Alkali-Aggregate Reaction

Map/Pattern Cracking Resulting from an Alkali-Aggregate Reaction
Figure 1. Map/Pattern Cracking Resulting from an Alkali-Aggregate Reaction

Alkali-aggregate reaction is the expansive reaction that takes place in PCC between alkali (contained in the cement paste) and elements within an aggregate. The most common is an alkali-silica reaction. This reaction, which occurs to some extent in most PCC, can result in map or pattern cracking (see Figure 1), surface popouts and spalling if it is severe enough. The mechanism for this alkali-silica reaction proposed by Diamond is as follows (Mindess and Young, 1981[1]):

  1. Initial alkaline depolymerization and dissolution of reactive silica. Cement (a high-alkali substance) can increase the solubility of non-crystalline silica and the rate at which it dissolves. Additionally, the cement will raise the pH of the surrounding medium which will affect the crystalline silica.
  2. Formation of a hydrous alkali silicate gel. The initial dissolution of reactive silica then opens up the aggregate pore structure and allows more silica to dissolve into solution. The end result is alkali-silica gel that is formed in place. This gel formation is not expansive itself but it does destroy the integrity of the aggregate particle.
  3. Attraction of water by the gel. The gel attracts considerable amounts of water and expands. If the expansion is great enough, the resulting stress will crack the now-weakened aggregate and surrounding cement paste.
  4. Formation of a gel colloid. After the gel ingests enough water, the water takes over and the substance becomes an alkali-silica gel disbursed in a water fluid. This fluid then escapes to surrounding cracks and voids and may partake in secondary reactions.

This reaction can be controlled by:

  • Avoiding susceptible aggregates. Local experience may show that certain types of rock contain reactive silica. Typically rock types that may be susceptible are: siliceous limestone, chert, shale, volcanic glass, synthetic glass, sandstone, opaline rocks and quartzite. River rock is also typically susceptible.
  • Pozzolanic admixture. By reacting with the calcium hydroxide in the cement paste, a pozzolan can lower the pH of the pore solution. Additionally, the silica contained in a pozzolan may react with the alkali in the cement. This reaction is not harmful because it essentially skips the expansive water attraction step.
  • Low-alkali cement. Less alkali available for reaction will limit gel formation.
  • Low water-cement ratio. The lower the water-cement ratio, the less permeable the concrete. Low permeability will help limit the supply of water to the alkali-silica gel.

In sum, alkali-silica reactions are expansive in nature and occur in most PCC. If the reaction is severe enough it can fracture aggregates and surrounding paste resulting in cracking, popouts and spalling. There are several ways of avoiding this reaction, the simplest of which is just avoiding susceptible aggregate.

Footnotes    (↵ returns to text)
  1. Mindess, S. and Young, J.F.  (1981).  Concrete.  Prentice-Hall, Inc.  Englewood Cliffs, NJ.