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MICA IN CONCRETE

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Advanced Engineering Solutions Journal Vol. 1 /2021

MICA IN CONCRETE

Maregesi, Mr Gerald Roosevelt

Email: gerald.maregesi@aesl.co.tz

ABSTRACT texture, grading, and strength of aggregates

are known to affect the compressive
The Udzungwa escarpment is located in strength of concrete. These properties not
Iringa region, extending its boundary to only influence the strength of the concrete,
Morogoro region (Tanzania). The fine and they also affect the workability and the
coarse aggregates produced from the rocks durability of the concrete.
available within these scarps contains an
abundant amount of mica. The pit-run sand Mica in both forms of biotite [black mica] and
which is formed from mechanical and muscovite [white mica] is one of the known
chemical disintegration of the parent rocks deleterious minerals which affects both
available within these scarps are equally compressive strength and water demand of
contaminated with mica. Depending upon the concrete [Dewar, 1963]. The presence of
some geological formation, the mica content mica in soil has also been reported to
[biotite, muscovite] of Udzungwa scarp decrease the unconfined compressive
aggregates is invariably high. In this paper, strength of the soil [Mshali et al., 2012]. The
the effect of mica on compressive strength reduction of the compressive strength and
and on the water demand of the concrete increase in water demand of the concrete
made using fine aggregates from these due to presence of mica was reported by
scarps is presented. From the laboratory test Muller [1971], who showed that the biotite
results, it was established that the presence has a negligible effect on the compressive
of mica in fine aggregates causes a reduction strength of the concrete, while 6% the
of the compressive strength in the range of muscovite in the sand was found to have a
8-23% while the water demand of the noticeable reduction in compressive
concrete was found to increase in the range strength. The muscovite was reported to
of 8-16% [17-33 l/m3]. reduce the slump of the concrete by half.
The decrease in compressive strength with
INTRODUCTION increase in mica content was reported by
Leemann et al., whereby the decrease in
Aggregate takes about 75% by volume of compressive strength was reported to
concrete ingredients and are included in decrease by 20% for 2% inclusion of
concrete as bulking to reduce the cost but muscovite mica in the sand. The reduction in
also, they can improve the concrete a slump which implies an increase in water
volumetric properties such as shrinkage, demand was also reported.
thermal movements and abrasion resistance.
The aggregate is considered as inert filler but In this paper, the negative impact of the
is a crucial ingredient in the concrete since it mica in reducing the compressive strength of
dictates thermal properties, elastic the concrete and effect of increasing the
properties as well as dimensional stability of water demand is presented. The test results
the concrete. Furthermore, the compressive presented in this paper is based on the
strength of the concrete is governing the investigation of aggregates for concrete
choice of the aggregates to be used, i.e. high production carried out during the
strength concrete with compressive strength construction of Lower Kihansi dam.
of more than 50 MPa requires aggregate
possessing higher mechanical strength. The AREA OF STUDY
compressive strength of the concrete is
affected by physical and mineralogical The data used for the analysis of both
properties of the aggregates. The shape of compressive strength and water demand
the aggregates, mineralogical composition, were collected during the exercise of
Advanced Engineering Solutions Journal Vol. 1 /2021 2

carrying out concrete mix design of concrete compressive strength and increasing the
used for the construction of Lower Kihansi water demand.
dam which occupies a segment of Udzungwa
escarpment in Iringa region – Tanzania. The
coarse aggregates used for concrete mix
design was crushed from “mucking” gneissic
stone drilled and blasted during tunnelling.
The fine aggregates used during this study
were from the investigated nearby sources
of natural sand namely Kalengakelo [19 Km
South of Kihansi], Chita [17 Km East of
Kihansi], Ngwasi [17 Km South of Kihansi],
Mpanga [44 Km Southwest of Kihansi],
Chisano [9 Km South of Kihansi], Ikule [31
Km Northeast of Kihansi] and Kimbi [40 Km Figure 1: Area of study
Northeast of Kihansi]. The rock formation in
this area is characterised by a high content GEOTECHNICAL PROPERTIES OF THE
of both white and black mica [biotite and AGGREGATES
muscovite] which affects the concrete
making properties of the aggregates The rock available from Kihansi [area of
available within this area. study] is mainly granitic gneiss with a high
percentage of mica primarily in the form of
The presence of mica in the fine aggregates biotite. The summary of the geotechnical
in this area is not only an engineering properties of the fine aggregates as
problem but an economic problem as well. determined in the laboratory is shown in
Mica has serious and harmful implications on Table 1.
concrete production by reducing the

