Admixtures Modifying Paste and Concrete Properties

Solubility of COj depends also on its partial pressure in the atmosphere surrounding concrete and on temperature. These relations, as it is known, will have an inverse effect it means that the CO2 solubility will decrease with temperature and increase with rising partial pressure of this gas. 

Dow and Glasser [356] advance a hypothesis that the water layer on a concrete surface is indispensable for carbonate efflorescence formation. Therefore they propose the following model the process is initiated when moisture appears on the surface of concrete in which alkalis will dissolve. Sorption of CO2 is then accelerated and its solubility markedly increased. CO3 removed from solution by CaCOj precipitation is supplemented rapidly. 

As aforementioned, the efflorescence is not harmful for the durability of concrete. When the mechanism involved in efflorescence formation is known, the remedies of effective preventing its occurrence are also possible. For example cements with mineral additions can be used in the production of concrete elements. Silica fume has particularly beneficial effect, which prevents efflorescence already from 5 % addition. The groimd granulated blastfurnace slag or siliceous fly ash are good additives too. The latter one should be groimd when added to concrete or taken from the last section of electrical precipitator, where the finest fractions are collected. 

In Sect. 4.1.3 the setting and hardening controlling admixtures have been discussed this section refers to the rheological properties and influencing concrete performance after hardening. 

Application of admixtures is as old as the history of mortars arrd concrete is. The ancient Romans used the animal blood, which was poprrlar also in Irrdia. According to Chandra [359], about 3,000 years BC the natmal admixtures of variorrs type were in use in India, for example molasses, fruit juices, bananas, olive oil, eggs, milk products and maity others. The natural admixtures to lime mortars in Irrdia can be classified, based on their specific role, as follows  

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