Alkalis are soluble bases, which means they have a pH greater than 7. They are commonly used to dissolve fats and can be found in various cleaning solutions.
But what impact does this have on precast concrete? Although potassium and sodium oxides are minimal in cement clinker, they are essential sources of alkalis in the finished product. This means that alkalis may be present in precast concrete as well, and they can substantially influence its performance. It is good practice to keep an eye on alkali levels and any variations to mitigate potential problems in precast manufacturing.
Establishing alkali content
You can find out how much alkali is in your cement by looking at the certificate from the cement plant. The presence of alkali will most often be indicated by the symbol Na2Oeq, which is an alkali equivalent based on the oxides of potassium (K2O) and sodium (Na2O).
Generally speaking, studies have found that alkali concentration increases the early hydration of concrete, which is good news. The type of the alkalis may impact this outcome (whether they are hydroxides or sulfates). Examine your alkaline levels if you notice irregularities with your set times.
Keep an eye on the alkali levels
Pre-casters who use air-entraining admixtures should keep track of the amount of alkali in the cement they use. Increases in alkali levels result in a rise in the production of air-entraining admixtures and vice versa. In general, the higher the alkali content of your admixture, the more significant the influence of even minor alterations on its efficacy.
Alkali levels can also influence the reactivity of additional cementitious materials, particularly class F fly-ash, which is used by pre-casters in supplementary cementitious materials. Because alkalis in cement often elevate the mix water’s pH, the reactive glasses in the ash are affected, resulting in further chemical reactions that may boost strength while decreasing permeability in the concrete.
It’s all about quality control
An essential aim of an extensive quality control program is to eliminate variability in test results. This necessitates the monitoring of certain vital variables, such as alkali levels. Closely watch variations in alkali concentration in aggregates when alkali-silica reactivity is a concern.
An alkali-silica reaction (ASR) in concrete might take years to manifest. However, it splits your concrete once it does, inflicting considerable damage. This harmful swelling response occurs over time and is aptly dubbed “concrete cancer.” It is an example of an alkali-aggregate reaction.
What causes ASR?
Three major factors cause ASR. These are:
- Reactive Aggregates
- High pH levels
- Water presence
How can one prevent ASR?
Non-reactive aggregates are one alternative. However, finding non-reactive aggregates and transporting them might be inconvenient and costly.
Alkali Loading: Instead, you can reduce the alkali loading to target the pH level. Because alkalis are derived from cement, your best bet is to use less cement by reconfiguring your combination. You may also use cement with a reduced alkali concentration, although this depends on how much cement you need for the job.
SCMs (Supplementary Cementitious Material): Instead of cement, use SCM. This means less cement, and less cement translates into less alkali. They are also capable of binding alkalis in their hydration products, lowering the pH of concrete, and can be found in the following contexts:
- Slag cement
- Silica fume
- Fly ash
Always use class F fly ash instead of class C fly ash. Class F fly ash has been found to lower pH by interacting with calcium hydroxide. In contrast, class C fly ash has been reported to aggravate ASR in certain situations.
Reduce permeability: Here are a few strategies for lowering the permeability of your water:
- reduce the water-to-cement ratio
- excellent consolidation
- good curing
- employing SCMs
Another method is to utilize a lithium-based admixture, which makes the ASR gel non-expansive.
After construction, it might take up to ten years for ASR symptoms to start showing. However, concrete containing highly reactive aggregate, which is subjected to a hot and humid climate, may exhibit evidence of cracking within two years of construction.
For any further questions on alkalis in precast concrete, reach out to our team of experts at TKL Group.