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Choosing the 'Right Stone' for the Job

Taking time to learn about some of the important properties of stone will go a long way to ensuring you choose the ‘right’ stone for your home.  Jim Mann, a stone consultant and principal of Stone Initiatives, provides some expert insights.

English philosopher Francis Bacon once said “Knowledge is power”. For today’s consumer, the adage should be “knowledge is buying power”. 

We have all been bamboozled by salespeople trying to explain the virtues of the latest product – whether it be a DVD player or a car.

Choosing stone for your home need not be the same experience. By learning some simple selection methods and understanding stone’s basic properties you can make an educated decision when you are selecting stone for your next project.

The knowledge you gain can give you the confidence to be a little bolder when selecting from the vast array of stone available. It can also save you money and heartache by making sure you have chosen the ‘right’ material for the job by ensuring that it is fit for its intended purpose.  

The ‘right stone’ needs to meet requirements based on appearance and performance. Selecting suitable flooring, for example, is firstly a matter of personal taste. One of the appealing aspects of using stone is how its unique character can be used to display your own distinctive personality. 

The range of colours, textures and finishes available in stone now rivals the range available in more ‘traditional’ floor coverings. Like these coverings, choosing a stone that is durable and resistant to staining and wear is important. The process of picking the right stone based on performance can be perplexing, but the following selection methods can assist you to make the right decision. 

History of Use
All stone has a story to tell, whether it’s the iconic Ayer’s Rock in Australia, in a two-hundred-year-old church or a recently installed granite bench top.

If you have a basic ‘style’, colour or texture in mind, it is worthwhile to take note of the use of similar material around you. Inspecting stone tiles used in a friend’s home, a hotel lobby or even on the table tops in McDonald’s can all be used to give you a feel for what could be right for your home.

A cursory inspection for stains, scratches, loss of polish, cracks or general colour variation is useful in assessing a stone’s suitability for its environment and its likely performance in your home. 

Physical Properties
As well as examining stone in use, a stone’s physical properties are a useful indicator of performance. Most dimension stone quarried in Australia will have been tested to determine its basic physical properties. Information on imported material may be harder to come by, but a stone merchant should be able to provide you with basic data for the more popular materials. The data provided is often not presented in a consumer-friendly manner but comparison of the basic properties of your short-listed stones can be a great help in finding the right product for you.

There are four basic properties that can be used to determine the likely performance of various stone types:

Stain Resistance
Water absorption is a useful indicator of stain resistance. High water absorption suggests that the stone will be more susceptible to staining from liquids, as these materials have a greater capacity to hold the staining agents.

Water absorption is usually expressed as ‘weight %’, which is a measure of the weight of water that the stone has absorbed compared to the weight of the sample.  The water absorption capacity of stone can vary greatly – granite and marble can have a water absorption of less than 0.1% while some sandstones and limestones can have a water absorption of more than 10%.  It is useful to compare the range and standard requirement for various stones as these values can be useful as a benchmark to determine the quality of a material.

Stone Type

Water Absorption (weight %)

Typical Range

Standard

Granite

0.8 – 0.01

0.40

Marble

0.8 – 0.04

0.20

Sandstone

12 – 0.3

8.0

Limestone

26 – 0.08

3.0

Slate

6 – 0.3

0.45

The table shows that marble generally has a low water absorption and therefore would appear to highly stain resistant - but this is only half the story.

Composition also plays a major role in determining the degree of stain resistance of a stone. Some stains are caused by a chemical interaction between the stone and the staining agent. For example, most limestone and marble types contain calcite and are therefore sensitive to acidic solutions. When a substance such as wine is spilt on these stones, staining occurs not only through the absorption into the pores, but also by the etching of the surface. Stone composed predominantly of chemically inert (stable) minerals such as quartz and feldspar (as found in granite) are more likely to resist etching and therefore are more popular for bench tops or entertaining areas. 

The general appearance of the stone can also affect its ‘apparent’ stain resistance.  A highly figured and variegated granite has more potential to hide a stain than a plain white stone with similar properties (like trying to hide a gravy stain on a white shirt!).

In summary, if stain resistance is very important to you, you should choose a stone that is low in water absorption and is not composed of acid-sensitive minerals such as calcite. You might want to consider a material that has a variegated appearance.  Stone types that meet these criteria include a wide range of granites, slates and some dolomitic or serpentine ‘marbles’. Other stone types could be used if sealed and maintained correctly.

