**2. References**

#### **2.1 Effect of curing temperature on cement hydration**

Portland cement hydration is affected by many variables, including chemical composition, the water/cement ratio, the presence of mineral additions, and fineness. Yet another variable, however, is regarded to play a key role, bearing on early hydration kinetics and the properties of the hardened cement paste: that variable is temperature [6].

Parry-Jones et al. studied hydration in cement pastes cured for up to 31 days at temperatures ranging from 20 to 80°C. These authors calculated the degree of hydration with 29Si MAS NMR. For pastes hydrated from 20 to 55°C, strength and degree of hydration were linearly correlated, but for a given degree of hydration, the pastes hydrated at 80°C had perceptibly lower strength than the pastes cured at lower temperatures [7]. Curing temperature affects both the inner and outer calcium silicate hydrate (C-S-H) gel structure. Regourd and Gautier reported that the outer C-S-H formed at 80°C was much more fibrous, exhibiting morphology reminiscent of pastes hydrated with calcium chloride accelerators [8].

Kjellsen et al. reported thicker inner C-S-H rims than pastes hydrated at low temperatures. Such brightness is associated with several developments: an increase in the average atomic number, a decrease in the water content or both, or highsulfate concentration in C-S-H gels formed at high temperatures [9, 10].

Temperature affects the interaction between additions and cement compounds. Alite hydration was found to rise sharply from a very early age in the presence of slag and volcanic ash but much less abruptly when fly ash was added. Belite hydration was delayed in the presence of fly ash at 40 and 60°C but was somewhat enhanced at lower temperatures [11–13].

Escalante et al. noted that for all blended cement pastes, an increase in curing temperature led to greater porosity, with the most prominent differences appearing between 10 and 60°C. The same authors confirmed that in fly ash-based pozzolanic cement pastes, Ca(OH)2 was almost absent in pastes cured at 60°C, whereas at 10°C, clusters of Ca(OH)2 were visible in the microstructure [12].

**107**

Ca(OH)2CaO [18].

the average pore size [15].

*Mechanical Behavior of High-Performance Concrete under Thermal Effect*

50–60%, and the concrete is considered fully damaged [14].

**2.2 Effects of high temperature on the residual performance of concretes**

The increase in temperature results in water evaporation, C-S-H gel dehydration, calcium hydroxide and calcium aluminate decomposition, etc. Along with the increase in temperature, changes in the aggregate take place. Due to those changes, concrete strength and modulus of elasticity gradually decrease, and when the temperature exceeds ca. 300°C, the decline in strength becomes more rapid. When a 500°C threshold is passed, the compressive strength of concrete usually drops by

Many researches showed a heating of cement paste results in drying. Water gradually evaporates from the material. The order in which water is removed from heated concrete depends on the energy that binds the water and the solid. Thus, free water evaporates first, followed by capillary water and finally by physically bound water. The process of removing water that is chemically bound with cement hydrates is the last to be initiated. The mechanical properties of cement paste are strongly affected by chemical bonds and cohesion forces between sheets of calcium silicate hydrate (C-S-H) gel. It is assumed that approximately 50% of cement paste strength comes from cohesion forces (important C-S-H gel sheet area); therefore, the evaporation of water between C-S-H gel sheets strongly affects the mechanical

According to the work of Verbeck et al. [17], in the process of simultaneously exposing the material to high pressures and temperature, it may activate the changes in the microstructure of hydrates and often increases cement paste strength. The nature of the phase changes will depend upon the mineralogical composition of the cement, its C/S ratio (mol of lime per mol of silica; CaO/SiO2), the amount of fine particles (quartz or silica fume), and the temperature and pressure levels that have been reached. Heating the cement paste with a C/S ratio around 1.5 to temperature above 100°C produces several forms of calcium silicates, in general highly porous and weak. When the C/S ratio is close to 1.0 and the temperature is above 150°C, a 1.5–1.0 to bemorite gel can form. At temperature between 180 and 200°C, other silicates such as xonotlite and hillebrandite may be formed [17].

