Among the shotcrete mixture additives is the hardening accelerator, which is added through an additive pump, and is synchronized with the concrete flow.
Hardening accelerators may be alkaline, or not, which is commonly known as alkali-free.
In 1960, the first alkaline aluminate accelerators appeared. These accelerators, which are normally liquids, are added at a rate of 3 to 6% of the binder content in the mix.
Later, in 1980, the first non-alkali (also called alkali-free) aluminum sulphate accelerators reached the market. The dosage of the accelerator was increased from 4 to 7% of the binder content.
Characteristics and advantages of alkali-free additives
Alkali-free accelerators are becoming increasingly popular in shotcrete mixtures worldwide.
They affect shotcrete projection through two of their chemical components:
- Alkalinity, which refers to its basicity or PH value. For human tissue, a safe PH is between 3 and 8. Alkali-free additives are PH 3, compared to the PH 13 in alkaline additives; therefore they are much safer for the operator when handling them.
- They contain alkali ions, such as sodium and potassium, which affect the shotcrete mixture’s properties. Alkali-free additives, which are lower in ion content, have a lower effect on ultimate strength and durability.
Other advantages of the alkali-free additives are that they improve the concrete’s adhesion to the substrate, decrease rebound, and facilitate vertical projection.
They are also more environmentally friendly and are not corrosive. Like the alkalis, they are dosed accurately and improve the shotcrete’s early resistance.
Is it more expensive to use shotcrete mixtures that are alkali-free?
Alkali-free accelerators are more expensive than alkaline, which leads to the belief that the total cost of the mix is more expensive.
However, keep in mind that accelerator dosing is done according to the amount of cement in the mix.
When using an alkali-free additive, less cement is required to obtain the same final resistance as the alkaline additives. Therefore, the extra cost of the alkali-free additive is offset by the lower cost of the cement, thus the total cost of the mixture is almost the same.
Would you like to know more about the composition of shotcrete mixture? In this infographic you will find all the components.