The importance of the right aggregate

The right aggregate and aggregate/binder/water proportions can make all the difference between success and faliure. The best lime putty mixed with a poor aggregate or inappropriate proportions of aggregate or too much water, will result in an underperforming mortar.

You often read or hear the quote: "a clean, sharp, well graded sand" but what does that really mean? Your local masonry supplier will simply point you to the piles in their yard or perhaps their bagged sand. 

Keep in mind that ASTM standards for OPC mortar do not pair well for lime mortar, in most applications.

To find out if your locally available aggregates are of good grading and quality you can start by testing the sands from your local suppliers. 

Take several samples of sand from your choice of local suppliers and run these simple, basic tests:

CLEAN:  put some sand in a jar (half full) add water to the top, cap and shake, than place on a flat surface. Is the water clear, cloudy or murky? how long does it take to clear up?

A clean sand would obviously leave the water clear, or sligtly cloudy or should clear up (settle) in a very short time

If it is murky, allow it to settle (decant). once the water is clear you may notice a layer of fine sediments sitting on the surface of the sand. These are undesirable clays, soils or very fine particles. This quick test will help you determine which sand is CLEANER and select the best sand from your choice of sources accordingly.

SHARP: Sand harvested from sand pits may vary from location to location, it is graded and washed and it is usually sharp, meaning that the sand particles are sharp edged, resembling crushed stone rather then rounded, like beach pebbles or marbles.

When rubbed between your hands it should make a "squeeky" sound.

These sands are more suitable for masonry construction.

Sand harvested in rivers or lakes has been rounded by rolling downstream, much like beach sand which is rounded by the constant wave action.  These sands are usually more suited for stucco and plasters. 

Commonly coarse"concrete" sand is sharp, and fine "mortar" sand is more rounded but not necessarily.

WELL GRADED: The grading of sand is extermely important as it varies for specific applications and also affects workability and many other traits of the mortar. It is important that an aggregate comprises particles of varied size so that the voids between the larger particles are filled by smaller particles down to the finest. This is referred to as "particle size distribution".

AGGREGATE GRADATION MAY AFFECT THE FROST RESISTANCE OF LIME MORTAR !

Particle size distribution is very important for specific applications and can make a big difference in the end result.

There are several studies which deal with particle size distribution and gradation curves at length. This information will enable you to make an informed choice of the appropriate aggregates you will be using on your particular project.

 Visit the "Links" page on this site for more information, or surf the internet for in-depth aricles on the subject.   

Since aggregates with a specific gradations curve may not be available, it may become necessary to determine the size and granulometric curve of an available aggregate and amend it accordingly to your particular need. To determine the grading of an aggregate you will need a series of test sieves.  By separating the different size particles you can determine the percentage amounts of fines to coarse particles and draw an accurate gradation curve. 

An aggregate which includes larger and smaller particles increases mechanical properties of lime mortar. A coarse sharp "concrete" sand sifted through a 5 mm. mesh is more suitable for a bedding mix, repointing, scratch coats, brown coats or a coarse stucco then a rounded fine sand, normally used with OPC mortar for the same task.

Blending aggregates of different sizes and properties will allow you to produce your own specific use blends.

A series of  large sifters are extremely useful in grading and blending your aggregates. Sifting your sand will also save time and aggravation by removing the occasional oversized particles which would otherwise complicate application.  

Passing your mixed mortar through a sifter will help better blend your mortar and remove any lumps or "balls" from your mix.

BINDER/AGGREGATE RATIO: The binder to aggregate ratio can vary depending on the size and shape of the aggregate, gradation and can also be modified for specific applications.

It must be understood that the binder is the weakest link in a masonry structure. The aggregates are harder then the binder, which is merely the "glue" which holds it all together. Too much or not enough binder will result in a mortar which may fail prematurely.

As a rule of thumb, the minimum amount of binder in a mix must be sufficient to fill the voids between the aggregate particles and surround each aggregate particle.

A high aggregate ratio, would result in a stiff, less plastic mix and a less cohesive mortar subject to segregation. A low aggregate ratio would result in a soft plastic mortar with lower compressive strength, not suitable to support heavy loads.

To determine a suitable mix ratio in relation to a specific aggregate, a simple test can be performed utilizing two measuring containers. A container filled with a measured amount of sand and a container filled with a measured amount of water. Slowly add water to the container with sand until the water reaches slightly above the sand level. The volume of water used is the void ratio in your aggregate. 

The amount of water used will give you the "minimum binder to aggregate ratio" for that paticular aggregate.

You generally want to slightly exceed this amount.

As a rule of thumb, the thickness of a lime mortar bed to be applied at any-one time sould not exceed 1.5 to 3 times the size of the largest aggregate particles in the mix. For example a stucco mix with an aggregate with particles sizes up to 4 mm. would allow you to lay a coat between 6 mm.  and 12 mm. 

Thick layers should be left with a rough surface and allowed to cure and carbonate for weeks before additional coats are applied.

Large voids in masonry work should not be filled with trowel-fulls of mortar but instead should be filled with stone or brick pieces laid in a mortar bed, keeping the bedding mortar more even in thickness. Doing so will diminish the amount of mortar used, increase structural strength and allow for an faster, more even setting and carbonation and less cracking. 

THE ROMAN EXPERIENCE:

Virtuvius and Plinius, left us some clues on their choice of aggregates and specifics of use gained through their experience while conducting their massive construction projects throughout Europe and their expansive Empire. Some of these structures are still standing today more than 2000 years later.  Some Roman structures like he "Pantheon" in Rome  and the "Pont Du Gard" in France have been in use since their completion and are still in use today.  We unfortunately don't have the resources and complete knowledge the Romans enjoyed. However we can make some basic but very effective conclusions from their experience and teachings left to us though their manuscripts.

Modern research and testing have also proved that the Roman guidelines make good sense and are correct.

Virtuvius specifies the different properties of aggregates, their specific application and binder ratios. 

Here are some parameters from Virtuvius Books:

Coarse, sharp sand mined from sand-pits should be mixed at a 3 to 1 binder ratio, it should be freshly dug, clean and kept covered. It should not be left exposed to the elements as sun, rain, wind and frost would deteriorate it.

Testing by putting a handful of sand on a white cloth, and lightly shaking it out should not leave any dirt or color on a white cloth.

This type of sand is recommended by Virtuvius for all general construction and vault construction. 

Sand from rivers should be harvested below waterfalls as it would be cleaner then if harvested from slow flowing areas (silt). It should be mixed at a 2 to 1 binder ratio (because of the higher void ratio of the sand)

This type of sand is recommended for stucco finishes and plaster work.

Beach sand was not recomended or must be washed prior to use as salt would be detrimental.

The Romans also made extensive use of man made aggregates such as marble and other stones like pumice, for light weight cement.

Natural pozzolanic aggregates and man-made pozzolans (chrushed soft fired brick) were also utilized by the Romans to formulate hydraulic cement for specific uses such as thick wall infill, underwater construction, moist environments, waterproofing aqueducts, water cisterns, bath houses and sewer systems.

The "Ten books on Architecture" written by Virtuvius have been translated in English and can be found through an online search.


Visit the links pages on this site for more information, articles and studies. Or don't hesitate to call us!

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