Mica in Concrete

Advanced Engineering Solutions Journal Vol. 1 /2021 MICA IN CONCRETE Maregesi, Mr Gerald Roosevelt Email: [email protected] ABSTRACT The Udzungwa escarpment is located in Iringa region, extending its boundary to Morogoro region (Tanzania). The fine and coarse aggregates produced from the rocks available within these scarps contains an abundant amount of mica. The pit-run sand which is formed from mechanical and chemical disintegration of the parent rocks available within these scarps are equally contaminated with mica. Depending upon some geological formation, the mica content [biotite, muscovite] of Udzungwa scarp aggregates is invariably high. In this paper, the effect of mica on compressive strength and on the water demand of the concrete made using fine aggregates from these scarps is presented. From the laboratory test results, it was established that the presence of mica in fine aggregates causes a reduction of the compressive strength in the range of 8-23% while the water demand of the concrete was found to increase in the range of 8-16% [17-33 l/m3]. INTRODUCTION Aggregate takes about 75% by volume of concrete ingredients and are included in concrete as bulking to reduce the cost but also, they can improve the concrete volumetric properties such as shrinkage, thermal movements and abrasion resistance. The aggregate is considered as inert filler but is a crucial ingredient in the concrete since it dictates thermal properties, elastic properties as well as dimensional stability of the concrete. Furthermore, the compressive strength of the concrete is governing the choice of the aggregates to be used, i.e. high strength concrete with compressive strength of more than 50 MPa requires aggregate possessing higher mechanical strength. The compressive strength of the concrete is affected by physical and mineralogical properties of the aggregates. The shape of the aggregates, mineralogical composition, texture, grading, and strength of aggregates are known to affect the compressive strength of concrete. These properties not only influence the strength of the concrete, they also affect the workability and the durability of the concrete. Mica in both forms of biotite [black mica] and muscovite [white mica] is one of the known deleterious minerals which affects both compressive strength and water demand of the concrete [Dewar, 1963]. The presence of mica in soil has also been reported to decrease the unconfined compressive strength of the soil [Mshali et al., 2012]. The reduction of the compressive strength and increase in water demand of the concrete due to presence of mica was reported by Muller [1971], who showed that the biotite has a negligible effect on the compressive strength of the concrete, while 6% the muscovite in the sand was found to have a noticeable reduction in compressive strength. The muscovite was reported to reduce the slump of the concrete by half. The decrease in compressive strength with increase in mica content was reported by Leemann et al., whereby the decrease in compressive strength was reported to decrease by 20% for 2% inclusion of muscovite mica in the sand. The reduction in a slump which implies an increase in water demand was also reported. In this paper, the negative impact of the mica in reducing the compressive strength of the concrete and effect of increasing the water demand is presented. The test results presented in this paper is based on the investigation of aggregates for concrete production carried out during the construction of Lower Kihansi dam. AREA OF STUDY The data used for the analysis of both compressive strength and water demand were collected during the exercise of 2 Advanced Engineering Solutions Journal Vol. 1 /2021 carrying out concrete mix design of concrete used for the construction of Lower Kihansi 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 Northeast of Kihansi]. The rock formation in this area is characterised by a high content of both white and black mica [biotite and muscovite] which affects the concrete making properties of the aggregates available within this area. The presence of mica in the fine aggregates in this area is not only an engineering problem but an economic problem as well. Mica has serious and harmful implications on concrete production by reducing the compressive strength and increasing the water demand. Figure 1: Area of study GEOTECHNICAL PROPERTIES OF THE AGGREGATES The rock available from Kihansi [area of study] is mainly granitic gneiss with a high percentage of mica primarily in the form of biotite. The summary of the geotechnical properties of the fine aggregates as determined in the laboratory is shown in Table 1. 