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Petrographic examination of mortar and render

Determination of mix proportions

Cement and aggregate contents of render and some mortars can be determined petrographically by point counting of thin sections using a petrological microscope. Where the mortar is particularly friable and weak it may be appropriate to test for cement content in accordance with BS 4551. Lime contents cannot be determined petrographically. However using petrographic techniques it is possible to determine the quantity of limestone in the aggregate and this figure can be used to correct the results of conventional BS 4551 testing for the contribution of the aggregate to acid-soluble CaO. Cement replacement materials such as GGBS and PFA can be readily identified petrographically and can be quantified by chemical analysis with the electron microprobe.

Deterioration of mortar

The most common form of chemical attack of mortar is sulphate attack and this commonly results from the migration of sulphate compounds from clay bricks in to the matrix of the mortar resulting in the expansive formation of compounds such as ettringite and gypsum. In extreme cases, the mortar may expand to such an extent that it protrudes from the face of the brickwork. Petrographic analysis can be used to test for the early stages of sulphate attack.

Deterioration of render

The delamination and spalling of render can result from a wide range of causes that include:

  • The penetration of moisture along the interface between the render and the substrate
  • Problems with the bond between the substrate and render
  • Shrinkage
  • Incompatibility between the individual layers of the render system
  • Frost damage
  • The build up of sulphate compounds at the interface between the render and the substrate

Petrographic techniques can be applied to investigate the various possibilities.