Table 1. Geotechnical properties of coarse and fine aggregates

Absorption

Clay/dust
Sulphate
Chloride
Source

Water

LAAV
SSS

TPF
SG

Kihansi 0.02 2.69 0.25 1.91 120kN 42%

Mpanga 0.005 3.2%
Chisano 0.004 8.6%
Ngwasi 0.004 2.3%
Chita 0.006 0.08 4.8%
Ikule 0.006 0.06 2.61 0.84 1.0%
Kimbi 0.004 0.03 2.69 0.33 3.7%

From the test results of the aggregates as in Table 1 and Figure 2, it is evident that the
provided in Table 1, it can be seen that both coarse and fine aggregates used during this
fine and coarse aggregates used during this study are good aggregate which can be used
study is complying with national and to produce concrete of satisfactory quality.
international standard specifying aggregates
for concrete making. The grading plots of all
fine aggregates samples except one sample
which is narrowly out of the envelope, are
within the envelope of overall limits of fine
aggregates as provided in BS 882:1992.
Generally, from the test results as presented
3Advanced Engineering Solutions Journal Vol. 1 /2021

project area is characterised by high biotite

[black mica] content. The most predominant
rock within the project area was found to be
biotite gneiss, although some tested sample
showed a granitic composition. The main
minerals present within the rock is quartz,
feldspar and biotite, which in most cases was
found to be the main constituent.

The mineralogical analysis of nearby natural

sand sources showed that these sources are
equally contaminated with mica mainly in the
form of biotite [black mica]. Presence of
biotite mica in the sand suggests that the
sand available within the project area were
Figure 2: Grading of the pit run sand formed from mechanical disintegration of
the gneissic parent rock, which contains an
excessive amount of black mica. Presence of
MINERALOGICAL/PETROGRAPHIC muscovite [white mica] in the sand was also
ANALYSIS OF AGGREGATES noted. The content of white mica was more
noticeable in pit-run sand from Kimbi in
The rock samples, as well as the crushed fine which the percentage of white mica was
aggregates and natural sand from various found to be as high as 16%. The percentage
sources available within the Kihansi of minerals from different sources of fine
hydropower project area, were sampled and aggregates located within the proximity of
its mineralogical composition analysed. It the Lower Kihansi hydropower project are
was found that the rock available within the shown in Table 2.

Table 2: Mineralogical composition of coarse and fine aggregates

Source quartz feldspar Biotite Muscovite
Kihansi [crushed] 20-80% 15-55% 25-60% Trace to 8%
Ikule [natural] 55-60 20-25 5-10 Trace to 5%
Kimbi [natural] 53 25 10-16 Trace to 16%
Mgungwe [natural] Trace to 10% Trace to 10%

DESIGN OF EXPERIMENT The control and experimental mixes were

made at a fixed water-cement ratio of 0.6
The effect of the presence of mica on and an aggregate cement ratio of 5.1. The
compressive strength concrete was water-cement ratio of 0.6 was selected to
evaluated using a factorial experiment minimise the effect of aggregate strength on
design. The same water-cement ratio, the resultant compressive strength of
coarse aggregates, amount of mixing water, concrete. Based on previous test results, it
cement type CEM II 42.5, cement content, was established that the water-cement ratio
and source of water and the aggregate- of 0.6 produces a concrete having cube
cement ratio were used for the production of compressive strength of less than 50 MPa.
both control and experimental mixes. The This selection of water-cement ratio was
crushed coarse aggregate was used for both dictated by the fact that the Ten-per cent
control and experimental mixes. The variable Fine of the aggregate used during this study
factors were the fine aggregate of which the was found to be 120 kN. Aggregates with a
non-micaceous sand from Mpji was used for ten-percent fine of 120 kN can be
the control mix while the micaceous sand categorised as moderate, strong aggregate
from different sources within the project which can be used for the production of a
vicinity was used for experimental mixes. concrete grade of not more than 50 MPa.
Advanced Engineering Solutions Journal Vol. 1 /2021 4

