Page 408 - Handbook of Thermal Analysis of Construction Materials

386 Chapter 9 - Non-Portland Cement Binders and Concrete

Factors affecting the compressive strength include aggregate type
and curing environment. Sand aggregate, in lieu of dolomite, significantly
reduces early strength (approximately 25% at 7 days); the 28 days strengths
are unaffected. [69] Moist curing results in a significant decrease in strength
(25% after 24 hours).

7.3 Calcium Phosphate-Based Materials

Calcium phosphate-based systems have wide applications in bio-
medical areas. Brown has outlined the similarities between the hydration of
calcium silicates and calcium phosphates. [70] The hydration products in
both systems have high surface areas, variable composition, and poor
crystallinity. Pozzolanic reactions and Hadley-like grains form in both
systems. The primary cement-water reactions for C S and tetracalcium
3
phosphate are as follows:

Eq. (18) 3CaO•SiO + ~ 5.3 H O
2
2
→ ~ 1.7 CaO•SiO •4H O + ~ 1.3 Ca(OH) 2
2
2
Eq. (19) 3[4CaO•P O ] + 3H O
2
5
2
→ Ca (PO ) (OH) + 2Ca(OH) 2
4 6
2
10
The reaction products in the phosphate system are stoichiometric
hydroxyapetite (HAp) and calcium hydroxide (CH).
The above reactions are specific to one composition of C-S-H or
HAp. It is known that C-S-H can exist over a range of compositions (c/s
extending from 0.83 to 2.0). The Ca/P ratio varies from ~ 1.40 to 1.84.
In the pozzolanic reaction analog, tetracalcium phosphate is the
source of Ca(OH) . A variety of acidic calcium phosphates can be consid-
2
ered as the SiO analog. The analogy is extended by the following reaction:
2
Eq. (20) 6CaHPO (2H O) + 4Ca(OH) → Ca (PO ) (OH) + (nH O)
10
4 6
2
2
2
2
4
where n = 5 or 18.
The hydrolysis reactions of tetracalcium phosphate are coupled
and solid CH actually does not form. Calcium deficient HAp then forms
through the following reaction: