Page 177 - Handbook of Thermal Analysis of Construction Materials

160 Chapter 4 - Introduction to Concrete Admixtures

Table 4. Setting Times of Concrete Containing Calcium Chloride and
Calcium Nitrite

Cement Brand Admixture Setting Time (hr: min)
Initial Final

A ----- 8:45 12:21
A Calcium Chloride 4:20 7:30
A Calcium Nitrite 6:00 10:20
B ----- 8:38 ---
B Calcium Chloride 3:16 5:00
B Calcium Nitrite 5:24 9:04

(Reprinted with permission from ASTM.)

Table 5. Effect of Calcium Nitrite on Strength Development

2
Admixture % Compressive Strength (lbs/in )
1 Day 7 Days 28 Days

----- 1299 3401 5032
2 1615 4538 5732
3 1959 4964 5907
4 2294 5337 6383
5 2367 5324 6493

(Reprinted with permission from ASTM.)

The relative acceleratory effects of calcium chloride, calcium
nitrate, and sodium thiocyanate on cement have been studied. [61] The total
heat is higher in the presence of all accelerators. Calcium chloride addition
results in the maximum amount of heat. Sodium thiocyanate accelerates
better than calcium nitrate. Calcium nitrate works well at early stages, but
sodium thiocyanate is more effective at later ages. Sodium molybdate and
trioxide also have an acceleratory effect. [62][63]
Generally, the α hydroxycarboxylic acids are regarded as retard-
ers, but lactic acid behaves differently. [64] In dosages of 0.1 to 0.4%, it
reduces both the initial and final setting times of cement. The compres-
sive strength at 28 days is higher than the reference by about 10–15%.