Silica Aggregate

Preparing the mix

• mix the lime and aggregate by volume (check the specification for exact mix ratio for the job or refer to the table below), measuring of material must always be with a gauging box or bucket. The sand/aggregate should be sharp, well-graded and washed.
• a conventional cement mixer can be used although for larger projects a roll pan or paddle mixer is preferable. Lime mortars mixed in drum mixers can be prone to balling but use of particular mixing techniques can reduce this. Switch the mixer on and dampen down the inside of the drum as this will help to reduce the amount of dust and prevent the lime from sticking too much. Switch the mixer off before adding two thirds of the water and half of the sand followed by all of the lime. Switch the mixer on, allow the water to thoroughly disperse throughout the mix (15-20 minutes) before adding the rest of the sand and more water if required. You will have to experiment for the first mix as the quantity of water will vary with sand moisture content – but be CAUTIOUS, for a 3:1 mix, initially add 6 litres of water, 30 litres of sand and then 20 litres of lime, mix, then add the remaining 30 litres of sand and adjust the water.
• add hair or synthetic fibre (approx 1.5kg per tonne) to improve the strength and durability of the render, tease it in towards the end of the mixing.
• use the mix within 2 hours.

Application

• after damping the surface, the plaster/render can be laid on using the normal technique in two coats of approx. 8-10mm.The first coat should be left to stiffen up and then a float used to compress the render over the whole area. Applying the second coat too early (i.e. before the material has stiffened) can weaken the render. The result may be voids deep in the render which make the render vulnerable to frost damage. Within an hour or two, the surface of the first coat should be scratched over using a suitable wire comb.
• once the first coat has hardened enough a similar technique is used for the second. The time interval between the two will vary depending upon the temperature. As little as 24 hours in warm conditions but several days if cool during this time, it may need damping down if drying too quickly.
• if a top coat is required, the mix will be different, it is usually a weaker mix than the background mixes, (refer to specification table) proceed as above, it should be ‘floated up’ as the surface stiffens, in a similar way to the previous coats.

To be used in conjunction with full application guide.