Resistance to Abrasion
This is an important property, particularly for internal stone. Low abrasion resistance will result in significant loss of polish, scratching and in cases of high traffic, significant change in surface profile over time. Abrasion resistance is a property based on the hardness of the minerals that make up the stone and the strength of the bond of the grains within the stone. A good example of this is to compare a typical sandstone with a granite. Both stones usually contain a significant proportion of quartz, which is considered a hard and durable mineral, but granite is considered to be much more abrasion resistant than sandstone. The main reason for this difference is that the mineral grains in granite are tightly packed, with the grains literally ‘welded’ to each other. In sandstone the grains are usually cemented together with a softer and less durable clay mineral. When sandstone is subject to abrasive wear the quartz grains can actually pluck out of the stone, while the tight bonding of granite will only allow the surface of the grains to wear away. 

In Australia, abrasion resistance is usually evaluated by determining its ‘Resistance to Abrasion Index’. This index is achieved by subjecting the stone to an abrasive medium and the volume of stone lost during the test is determined. The lower the index number, the lower the resistance to abrasion. Typical figures for some stone types are shown below:

Stone Type

Typical Abrasion Resistance Index (Ha)

Granite

50 - 150

Marble

15 - 50

Sandstone

4 - 24

Limestone

< 1 - 20

Slate

4 - 20

Stone with an index number greater than 8 is usually considered to be sufficiently resistant to abrasion to allow its use as interior domestic paving. It is recommended that, where the stone will have a polished finish or will be used for general commercial use, it should have a minimum index number of 12. This will reduce the loss of surface finish. Care should be taken when choosing stone for restricted areas, such as entrances, as these concentrate traffic and are subject to higher wear. 

Polished surfaces are more sensitive to abrasion and tracking can be a problem, particularly on surfaces with dark uniform colours.  A high ‘Resistance to Abrasion Index’, along with a regular cleaning regime to remove fine abrasive gritty particles, will minimise the risk of tracking paths in high-traffic areas.

If resistance to abrasion is important, then you should choose a stone that has a high index number. In high traffic areas, consider using a stone that is less likely to show tracking paths, such as stones with honed, sandblasted, antiqued or sawn finishes.

Strength
Strength is an inherent property that relates to the force the material can withstand per unit of cross sectional area. The standard unit for strength is the Mega Pascal (MPa), which describes the force (Newtons) required to cause failure of the material for each mm2 of its cross section (i.e. thickness x width). The term ‘strength’ may relate to either the compressive (crushing) or flexural (bending) strength of the material. Inadequate compressive strength is rarely a problem in domestic construction, but it is useful to compare the strength of a range of materials.   

Stone Type

Typical Compressive Strength Range (MPa)

Granite

300 -100

Marble

150 -80

Sandstone

120 - 10

Limestone

150 - 4

Slate

200 - 25

As an example of the strength of stone, if we tried to crush a brick-sized (230x110mm) piece of limestone that has a compressive strength of 4MPa, we would need to apply a force of 101,000 Newtons - equivalent to load of 10.3 tonnes.

Flexural strength, however, is a property that is highly relevant to the adequate performance of a dimension stone. Modern processing facilities can now produce stone in larger and thinner slabs, which can be used to span greater distances with relatively lightweight units. Products such as large flagstones, veneer panels and large bench tops must have an adequate flexural strength to prevent failure. The typical range of flexural strength for some stone types is presented below:

Stone Type

Typical Flexural Strength Range (MPa)

Granite

30 - 6

Marble

22 - 6

Sandstone

15 - 3

Limestone

21 - 2

Slate

50 - 15

As you can see from the table, all stone types listed vary in strength considerably, but what the end user of the stone product really wants is to be sure that the stone will not fail or break in service. Unlike strength, breaking load is also determined by the actual dimensions of the finished unit. 

