano-TiO2**

**applications**

clearly limited by the photocatalyst surface conditions.

**Figure 5.** Illustration of cement-mediated charge recombination on nano-TiO<sup>2</sup>

Hydrated cement is a continually evolving material: even after the hydration process has reached its end, the material keeps experimenting changes due to both increased hydration of the cement particles and changes in the microstructure of the products after they form. These changes include increases in the specific surface area, changes to the pore size distribution and a continued increase in stiffness. Likewise, the evolution of the C-S-H gel during hydration time implies changes in its density. Therefore, understanding the formation of the gel nanostructure is crucial to predict its mass/volume ratio.

Among the many different structural models suggested for C-S-H, the colloidal models proposed by Jennings [13] successfully explain various bulk properties of C-S-H found in hydrated cement pastes. The basic of these models is the existence of a 5-nm-diameter building block. These basic units pack together to form the microstructure of C-S-H. However, more recent studies [14] propose that the basic building block is a unit of C-S-H that is roughly spherical and approximately 2 nm across with a specific surface area of about 1000 m<sup>2</sup> /g. These building blocks flocculate to form larger units. This model has been already validated by many experimental works [15], where spherical nanostructures of C-S-H gel have been found. According to this model, the density of the smallest unit that can be used to build the nanometer structure of C-S-H is taken as 2450 or 2800 kg/m<sup>3</sup> , which are typical of the values for densities reported in the literature. Taking into account the high density of the C-S-H basic unit, the probability of a nanoparticle falling within a volume close to the gel is reasonable. Furthermore, according to this model, water fills the pore space starting with the finest, which impulses the idea of placing the TiO<sup>2</sup> nanoparticles in capillary pores that might be directly connected or closely near to fine deposits of water.

#### **4.3. Cement as substrate-mediator for TiO<sup>2</sup> photocatalysis**

Before discussing the validity of cement microstructure as support for photocatalysis of nano-TiO<sup>2</sup> , let us expand the implications of the photoinduced reactions that take place during photocatalysis. Once the UV photon has promoted the electron to the conduction band, leaving a gap in the valence band, this electron–hole pair is "trapped" and should proceed with the interfacial charge transfer process to directly oxidize/reduce contaminants or generate reactive oxidants.

present. Moreover, large pores could be connected to gel pores through the fine capillary pores. Cement porosity is therefore directly related to the water present in the mix. The classification of the state of water in cement paste is generally based on the location where it is held and the nature of its bonding with the solid structure. Apart from the water vapor in the pores and the capillary water, it is suggested that water can also exist as adsorbed water (held by hydrogen bonds on the surface of the hydrated particles) and interlayer water [12]. Thus, the inherent availability of water in the vicinity of capillary pores leads to consider these porous structures

Since the C-S-H gel is the main solid reaction product that is formed during the hydration process, its presence must necessarily be compatible with the nanoparticle housing. As we have seen, nanoparticles not only do not interact negatively with the gel, but they are able to promote its development; therefore, the nanostructure of the C-S-H must be a clear reference

Hydrated cement is a continually evolving material: even after the hydration process has reached its end, the material keeps experimenting changes due to both increased hydration of the cement particles and changes in the microstructure of the products after they form. These changes include increases in the specific surface area, changes to the pore size distribution and a continued increase in stiffness. Likewise, the evolution of the C-S-H gel during hydration time implies changes in its density. Therefore, understanding the formation of the gel nano-

Among the many different structural models suggested for C-S-H, the colloidal models proposed by Jennings [13] successfully explain various bulk properties of C-S-H found in hydrated cement pastes. The basic of these models is the existence of a 5-nm-diameter building block. These basic units pack together to form the microstructure of C-S-H. However, more recent studies [14] propose that the basic building block is a unit of C-S-H that is roughly spherical and approximately

form larger units. This model has been already validated by many experimental works [15], where spherical nanostructures of C-S-H gel have been found. According to this model, the density of the smallest unit that can be used to build the nanometer structure of C-S-H is taken as

into account the high density of the C-S-H basic unit, the probability of a nanoparticle falling within a volume close to the gel is reasonable. Furthermore, according to this model, water fills

Before discussing the validity of cement microstructure as support for photocatalysis of nano-

, let us expand the implications of the photoinduced reactions that take place during photocatalysis. Once the UV photon has promoted the electron to the conduction band, leaving a gap in the valence band, this electron–hole pair is "trapped" and should proceed with the interfacial charge transfer process to directly oxidize/reduce contaminants or generate reactive oxidants.

 **photocatalysis**

in capillary pores that might be directly connected or closely near to fine deposits of water.

the pore space starting with the finest, which impulses the idea of placing the TiO<sup>2</sup>

, which are typical of the values for densities reported in the literature. Taking

/g. These building blocks flocculate to

nanoparticles

as possible inert domains for hosting the photocatalytic nanoparticles.

