Expansion producing admixtures

Under conditions of internal restraint, the expansion produced is proportional to the ratio of steel to concrete and the dosage of the admixture. Special care should be taken to ensure that reinforcement is located in its proper position during placement and consolidation so that adequate restraint and good bond to steel is obtained. Restrained expansion increases the density of the matrix and produces concrete or mortar with a lower coefficient of permeability than that of corresponding Portland cement concretes and mortar. 

Probably a majority of practical expansive cements have depended on the modification of a Portland cement in such a way as to increase the formation of ettringite. Single expansive cement clinkers can be made, but it has been more usual to produce admixtures that are blended or interground with a normal Portland cement or clinker. Blending has the advantage that the... 

The silica fume is said to decrease the formation of ettringite by reducing the concentration of calcium (Ca" +) and hydroxyl (OH ) ions involved in the formation of ettringite [146]. The expansion of lime-based admixtures is not affected by water-reducing admixtures to the same extent as observed for CSA-based admixtures. However, set retardation may result due to the increased amounts of calcium hydroxide produced in the presence of the WRA. 

These admixtures control settlement and provide expansion in the plastic state. In the plastic state, metallic Al, Mg, or Zn produce gases. Admixtures that produce expansion both in plastic and hardened states are calcium sulfoaluminate and lime based materials. Those which produce expansion only in the hardened state are granulated iron fdings and chemicals that promote the oxidation of iron and offset volume changes due to shrinkage. 

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