0.08 0.06 0.03 2.61 2.69 0.84 0.33 From the test results of the aggregates as provided in Table 1, it can be seen that both fine and coarse aggregates used during this study is complying with national and 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 3.2% 8.6% 2.3% 4.8% 1.0% 3.7% LAAV 0.25 TPF 2.69 SSS Water Absorption 0.02 Clay/dust SG 0.005 0.004 0.004 0.006 0.006 0.004 Sulphate Kihansi Mpanga Chisano Ngwasi Chita Ikule Kimbi Chloride Source Table 1. Geotechnical properties of coarse and fine aggregates 1.91 120kN 42% in Table 1 and Figure 2, it is evident that the coarse and fine aggregates used during this study are good aggregate which can be used to produce concrete of satisfactory quality. 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. Figure 2: Grading of the pit run sand MINERALOGICAL/PETROGRAPHIC ANALYSIS OF AGGREGATES The rock samples, as well as the crushed fine aggregates and natural sand from various sources available within the Kihansi hydropower project area, were sampled and its mineralogical composition analysed. It was found that the rock available within the 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 formed from mechanical disintegration of the gneissic parent rock, which contains an excessive amount of black mica. Presence of muscovite [white mica] in the sand was also noted. The content of white mica was more noticeable in pit-run sand from Kimbi in which the percentage of white mica was found to be as high as 16%. The percentage of minerals from different sources of fine aggregates located within the proximity of the Lower Kihansi hydropower project are shown in Table 2. Table 2: Mineralogical composition of coarse and fine aggregates Biotite Source quartz feldspar Kihansi [crushed] 20-80% 15-55% 25-60% Ikule [natural] 55-60 20-25 5-10 Kimbi [natural] 53 25 10-16 Mgungwe [natural] Trace to 10% DESIGN OF EXPERIMENT The effect of the presence of mica on compressive strength concrete was evaluated using a factorial experiment design. The same water-cement ratio, coarse aggregates, amount of mixing water, cement type CEM II 42.5, cement content, and source of water and the aggregatecement ratio were used for the production of both control and experimental mixes. The crushed coarse aggregate was used for both control and experimental mixes. The variable factors were the fine aggregate of which the non-micaceous sand from Mpji was used for the control mix while the micaceous sand from different sources within the project vicinity was used for experimental mixes. Muscovite Trace to 8% Trace to 5% Trace to 16% Trace to 10% The control and experimental mixes were made at a fixed water-cement ratio of 0.6 and an aggregate cement ratio of 5.1. The water-cement ratio of 0.6 was selected to minimise the effect of aggregate strength on the resultant compressive strength of concrete. Based on previous test results, it was established that the water-cement ratio of 0.6 produces a concrete having cube compressive strength of less than 50 MPa. This selection of water-cement ratio was dictated by the fact that the Ten-per cent Fine of the aggregate used during this study was found to be 120 kN. Aggregates with a ten-percent fine of 120 kN can be categorised as moderate, strong aggregate which can be used for the production of a concrete grade of not more than 50 MPa. 4 Advanced Engineering Solutions Journal Vol. 1 /2021 The effect of mica on the water demand was evaluated using the same procedures as for compressive strength with the exception that the amount of water was varied until the desired slump of 75±25 mm was achieved. The water content recorded for achieving this desired slump of 75±25 mm was termed as standard water requirement for particular sand. COMPRESSIVE STRENGTH The concrete trial mixes for determination compressive strength of the mixes were made at a constant water-cement ratio of 0.6 and a fixed aggregate-cement ratio of 5.1 in line with experiment design. The water content was fixed at 205 l/m3, while the cement content was fixed at 345 kg/m3. The control and experimental concrete mixes were made using identically the same mix having the same cement content, same water content and the same aggregatecement ratio. Due to the variation in water demand, the slump was found to be varying depending on the quality of fine aggregate used. Neglecting other secondary effects which make the strength of concrete vary, theoretically, the concrete made using the same batch of cement at the same watercement ratio is supposed to have the same compressive strength. Therefore, any reduction in compressive strength below that achieved by the control mix was deemed to be caused by the presence of mica in the fine aggregates. The comparison of the achieved cube compressive strength of control mix and the experimental group mixes is shown in Table 3, from it can be seen that the 28 days cube compressive strength of the concrete made using micaceous sand exhibited low compressive strength in the 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 (%) cube cube (% of (% of Biotite Muscovite strength strength control) control) (Mpa) (Mpa) Control Kimbi Kihansi Ikule Mgugwee 0.6 0.6 0.6 0.6 0.6 23.9 17.9 22.0 22.1 20.5 32.9 25.2 29.9 30.3 28.7 100 75 92 92 87 WATER DEMAND OF CONCRETE For the purpose of this study, the standard water demand was defined as