DTA Characteristics of Ancient Mortars
From the DTA results of historic mortars there are several distinct features that regularly occur, and can be classified into four broad groups although this does not mean that all mortars with a similar profile are the same. Mortars from different sources have their own unique profile, albeit that some of the differences are trivial. These groups could be further sub-divided into additional groups.
The broad classifications are:
Group 1
The main distinguishing feature in the thermograms of mortars that fall within this group is that there is no evidence of endothermic reactions seen in the temperature range between 800C to 3000C.
This is clearly evident in the 3 examples shown in Figures 1.1 to 1.3:
Figure 1.1 1642, The Cage, Lyme Park.
Figure 1.2 1657, Mells Manor.
Figure 1.3 1639, Burgate Manor.
The calcium and soluble silica test results for these examples are:
Test Figure 1.1 Figure 1.2 Figure 1.3
The Cage Mells Manor Burgate Manor
CaO% 31.1 42.2 10.7
SiO2% 0.7 1.4 1.1
The DTA shows that calcium silicate hydrates or complex sulphate salts are absent but significant soluble silica results could suggest some evidence of hydraulicity during the history of these mortars. Microscopic examination of 1642 and 1657 showed that ash and clay are present, and this is shown by the exothermic activity in the thermograms. These profiles include non-hydraulic and aged (possibly weakly hydraulic) lime mortars.
Group 2
In this group there is clear evidence of a single endothermic peak in the region between 900 C and 2000 C. From the data it is clear that the single peak occurs either around 1000C to 1250C or between 1500C to 1800C.
Typical examples are shown in Figures 2.1 to 2.3:
Figure 2.1 1611, Woodchester Mansion.
Figure 2.2 1718, St. Peter’s, Exton.
Figure 2.3 1607, Queen Anne’s Gate.
The chemical test results for these examples are:
Test Figure 2.1 Figure 2.2 Figure 2.3
Woodchester St Peters Queen Anne
CaO% 43.9 28.0 35.0
SiO2% 2.0 1.4 1.1
SO3% <0.1 0.1 0.8
The peak observed at 1000C to 1300C is only seen in samples with a significant water and soluble silica content. The evidence therefore suggests that this could be CSH but it is also possible that there is a high clay content and this is physically bound water within the clay. Likewise there is a clear correlation between the endothermic peak seen between 1500C and 1800C and the concentration of sulphate measured in the sample. It is not clear whether this peak is the hemihydrate salt of calcium, a more complex salt, or a poorly defined gypsum peak. Comparing the data with Group 1 mortars, it is clear that for accurate interpretation of the results, further analysis such as XRD or SEM may resolve the identities of the compounds detected.
Group 3
Group 3 differs from Group 2 in that there is evidence of a more complex hydrate structure as two or more endothermic peaks are observed in the temperature range between 1000C and 3000C. These peaks indicate the remnants of compounds of hydrated calcium silicates and aluminates, but these are not distinguished in this broad classification. This can be seen in the examples in Figures 3.1 to 3.3.
Figure 3.1 1594, Greys Bridge.
Figure 3.2 1605, Winchester Castle Hill.
Figure 3.3 1725, Warminster Bedding.
The chemical test results for these examples are:
Test Figure 3.1 Figure 3.2 Figure 3.3
Greys Bridge Winchester Warminster
CaO% 8.5 19.2 17.1
SiO2% 3.7 6.4 2.9
The mortars that fall within this category include Portland and early cements, natural cements and hydraulic limes, and lime/pozzolana mortars.
The sample from Greys Bridge (Figure 3.1) is a typical aged Portland cement, the sample from Winchester Castle Hill (Figure 3.2) is of a type known as ‘Roman Cement’, and the Warminster sample (Figure 3.3) is a lime mortar containing brick dust which has clearly reacted.
Group 4
The mortars that are categorised in this Group clearly contain calcium sulphate dihydrate or a proportion of calcium hemi/dihydrate sulphate in the samples. The concentration of sulphate (SO3) ranges from <1.0% to 45%. These are gypsum mortars and gypsum:lime hybrids.
This can be seen in the 3 examples in Figures 4.1 to 4.3:
Figure 4.1 1676, Somerset House.
Figure 4.2 1717, Stoneleigh Abbey.
Figure 4.3 1626, Hampton Court Palace.
The chemical test results for these examples are:
Test Figure 4.1 Figure 4.2 Figure 4.3
Somerset House Stoneleigh Abbey Hampton Court
CaO% 22.0 50.0 23.8
SiO2% 0.7 0.2 1.4
SO3% 32.4 25.8 3.6
This group is distinguished from Group 2 in that there is a distinct double endothermic peak around 1600 C to 1850C that can be assigned to the calcium sulphate dihydrate/hemihydrate. The samples that contain less than 5% sulphate (SO3) are of more academic interest, and often indicate the results of polluted environments.
