Sand properties. Sand processing for preparing concrete
Pure quartz sand has the best properties for preparing mortars. Good sand for preparing mortars and concrete shall:
- have not too polished grains;
- have specific gravity close to standard;
- not be smudging (i.e. it shall not contain any foreign inclusions);
- when shaken with water it shall quickly settle on bottom and not make water turbid;
- calcined quartz sand shall not be visibly dissolved in hot hydrochloric acid.
Grain size shall be determined by consecutive sieving of a definite amount of sand through sieves with different mesh size. Usually 1.2 and 0.3 millimetre mesh size is used. 2 kg of sand is screened. If more than 1 kg of sand is left on the first sieve, then it is considered to be coarse.
If more than 1 kg of sand passes through 0.5 mm mesh size sieve, then sand is considered to be fine.
Presence of admixtures in sand.
To assess the quantity of organic admixtures, sand shall be shaken with equal volume of 3% sodium hydroxide, and left for settling for 24 hours: if the liquid remains colourless or becomes slightly yellowing – sand is satisfactory; if the liquid colour is dark brown, then sand is suitable neither for solutions, nor for concretes.
To determine interstices, 0.4 l of water is poured into a large measuring glass (1.5 litre capacity). Then a precisely measured volume – 1 litre – is poured as a thread of sand.
The difference between the formed volume and the volume of added sand shows the volume of interstices between sand grains occupied by water.
Any admixtures occurring in sand can be removed:
course mechanical admixtures – stones, chips, etc. are removed by sieving. A screen is usually applied for sieving.
Fine admixtures – clay, etc. are removed by washing with water. Sand can be washed by arranging a tray with flowing water and throwing sand with a shovel or rabbling it with a rake towards the water flow direction.
Calcining sand as a measure to remove organic substances is used very seldom.
The permissible content of rocks and minerals considered to be harmful components and admixtures in the sand used as a concrete and mortar filler shall not exceed the following values:
- amorphous phase of alkali-soluble silicon dioxide (chalcedony, opal, flint stone, etc.) - no more than 50 mmol/l;
- sulphur, sulphides, except pyrite (cellular pyrite, magnetic pyrite, etc.) and sulphates (gypsum, anhydrite, etc.) in equivalent SO3 – no more than 1.0%; pyrite in equivalent SO3 – no more than 4% by weight;
- mica – no more than 2% by weight;
- halogen compounds (halite, sylvin, etc.) including water-soluble chlorides in equivalent chlorine ion – no more than 0.15% by weight;
- coal – no more than 1% by weight;
- organic admixtures (humic acids) – less than quantity imparting sodium hydroxide solution (colorimetric test according to GOST 8267) the colour corresponding to standard colour or darker than it. Using sand not complying with such a requirement is only permitted after obtaining positive results of durability test of sand in concrete or mortar.
- The allowable percentage of zeolite, graphite, combustible shales shall be determined based on the study of influence of sand on concrete or mortar durability.
A tubular ball mill is used for processing sand in addition to vibrating sieves and screens. Sand is ground to the required size in the ballmill, and simultaneously it can be subjected to some additional processes – drying, mixing with other substances and minerals, moistening. The Samara Strommashina plant manufactures specialised industrial equipment for processing sand.