Knoop micro hardness: UM 9724/6
The test is based on the determination of the measurement of a series of 20 imprints (or 40 for materials with uniform grain structure) produced by a diamond punch pressed with a specific load on the highly polished surface of a sample under examination.
The Knoop microhardness HK is expressed in MPa (1MPa = 0.102 kgf/mm^2) from the following formula:
HK = (14.23 p) / (l^2)
in which
P = the load on the punch, expressed in N; l = the length of the longest diagonal of the imprint left by the punch, expressed in mm.
The values of microhardness, calculated for a series of examined points are plotted on a graph in increasing order (hardness graph) which has on the X - axis of the graph the points measured and on the Y - axis the corresponding values of microhardness.
Porphyry - Knoop microhardness, average value: 5416 MPa.
Coefficient of absorption
To determine the coefficient of absorption the constant mass sample is first dried at 105° and the mass is measured to within an error margin of 0.01 g. It is then immersed in distilled water until it reaches a stable weight +/- 0.01g.
This is an indication of the maximum compactness, resistance and durability of the rock and is of particular importance for rock materials used in construction, especially for roofing, paving and cladding.
The coefficient of absorption is the measure of the maximum quantity of water absorbed by the rock material when it is immersed in de-ionised water at ambient temperature and pressure, expressed as a percentage of the dry mass of the sample.
The permeability of rock materials is the possibility of movement of fluids within the rock through the internal pore spaces.
The permeability is related to the shape and size of the spaces rather than the actual porosity which expresses the amount of spaces themselves. Its low coefficient of absorption has led to porphyry being used where it is in direct and continuous contact with water, on the bed and banks of rivers for example and exterior paving.
Porphyry - coefficient of absorption, average value: 6.53%ο.
Freeze/thaw behavior
The breaking load is measured under simple compression according to article 8 of R.D. 16.11.1939 no. 2232 after a cycle of 20 immersions in water at 35°C alternated with periods in a refrigerator at -15°C.
The material is considered resistant if after the freeze/thaw test the results are not more than 25% lower than the average tests carried out on dry samples. Rocks are considered not resistant if they crumble under the effect of freezing, i.e. due to the increase in volume of the absorbed (pore) water and consequent pressures on the walls of the cavities with the progressive deterioration of the rock. The resistance to freezing and the coefficient of linear thermal expansion are therefore extremely important for the selection of rock materials for exteriors and related technical uses. Materials must be able to withstand the action associated with climatic conditions including considerable temperature variations and low temperatures.
Porphyry demonstrates a level of breaking under compression after the freeze/thaw cycle among the highest for rock materials. This characteristic of particular importance in cold climates, ensures a high level of resistance even after freezing and thawing for all those places and structures which are subjected to forces, paving, external cladding, load bearing walls etc.
Porphyry - monoaxial compression after freeze/thaw. average value: 202.6 mPa.
