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

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 properties of the cement paste [15, 16].

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:

$$\text{Ca(OH)}\_{2} \rightarrow \text{CaO} + \text{H}\_{2}\text{O}$$

At 550°C, the peak corresponding to the decomposition of the free limestone Ca(OH)2CaO [18].

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 the average pore size [15].

Reinforced concrete structures exposed to the environment require durable concretes to provide long-lasting performance with minimal maintenance. Low permeability is an important characteristic of durable concretes and may be obtained by lowering the water/cementitious material ratio (W/C) and using pozzolans (fly ash and silica fume) or slag as a portion of the cementitious material [15, 18].
