Page 407 - Handbook of Thermal Analysis of Construction Materials

Section 7.0 - Magnesia Phosphate Cement Binders 385

7.2 Additives

Additives such as sodium tetraborate decahydrate (borax), and
ammonium pentaborate can be used to control the setting time and rate of
strength development of phosphate cement concrete made with ammonium
phosphate solutions. [68] The early strength is reduced with the addition of
1% borax and the setting time is increased from 9.5 minutes to 15 minutes.
The effects of various additives on the setting times of the phosphate cement
are given in Table 4. [68] The compressive strengths at 28 days for phosphate
cement incorporating some additives are given in Table 5. [68]

Table 4. Effect of Additives on Setting Times of Phosphate Cement a

Setting time (min)
Concentration (%)
Additive 0 0.5 1.0 1.5 2.0 3.0

Borax 9.5 15.0 23.0 30.0
Ammonium pentaborate 7.5 14.5 22.0
Boric acid 7.5 12.5
Trimethylborate 7.5 13.5

a
Typical mix contains 1335 parts sand, 150 parts fly ash, 495 parts MgO, 198 parts
ammonium phosphate and 150 parts water.

Table 5. Effect of Additives on 28 day Compressive Strength

28 day Compressive Strength (MPa)
Concentration (%)
Additive 0 0.5 1.0 1.5 2.0 3.0

Borax 55.5 60.0 50.0 45.1
Ammonium pentaborate 65.0 69.3 63.5
Boric acid 40.4 52.3