.

**4.2. Nanostructure of C-S-H gel**

56 Cement Based Materials

in the photocatalytic capacity of TiO<sup>2</sup>

2450 or 2800 kg/m<sup>3</sup>

TiO<sup>2</sup>

structure is crucial to predict its mass/volume ratio.

2 nm across with a specific surface area of about 1000 m<sup>2</sup>

**4.3. Cement as substrate-mediator for TiO<sup>2</sup>**

**Figure 5.** Illustration of cement-mediated charge recombination on nano-TiO<sup>2</sup> in the photocatalysis conversion process.

The transfers of trapped electrons and trapped holes should be fast enough to compete against electron-hole recombination, which is the main process that limits the overall photocatalysis performance. All this occurs on the surface of TiO<sup>2</sup> ; therefore, the charge transfer reactions are clearly limited by the photocatalyst surface conditions.

In the cement microstructure, the TiO<sup>2</sup> nanoparticles are naturally placed in the capillary pores, since these locations are surrounded by vapor water and are thermally stable. In this situation, the nanoparticles exhibit high surface specific area and have defects around them that act as traps for the adsorbents. Moreover, the capillary pores are connected to each other by capillary water, which ensures the provision of water molecules essential for the photocatalytic process to take place.

In this picture, we now add the presence of C-S-H gel nanostructures as nanosphere clusters, in addition to portlandite crystals that prefer to nucleate in the surface pores. We must remember that the density of the C-S-H gel is very high, and it is also surrounded by negative charge. On the other hand portlandite, namely calcium hydroxide, tends to react with oxygen and carbon dioxide to form calcite. These facts ensure the presence of molecular oxygen in the vicinity of nano-TiO<sup>2</sup> . The situation is illustrated in **Figure 5**.

Owing to the negative charges of the C-S-H gel nanostructure framework, it is expected that the trapped electron feels the Coulomb repulsion force that pushes the electron away favoring its reaction with adsorbents. The overall effect results in providing charge reservoir sites which promote the interfacial charge transfer mechanism. Cement microstructure can therefore effectively mediate in TiO<sup>2</sup> photocatalytic performance.

#### **5. Nano-TiO2 -cement composites preparation for photocatalytic applications**

When preparing photocatalytic cement, it is crucial to take into account some key factors that will determine both the compound photodegradant capacity and its final performance as structural material. On the one hand, the choice of photocatalytic dosage and insertion method usually depends upon the composition of the host cement-based composite. On the other hand, TiO<sup>2</sup> photocatalyst can be used either as freestanding particulate or as coating on a substrate. However, much more insight is needed from engineering design and modeling point of view, for successful application of the laboratory-scale techniques to large-scale operation. Questionless, there exist some beneficial effects on the environment derived from inclusion of TiO<sup>2</sup> nanoparticles to cement production. But on its own, cement is a highly efficient material in terms of energy consumption and welfare that generates; therefore we should aim to respect its identity in attempting to develop a more sustainable material.

The water-cement ratio directly influences the permeability properties of the material. Thus, a water-cement ratio of cement paste above 0.4 likely leads to having prohibitive sedimentation and bleeding. At the same time, the "Adsorb and Shuttle" (A&S) effect explained in previous sections implies that cement particles need to be kept in suspension before setting and hardening, in order to allow the adsorbents to be located in the microstructure. On the other hand, an excessive amount of cement would give rise to a material with significant hydration defects, since the available water would be almost entirety consumed, without leaving enough water molecules in the environment of the nanoparticles, in addition to a significant amount of clinker

Taking into account the previous comments as well as the experimental results reported in the literature, given that the water cement ratio is directly involved in the porosity of the resulting mixture, it seems essential to estimate what percentage of porosity will be in the cementitious matrix manufactured with a reasonable water-cement ratio. In this regard, 0.5 is the ratio most widely used in literature related to the manufacture of photocatalytic cements. Such water-cement ratio produces about 32.3% porosity in the cement matrix [18], along with a

/g and bulk density of 1448 kg/m<sup>3</sup>

through a Monte Carlo approach [19], resulting in an increase of the amount and distribution of adsorbents within the exposed area. This likewise explains the effect of higher reaction rate

This chapter concludes that the singularities and the qualities of the hydrated cement microstructure enhance the photocatalytic processes, driven by titanium dioxide, to create environment-friendly cement. We have exposed the microstructural characteristics of cement and those aspects that make possible the promotion of photocatalytic activity. The cement porosity and the nanostructure of the C-S-H have been identified as surface modificators

performance. Suggested directions have been provided for the preparation of photocatalytic

of the cementitious material, according to the most recent trends reported. This framework is also a starting point for future studies that seek to improve the photocatalytic response of titanium dioxide inserted in the cement matrix as well as to provide implications for the appli-

providing charge reservoir sites which promote the interfacial charge transfer

have been used to predict the photocatalytic performance of nano-TiO<sup>2</sup>

constant obtained when 0.5 water-cement composite is present as the TiO<sup>2</sup>

mechanism. Cement microstructure can therefore effectively mediate in TiO<sup>2</sup>

cation of photocatalytic cement technology in the construction materials industry.

mixtures, taking into account both the suitable content of nano-TiO<sup>2</sup>

The author declares that there was no conflict of interest.

