**4. Results and discussions**

The main problem of ensuring the conditions for the effective development of the process of cement hydration according to the criteria *E* and τ. in the case of nano-modification in the determining measure consists in the optimal combination of the kinetic and diffusion components of structure formation [26]. The kinetic component controls the evolutionary route of the formation of the solid phase during the hydration of cement as far as possible the realization of all the phenomena (stages, transitions, etc.) of the nucleation of particles of the new phase and their development; the diffusion component acting as opposed to the kinetic one controls the dynamics of the evolutionary route phenomena. With the optimal combination of these types of control, it is possible to ensure minimization of the energy intensity of the formation of the cementing bond and the duration of the hardening process.

The following conclusions about the energy efficiency of nano-modification are based on the results we obtained on speed change, temperature coefficient of speed, effective activation energy of cement hydration process under the influence of factors of introducing nano-additives, and changes in temperature conditions presented in **Tables 4** and **5** and in **Figure 2**.

The generalization of the experimental data shows that in systems with nanomodifying additives, the cement hydration process is substantially (**Table 4**) accelerated: to the daily duration of hardening at 20°C (293 K), the degree of hydration reaches at least 70–75%.

Such a result for the control "additive-free" system is recorded only by 28 days. And thus, in this example, hydration is accelerated by criterion τ. almost 30 times.

The effect of the accelerating action of additives is valid for all variants of the composition and temperature conditions of hydration, differing, however, in the magnitude and timing of the achievement of the limiting values of the hydration degree. For the options of low temperatures (0°С, 273 К), the hydration degree of 70–75% is reached by 7 days, if the additive of pressure coefficient is used, and by 14 days, if the additive of CNT is used. At the same time, in the "additive-free" system, such a hydration degree is not achieved at all. By the 28th day, it makes only 50% of the possible maximum value.

For elevated temperatures (40–60°C, 303–333 K), in systems with the considered nano-additives CND, the degree of hydration already reaches 80–85% by the hardening for 1 h; by 7 h it is 90%, and by the first day up to 95%. Based on this data, it can be concluded that, firstly, in the presence of nano-additives, the main part of the evolutionary route of cement hydration takes place almost within the first hour and, secondly, that the hydration of cement can be completed within the first day.

The effects of such acceleration of hydration are explained by the achieved decrease in the effective activation energy of the process under the conditions of nano-modification: the effective activation energy required for the process "start" decreases 2.3–2.8 times as compared with the control "additive-free" system.


#### *Nano-Modification of Building Composite Structures DOI: http://dx.doi.org/10.5772/intechopen.86388*

#### **Table 4.**

*The hydration degree of cement depending on temperature for cement paste modified by nano-additives (0.01% mass cement).*


#### **Table 5.**

*Kinetic parameters of hydration process of cement paste modified by nano-additives (0.01% mass cement).*

It is worth emphasizing that the experimentally obtained lowest EEA value (61.7 kJ/mol) corresponds to the use of a complex modifying additive based on nano-sized SiO2 particles in combination with a superplasticizer. This is explained by the fact that, unlike carbon nanotubes, SiO2 nanoparticles are related to the mineral phases of hardening cement neoplasms with a crystalchemical structure that facilitates the formation of molecular clusters and nuclei on SiO2 nanoparticles as active crystallization centers. We draw attention here to the fact that the accelerating role of the additive takes place, despite the fact that the surfactant present in the complex additive slows down the hydration processes.

Analyzing the results of kinetic studies, it is necessary to address the question of the feasibility of combining the factor of introducing additives of nano-modifiers with the temperature factor accelerating the process of cement hydration. And in this regard, we will review the data obtained on the values of the temperature coefficient of the reaction rate (α(Т)) (see **Figure 2**).

#### **Figure 2.** *The temperature coefficient of the reaction rate.*

First of all, we note that only for the control "additive-free" system at lower temperatures of the hydration process, the situation of its kinetic control is realized. Indeed, the temperature coefficient of the reaction rate, up to the duration of hydration of 1 day, has a value close to 2 m (α(Т) = 1.85–2.25). However, already after the 3 days, the coefficient α(Т) decreases to values of 1.25 and subsequently (for durations of 7, 14, and 28 days) remains at the indicated level. This indicates the transition of the process to the area of the diffusion control, which explains the relatively low (in the limit of about 50%) values of the degree of hydration, which are fixed for the standard system (see **Table 4**).

The use of nano-modifying additives fundamentally changes the kinetic picture of the process. It accelerates so much that even at low temperatures, in the first hour, it goes (see **Figure 2**) to the mixed (for the CNT α(Т) = 1.46) and diffusion (for the CND α(Т) = 1.27) control. The transition of the process kinetics from the kinetic control region to the diffusion region in systems with nano-additives is observed at a much earlier time than in the reference system.

In the cumulative effect of the introduction of nano-additives and temperature conditions, the manifestation of the first factor is much stronger. Therefore, the use of technologies that provide for raising the temperature of cement hardening above 30°C is not relevant but more unacceptable. Indeed, for all systems with additives, an increase in temperature leads to a decrease in the α(Т) coefficient to values of 1.0–1.1, that is, to the transition of the hydration process to the region of strict diffusion control.

In contrast, for the reference "additive-free" system, the temperature increase is a positive factor for the kinetics of the process. In this case, we can speak about the phenomenon of the so-called "temperature breakdown," when the diffusion of anions and cations in the "inter-grain" volume through the "subsurface" volume is accelerated, with a layer of shielding surface of the cement grains. For this reason, at lower temperatures, the development of the process from the diffusion control region can pass into the mixed control region (the coefficient α(Т) increases from 1.0–1.05 to 1.3).

