Stokesay Castle

Mortar Analysis

Figure 1. The particle, centre left, contains belite (C2S). The surrounding paste is mainly C-S-H and ettringite (e); ettringite has also formed within the pore at upper right.


Figure 2. X-ray spectrum of belite crystal in Fig 1. Ca/Si ratio was 2.0.


Figure 3. X-ray spectrum of C-S-H in Fig 1. Note the small peak due to chlorine.


Figure 4. X-ray spectrum of paste in Fig 1. Mainly C-S-H; also some feldspar, minerals containing Al, Si, Fe in varying proportions, and some iron-rich particles.

It is now accepted that a detailed understanding of existing mortars is essential in informing and assisting the specification of replacement or repair materials. The great majority of historic mortars were based on lime, but traditional limes varied depending on many factors including the particular limestone burnt, kiln design, fuel type and kiln temperature. Binder: aggregate ratios vary and pozzolanic reactions may have occurred, or deleterious processes taken place. Aggregate type and grading, fines proportion and void ratio also affect performance. The picture becomes complex from 1780 with the development of Roman cement, artificial hydraulic limes, early cements and eventually Portland cement by about 1840.

We can assess the hydraulicity and identify the constituents and proportions of all mortars, plasters and renders. Non-hydraulic lime; hydraulic lime; natural cements; roman cements; early cements and Portland cement; cement/lime blends; gypsum and gypsum/lime blends are all determined.

Our Standard Analysis Procedure comprises:

  • Microscopic evaluation, laboratory observations, and reaction comments.
    As-received moisture content and phenolphthalein carbonation test.
  • Chemical 'dissolution' tests in accordance with BS4551:2005 +A2:2013 to determine:
    Proportions of acid-soluble calcium, magnesium, iron, sulphur, aluminium, acid and alkali soluble silica, and acid insolubles.
    Identification of insoluble sands, aggregates and additives.
    Determination of acid-soluble calcareous aggregate.
    Dry-sieving to determine fines proportion and microscopic examination of insoluble residue
  • Report/ Interpretation of results to determine:
    Binder and aggregate type.
    Hydraulicity of binder and evidence of pozzolanic reactions.
    Calculation of original volumetric mix proportions.
    Suggestions for a matching repair or replacement mortar mix recipe on a 'like-for-like' basis.
    Possible explanations for failure, poor performance or deleterious reactions (if applicable)

This Standard Procedure gives sufficient information on the vast majority of samples. Further additional instrumental techniques - SEM (Scanning Electron Microscopy), X-ray microanalysis and DTA (Differential Thermal Analysis) can be employed if necessary.

Water-soluble salts analysis can also be undertaken.

A representative sample of at least 40g is required, and contextual information is always useful. Orders and cheques should be made out to Rose of Jericho Ltd and samples sent to Horchester Farm. Mortar analysis costs £265 + VAT per sample and normally takes three to four weeks. Please note that for example two-coat plaster is two samples if both coats are to be tested.

Recent projects include the analysis of roughcast render samples from the late 13th century Stokesay Castle.