the water demand, which gives a slump of 75±25 mm. The mix design was carried out by adjusting the water content until the desired slump of 75±25 mm was achieved. The concrete mixes were made in replicates to improve the reliability of the test results. The water demand to achieve the standard slump of 75±25 mm was determined for both nonmicaceous [control sand] and micaceous sand [experimental group]; the increase in water demand above the amount used in control mix was deemed to be caused by the presence of mica within the sand. It is worth noting that the water demand is affected by grading of the sand, shape and surface area 100 77 91 92 87 0 10-16 25-60 5-10 0-10 Sand type 0 0-16 0-8 0-5 0-10 Natural Natural sand Crushed sand Natural sand Natural sand of the aggregates. The shape of the aggregates was pustulated to be uniform because only pit-run sand was used for making both control and experimental mixes. The effect of grading was not taken into consideration since the grading of all sources of sand used for making experimental mixes were found to be within a narrow envelope [Figure 2]; therefore its impact on the water requirement of the mixes was postulated to be negligible. The water content required to achieve the standard slump of 75 ±25 mm for sand sources investigated is given in Table 4, from which it can be seen that presence of mica in the fine aggregates increases the water demand in the range of 17-33 litres/m3 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 litres/m3 Biotite Muscovite Control [natural] 205 0 0 Kimbi [natural] 238 10-16 0-16 Ikule [natural] 222 5-10 5-10 Mgugwe [natural] 225 0-10 0-10 DISCUSSION OF THE RESULTS Several researchers have reported that the compressive strength of concrete is greatly affected by the presence of mica in fine aggregates. While mica is less likely to cause problems when incorporated in the stone portion of the mix, in the sand, the presence of mica can influence the water demand, compressive strength and flexural strength. The research has shown that 5% of biotite content resulted in 6% decrease in compressive strength while 10% content resulted in a 10% loss of strength [Hoon, R.C and Sharma, K.R] . Davis at el reported that the compressive strength could be reduced in the order of 20 -30 % due to the presence of mica in the fine aggregate. Research carried out by PCI in South Africa reported that the presence of muscovite mica in the sand could reduce the concrete compressive strength in the order of 35% for 5% inclusion and 60% for 10% inclusion. Based on the compressive strength test results as presented in Table 3, it can be seen that the presence of mica in the fine aggregates available within the Udzungwa scarp can reduce the cube compressive strength of the concrete in a range of 823%. The compressive strength of the concrete reported in Table 3 indicates that the compressive strength of the concrete made using Kimbi pit-run sand is 23% less than the control mix. Table 2 gives the mineralogical composition of sand source, which shows that Kimbi pit-run sand has muscovite mica as high as 16%. Therefore, the substantial reduction of the compressive strength of the concrete made from Kimbi sand can be Increase % increase (l/m3) 33 17 20 16.1% 8.1% 9.8% attributed to the presence of high muscovite [white] mica in the sand. This result supports earlier researches which concluded that presence of muscovite [white mica] within the sand is more injurious on compressive strength of the concrete compared to biotite [black mica]. From the test results given in Table 3, it is seen that to achieve the desired targeted mean strength of concrete during mix design as well as during concrete production using micaceous sand, additional cement content is required. The approximate relationship between water-cement ratio and cube compressive strength of the concrete for the cement content used during this study is shown in Figure 3, from which it can be seen that to achieve cube compressive strength of 32.9 MPa achieved by the control mix, additional cement content is required, i.e. lower water-cement ratio. For Kimbi and Ikule sand, it can be seen that the water-cement ratio of 0.6 yields a 28 days cube compressive strength of 25.2 MPa and 30.2 Mpa respectively. For Ikule sand, the achieved compressive strength is 2.6 MPa, and for Kimbi sand is 7.7 MPa less than the compressive strength produced by the control mix. Therefore, to achieve the same compressive strength of 32.9 MPa, the targeted compressive cube strength needs to be increased to 40.6 MPa (+7.7 MPa) for Kimbi sand, which translates to an approximate water-cement ratio of 0.48. For Ikule sand, the targeted compressive cube strength needs to be increased to 35.5 MPa [+2.6 MPa] which translates to a watercement ratio of water-cement ratio of 0.53. The water content of the control mix was fixed at 205 litres/m3. Therefore, the 6 Advanced Engineering Solutions Journal Vol. 1 /2021 estimated required cement content to produce concrete with cube