The right aggregate and aggregate/binder proportions can make all the difference between success and faliure. The best binder mixed with a poor aggregate or inappropriate proportions will result in an underperforming mortar.
You will often read the quote "a clean, sharp, well graded sand" but what does that mean? Your local masonry supplier will simply point you to the pile out in the yard or perhaps their bagged sand.
To find out if your local aggregates are of good quality and formulate your aggregates and proportions for a specific project you can start by testing the sands from your local suppliers.
Take several samples of sand from your 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 clear up 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 settled on the surface of the sand. These are undesirable clays, soils or very fine particles. This test will help you determine which sand is of better quality and choose from your 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.
When rubbed between your hands it should make a squeeky sound.
Sand harvested in rivers has been rounded by the water and rolling downriver, much like beach sand is rounded by the constant wave action.
Commonly coarse"concrete" sand is sharp and fine "mortar" sand is more rounded.
GRADED: The grading of sand is extermely important as it will vary between specific applications affecting workability, and many other traits of your final mix. It is important that an aggregate comprises particles of varied size so that the voids between the larger particles are filled by the smaller particles down to the finest particle size. This is referred to as "particle size distribution".
Particle size distribution is important for specific applications and can make a big difference in the end result. Analizing the percentage of the different particle sizes present in an aggregate will enable you to draw out it's gradation curve.
There are several studies which deal with particle size distribution and gradation curves at length available on line.
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 obtain a customized aggregate. To determine the grading of an aggregate you will need a series of sieves. By separating the different size particles you can determine the percentage amounts of fines to coarse particles and draw an accurate gradation curve.
PARTICLE SIZE: Slaked lime has a much finer particle size and much higher viscosity then portland based mortars resulting in a mortar with higher plasticity and workability containing the least possible amount of water content.
A lime mortar would benefit from the inclusion of larger and smaller particle size within an aggregate, and a higher aggregate-binder ratio. For example: a coarse sharp "concrete" sand sifted through a 4mm. mesh may be more suitable to be used to mix a lime mortar for a bedding mix, a base coat, a coarse stucco then a rounded fine "mortar" sand normally used for a portland based mortar used for the same task.
A series of sifters are extremely helpful in grading your aggregates for each specific use. Sifting your sand will save time and aggravation by removing the occasional oversized particles which would otherwise hinder application and provide you with the right particle size aggregate for each specific job.
BINDER/AGGREGATE RATIO: The binder to aggregate ratio can vary depending on the size and shape of the aggregate and also 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 to surround each particle.
A high aggregate ratio would result in a less plastic mix and a less cohesive mortar subject to segregation. A low aggregate ratio would result in a plastic mortar unable to support heavy loads.
To assess 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 dry sand and a container filled with a measured amount of water. Slowly add water to the sand until it reaches slightly above the sand level. The volume of water used is the void ratio of your aggregate.
Thamount 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 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. Thicker layers should be allowed to carbonate for weeks before additional coats are applied.
Large voids in masonry work should not be filled with trowelfulls of mortar but instead should be filled with stone or brick pieces laid in a mortar bed, keeping the bedding mortar even in thickness. Doing so will diminish the amount of mortar used, increase structural strength and allow for an faster setting and carbonation.
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 decisions from their experience and teachings.
Modern research and testing have proven the roman guidelines to be correct.
Virtuvius specifies the different properties of aggregates, specific uses and binder ratios. These guidelines can aid us on choosing the proper aggregates, it's size and binder ratios for specific applications.
These are some parameters from Virtuvius Books:
Coarse, sharp sand mined from sandpits 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 and frost would deteriorate it.
Testing by putting a handful of sand on a white cloth, and lightly shaken out should not leave any dirt or coloring on the 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. It should be mixed at a 2 to 1 binder ratio (because of the larger 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 aggregates from other sources such as marble and stone aggregates (derived from their stone quarries) , aggregates with pozzolanic properties and man-made pozzolans (chrushed soft fired brick) to formulate hydraulic concrete and mortars to be used for underwater construction, moist environments, waterproofing aqueducts, water cisterns, bath houses, sewer systems, etc.
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