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Evaluation of mortar samples obtained from UK houses treated for rising damp

Dr Eric Rirsch

Safeguard Europe Ltd, Redkiln Close, Horsham, West Sussex, RH13 5QL, UK

James MacMullen, Dr Zhongyi Zhang

Advanced Polymer and Composites (APC) Research Group, Department of Mechanical and Design Engineering, University of Portsmouth, Portsmouth, Hampshire, PO1 3DJ, UK


Mortar samples were obtained from a variety of dwellings in the UK with the majority from houses with rising damp. This paper aims to evaluate the mortar attributes and their influence on rising damp. The samples were characterised in the laboratory in terms of pH value and water absorption characteristics and examined using scanning electron microscopy to reveal the microstructure. It was found that the water absorption characteristics varied considerably, with older mortars having a higher sorptivity and higher concentration of soluble salts. The majority of building mortars treated for rising damp in this survey were approximately 100 years old and had a typical pH value of 9. A good understanding of the relationship between rising damp and mortar characteristics was developed, which may be practically employed to assess and mitigate rising damp problems.

1. Introduction

Rising damp describes the movement of moisture upwards through permeable building materials by capillary action. The dampness and associated salt leaching can cause aesthetic degradation and/or structural damage to exterior building façades. Serious rising damp can lead to a building becoming inhospitable due to mould growth, paint blistering, plaster crumbling and wallpaper separation [1]. It is a vexatious and persistent problem requiring a great deal of effort and financial resources in addressing its manifestation, typically with varying degrees of success. In addition, thermal insulation properties for bricks and mortar are generally considered to be attributed to the porosity of the building materials. When water is absorbed, the effectiveness of heat retention and thermal insulation of building materials is significantly reduced due to the considerably higher heat capacity and thermal conductivity of water compared to air [2,3].

Mortar is a paste used to bind construction blocks together and fill the gaps. It becomes hard on setting, resulting in a rigid aggregate structure. Mortar is typically made from a mixture of sand, a binder such as cement or lime, and water. Rising damp in buildings is dictated by the nature of the particular brick and mortar. It has been widely recognised that characteristics of the mortar have a strong influence on rising damp problems and solutions [4]. Therefore, an understanding of the characteristics of mortar is of importance and significance in combatting rising damp problems in existing buildings. According to the Sharp Front model, the height of rise of the damp front is governed by the following equation [5];

Rising damp equation

Where H is the height of the rising damp front, s the sorptivity (the suction of water into the mortar), b the wall thickness, e the rate of evaporation per unit area of the wetted surface, and θ is the moisture content of the wetted region (the volume of water per unit volume material).

As can be seen in Equation 1 (above), when the sorptivity of the material and the thickness of the wall increases, so does the height of rise. With consideration of the Sharp Front model variables, it was decided that a range of mortar samples characteristic of the types found when treating dwellings against rising damp would be tested. It is known from the Sharp Front model that sorptivity is a key variable that needs to be considered when investigating rising damp. The sorptivity is a measure of the suction or absorption of water into the material and has a strong influence on the height of the rising damp front. Additionally, the pH of the mortar is an important parameter regarding the subsequent risingdamp treatment of the wall and this was also investigated in the study.

2. Experimental

All samples obtained for this investigation are listed in Table 1. Fig. 1 shows the different locations in the UK where the mortar samples were sourced. It should be noted that it is not always possible to obtain a sample piece of mortar from the wall jointing. In some instances, only drilling dust or render pieces were available for this study.

An aqueous suspension of each mortar was produced by first crushing 5 g of mortar and dispersing the powder in 50 g of de-ionised water. The suspension was then allowed to stand for 24 h and the pH measured (Hanna Instruments; pH 209). The different suspensions from the pH study were then filtered and analysed for water soluble salts by Inductively Coupled Plasma (ICP) analysis.

The water uptake of the samples was determined by the sorptivity, which is defined as the gradient of the graph of volume of water absorbed per unit area with regard to the square root of time.

Sorptivity measurements were made by placing pieces of dried mortar in contact with water on the lower surface only. The mortar was dried at 55°C to constant weight prior to this measurement. The weight increase as a function of time was then measured and a plot of the volume of water absorbed per unit area was produced. The test was carried out using flat pieces of mortar with dimensions of approximately 2 cm × 2 cm as they were the largest pieces that could be collected (it was difficult to make sorptivity measurements on smaller sized samples).

The pore structure was assessed by scanning electron microscopy (SEM) (JEOL; JSM 6100). All specimens were sputtered with approximately 10 nm thick layers of gold and palladium consecutively prior to examination.

Locations of mortar samples taken in the UK

Fig 1. Location map of mortar samples in the UK

List of mortar samples

Table 1. A list of mortar samples collected for this study

3. Results

3.1 pH value measurement

The pH results in Fig. 2 show a reduction in the pH value of the mortar with an increase of the age of the property. The pH eventually reaches a value of approximately 9. The pH values exhibit a large degree of scatter between samples. This could be attributed to differences in local materials used for house building and differences in the extent of carbonation of the cement and lime-based mortars. The general reduction of pH with time can be attributed to carbonation [6,7].

