Material properties and workability characteristics of plaster mortar
by R. Vogel, F. Scharfe, C. Andratschke, G.-E. Vogel


Results of tests on plaster mortars and assessments of workability characteristics based on practical plastering tests are compared to rheological measurements.

The comparison shows that the properties of fresh mortar determined by newly de-veloped equipment correspond to practical experience gathered in the application and treatment of this material.

Besides, the rheological parameters give useful advice regarding quality control and the development of new plaster mortars.


The decisive criteria in the assessment of the workability characteristics of plaster mortar are: good staying power during spraying, easy screeding during leveling, plasticity and no pilling during finish-ing.

There is no standardised instruction for the description or measurement of these characteristics. Therefore, workability characteristics and consequently also material properties are determined by spraying the plaster mortar on test walls, treating it individually and finally assessing the fresh plaster from experience. In addition to such subjective assessment of plaster mortar, agitating tests are also carried out by which a speed-depending torque is related to specific properties of the product.

Lastly, the assessment of the plaster mortar by means of the tests described in combination and inter-action with the respective mortar-technological tests acc. to DIN-EN forms the basis for making-up the mortar mix designs.

In order to assess material properties and workability characteristics of plaster mortars more objec-tively than to date, a procedure and the required equipment have been developed which will be re-ported on in the present paper.

Introduction into the approach

With regard to character and behaviour plaster mortar is no fluid in the classical sense, but it is also no bulk material. Neither can this mortar be put into the group of suspensions, as the bulk properties predominate. Its properties and its behaviour under exterior forces are determined by both the fluid and the bulk phase. With regard to its structure plaster mortar may be defined as a fine-grained bulk material whose particles are wetted with a fluid.

From the rheological point of view, the treatment of fresh mortar on the wall, e.g. by screeding, takes place in a range of very small rates of shearing and its appearance resembles a solid rather than a fluid. Therefore, contrary to the common characterisation of mortars by flow curves other well-known rheological experiments are used for the description of the properties of plaster mortar. These are the creep test, shear strain test and residual deformation test. The characteristic features of these tests can be seen in the following figures.

Fig. 1 Creep test


In the creep test the sample is loaded for a given time (t1) with a constant moment T (first dis-continuity of moment), then the load is abruptly removed (second discontinuity of moment). Re-cordings or measurements are made of the dis-placement caused by the load - in this case the distorsion of the sample (section I). The following removal of load (section II) leads to a partial recovery. The mortar is assessed with given TIII by means of the absolute values of A, B and C and on the basis of the relative values formed with the maximum angle of distorsion jI,1.

The three j-angle values and the shape of the curve in section II furnish detailed information about the composition of the sample.

Fig. 2 Shear strain test


The shear strain test serves to determine the yield moment TFl. For this purpose a moment ramp T(t) is fixed, realised and the angle of distortion j(t) of the sample is measured. The formation of a quotient (which corresponds to an equivalent of viscosity) leads to a maximum value enabling the determination of the yield moment.

Fig. 3 Residual deformation test


In the residual deformation test an angle ramp j(t) is realised and the moment required for the distorsion of the sample is measured. This test procedure makes it possible to measure the moment TB, which indicates the destruction of the bulk structure in the sample. Due to the destruction of the structure the moment rapidly decreases with every further rotation of the measuring sensor. Certain properties of the material may be related to the further shape of the curve.

In some cases of material testing a coupling (serial arrangement) of at least two tests of this kind seems to be possible. Because of the irreversible structure of the plaster mortars care must be taken that the critical ranges of the angle of distortion are not too close to each other (e.g.  jI,1 <<   jB ) .

Test procedure

The aim of the tests was to find out whether information on the workability characteristics of plaster mortars can be obtained by means of rheological investigations. On the one hand, according to present practice, the usual technological small-scale tests were carried out, which were, however, supple-mented by rheometrical measurements, on the other hand, separate tests on plaster mortar were made on laboratory-scale according to an adapted test procedure.

A.  Technological small-scale tests combined with laboratory tests

Walls (4 x 1,75 2) made of vertically perforated bricks (500 x 240 x 110 mm3) served as plaster base. The plaster mortars were applied by means of a plastering machine of the type solomix by mtec. The ambient temperature in the test facility was about 20 C.

Three plasterers independently applied and assessed the plaster mortar.

Parallel to every operation of the plasterers measurements were made in the laboratory. After about half of the mortar mass had been used up, while the plaster mortar was being applied, 2-liter samples were sprayed into a vessel and the flow diameter was determined. After the plaster mortar had been applied a brick was removed from the upper part of the wall and simultaneously subjected to rheological investigations while the plaster mortar was subjected to further treatment by the plaster-ers.

