Page 189 - Handbook of Thermal Analysis of Construction Materials

172 Chapter 4 - Introduction to Concrete Admixtures

C A. [76]–[78] Hydration of C S is known to be retarded by the superplasti-
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cizer. [79][80] There is also evidence that the C/S ratio of the C-S-H phase is
changed. In cements, the hydration of the C S phase is retarded, but to a
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lesser extent than when hydrated in the pure form because part of the
admixture is adsorbed by the C A phase. Formation of ettringite may be
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accelerated or retarded by the superplasticizer, depending on the amount of
alkali sulfates present in cement.
The amount of water reduction achievable with a particular super-
plasticizer depends on the dosage and initial slump. Some results are given
below (Table 10). [40] There is evidence that beyond a particular dosage,
further water reduction is not possible. In all types of cements, water
reduction occurs to different extents.

Table 10. Water Reductions in Concrete with an SNF Superplasticizer

Dosage w/c Ratio Water Reduction Slump

Reference 0.60 0 100
Normal 0.57 5 100
Double 0.52 15 100
Triple 0.48 20 100

Reference 0.55 0 50
Normal 0.48 13 55
Double 0.44 20 50
Triple 0.39 28 45

In flowing concrete in the presence of a superplasticizer, the 28 day
strength is equal or greater than the corresponding strength of the reference
concrete. The strength for flow and water-reduced concrete made with three
types of cement is generally commensurate with the values expected from
the w/c ratios. [81] Figure 12 gives the strengths developed by concrete
containing an SMF superplasticizer. [40][81] Strengths are always higher in
the presence of the superplasticizer at any particular w/c ratio. In addition,
strengths are higher at lower w/c ratios.