What is alkali content?
The alkali content refers to the content of Na2O and K2O in cement. The condition for concrete to conduct the alkali-aggregate reaction is that the cement must contain alkali. When the active aggregate material is used, the cement with low alkali content should be used.
What is the percentage of alkali in OPC?
3. ALKALI COMPOUNDS. The alkalis, sodium and potassium (Na2O , K2O), in Portland cement clinker are derived mainly from the clay components present in the raw mix and coal; their total amount, expressed as Na2Oe equivalent (Na2O + 0.658K2O), contain no more than 0.60 percent equivalent alkali.
What are alkali in cement?
The alkali–silica reaction (ASR), more commonly known as “concrete cancer”, is a deleterious swelling reaction that occurs over time in concrete between the highly alkaline cement paste and the reactive amorphous (i.e., non-crystalline) silica found in many common aggregates, given sufficient moisture.
Is sand a reactivity?
One control used manufactured sand as the sole aggregate, which is a commonly used fine aggregate and typically not reactive (in terms of ASR).
Why alkali is used in cement?
An increase in the alkali content of cement delayed the time of final setting, reduced the compressive strength, and increased the rapid chloride ion permeability, drying shrinkage and volume of permeable voids of HPCM.
What is low alkali?
Low-alkali cements are portland cements with a total content of alkalies not above 0.6 percent. These are used in concrete made with certain types of aggregates that contain a form of silica that reacts with alkalies to cause an expansion that can disrupt a concrete.
Is concrete an alkali?
In most concrete, aggregates are more or less chemically inert. However, some aggregates react with the alkali hydroxides in concrete, causing expansion and cracking over a period of many years. This alkali-aggregate reaction has two forms: alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR).
What is alkaline silicate?
Finally, 29Si NMR quantifies these various entities. In this study, various sodium and/or potassium alkali-silicate solutions were investigated, as referenced in Table 1. The main parameters studied are the type of cation and the Si/M molar ratio with M = K and/or Na (0.5, 1.0 and 1.7).
Is alkali basic?
Alkali is a base. It is a base that dissolves in water. It is a basic salt alkali earth metal or alkali metal. On adding alkali to acid the pH of the mixture increase.
What is the composition of sand?
The most common component of sand is silicon dioxide in the form of quartz. The Earth’s landmasses are made up of rocks and minerals, including quartz, feldspar and mica. Weathering processes — such as wind, rain and freezing/thawing cycles — break down these rocks and minerals into smaller grains.
What are the properties of an alkaline soil?
Alkali, or Alkaline, soils are clay soils with high pH (> 8.5), a poor soil structure and a low infiltration capacity. Often they have a hard calcareous layer at 0.5 to 1 metre depth. Alkali soils owe their unfavorable physico-chemical properties mainly to the dominating presence of sodium carbonate,…
What does the alkali content of cement mean?
The alkali content refers to the content of Na 2 O and K 2 O in cement. The condition for concrete to conduct the alkali-aggregate reaction is that the cement must contain alkali. When the active aggregate material is used, the cement with low alkali content should be used.
What causes an increase in alkali in concrete?
In the areas of cold weather, de-icing salt containing sodium compounds which may increase the alkali content on the surface layer of concrete. Soils containing alkali may also increase alkali content on the surface of concrete.
What happens to alkalis with high water content?
High water content will lead to the hydrolysis of AGs to FFAs, followed by saponification of the FFAs and thus reduce the concentration of the alkali ( Komers et al., 2001 ). For low FFA waste oils, alkali catalyst still exhibits good performance and can attain high BD yield.