Alkalis are soluble bases, meaning they possess a pH higher than 7. They are often used to dissolve fats and are found in many cleaning products.
But how does this affect you as a precaster? Potassium and sodium oxides are minor constituents in cement clinker, but are substantial sources of alkalis in cement. This means alkalis are also found in precast concrete, and can have a significant impact on the concrete’s performance. Keeping an eye on alkali levels and any fluctuations helps offset potential issues in precast production.
Precasters can refer to cement mill certificates to determine alkali levels in their cement.
To determine how much alkali is in your cement, refer to your cement mill certificate. Alkali content will most likely be noted as Na2Oeq, which is an alkali equivalent based on potassium (K2O) and sodium (Na2O) oxides. More information on reading and recording data from cement mill certificates is available in our two-part Precast Inc. series.
Generally, studies indicate concrete will experience an increase in initial hydration with alkali content. This may differ based on the nature of the alkalis (whether they are hydroxides or sulfates). If you’re seeing inconsistencies with set times, check your alkali levels.
Precasters using air-entraining admixtures should track the alkali content in cement. As alkali levels increase, air-entraining admixtures generally produce more air, and vice versa. The higher your alkali content, the more impact even slight changes will have on the effectiveness of the admixture.
Additionally, for precasters who use supplementary cementitious materials – especially class F fly ash – alkali levels can impact the reactivity of the ash. Because alkalis in the cement tend to raise the pH of the mix water, the reactive glasses in the ash are attacked, creating additional chemical reactions that may increase strength and lower permeability.
If alkali-silica reactivity of aggregates is a concern, changes in alkali content must also be carefully tracked.
An essential goal of a thorough quality control program is to reduce variability. This requires tracking certain key indicators, including alkali levels.
References
Sprung, S., and W. Rechenberg. “Influence of Alkalis on the Hydration of Cement.” Effects of Alkalis on Properties of Concrete, Proceedings of a Symposium held in London. 1976.
Osbaeck, B. (1983). Alkalis and Cement Strength. In Proceedings of the 6th International Conference on Alkalis in Concrete (pp. 93-100).
Johansen, V. (1976, September). Influence of Alkalis on the Strength Development of Cements. In Proceedings of the Symposium on the Effects of Alkalis on Properties of Concrete, London (p. 81).
I did not think that alkali could have any affect on concrete. However, it makes sense that during the actual process of precasting, the improper ph could result in potential integrity issues in the finished product. I will have to do some research on concrete companies to make sure they are aware of this before hiring anyone. Thank you for the information.