Thus, it is clear that the use of nano-additives is accompanied by the change in the parameters of the development of the cement hydration process in terms of its kinetics and energy. The result of a possible reduction in the hydration time t is a consequence of the acceleration of the reactions due to a significant decrease in their effective activation energy. On the contrary, in the field of lower and possibly negative temperatures, the use of nano-additives seems to be very effective, providing a high temperature coefficient of the reaction rate, characteristic of the development of the hydration process in the kinetic region.

When modifying the structure of cement paste with nano-additives, the change in the parameters and indicators of the strength kinetics is decisive in terms of raising efficiency. These include the speed of curing, the duration of reaching the "tempering," and the achievable limit values of the strength of the hardened cement paste. Due to this fact, it was important to analyze the strength kinetics in relation to the type and dosage of nano-additives.

In the course of experiments, when *assessing the effect* of nano-modification of the structure on the strength, the above two additives in dosages of 0.01–1% by weight of cement were studied.

**Figures 3** and **4** reflect the effect of these additives on the strength kinetics; a conclusion is drawn about the acceleration of the curing of the hardened cement paste using the minimum and maximum doses of additives. In the case of the use of the complex additive of nanoparticles SiO2 in combination with a superplasticizer (СND), close to the limiting strength, values are achieved for compositions with different dosages by the third day of hardening. The use of CND allows you to provide 70% of the strength of the values achieved by 28 days, for 1 day. For the option of dosing an additive of 0.01%, an increase in the limiting values of strength up to 90 MPa against 50 MPa in the control "additive-free" system is noted. Such a difference is explained by the modifying effect of the additive on the dispersed and morphological composition of the cement stone neoplasms, which acquire a nano-dispersed and cryptocrystalline state. This is precisely in accordance with the

**Figure 3.** *Strength kinetics curves of cement pastes modifying by CND.*

**Figure 4.** *Strength kinetics curves of cement pastes modifying by CNT.*

well-known theory of "fine grain" of Rebinder P.A. and the pattern of Hall–Petch and determines the higher resistance of the modified structure to destruction.

A similar situation occurs when the carbon nanotube additives at a dosage of 0.01% are applied. However, the strength kinetics in this type of nano-modification is noticeably different in that in the early periods of hardening, strength is slower: achieving 70% strength from its values at 28 days old is provided in this variant for 7 days, and for 1 day cement stone gains only 20%. Another difference concerns the maximum achievable strength value, which is 80 MPa.

Based on this data, it can be observed that the introduction of an additive to carbon nanotubes is less effective in comparison with the addition of nanoscale particles of SiO2 and above all in terms of the kinetic parameters of hardening strength.

Based on the results of the experiments, we should specifically dwell on the question of the strength kinetics for the variants with the dosage of additives of 1% by weight of cement. With this dosage, an accelerated increase in strength in the initial (1–3 days) hardening periods is also noted; however, no increase in strength is noted in the subsequent periods, and it remains at 35–40 MPa, which is lower than the

#### *Nano-Modification of Building Composite Structures DOI: http://dx.doi.org/10.5772/intechopen.86388*

values for the control "additive-free" composition. It can be assumed that, at an elevated dosage, the so-called poisoning of the system occurs, when excessively accelerated hydration of cement and crystallization of new growths lead to the development of self-destructive crystallization pressure. This is true: in our experiments, which involved the introduction of a 5% dose of the additive of nano-particles SiO2, selfdestruction of cement paste samples was observed after the first day of hardening in normal temperature and humidity conditions. These experimental facts confirm the urgency of the problem of optimizing the dosages of additives recommended in the technology of nano-modification of the structure of cement stone.

Thus, it has been shown that the effect of nano-modification of cement paste in relation to *R* is that it hardens faster in the early stages of structure formation and becomes more durable when the curing process is completed. And this is the basis for a reduction in overall terms of hardening and refusal of heat treatment of hardening cement paste.

*The criterion assessment of nano-modification effectiveness* was made using comparative indicators, which allowed showing the measure of its action on the kinetic parameters of the hydration and hardening processes of cement paste (**Table 6**).

It has been established that the introduction of nano-modifying additives into the "cement + water" system changes the kinetic parameters of the evolutionary route development and accelerates cement hydration by 10–30 times, which is explained by a decrease in the activation energy of the process by 2–3 times.

Modification of the hardened cement paste structure according to the dispersion and morphology of the neoplasms is accompanied by an increase in the strength value at 28 days old by 45–65%, depending on the type of additive (at their dosage 0.01% by weight of cement).

The use of additives based on SiO2 nanoparticles, which are related in their crystal-chemical structure to the minerals of the cement stone neoplasms, ensures the completion of the hardening process almost on the first day. The efficiency coefficient of nano-modification to increase the specific strength of hardened cement paste (that is, in terms of the effectiveness of the realization of the potential of the activity of cement in the synthesis of strength under the conditions of nano-additives) in the first days of hardening is 2.5 times higher for additive CND compared to adding CNT.


#### **Table 6.**

*Criteria and efficiency ratios of cement paste nano-modification by the additives CND and CNT.*

By reaching the "mature" 28-day-old age, the magnitude of the efficiency coefficient of nano-modifying increases the specific strength of hardened cement paste which is 1.36 for the additive of CND and 1.25 for the addition of CNT, and this shows that the effect of nano-modifying with the introduction of additives is not just in the first periods, when their accelerating effect is manifested on the parameters of hydration kinetics and, accordingly, on quantitative accumulations of the neoplasm phase but also in regulatory terms (28 days), when the quality begins to manifest itself of the cement hydration products.
