Page 308 - Handbook of Thermal Analysis of Construction Materials

Section 5.0 - Cement 285

Table 7. Influence of Molecular Weight of SNF Superplasticizer on the
Hydration of Cement in Terms % Ca(OH) Formed
2

Mol. Wt. Average 8 hours 1 day 7 days 28 days

Reference 2.0 7.3 9.0 10.50
260 1.8 6.6 9.5 10.7
290 1.7 6.5 10.5 11.2
480 1.6 6.7 11.0 12.1
640 1.5 6.8 12.0 15.2

The type of cation associated with the superplasticizer may influ-
ence the rate of hydration of cement. For example, the time to attain the
maximum heat effect in Type I cement with SNF, having cations such as
NH , Co, Mn, Li, and Ni is 12.8, 11.5, 10.5, 10.3, and 9.3 respectively. The
4
exact mechanism for these variations is not understood. [34]
Type I portland cement concrete, made at a particular w/c ratio,
with or without the superplasticizer (SMF or SNF) does not show differ-
ences in strength values. It has been found, however, that superplasticized
Type V cement concrete develops lower strengths from early ages to 90
[5]
days. For example, at a w/c ratio of 4, the 1, 3, 7, 28, and 90 day strengths
of the reference concrete are 15.2, 24.5, 30.4, 40.6, and 52.1 MPa respec-
tively, and the corresponding values for the concrete containing 0.6% SNF
superplasticizer are 15.9, 22.3, 27.3, 36.0, and 44.6 MPa. The conduction
calorimetric curves for Type I and Type V cement pastes treated with a
0.6% SMF superplasticizer showed differences (Fig. 18). [5][35] A more
efficient retardation occurs in Type V cement. The causes leading to lower
strengths in Type V cement are explained by differences in porosity values
of the products and lower rates of hydration.
Ramachandran [36] found that by the addition of calcium gluconate
slump loss in superplasticized concrete could be reduced. As the addition
of gluconate also leads to a retardation of set, a quick method was needed
to assess the maximum amount of gluconate that can be added without
causing undue retardation of set. Conduction calorimetry was used for this
purpose. Figure 19 gives the calorimetric curves for four samples contain-
ing gluconate and SMF. Gluconate (curve 4), in excess of 0.1%, leads to the
peaks shifting to longer times, hence causing unacceptable retardation.