B.  Laboratory tests

Independently of the small-scale tests laboratory tests of a basic character were carried out. 2-kg samples acc. to DIN 18555 were mixed in a Hobart mixer and by means of a Hägermann flow table the flow diameter a was determined. The empirical value of 165 < a < 175 mm was fixed as the required range. Deviating samples were discarded.

In order to include the absorbing capacity of the plaster base in the investigations the glass plates of the flow table (see DIN 1060) were either covered with a double layer of filter sheets or replaced by split tiles.

For the rheological investigations a rheometer was available that had been developed after a draft by R.VOGEL Research Laboratory together with RHEOTEST Medingen GmbH. Details of the devices are dealt with elsewhere.



Fig. 4  Schematic drawing of mortar shear strain test arrangement  

1 shearing RING

2 shearing PLATE

3 mortar PAT

The patented measuring technique and the measuring sensor are shown in Fig. 4. The sensor is im-mersed in the mortar pat which is lying on the plate of the flow table or has been applied to / sprayed on another base. The mortar pat must not be subjected to any further disturbance. Then the behaviour of the plaster mortar as to creep, shear strain, and residual deformation is measured. The required precise measurement concerns in the first line the three magnitudes time t, torque T and angle of distorsion j in variable arrangement to each other. In certain cases an additional measurement of the difference in height is required.


Five different plaster mortars were investigated, see Table 1. For the small-scale tests sacked material was used. The laboratory tests were carried out with mixtures made after mix designs according to works standard and selected variations were tested.

Table 1  Description of factory-made dry mortars used



Mix composition

Lime-gipsum plaster


Multi-phase gipsums, calcium hydrate, aggregate, additives, retarder

Lime-cement plaster


Calcium hydrate, cement, aggregate, additives

Lime-cement light-weight plaster


Calcium hydrate, cement, aggregate, additives, polystyrene

System light-weight plaster


Calcium hydrate, cement, aggregate, additives, lightweight aggregates

Table 2 shows the most important results of the plastering tests.

In lines 1, 8 and 9 the judgement of the plasteres is quoted. In the language of these experts 'easiness' refers to the strength required in the operation of screeding, 'stickiness' means that the mortar adheres to the screeding board, and 'plasticity' refers to the behaviour of the plaster mortar in the second stage, the finishing. The flow diameters in line 3 may directly be related to these statements.

Since the mortar was tested under the same prerequisites the assessments given by the plasterers are comparable to the rheometer measuring results in lines 2 and 10 to 13.

Table 2
Technological small-scale tests made in the factory Franken-Maxit compared to the creep, residual deformation and shear strain tests made to establish workability characteristics








Plaster base










M. of fracture









Flow diameter









Flow diameter









Moment of fracture







glass/metal plate







split tile







double layer of paper



























jII,120 / jI,1









TV / >Dj1









TV / jII,120








Considering the data in lines 1, 2 and 3 it may be stated:

  • Judging by the scattering of the flow diameters the consistency of the sprayed fresh mortar varied considerably.
  • If we leave out of account that the moments of fracture of KZLP are higher than the evalua-tion 'medium' for KGPs, 'easiness' may be considered as synonym for moment of frac-ture. That means that 'easiness' can be described by a concrete numerical value. According to the present state of knowledge a scale of easiness should range from 30 to 150 mNm or from 350 to 1800 Pa.
    All moments of fracture in line 2 printed in bold letters belong to a flow diameter of 166 mm.


As a magnitude typical of the material the moment of fracture can also be determined for plaster mor-tar under defined laboratory conditions. These measuring results can be found in lines 4 to 7 of table 2. The data in line 5 were determined in a test arrangement acc. to Fig. 4, i.e. mortar pat on dense plaster base. The effect of a variation of the plaster base, e.g. for KZP, is shown in lines 6 and 7.

  • Understandably, the measurements for a dense plaster base (glass/metal plate as pat base) show smaller moments of fracture than those in line 2. But the trend of both corresponds.
  • If the pat base is absorbent the results of the laboratory tests come close to the moments of fracture in the technological small-scale tests. (In the tests with a double layer of paper about 10% of the mixing water was drawn off.)

As far as the plasterers' statements on stickiness and plasticity (see lines 8 and 9) are concerened the rheological investigations still leave some questions unanswered. The reason is that, on the one hand, deviations from the desired condition 'not sticky' and 'good plasticity' occur exclusively and si-multaneously with KGP 2 and that, on the other hand, a classification under the well-known physical terms is uncertain. Considering in this context the results of the creep tests in lines 10 to 13 we find an indication in line 11. The values given here for the irreversible deformation jII,120 / jI,1 for KGP 2 deviate noticeably from those for other plasters. It is, however, too soon to draw conclusions from this observation as this requires information on the exact composition of the material, which is unfor-tunately not available. The data in lines 10 to 13 let us assume that then a number of conclusions might be drawn from the creep tests.