having the same cement content, same

The effect of mica on the water demand was water content and the same aggregate-
evaluated using the same procedures as for cement ratio. Due to the variation in water
compressive strength with the exception that demand, the slump was found to be varying
the amount of water was varied until the depending on the quality of fine aggregate
desired slump of 75±25 mm was achieved. used. Neglecting other secondary effects
The water content recorded for achieving which make the strength of concrete vary,
this desired slump of 75±25 mm was termed theoretically, the concrete made using the
as standard water requirement for particular same batch of cement at the same water-
sand. cement ratio is supposed to have the same
compressive strength. Therefore, any
COMPRESSIVE STRENGTH reduction in compressive strength below that
achieved by the control mix was deemed to
The concrete trial mixes for determination be caused by the presence of mica in the fine
compressive strength of the mixes were aggregates. The comparison of the achieved
made at a constant water-cement ratio of cube compressive strength of control mix
0.6 and a fixed aggregate-cement ratio of and the experimental group mixes is shown
5.1 in line with experiment design. The water in Table 3, from it can be seen that the 28
content was fixed at 205 l/m3, while the days cube compressive strength of the
cement content was fixed at 345 kg/m3. The concrete made using micaceous sand
control and experimental concrete mixes exhibited low compressive strength in the
were made using identically the same mix range of 8-23%.

Table 3: The compressive strength of micaceous sand and non-micaceous

W/C 7 days’ 28 days’ 7 days 28 days Mica content (%) Sand
cube cube (% of (% of Biotite Muscovite type
strength strength control) control)
(Mpa) (Mpa)
Control 0.6 23.9 32.9 100 100 0 0 Natural
Kimbi 0.6 17.9 25.2 75 77 10-16 0-16 Natural
Kihansi 0.6 22.0 29.9 92 91 25-60 0-8 sand
Crushed
Ikule 0.6 22.1 30.3 92 92 5-10 0-5 sand
Natural
Mgugwee 0.6 20.5 28.7 87 87 0-10 0-10 sand
Natural
sand
WATER DEMAND OF CONCRETE of the aggregates. The shape of the
aggregates was pustulated to be uniform
For the purpose of this study, the standard because only pit-run sand was used for
water demand was defined as the water making both control and experimental
demand, which gives a slump of 75±25 mm. mixes. The effect of grading was not taken
The mix design was carried out by adjusting into consideration since the grading of all
the water content until the desired slump of sources of sand used for making
75±25 mm was achieved. The concrete experimental mixes were found to be within
mixes were made in replicates to improve a narrow envelope [Figure 2]; therefore its
the reliability of the test results. The water impact on the water requirement of the
demand to achieve the standard slump of mixes was postulated to be negligible.
75±25 mm was determined for both non-
micaceous [control sand] and micaceous The water content required to achieve the
sand [experimental group]; the increase in standard slump of 75 ±25 mm for sand
water demand above the amount used in sources investigated is given in Table 4, from
control mix was deemed to be caused by the which it can be seen that presence of mica
presence of mica within the sand. It is worth in the fine aggregates increases the water
noting that the water demand is affected by demand in the range of 17-33 litres/m3
grading of the sand, shape and surface area
5Advanced Engineering Solutions Journal Vol. 1 /2021

which translates to 8.1%-16.1% increase in

water demand.

