Civil Engineering Reference
In-Depth Information
Mg(HPO
4
)
2
•4H
2
O, also forms initially. Minor amounts of dittmarite,
NH
4
MgPO
4
•H
2
O, and/or stercorite, NaNH
4
PO
4
•4H
2
O, were also detected.
DTA indicated the presence of quartz (endotherm at 573°C, Fig. 28b). A
double endotherm at approximately 95°C and 110°C is due to the presence
of both struvite and schertalite. The appearance of an apparent double
endotherm is due to the superposition of an exotherm at about 100°C
superimposed on a large endotherm due to dehydration. The endotherm is
due to the formation of NH
4
MgPO
4
•H
2
O. Dittmarite is responsible for the
endotherm at 248°C. The formation of Mg
2
P
2
O
7
is responsible for the
exotherm at 705°C and Mg
3
(PO
4
)
2
for the one at 923°C.
In mortars originally formed with both ADP and STPP, there is a
small endotherm around 70°C. At early times, e.g., 5 minutes, the superpo-
sition of exothermic and endothermic effects (at about 107°C) results in a
double endotherm indicating the presence of schertalite. A small exother-
mic peak above 800°C may be due to the formation of Mg(PO
4
)
2
coincident
with an endothermal effect due to the melting of Na
4
P
2
O
7
formed from
STPP.
In mortars prepared at the low water/solid ratio (1:16), a small
endotherm at around 51°C is attributed to stercorite.
Phosphate additions on the hydration of portland cement were
investigated by Ma and Brown.
[28]
There was no apparent literature on the
subject prior to their investigation. Strength development in the CaO-SiO
2
-
P
2
O
5
-H
2
O system may be due to the formation of C-S-H and C-S-P-H gels.
The contribution of C-S-H may, however, be a prime factor.
Ma and Brown reported on the mechanical properties obtained with
the addition of sodium and calcium phosphates. The portland cement and
phosphate were mixed with deionized water and then pressed at 28 MPa.
The water/solid ratio varied from 0.176-0.250 and the phosphate/cement
ratio varied from 0.10-0.42.
Calorimetric curves for portland cement (OPC) and samples con-
taining (NaPO
3
)
n
and (NaPO
3
)
n
•Na
2
O are shown in Fig. 29 for the first four
hours of hydration. Phosphate addition increased the rates of hydration and
more heat was evolved at the beginning of hydration. The (NaPO
3
)
n
•Na
2
O
had a greater effect than (NaPO
3
)
n
. At times greater than 4 hours (Fig. 30),
OPC samples exhibit a second peak and exceed the total heat developed by
the phosphate-modified cements at about 30 hours. Generally, the phos-
phate-modified cement samples had higher flexural strengths [except
(NaPO
3
)
n
•Na
2
O]. In some cases, calcium phosphate-modified cements also
give superior strength results.
