3.
3.
Химические и минеральные добавки в цементы и бетоны - Синайко Н.П.
152 
Improvement of
methods to cast concrete in wintertime requires thorough studying of processes,
which take place within the cement stone at low temperatures. Solution of this
problem by achievements of physical chemistry and technology of binding is
closely linked to the processes of structure formation and properties of the
cement stone. The analysis of the processes of the structure formation can be
useful to specify the effect of influence of chemical admixtures on cement
hardening at temperatures below 0°C.
Chemical
admixtures is a very efficient mean to regulate the reactions of hydration and
structure formation when concrete hardens under winter conditions. Antifreezing
effect of the admixtures has several causes /1,2/. First of all, it is the
decrease of the water freezing point, formation of the small-size porosity and,
as a result, decrease of the freezing point pore liquid, adjustment of negative
influence of frost on the kinetic processes of hydration and heat evolution,
decrease of water content in the concrete.
In our previous
publications /3,4/ we have shown that the structure of the cement stone is a
result of interaction of the three phases: liquid, gas and solid. The influence
of temperature while doing the concrete casting in wintertime is a prime cause
to intensify the hardening of concrete not only due to the decrease of the water freezing
temperature but also due to the increase of the heat evolution during the
cement hydration.
Object and methods of the
research
The technology of winter concreting efficiently utilizes
individual and complex admixtures of various types /1,2/. The antifreezing
admixtures of “Relaxol” system (R2.3) are among them. They are of complex type
and consist of hardening accelerator (a), which is a result of coke cleaning,
and stabilizer (c), which is an organic composition modified with acids.
The object of the research is cement stone based on portland
cement of the following mineral composition: (C3S - 57%, C2S - 23%, C3A - 8%, C4AF - 12%). The efficiency of antifreezing admixtures has been evaluated
by the method of low-temperature scanning microcalorimetry (DSC) /3/. Temperature of freezing of and types of
binding in water have been determined. The type of ice formation and that of microstructure have been evaluated.
To analyze the
influence of chemical admixtures at early stages of cement hardening, the
specimens were placed into a refrigerator at - 15°C. The mode! specimen was
hardening under normal conditions.
Discussion
The sharp
decrease of the water freezing temperature during first 4 hours of hardening is
governed by solvation processes. The growth of the volume of micropores after 6
hours of hardening without admixtures is due to formation of calcium
hydrosulphoaluminates, and ettringite at the first rate. This brings to linear
and spacious expansion of the cement stone within 4 to 5 % and to growth of
micropores’ volume. This fully interfaces with data obtained by P. Metha /6/ as for the time
of formation of ettringite-3CaO-Al203-3CaS04-30-32H20 in
the cement stone (Fig. 3). Further decrease of crystallization temperature is
governed by the processes of formation of porous structure and decrease of the
pores’ dimensions.