Table 4: Water demand to achieve a slump of 75±25 mm

Source Water Demand, Mica content Increase % increase
litres/m3 Biotite Muscovite (l/m3)
Control [natural] 205 0 0
Kimbi [natural] 238 10-16 0-16 33 16.1%
Ikule [natural] 222 5-10 5-10 17 8.1%
Mgugwe [natural] 225 0-10 0-10 20 9.8%

attributed to the presence of high muscovite

DISCUSSION OF THE RESULTS [white] mica in the sand. This result supports
earlier researches which concluded that
Several researchers have reported that the presence of muscovite [white mica] within
compressive strength of concrete is greatly the sand is more injurious on compressive
affected by the presence of mica in fine strength of the concrete compared to biotite
aggregates. While mica is less likely to cause [black mica].
problems when incorporated in the stone
portion of the mix, in the sand, the presence From the test results given in Table 3, it is
of mica can influence the water demand, seen that to achieve the desired targeted
compressive strength and flexural strength. mean strength of concrete during mix design
The research has shown that 5% of biotite as well as during concrete production using
content resulted in 6% decrease in micaceous sand, additional cement content
compressive strength while 10% content is required. The approximate relationship
resulted in a 10% loss of strength [Hoon, R.C between water-cement ratio and cube
and Sharma, K.R] . Davis at el reported that compressive strength of the concrete for the
the compressive strength could be reduced cement content used during this study is
in the order of 20 -30 % due to the presence shown in Figure 3, from which it can be seen
of mica in the fine aggregate. Research that to achieve cube compressive strength of
carried out by PCI in South Africa reported 32.9 MPa achieved by the control mix,
that the presence of muscovite mica in the additional cement content is required, i.e.
sand could reduce the concrete compressive lower water-cement ratio.
strength in the order of 35% for 5%
inclusion and 60% for 10% inclusion. For Kimbi and Ikule sand, it can be seen that
the water-cement ratio of 0.6 yields a 28
Based on the compressive strength test days cube compressive strength of 25.2 MPa
results as presented in Table 3, it can be and 30.2 Mpa respectively. For Ikule sand,
seen that the presence of mica in the fine the achieved compressive strength is 2.6
aggregates available within the Udzungwa MPa, and for Kimbi sand is 7.7 MPa less than
scarp can reduce the cube compressive the compressive strength produced by the
strength of the concrete in a range of 8- control mix. Therefore, to achieve the same
23%. compressive strength of 32.9 MPa, the
targeted compressive cube strength needs
The compressive strength of the concrete to be increased to 40.6 MPa (+7.7 MPa) for
reported in Table 3 indicates that the Kimbi sand, which translates to an
compressive strength of the concrete made approximate water-cement ratio of 0.48. For
using Kimbi pit-run sand is 23% less than the Ikule sand, the targeted compressive cube
control mix. Table 2 gives the mineralogical strength needs to be increased to 35.5 MPa
composition of sand source, which shows [+2.6 MPa] which translates to a water-
that Kimbi pit-run sand has muscovite mica cement ratio of water-cement ratio of 0.53.
as high as 16%. Therefore, the substantial
reduction of the compressive strength of the The water content of the control mix was
concrete made from Kimbi sand can be fixed at 205 litres/m3. Therefore, the
Advanced Engineering Solutions Journal Vol. 1 /2021 6

estimated required cement content to water content used during the mix design of
produce concrete with cube compressive the control mix was 205 l/m3. The
strength of 32.9 MPa using micaceous sand experimental mixes were produced using
from Kimbi is estimated to be 205/0.48=427 micaceous sand. The water demand for the
kg/m3. For Ikule sand, the estimated cement experimental mixes was found to increase
content required is 205/0.53= 387 kg/m3. from 205 l/m3 [for the control mix] to 222-
Therefore, the increase in cement 238 l/m3 [17-33 l/m3 increase in water
requirement due to the presence of mica in demand]. Due to an increase in water
the soil is estimated to be in the range of 42- requirements, the cement content required
82 kg/m3. to produce the control mix at a water-
cement ratio of 0.6 needs to be increased
from 345kg/m3 used for the control mix to
370-397 kg/m3 to maintain the water-
cement ratio of 0.6. Therefore, the increase
in cement content due to the presence of
mica within the fine aggregate is estimated
to be in the range of 25–52 kg/m3.

