Accelerating admixtures for concrete
TABLE OF CONTENTS
1 Introduction ………………………………………………………………………………………4
2 Background and accelerators in general………………………………………………4
2.1 Classification of accelerators………………………………………………4
2.2 Benefits provided by accelerators………………………………………………6
2.3 Consumption of accelerators in Norway……………………………………..6
3 Chemical overview of accelerators………………………………….9
4 Calcium chloride……………………………………………………………….10
5 Calcium nitrate and other nitrate/nitrite salts……………………………11
6 Thiocyanates……………………………………………………………………..13
7 Alkali carbonates …………………………………………………………..14
8 Carboxylic acids and their salts……………………………………………………..15
9 Alkanolamines…………………………………………………………………..17
10 Antifreeze admixtures ………………………………………………………….20
11 Shotcrete accelerators…………………………………………22
12 Commercial accelerators for normal concrete……………………………..25
13 Commercial shotcrete accelerators……………………………..25
14 Accelerators for calcium aluminate cements……………………………….28
15 Recommendations for future R&D………………………………….28
16 Conclusions ………………………………………………………..30
17 References ………………………………………………….31
Among the first group, calcium formate is a well known accelerator, while triethanolamine (TEA) is the most common among the alkanolamines. Alkanolamines are typically used as ingredients in accelerator blends, and rarely, if ever, as a sole ingredient. Probably, the most common use of TEA is to compensate for the retarding effect of some plasticizers.
Accelerating properties have been reported for the lower monocarboxylic acids (C1 to C4) and their salts (formic, acetic, propionic and butyric), while higher carboxylic acid homologes (>C4) are retarders [5]. It seems that those belonging to C1 are the most efficient ones, and that the calcium salt of formic acid, calcium formate (Ca(HCOO) 2 ), are the most common. Dodson [8] began research on Ca(HCOO) 2 in early 1960s and found that although Ca(HCOO) 2 was not as effective as CaCl 2 , it does produce satisfactory increases in early strength as well as that of later ages. According to Dodson [8] Ca(HCOO) 2 also accelerates setting of OPC. Heikal [34] found that the initial and final setting times of both OPC and pozzolanic cement pastes are shortened with the addition of Ca(HCOO) 2
A lot of potential chemicals have already been tested as accelerators for concrete. Future R&D efforts should focus on, in a systematic way, synergistic effects occurring when several chemicals are mixed. For instance, a synergistic effect is reported with a mixture of TEA and calcium formate
Similar approach can be utilized for developing soluble non-alkali salts of carboxylic acids, e.g. formic and oxalic acids. The effect of non-calcium formate on the hydration of OPC with different C 3 A/SO 3 ratios should be compared to the C 3 A/SO 3 sensitivity reported for calcium formate.
In Europe calcium nitrate has replaced calcium chloride as a setting accelerator, while sodium thiocyanate has replaced calcium chloride as hardening accelerator. In North America calcium chloride is partly replaced by calcium and sodium nitrite and calcium formate (in powder form). Alkali shotcrete accelerators have partly replaced the conventional alkali silicates and aluminates, particularly in Northern part of Europe.