Building Materials, Third Edition

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condition of silica being in particular proportion and fineness is disturbed and the aggregates
turn to be innocuous.

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2e2i—2eX The alkali-silica-gel imparts osmotic pressure over the set cement
gel and this is mainly responsible for formation of cracks. When air entraining agents are
added they absorb the osmotic pressure and control the expansion.

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The thermal properties of coarse aggregate are specific heat, thermal conductivity and coefficient
of expansion. The first two are detrimental in case of mass concrete. Also these properties are
of concern in case of light weight concrete used for thermal insulation purposes. The third one
affects the concrete in general since the coefficient of thermal expansion of concrete increases
with that of coarse aggregate. Any appreciable difference in the coefficients of coarse aggregate
and cement paste may break the bond between the two. Freezing and thawing effect may be
prominent if the difference in the two is more than 5.4 × 10-6 per °C.

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Sand (> 0.07 mm) is used as a fine aggregate in mortar and concrete. It is a granular form of
silica. Sand used for mix design is known as standard sand (IS: 650). In India Ennore Sand is
standard sand and in U.K. it is Leighton-Burrard Sand. The standard sand should be obtained
from Ennore, Tamil Nadu. It should be quartz, light grey or whitish variety and should be free
from silt. It should (100%) pass through 2-mm IS sieve and should be (100%) retained on 90-
micron IS sieve with the following distribution,

Particle Size Per cent

Smaller than 2 mm and greater than 1 mm 33.33

Smaller than 1 mm and greater than 500 micron 33.33

Smaller than 500 micron but greater than 90 micron 33.33

Sand used in mortars for construction purposes should posses at least 85 per cent of the
strength of standard sand mortars of like proportions and consistency.

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X Sand may be classified on the basis of source, mineralogical composition, size
of the particles and particle size distribution. Depending upon the source sand may be classed
as natural sand—resulting from natural disintegration of rocks or deposited by streams; crushed
stone sand—produced by crushing hard stones and, crushed gravel sand—produced by crushing
natural gravel. Based on mineralogical composition, sand is divided into quartz, felspar and
carbonaceous varieties. Depending upon its size sand is classified as coarse sand—fineness
modulus (F.M.) 2.90-3.20; medium sand—F.M.: 2.60-2.90 and; fine sand—F.M. : 2.20-2.60. Based
on particle size distribution fine aggregate have been divided in four grades from grading zone
I to grading zone IV as given in Table 6.2.