SUMMARY AND CONCLUSION

The study has shown that the compressive

strength of concrete made using micaceous
sand is reduced compared to concrete made
from non-micaceous sand. Based on the
compressive strength test results obtained
using pit-run micaceous-sand from
Udzungwa scarp, the compressive strength
Figure 3: The approximate Relationship is reduced in the range of 8-23% as
between water-cement ratio and determined from concrete mixes with cube
compressive strength compressive strength of 32.9 MPa at the
water-cement ratio of 0.6. It can be inferred
that the injurious effect of the presence of
EFFECT OF MICA ON WATER DEMAND mica on the compressive strength is more
appreciable when the content of white
The presence of mica in fine aggregates is [muscovite] mica is predominant. When
one of the known factors which affects the black mica [biotite] is present the reduction
water demand. Research conducted by PCI of compressive strength is noted but not so
of South Africa reported that the water pronounced compared to when muscovite
demand is increased by about 6 litres/m3 for mica is present as evidenced by the test
every 1% of muscovite contained in the results shown in Table 4 which shows that
sand. Kimbi sand affected the compressive
strength appreciably as compared to other
From Table 3, it can be seen that the sources of fine aggregate because it has an
presence of mica in the fine aggregate leads appreciable amount of free white mica. It
to an increase in water demand in the range was established that the presence of mica in
of 17-33 litres/m3. Because the strength of the sand is likely to increase the cement
the concrete is primarily governed by the requirement in the range of 42- 82 kg/m3
water-cement ratio; therefore, an increase in based on the test result of compressive
water demand is normally associated with an strength at water-cement ratio 0.6.
increase in cement content to produce the
same grade of concrete. Based on the water It was also noted that the water demand of
demand given in Table 4, the increase in concrete made using micaceous sand is
cement content caused by the increase in higher than that made from non-micaceous
water demand was computed. The standard sand. The water demand was found to
 7Advanced Engineering Solutions Journal Vol. 1 /2021

increase in range of 17-33 l/m3 as tested 2. Hoon, R.C. and Sharma, K.R. The
using 20 mm nominal size of aggregate selection, processing and specification of
which translates to increase in water content aggregates for concrete for large dams;
in terms of percentage in the range of 8.1- effect of employing micaceous sand as
16.1%. The test results suggest that when fine aggregates fraction on the
the natural sand contains significant amount properties of cement mortar and
muscovite mica, more water is required to concrete. 7th international congress on
achieve the desired workability compared to large dams, Rome, 1961, Vol.1, pp.363-
when black mica [biotite] is present. Due to 379.
an increase in water demand, it was 3. Fulton’s Concrete Technology,
established that the cement content Aggregates for concrete, pp 56-57,
increases in the range of 25-52 kg/m3 to Seventh Edition 1994.
maintain the same water-cement ratio. 4. Mshali, M.L and Visser A.T, Influence of
mica on unconfined compressive
The combined effects of mica on strength of cement-treated weathered
compressive strength and, on the water granite gravel, Journal of the South
demand can increase the cement content in African Institution of Civil Engineering,
the range of 67-134 kg/m3. Vol 54 No.2, October 2012, pages 71-77,
paper 803
Based on the test result recorded during this 5. Muller, O.H, Some Aspects of the Effect
study, the compressive strength, and the of Micaceous Sand on Concrete, The Civil
water demand, is negatively impacted by the Engineer in South Africa, September
presence of both forms of mica in the sand. 1971
The presence of muscovite [white mica] in 6. Leeman, A and Holzer, L, Influence of
the sand was found to be more injurious to Mica on the Properties of Mortar and
both compressive strength and water Concrete,
demand compared to black mica. https:/www.reaserchgate.net/publicatio
n/304181219, 2001
REFERENCES: 7. Dewar, J.F, Effect of mica in the fine
aggregates on the water requirements
1. Davis, D.E. and Alexander, M.G. and strength of concrete, Techn. Rep.
Properties of aggregates in concrete. Cem. concr. Asstc., London 1963
Part 1, Sandton: Hippo Quarries, 1989.

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