As an example, imagine you have a granite flagstone that measures 300mm wide x 1200mm long x 10mm thick and has a flexural strength of 10MPa. You now lay the flagstone over two bricks that are 1100mm apart (the span) and you start loading lead blocks on top. In this test, you will find that you only need to load about 36kg of lead onto the stone to cause it to break in half. If you had used a flagstone with the same flexural strength, but 20mm thick instead of 10mm, the stone would fail at around 148kg. This example is an unrealistic situation as, in practice, the stone would be laid on a rigid base, but it shows that the breaking load of the unit is affected by the dimensions of the unit as well as its strength. Some basic rules that link breaking load to the dimensions are listed below:

Doubling the width of the ‘tile’ doubles the breaking load
Halving the span will double the breaking load
Doubling the thickness will increase the breaking load fourfold
Doubling the strength will double the breaking load

If flexural strength is important to you. For example, if you are laying pavers in a driveway, then you can increase the breaking load by choosing:

A material with a higher flexural strength
A thicker material
A unit size where the length and width have approximately equal dimensions
A strong bedding and sub-base that will adequately support and minimise flexing of the units.

If you want to use units where the length is much greater than the width, ensure that the thickness and flexural strength will adequately compensate for the larger span. If you are about to outlay a considerable sum for new paving, you may want to have the typical breaking load of your flagstone determined by an independent stone expert. The relatively small additional cost may save you time and money as well as give you confidence in the performance of your selection.

Durability
Durability has always been an issue for all materials. The survival of iconic wonders such as the Parthenon and the Pyramids show that stone can be a durable material that ages gracefully.

The wide use of stone can result in exposure to aggressive environments such as acid rain and freeze-thaw cycling. In Australia, the most common aggressive environments are related to rising damp and salt attack. A stone’s durability in these environments can be assessed by determining its ‘Resistance to Salt Attack’. This test is specifically designed for sandstones, but is useful for testing other stone types such as low density limestone.

In this test, the stone is soaked in an aggressive salt solution and then oven-dried. The stone is subjected to 15 soaking cycles and, on completion, the amount of weight lost by the specimen is determined by drying and weighing the collected residue. The cycling not only gives an indication of the effect of salt crystallisation on the stone, but it can also give an indication of the effect of cyclic wetting and drying. The results are presented as a percentage weight loss for the specimen. It is useful to classify the results into the various grades shown below:

Weight Loss (%)

Grade

Environment Suitability

<0.1

AA

Aggressive environments, constant wetting and drying and exposure to salt attack

0.1 - 1

A

Exposed to continual wetting and drying or moderate salt attack

1- 5

B

Exposed to intermittent wetting and drying or moderate level salt attack

6 – 10

C

Exposed to intermittent wetting and drying or low level salt attack

> 10

D

Suitable only for use in sheltered locations free from exposure to salt attack. Additional engineering practices may be required to protect stone

Grade A stone is usually specified for stone to be used as curtain wall cladding and is considered suitable for use as paving around pools. Stone used as coping at the edge of a pool needs to be very durable and therefore should be Grade A or better.

Durability is also associated with strength. Both compressive and flexural strength are related to the bonding of the minerals that make up the stone. High bond strength between the individual grains makes the stone more resistant to the disruptive mechanical forces that are created by crystallising salts and freezing water as they apply pressure to these bonds. If these forces exceed the bond strength within the stone they can cause the stone to deteriorate and crumble.

Water absorption can also be used as an indicator of durability. High water absorption can allow aggressive solutions, such as salt laden or acidic water, into the stone where it can physically or chemically disrupt the structure of the stone. There are exceptions to this rule as durability is also related to pore-size distribution. But, if durability (resistance to salt attack) is important to you, choose a stone with a low weight loss after salt attack testing, a low water absorption capacity and high strength.

Summing Up …
Hopefully, you now have a basic understanding of some of the important properties of stone that will enable you talk to stone merchants in their own language. You may now also have a grasp of some guidelines that will assist you in the selection of a material that is ‘fit for purpose’. 

Alas, the physical properties for all stone types on the market are not readily available, so evaluation of a material from published data is not always possible. The next time you are confronted with a plethora of stone to select from, and you have limited information to choose what’s ‘right’, remember, it’s your consumer dollar that drives the industry and creates demand. 

It is up to you to demand useful information that will assist you in the selection process. After all, it’s through similar demands on the food industry that we now know that a Big Mac contains 24 grams of fat (that’s one fact I didn’t want to know)! 

As specified in the relevant ASTM standard specifications