OTHER ADMIXTURES

Many chemicals and waste products may act as retarders. Thermoanalytical techniques are very useful to assess the effect of these products on the hydration of cements. A few examples are described below. 

Copper refining wastes containing Cr and Sb were added to cements and the rate of hydration was followed by DTA, TG, and DTG. There was a significant decrease in the amount of Ca(OH)2 in pastes hydrated with the copper refining waste, suggesting that it is a good retarder.I l 

The metal plating waste products from a zinc plating process were added to portland cement and the hydration was followed by conduction calorimetry. At an addition level of 3-10%, the rate of hydration was greatly reduced, 

In an investigation of various pol5miers, it was found that sodium carboxy methyl cellulose and versicol acted as strong retarders. For example, with the reference portland cement pastes, the peak in the conduction calorimetry occurred at 11 hours, and with versicol, the peak shifted to 20, 29, and 34 hours at dosages of 0.25, 0.50, and 0.75% respectively, 

Zinc oxide, in small amounts, retards the setting of cement very effectively. Calorimteric investigation has shown that, at an addition of 0.25% ZnO, hydration is almost nil up to 12 hours, and, at 1% addition, hydration is not initiated even up to 2 days. l 

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Nawa, T. et al. (1989). Third International Conference of Superplasticizers and Other Admixtures, Canada, ACI SP-119, 405-24. 

Shonaka, M. et al. (1997) Fifth Canmet/ACI Conference on Superplasticizers and Other Admixtures in Concrete, Italy, 613. 

It is normally used as a component in other admixture formulations and rarely, if ever, as a sole ingredient. 

Compatibility with other admixtures (such as WRAs and superplasticizers) so that rheology of the mix is not significantly altered, set is not drastically extended, or heavy air-entrainment and foaming is not produced. 

Compatibility with other admixtures due to the inherent surfactant properties exhibited by most of the materials in classes A, B and C, their use with other admixtures such as water reducers should be evaluated in terms of the side effects on the plastic and hardened concrete, viz. severe set retardation and excessive air entrainment. The molecules of materials with fewer surfactant properties compete with the superplasticizer for adsorption sites on the cement particle. This will result in a higher dosage of the superplasticizer. 

Be compatible with other admixtures used in concrete. 

The expansion achieved in concrete containing these admixtures is dependent on the type of aggregates used in the mixture. Thus the desired level of shrinkage compensation should take into account the modulus of the aggregate to be used on the job. The effect of cement and water contents on the extent of expansion should also be considered. Minimum cement content required to achieve desired expansion should be determined through mix trials. Compatibility with other admixtures may also need trials and the manufacturer s recommendation should be followed [80]. 

Two factors affect the reaction of both the wet and dry processes, viz. (1) mix design and (2) the ambient conditions prevailing during the spraying of the concrete. Mix design factors include cement type and content, water- cement ratio, cement-admixture compatibility and the presence of other admixtures. Cements with higher C3A contents such as Types I and III give faster reactions than cement with low C3A contents such as Types IV... 

Atzeni, C., Mantegazza, Massida, L. and Sanna, U. (1989). Proceedings 3rd Conference Superplasticizers and Other Admixtures in Concrete, ACI SP-119, 457-70. 

Mix proportion aspects that should be considered in the design of highly flowable mixtures include (1) cement content (2) fines content (3) type of superplasticizer (4) the presence of other admixtures in the mix (5) type of cement or cementitious material (6) dosage of the admixture (7) sequence... 

Factors which affect the dosage rate are concrete temperature, initial slump (i.e. slump before the addition of the superplasticizer), cement type and content, the presence of other admixtures in the mix prior to the addition of the superplasticizer and the sequence of addition to the mix. The... 

Admixtures should be purchased under specifications that stipulate the desired properties, exclude adverse effects and provide evaluation of uniformity of the admixture from batch to batch. The user should ensure that the admixtures being considered conform to ASTM or other applicable specifications and the manufacturer of the admixture should be required to certify that individual lots meet the requirements of applicable standards or specifications. The following information should be provided by the admixture manufacturer or supplier (1) composition of the admixture in terms of the generic type of its main constituents (2) incompatibility with other admixtures or special cements (3) typical dosage (4) detrimental effects of overdosage or underdosage (5) chloride content and (6) whether air is entrained at the recommended dosage. 

Two types of calcium nitrite-based corrosion inhibitors are currently marketed, viz. a set- and strength-accelerating type and a normal-setting type. The former increases the early strength development in concrete. This effect increases with the dosage. Both admixtures are compatible with all types of Portland cements and other admixtures, although both may moderately reduce the amount of entrained... 

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