During heating, ettringite decomposes first, even before the temperature reaches 100°C. C-S-H gel dehydration is progressive and takes place from the very beginning of material heating. In this state the structure of the cement paste is partially damaged due to dehydration at a temperature of 105°C, which is standard for the drying of materials. As soon as cement paste is heated to temperature of 500–550°C, the portlandite content rapidly decreases, as it decomposes according to the following reaction:

Ca(OH)2 → CaO + H2O

Hager noticed that the CaO created in this reaction makes the elements made of the portland cement practically redundant after cooling. The dehydration process of the C-S-H gel reduces its volume, which in turn increases the porosity of the cement matrix. Moreover, during heating, the cement paste experiences a slight expansion up to temperature of approximately 200°C although the intense shrinkage begins once this temperature is exceeded. This significantly contributes to the porosity evolution of the cement paste. Due to heating total pore volume increases, as does

At 550°C, the peak corresponding to the decomposition of the free limestone

Reinforced concrete structures exposed to the environment require durable concretes to provide long-lasting performance with minimal maintenance. Low

*DOI: http://dx.doi.org/10.5772/intechopen.89916*

properties of the cement paste [15, 16].

*Compressive Strength of Concrete*

high temperature.

**2. References**

temperature [6].

1100°C in buildings and even up to 1350°C in tunnels, leading to severe damage in a concrete structure. When concrete is heated under conditions of fire, the increase in temperature in the deeper layers of the material is progressive, but because this process is slow, significant temperature gradients are produced between the concrete member's surface and core inducing additional damage to the element. Fundamental issues related to the impact of high temperature on concrete involve identification of the complex changes that take place in concrete while heated. This concerns both the physical and chemical changes taking place in the cement matrix, as well as the phenomena involved in mass movement (gases and liquids) [4].

The analysis is complicated due to the fact that cement concrete is a composite consisting of two substantially different constituents: cement paste and aggregates. The effects of the various changes taking place in heated concrete are the alterations

Many research have demonstrated that changes in the strength of concrete as a function of temperature are related to, inter alia, concrete composition, the type of aggregate used, the water/cement ratio, the presence of pozzolana additives, etc. Important factors are also the rate of heating and the time of concrete exposure to

Portland cement hydration is affected by many variables, including chemical composition, the water/cement ratio, the presence of mineral additions, and fineness. Yet another variable, however, is regarded to play a key role, bearing on early hydration kinetics and the properties of the hardened cement paste: that variable is

Parry-Jones et al. studied hydration in cement pastes cured for up to 31 days at temperatures ranging from 20 to 80°C. These authors calculated the degree of hydration with 29Si MAS NMR. For pastes hydrated from 20 to 55°C, strength and degree of hydration were linearly correlated, but for a given degree of hydration, the pastes hydrated at 80°C had perceptibly lower strength than the pastes cured at lower temperatures [7]. Curing temperature affects both the inner and outer calcium silicate hydrate (C-S-H) gel structure. Regourd and Gautier reported that the outer C-S-H formed at 80°C was much more fibrous, exhibiting morphology

Kjellsen et al. reported thicker inner C-S-H rims than pastes hydrated at low temperatures. Such brightness is associated with several developments: an increase in the average atomic number, a decrease in the water content or both, or highsulfate concentration in C-S-H gels formed at high temperatures [9, 10].

Temperature affects the interaction between additions and cement compounds.

Escalante et al. noted that for all blended cement pastes, an increase in curing temperature led to greater porosity, with the most prominent differences appearing between 10 and 60°C. The same authors confirmed that in fly ash-based pozzolanic cement pastes, Ca(OH)2 was almost absent in pastes cured at 60°C, whereas at

Alite hydration was found to rise sharply from a very early age in the presence of slag and volcanic ash but much less abruptly when fly ash was added. Belite hydration was delayed in the presence of fly ash at 40 and 60°C but was somewhat

reminiscent of pastes hydrated with calcium chloride accelerators [8].

10°C, clusters of Ca(OH)2 were visible in the microstructure [12].

enhanced at lower temperatures [11–13].

of its physical, thermal, and mechanical properties [5].

**2.1 Effect of curing temperature on cement hydration**

**106**