. These microstructural values

Cement Microstructure: Fostering Photocatalysis http://dx.doi.org/10.5772/intechopen.74365 59

substrate.


photocatalytic

as well as the formulation

that would remain anhydrate.

specific surface area of 134 m<sup>2</sup>

**6. Conclusions**

of nano-TiO<sup>2</sup>

**Conflict of interest**

#### **5.1. Optimal content and inclusion method of nano-TiO<sup>2</sup> in cement**

Initially, we focused on the optimum weight fraction of TiO<sup>2</sup> nanoparticles that should be added to the cement to develop a sufficiently efficient photocatalytic activity. Many works reported in literature ensure that the minimum fraction of nanoparticles to be added to cement in order to obtain a minimum photocatalytic activity must exceed 1% by mass of cement [16]. In fact, the photocatalytic properties begin to be significant from the eco-efficient point of view when the percentage of nanoparticles included in cement is close to 3%. This percentage not only provides photocatalytic activity to the material, but also favors the development of hydration products, leading to improvements of up to 62% in mechanical properties for long ages [17].

Regarding the preparation method, the TiO<sup>2</sup> nanoparticles can be synthesized by chemicalphysical methods, such as sol-gel, using different precursors based on titanium oxides, such as titanium tetrabutoxide and titanium tetraisopropoxide (TTIP). These methods have proven to be suitable for the formation of spherical nanoparticles with controlled particle size. In addition, the so obtained nanostructured material features high specific area, which is an essential requirement to obtain adequate rates of photocatalysis.

At the end of the synthesis procedure, the nanoparticles can be kept in liquid medium or be subjected to a calcination process in order to increase their crystallinity and achieve a particulate system. Regardless, the most appropriate way to include the nanostructured TiO<sup>2</sup> in the cement matrix is found to be adding it directly to the hydration water. Thus, the nanoparticles will directly occupy the capillary pores that remain as the water is consumed due to the formation of products. In addition, the aqueous environment around the nanoparticles is guaranteed.

#### **5.2. Suitable fabrication parameters of the cementitious matrix**

Another key factor when preparing photocatalytic cement with adequate structural characteristics is the correct choice of manufacturing parameters for the cement mixture. Many researchers have used white cement to prepare their photocatalytic mixtures. The choice of white cement basically supports esthetic reasons, since from its preparation, it already seems a "clean" material and therefore more conducive to integration in a pollutant-free environment. In any case, other types of cement are also valid to form the cementitious matrix in which the nanoparticles are going to be housed. The requirement is so that the matrix must be properly hydrated and provide an adequate porous structure. In this sense, the water-cement ratio plays an extremely important factor.

The water-cement ratio directly influences the permeability properties of the material. Thus, a water-cement ratio of cement paste above 0.4 likely leads to having prohibitive sedimentation and bleeding. At the same time, the "Adsorb and Shuttle" (A&S) effect explained in previous sections implies that cement particles need to be kept in suspension before setting and hardening, in order to allow the adsorbents to be located in the microstructure. On the other hand, an excessive amount of cement would give rise to a material with significant hydration defects, since the available water would be almost entirety consumed, without leaving enough water molecules in the environment of the nanoparticles, in addition to a significant amount of clinker that would remain anhydrate.

Taking into account the previous comments as well as the experimental results reported in the literature, given that the water cement ratio is directly involved in the porosity of the resulting mixture, it seems essential to estimate what percentage of porosity will be in the cementitious matrix manufactured with a reasonable water-cement ratio. In this regard, 0.5 is the ratio most widely used in literature related to the manufacture of photocatalytic cements. Such water-cement ratio produces about 32.3% porosity in the cement matrix [18], along with a specific surface area of 134 m<sup>2</sup> /g and bulk density of 1448 kg/m<sup>3</sup> . These microstructural values have been used to predict the photocatalytic performance of nano-TiO<sup>2</sup> -cement composites through a Monte Carlo approach [19], resulting in an increase of the amount and distribution of adsorbents within the exposed area. This likewise explains the effect of higher reaction rate constant obtained when 0.5 water-cement composite is present as the TiO<sup>2</sup> substrate.
