Page 392 - Handbook of Thermal Analysis of Construction Materials

370 Chapter 9 - Non-Portland Cement Binders and Concrete

5.0 PORTLAND CEMENT–CALCIUM
ALUMINATE CEMENT BLENDS

5.1 Introduction

The development of OPC-CAC based expansive cements can be
traced back to the 1920s. [36]–[38] Fast set control was a problem that limited
the actual application of these systems. The setting time varies with the
proportion of the two cements. The optimum proportion resulting in
minimum setting time is about 70% CAC and 30% OPC. [39] Methods for
controlling rapid set involving the use of pre-hydrated CAC (H-CAC) have
been recently proposed. [40] Satisfactory workability can be attained without
loss of expansive and mechanical properties. Understanding of the hydra-
tion mechanisms and the microstructural development in these expansive
cement systems is important to optimize potential application. This in-
volves evaluation of factors affecting: (i) fast set behavior of the OPC-CAC
system and (ii) the hydration mechanism of the OPC-H-CAC system.

5.2 Hydration

Electrochemical methods appear to have distinct advantages in the
study of cement hydration. Methods involving potential measurement
(including pH, zeta potential, and selected ion potential), conductivity
measurement, and a.c. impedance measurement provide useful information
related to both ion concentration of pore solution and microstructural
change in hydrating cement paste. The early hydration and setting behavior
of OPC-CAC and OPC-Hydrated-CAC paste systems can be determined
using these techniques.
It appears that the impedance behavior of the OPC-CAC paste
system is determined by the following microstructural events: [41]
i) Nucleation of ettringite accompanied by significant con-
sumption of free water and fast set. This process is
concomitant to the first rapid increase in impedance
value at about 20 minutes (curves 2–5, Fig. 5).
ii) An “induction” period while hydration continues at a
relatively slow rate. Accumulation of hydration prod-
ucts (mainly C-S-H, CAH , C AH ) on the unhydrated
10
2
8
particle surfaces are observed. The impedance value
remains relatively constant.