Expansive cement
A characteristic feature of the expansion cement is the increase in the volume of the grout during setting and hardening. This phenomenon occurs as a result of the reaction between calcium aluminate and gypsum. Then the tricalcium sulphate aluminate is formed with a volume increased by 130% in relation to the sum of the volumes of its components in the starting station. The expansion of the grout from this cement is from 0,5 do 0,7%. The swelling does not damage the concrete, as this process is slow and steady throughout the setting and initial hardening period: practically from 1 do 3 (exceptionally to 7 days) from the implementation of concrete, when it is bonded, but flexible enough, that it will not break. To be safe, a swelling stabilizer is sometimes used, e.g. blast furnace slag, absorbing excess sulfate.
Expansive cement is mainly used in mining, mainly for filling the space between the tubings and the rock mass, ensuring good stiffening, sealing and bonding the tubings to the rock mass.
In addition, it is also used to supplement concrete in structures, to the so-called. tamping walls and foundations, for self-compressing structures. It is also used for reinforced road surfaces (highway) and airport runways.
Since the natural shrinkage of cement products, as a rule, nothing can be avoided, therefore, this shrinkage should be taken into account when determining the degree of expansion, which must be correspondingly larger.
As far as the behavior of the element is concerned without being shortened compared to the original quantity, then the amount of expansion should be taken equal to the expected contraction, as a rule, for a grout from 0,2 do 0,3%, and concrete from 0,02 do 0,03% (0,2 do 0,3 mm/m).
The expansive and shrink-free cement is also obtained by the separate production of expansive clinker, the expanding component of which is tetra-calcium sulphate aluminate. which is the source of the formation of etryngite. By grinding this expansive clinker with the right amount of Portland clinker and gypsum, the desired amount of expansion can be achieved.
Recently, the method of adding to Portland cement has been developed (plain or mixed), when mixing concrete mix in a concrete mixer, special, that is, condensed admixtures, with a patented composition, causing a uniform expansion of the concrete made.
Because swelling only occurs when wet, therefore, always remember to follow the care conditions (humidity and temperature).
Small amounts of expansion binder (for immediate needs), can be obtained by adding S to Portland cement 10% a mass of semi-water gypsum (50%) and kaolinite (18%) and hydrated lime (32%).