Stable Microstructure of Hardened Cement Paste
One of the main problems in modern concrete science is the low durability of reinforced concrete structures and constructions, especially those located in harsh climatic conditions and saturated with marine or mineralized water. Existing standards guide designers and builders to increase the density of concrete with increasing degree of aggressive impact, as well as through cyclical freezing of concrete structures, which taken as the main indicator of durability, air entrainment is mandatory. The problem is not only the provision of high frost resistance of concrete, but also finding a method of controlling it, which, as a rule, takes quite a long time. The destruction of concrete during cyclical freezing occurs not only due to the formation of ice in the pores of concrete, but also as a result of temperature stresses in concrete with ice, as well as the washing out of portlandite and aging of the cement gel – the main structural element of hardened cement paste. This means that the durability of the concrete can be ensured by maintaining the finely dispersed structure of the hardened cement paste, preventing the crystallization of the gel from external influences or increasing its basicity. It is shown that only water reduction and pozzolanization, which provides residual portlandite content in Portland cement concrete stone 2–5 % by weight ensure the achievement of increased durability of concrete, reinforced concrete structures and constructions. A method is proposed to assess the stability of hydrate phases as well as the relationship between the durability and frost resistance of concrete.
The durability of concrete and reinforced concrete structures and constructions depends on the quality of the concrete and the environment in which they are located. In the northern regions, the most aggressive conditions occur as a result of mineralized water and cyclic freezing. The temperature in intertidal zones can drop to –20 ℃, while the air temperature can drop below –50 ℃.
According to normative documents EN-197, GOST (Russian Federation National Standard) 31384-2017 and GOST 26633-2015, the following operating environments in severe conditions for concrete structures of transport and hydraulic structures should be adopted:
XD-3 action of chlorides, except sea water;
XS-3 action of sea water in a zone of variable level;
XF-4 variable freezing and thawing with strong saturation with solutions of de-icing salts or seawater.
Under these operating environments, the national standards introduce the following restrictions on the composition and properties of concrete