Page 409 - Handbook of Thermal Analysis of Construction Materials

Section 7.0 - Magnesia Phosphate Cement Binders 387

Eq. (21) 6CaHPO (2H O + 3Ca (PO ) O
4 2
4
2
4
→ 2Ca (HPO )(PO ) (OH) + (nH O)
4
4 5
2
9
where n = 1 or 13.
7.4 Lime Silico-Phosphate Cement
Quick setting silico-phosphate cements have been considered for
use as pavement patching material. These are cement compositions which
have a sufficiently high and rapidly developed early strength to support
normal highway loads within one hour after application.
The fully developed compressive strengths are equivalent to those
obtained with conventional portland cement. The silico-phosphate cement
should be compatible with conventional portland cements. This is achieved
by reacting wollastonite powder with a buffered phosphoric acid to form a
lime silico-phosphate cement having a high early set strength. A typical
phosphate solution consists of the following: 918 ml H O, 1611 ml 85%
2
H PO , 510 g Zn (PO ), 456 g AlPO , and 21 g Mg (PO ) . [69][71]
4
4
4
3
3
4 2
3
Concrete nominally contains 1 part phosphate solution, 1 part
magnesia and 4 parts dolomite. Setting usually occurs in less than 30
minutes and up to 50 MPa compressive strength is developed within 4
hours. Factors affecting strength development are the particle size of the
wollastonite, the P O content of the liquid, and the liquid/powder ratio.
5
2
X-ray diffraction analysis of this cement system gives only those
peaks attributable to wollastonite. It is suggested that an acid-crystal
surface interaction occurs, reducing the apparent wollastonite surface area
and freeing the bonding constituents for matrix development. [71] Micro-
probe analysis shows that the matrix binding wollastonite grains contains
phosphorus, calcium, silicon, and some aluminum and zinc.
Typical strength development data of air cured silico-phosphate
cement indicate that this system develops about 30% of its 28 day compres-
sive strength in 2 hours. The tensile bond strength, compared with conven-
tional concrete substrates, is very poor. [69]
A minor amount of blast-furnace fines can be added to the wollas-
tonite-sand mixture to decrease the set time of the cement. A corresponding
reduction of compressive strength also results. The use of 20% blast-
furnace fines has resulted in satisfactory properties.