compressive strength of 32.9 MPa using micaceous sand from Kimbi is estimated to be 205/0.48=427 kg/m3. For Ikule sand, the estimated cement content required is 205/0.53= 387 kg/m3. Therefore, the increase in cement requirement due to the presence of mica in the soil is estimated to be in the range of 4282 kg/m3. water content used during the mix design of the control mix was 205 l/m3. The experimental mixes were produced using micaceous sand. The water demand for the experimental mixes was found to increase from 205 l/m3 [for the control mix] to 222238 l/m3 [17-33 l/m3 increase in water demand]. Due to an increase in water requirements, the cement content required to produce the control mix at a watercement ratio of 0.6 needs to be increased from 345kg/m3 used for the control mix to 370-397 kg/m3 to maintain the watercement 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 Figure 3: The approximate Relationship between water-cement ratio and compressive strength EFFECT OF MICA ON WATER DEMAND The presence of mica in fine aggregates is one of the known factors which affects the water demand. Research conducted by PCI of South Africa reported that the water demand is increased by about 6 litres/m3 for every 1% of muscovite contained in the sand. From Table 3, it can be seen that the presence of mica in the fine aggregate leads to an increase in water demand in the range of 17-33 litres/m3. Because the strength of the concrete is primarily governed by the water-cement ratio; therefore, an increase in water demand is normally associated with an increase in cement content to produce the same grade of concrete. Based on the water demand given in Table 4, the increase in cement content caused by the increase in water demand was computed. The standard 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 is reduced in the range of 8-23% as determined from concrete mixes with cube 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 mica on the compressive strength is more appreciable when the content of white [muscovite] mica is predominant. When black mica [biotite] is present the reduction of compressive strength is noted but not so pronounced compared to when muscovite mica is present as evidenced by the test results shown in Table 4 which shows that Kimbi sand affected the compressive strength appreciably as compared to other sources of fine aggregate because it has an appreciable amount of free white mica. It was established that the presence of mica in the sand is likely to increase the cement requirement in the range of 42- 82 kg/m3 based on the test result of compressive strength at water-cement ratio 0.6. It was also noted that the water demand of concrete made using micaceous sand is higher than that made from non-micaceous sand. The water demand was found to 7Advanced Engineering Solutions Journal Vol. 1 /2021 increase in range of 17-33 l/m3 as tested using 20 mm nominal size of aggregate which translates to increase in water content in terms of percentage in the range of 8.116.1%. The test results suggest that when the natural sand contains significant amount muscovite mica, more water is required to achieve the desired workability compared to when black mica [biotite] is present. Due to an increase in water demand, it was established that the cement content increases in the range of 25-52 kg/m3 to maintain the same water-cement ratio. The combined effects of mica on compressive strength and, on the water demand can increase the cement content in the range of 67-134 kg/m3. Based on the test result recorded during this study, the compressive strength, and the water demand, is negatively impacted by the presence of both forms of mica in the sand. The presence of muscovite [white mica] in the sand was found to be more injurious to both compressive strength and water demand compared to black mica. REFERENCES: 1. Davis, D.E. and Alexander, M.G. Properties of aggregates in concrete. Part 1, Sandton: Hippo Quarries, 1989. 2. Hoon, R.C. and Sharma, K.R. The selection, processing and specification of aggregates for concrete for large dams; effect of employing micaceous sand as fine aggregates fraction on the properties of cement mortar and concrete. 7th international congress on large dams, Rome, 1961, Vol.1, pp.363379. 3. Fulton’s Concrete Technology, Aggregates for concrete, pp 56-57, Seventh Edition 1994. 4. Mshali, M.L and Visser A.T, Influence of mica on unconfined compressive strength of cement-treated weathered granite gravel, Journal of the South African Institution of Civil Engineering, Vol 54 No.2, October 2012, pages 71-77, paper 803 5. Muller, O.H, Some Aspects of the Effect of Micaceous Sand on Concrete, The Civil Engineer in South Africa, September 1971 6. Leeman, A and Holzer, L, Influence of Mica on the Properties of Mortar and Concrete, https:/www.reaserchgate.net/publicatio n/304181219, 2001 7. Dewar, J.F, Effect of mica in the fine aggregates on the water requirements and strength of concrete, Techn. Rep. Cem. concr. Asstc., London 1963