Page 394 - Handbook of Thermal Analysis of Construction Materials

372 Chapter 9 - Non-Portland Cement Binders and Concrete

The hydrates CAH and C AH (Eq. 8) have hexagonal crystal
10
2
8
structures and are metastable with respect to conversion to the cubic hydrate
C AH and gibbsite (AH ). The microstructural changes associated with a
3
6
3
reduction in volume of solids result in an increase of porosity in the paste
matrix. This process is unlikely the reason for the decrease of impedance
after the maximum since OPC-CAC paste is still in a very early hydration
stage. Moreover, there was no decrease of matrix impedance in the pure
CAC paste (curve 1, Fig. 5). The second equation (Eq. 9) describes the
process of conversion of ettringite to monosulfoaluminate accompanied by
release of sulfate ions and free water. Free ions re-entering the liquid
phase reduce the matrix impedance of the paste; however, this is not
likely the main reason for the decrease of matrix impedance in OPC-
CAC paste systems. At room temperature, ettringite, (solubility product,
-40
1.1 × 10 ), is more stable than monosulfoaluminate, (solubility product,
-28 [42]
1.7 × 10 ). In addition, the amount of ettringite generated and its
conversion rate are negligible in the period studied. The most probable
explanation for the decrease of matrix impedance is the formation of CH
due to the OPC hydration as indicated in the third equation (Eq. 10). CH is
++
-
a source of Ca and OH ions for the pore solution and increases the
conductivity of the liquid phase.
5.3 Setting Behavior and Ettringite Nucleation

The fast set behavior due to the formation of ettringite, controlling
calcium, sulfate, and aluminate ions in the liquid phase is apparently the key
in delaying the setting of OPC-HAC pastes. [41] Use of pre-hydrated HAC
to control the aluminate ion concentration in the liquid phase provides
satisfactory results. Impedance behavior of three OPC-CAC paste systems
is illustrated in Fig. 6. The numbers 1–3 correspond to pastes containing 80,
50, and 20% of H-CAC, respectively. In contrast to OPC-HAC pastes in Fig.
5, there is no rapid increase of impedance value in all three pastes. The
behavior is closer to that of OPC paste. The pre-formed layer of hydration
products on the unhydrated CA particle surface is effective in delaying the
setting time through control of the aluminate ion concentration in the pore
solution. Dissolution of aluminate ions into the liquid phase is, therefore,
controlled by a slow diffusion process.