Page 400 - Handbook of Thermal Analysis of Construction Materials

378 Chapter 9 - Non-Portland Cement Binders and Concrete

5.6 Chemical Admixtures

The hydration process of OPC/CAC mixtures is affected by the
presence of admixtures. Phosphonate compounds have attracted attention
because of their super-retarding capability with respect to ordinary portland
cement and calcium aluminate cement hydration. [56] Phosphonate com-
pounds, aminotri (methylene-phosphonic acid) (ATMP), 1-
hydroxyethylidene-1, 1-diphosphonic acid (HEDP), and diethylene-
triaminepenta (methylene-phosphonic acid) (DTPMP), appear to be much
more efficient retarders than many others used in concrete practice. Low
dosages (0.05%) of phosphonate admixtures generally appear to have little
effect on the hydration reactions in OPC/CAC binary cement systems
associated with quick-setting phenomena, e.g., ettringite formation and
simultaneous hydration of CAC and OPC. Higher dosages of phosphonate
admixtures (0.2%) substantially reduce the rate of ettringite formation and
simultaneous hydration of CAC in OPC/CAC binary cement systems. OPC
hydration is significantly retarded. It is apparent that the phosphonate
compounds interfere with the gypsum-cement reactions especially in the
pastes containing 0.2% ATMP and HEDP resulting in portland cement
hydration (mainly C S and C S phases) retardation in an OPC/CAC
3 2
blended system.
The retarding action of phosphonates on CAC paste hydration
alone can be explained as follows. [57] Phosphonates may adsorb or form
complexes incorporating cations such as Ca and Al on the surface of
unhydrated cement particles and stabilize the very early hydration
product (CAH or C AH ) retarding further hydration. Poisoning nucle-
10 2 8
ation may also occur. The efficiency of retardation does not appear to relate
to the chelating capability of the phosphonates but rather to the spatial
+3
effects in the complex. This may be due to the small cations (Al in the form
-3
of AH or AH ) and large chelating molecules which render the larger
6 3
chelating molecule less effective. The efficiency of retardation is in the
order ATMP and HEDP > DTPMP in CAC hydration.
Lithium salts cause rapid setting and hardening of CAC and
promote very early strength development. [58] It has also been shown that the
delay of OPC hydration in OPC/CAC mixtures can be overcome by using
lithium-salt-based chemical admixtures. Table 3 lists the compressive
strength of the binary cement pastes containing 0.8% lithium salts prepared
at a water-cement ratio of 0.40. An improvement of strength is obtained in
the pastes containing LiOH at < 3 days. However, all the samples reach
about the same strength at 7 days hydration. The paste containing Li CO
2 3
had highest strength.