3.2 Soluble salts

A number of samples were analysed in order to develop an understanding of the types of soluble ions present and the extent of variation as listed in Table 2. There is quite a wide variation in the salt levels of the different samples. Mortar samples No. 4 and No. 15 have low levels of below 500 mg/l whereas samples No. 1 and No. 17 show values of over 2000 mg/l. The most abundant soluble ions were calcium, potassium, sodium, chloride and sulphate.

Looking at the relationship with time, there is a general tendency for salt levels to increase with the age of the property as shown in Figs. 3 and 4, indicating an increase in soluble calcium and total soluble salts. The results are plotted here as a set of spot measurements of data points.

An interesting observation is the high levels of salts found in sample No. 17. This was taken from a property near the coast and high levels of magnesium, sodium, sulphate and chloride were found – all abundant in seawater [8,9]. It has been observed elsewhere that increased salt levels are found in-land from the effects of a marine climate [10]. All of these parameters indicate that rising damp is a complex phenomenon which is affected by the component materials, their compositions and the geographical location of the structure [11–13].


3.3 Sorptivity

The sorptivity is a dictating factor in rising damp and it is determined by the gradient of the graph in Fig. 5 for all collected samples.

Fig.5 (Sorptivity)

It can be seen that the general trend is a linear progression during the first part of the graph and then a deceleration of water gain corresponding to the sample reaching saturation.

There is a wide difference in the water absorption rates of the different samples tested. The greatest rate was found in sample No. 21, a 160 years old mortar from a stone wall in Harrogate. Much lower sorptivity was found with some of the other samples, e.g. No. 0. This modern mortar consisted of a 5:1 ratio of building sand and ordinary Portland cement respectively, with a plasticiser which is typically used when laying bricks according to modern practice. The trend is that the older mortars generally show more rapid absorption of water, although there are some exceptions.

Measurements were also made during the test of the total amount of water absorbed in 24 h as shown in Table 3. The older mortars generally have greater values of 24 h water absorption, giving up to 25% weight gain. Measurements of density gave values of 1.9 to 2.1 g/cm3 for the mortar samples. Taking a mean density value of 2.0 g/cm3 gives a resulting range of volume porosity of 5%–50%.

Water absorbing properties of some mortars

3.4 Microscopy

The mortar microstructures observed by SEM for pore comparison are shown in Fig. 6 at magnification factor of 50 times. The following structural features can be observed:

Firstly the lower sorptivity newer mortars have a noticeable degree of internal porosity. This is shown in samples No. 0 and No. 15, representing a new mortar made at 5:1 sand cement ratio with a plasticiser and a 15 years old cement mortar respectively. Despite the visible porosity in the fractured section, the sorptivity of these two mortars was low.

Secondly, the higher sorptivity mortars were samples Nos. 21, 12, H and 8. These were taken from older buildings and therefore may be lime-based. Mortar No. 21, which demonstrated the highest sorptivity, shows a loosely held open structure. It is not immediately clear why the sorptivity should be significantly greater than, say, No. 0. At 500 times magnification as shown in Fig. 7, there is evidence of a more open structure with broad fissures or channels which may facilitate the flow of water.

SEM images of mortars at 50× magnification SEM images at 500× magnification

4. Discussion

The sorptivity results show a wide range of values from different mortars. In the extreme cases the sorptivity values varied from 0.2 to 14.0 mm min. If these values are put into Eq. (1) to predict the height of rise, the values for the minimum, typical and average cases can be determined as shown in Table 4. If a typical sorptivity value for an old mortar is 4 mm min, the predicted height of rise of the damp front is approximately 1.6 and 2.6 m for walls of thicknesses of 110 and 300 mm respectively.

Various mortar mixtures are used to replicate old mortars in an attempt to test the efficacy of rising damp remedial treatments. There are at least four types which are generally used for this purpose. Examples of these are the UK Agrément MOAT No. 39: 1988 and the German WTA Guideline 4-4-04/D. In all cases the main components are sand and lime as listed in Table 5.

The pH and sorptivity of some of these test laboratory mortars were measured in comparison with those found in the survey. As Fig. 8 shows, these recipes have a higher pH than the mortars found in properties with rising damp problems. The test mortars grouped in a box in Fig. 8 represent a more realistic representation of typical mortars. Looking at the wide range of behaviour seen in the survey, it may be advisable to use both a high and low sorptivity mortar in predictive testing.

Predicted values rising damp front. Lab test mortar recipes. Properties of mortars

5. Conclusions

  1. The pH values of most mortars collected from properties treated for rising damp were found to be in the range of 7–12 with a typical value of 9. New mortars have higher pH values and the reduction with time can be attributed to carbonation.
  2. The key parameter in controlling rising damp is sorptivity. As far as collected samples are concerned, the values of this parameter varied from 0.2 to 14 mm min. The higher sorptivity values were associated with older mortars, indicating the potential for more rising damp in these cases.
  3. There is an increase in the amount of soluble salts found in mortars with a greater age, as expected.
  4. Findings from this study have developed a better understanding of the nature and characteristics of different mortars. These findings can be utilised to develop replica mortars that can be used in the laboratory to simulate rising damp problems in existing buildings.


Many thanks to the invaluable help of Peter Cox Limited for their assistance with the collection of mortar samples.


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Construction and Building Materials

Journal - Construction Building Materials

This article is also published in Construction and Building Materials 25 (2011) 2845 –2850

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For more information call 01403 210204