Fig. 5

Typical shape of moment curve T( j) in residual deformation tests of the KGP series

A qualitative information on stickiness, i.e. a combination of adhesive and cohesive forces, is ob-tained from the shape of the moment curve in the residual deformation test . Fig. 5 shows one for both lime-gypsum plasters. With the same flow diameter of 166 mm both plasters have a moment of fracture of about 60 mNm. But in contrast to KGP 1, KGP 2 shows a much flatter shape of the moment curve after the structure had been destroyed. As was shown in tests with other materials e.g. dispersion adhesives, this behaviour is clearly due to adhesive properties. If the composition of the samples were known, it could be found out to which component these properties are ascribable.

Fig. 6 shows that among others the methyl-cellulose content also influences the shape of the curve. Plaster mortar without MC is very 'sharp' , the fracture resembles a brittle fracture. It can be seen that due to the addition of MC not only the moment of fracture is reduced but that the curve also becomes flatter. Consequently, too much MC leads to stickiness. Also for these results it holds true that a = 166 mm.

Fig. 6

Influence of MC-content on shape of moment curve for KZP

As a supplement to Table 2 comparable results of the laboratory tests are compiled in Fig. 7 . The dependence of the moment of fracture on the character of the mortar becomes evident, and also the trend of development after the destruction of the structure is about the same. The deviation for KGP 1 in the right last section of the curve indicates slight stickiness, which was confirmed by the plasterers' reports.

Fig. 7

Comparison of mortars investigated, a = 168 mm.

Fig. 8

Influence of setting time on moment of fracture in KGP 1

If the aim of a laboratory test is clear, a skillfull test arrangement will lead to a solution. For instance, in order to pursue the setting process the Hobart mixer was charged with an amount of premixed dry mortar sufficient for 4 samples. After mixing the samples were spread in rapid succession by the well-known procedure. At the beginning of the rheological investigations there were four equally treated mortar pats on hand. One result from this test series is seen in Fig. 8 . Obviously the time between end of flow table test and residual deformation test makes itself felt only for periods of more than half an hour. The trend of development of the moment of fracture remains unchanged. Only if the time between end of flow table test and residual deformation test amounts to two hours the material shows signs of brittle fracture. In this case the mortar should be discarded.

Since almost no quantitative data of the material composition of the mortars investigated are available the results of the creep test are of little value. Fig. 9 shows how much valuable information can therefore not be gained and shall give an incentive to carry on research work. In Fig. 9 the creep tests for the two mortars KGP and KZLP are compared. On the left side the absolute values are repre-sented and on the right a section of a non-dimensional plotting of the angle of distorsion in the form of j  /  jI,1 can be seen.

Fig. 9 Results of selected creep tests on KG and KZL plaster mortars

With equal flow diameter a = 171 mm and equal loading of the samples TV = TI..II = 15 mNm the differences in the materials are clearly seen. If we use for the assessment of the range of recovery an exponential function of the form

or in non-dimensional notation


we find that the creep test yields altogether 7 data, namely the direct measured values

jI,1;   jII,1;   jII,120 resp. jII,   and the indirect measured values

a ;  b ;  c ;  tanb.

The representation of the absolute measured values in Fig. 9 reveals the difference in the order of magnitude of the first group, the non-dimensional representation shows the differences in the second group of values.


Summary and conclusions

The paper reports on tests carried out on plaster mortars in a facility for small-scale tests and in the laboratory. Conventional plasterers' assessments of the workability of the plaster mortars are com-pared to rheological measurements. The properties of fresh mortar determined by a newly developed equipment correspond to practical experience in the application. The comparison between the rheological measurements and the judgements given by the plasterers shows that there are no princi-pal discrepancies. Besides, the rheological parameters give useful advice regarding quality control and the development of new plaster mortars.

In order to achieve more telling results of rheometer measurements, particularly regarding the creep test, it is necessary to concentrate on finding out material parameters by means of model mixtures.


Prof. Dr.-Ing. habil. Ruprecht Vogel
Dipl.-Ing. Gert-Erik Vogel

Research Laboratory for Flow and Bulk Engineering
Malerstieg 6
D 99 425 WEIMAR / Germany


Dipl.-Ing. Friedbert Scharfe
Dipl.-Ing. Christin Andratschke

Franken MAXIT GmbH & Co
Azendorf 63
D 95 359 KASENDORF / Germany