Polymer modified jute fibre as reinforcing agent controlling the physical and mechanical characteristics of cement mortar
Construction and Building Materials 49 (2013) 214–222
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Polymer modiﬁed jute ﬁbre as reinforcing agent controlling the physical and mechanical characteristics of cement mortar Sumit Chakraborty, Sarada Prasad Kundu, Aparna Roy, Basudam Adhikari, S.B. Majumder ⇑ Materials Science Centre, Indian Institute of Technology, Kharagpur 721 302, India
h i g h l i g h t s Methodology to disperse polymer modiﬁed jute ﬁbre homogeneously into the mortar. Signiﬁcant improvement of CCS, MOR, and FT in jute ﬁbre reinforced mortar. Substantial improvement in TI as well as the PCRE in modiﬁed mortar. Plausible mechanism to explain the improvement in mechanical properties.
a r t i c l e
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Article history: Received 5 September 2012 Received in revised form 26 July 2013 Accepted 18 August 2013 Available online 10 September 2013 Keywords: Cement Polymer Fibre reinforcement Mechanical properties Interfacial bonding
a b s t r a c t Polymer modiﬁed alkali treated jute ﬁbre as a reinforcing agent, substantially improves the physical and mechanical properties of cement mortar with a mix design cement:sand:ﬁbre:water::1:3:0.01:0.6. The workability of the mortar is found to increase systematically from 155 ± 5 mm (control mortar) to 167 ± 8 mm (0.2050% polymer modiﬁed mortar). The density of the mortar is increased from 2092 kg/ m3 to 2136 kg/m3 with a concomitant reduction of both water absorption and apparent porosity. Optimal polymer content in emulsion (0.0513%) is found to increase the compressive strength, modulus of rupture and ﬂexural toughness 25%, 28%, 387% respectively as compared to control mortar. Based on the X-ray diffraction and infra-red spectroscopy analyses of the mortar samples a plausible mechanism of the effect of modiﬁed jute ﬁbre controlling the physical and mechanical properties of cement mortar has been proposed. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Natural ﬁbres as reinforcing agent in cement matrix are nowadays being considered as effective alternative to steel and other inorganic synthetic ﬁbres [1,2]. Natural ﬁbres such as sisal, coconut, sugar-cane bagasse, hemp, jute are reported to yield improved mechanical strength of the cement based composites [3–7]. Additionally they also enhance the post-cracking resistance, yield high-energy absorption characteristics and improve the fatigue resistance of cement based composites [8–10]. Reviewing the literature, it remains difﬁcult to disperse the natural ﬁbre into cement matrix and also their long term durability in cement matrix is yet to be investigated [11–14]. The potential application of natural ﬁbre reinforced cement composites are limited to those area where energy must be absorbed or the areas prone to impact damage. Accordingly, natural ﬁbre reinforced cement composites are most suitable for shatter and earthquake resistant construction, foundation ﬂoor for machinery in factories, fabrication of light weight ⇑ Corresponding author. Tel.: +91 3222 283986; fax: +91 3222 282274. E-mail address: email@example.com (S.B. Majumder). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
cement based rooﬁng and ceiling boards, wall plaster, and construction materials for low cost housing . Variety of factors inﬂuences the physical and mechanical properties of natural ﬁbre reinforced cement composites. These factors may be grouped according to (i) the type and characteristics of reinforcing ﬁbres, (ii) nature of the cement based matrix and mix design, and (iii) way of mixing, casting and curing of the composites . Among these parameters, the compatibility between the ﬁbre and cement based matrix leading to a homogeneous distribution of the reinforcing ﬁbres remains one of the most dominating factors that inﬂuences the mechanical properties of these composites . The ﬁbre–matrix compatibility is dominated by the chemical composition of the reinforcing ﬁbre together with their surface properties. Due to the parametric dependence of so many factors, the wide scattering in the mechanical properties of natural ﬁbre reinforced cement composites as tabulated in Table 1 seem to be obvious. In the present work, we aim to investigate the effect of jute ﬁbre as a reinforcing agent to cement mortar. For homogeneous distribution of jute ﬁbre into the cement matrix we have modiﬁed both the chemical composition as well as surface properties of jute ﬁbre
S. Chakraborty et al. / Construction and Building Materials 49 (2013) 214–222 Table 1 Comparative study of mechanical behavior of different ﬁbre reinforced cement composites. Fibre type
Type of modiﬁcation
Mechanical properties a
Eucalyptus Hemp Jute Kraft banana E grandis Kenaf (1.2, %) Kraft Coconut husk Bagasse ﬁbre Jute a b c d e f
Compressive strength. Modulus of rupture. Flexural modulus. Fracture toughness. Toughness index. Post cracking resistance energy.
by a combined dilute alkali and polymer emulsion treatment. The effect of ﬁbre modiﬁcation on the physical and mechanical properties of cement mortar has been investigated. Moreover, the effect of chemical treatment of the reinforcing ﬁbres on their durability in highly alkaline cement environment has also been investigated. Finally, the plausible mechanism of such ﬁbre treatment controlling the physical and mechanical properties of cement mortar is elucidated. 2. Experimental 2.1. Preparation of alkali and polymer modiﬁed jute ﬁbre reinforced cement mortar Portland pozzolana cement conﬁrming with IS 1489-1991 (reafﬁrmed 2005) (Ambuja cement)  was used as the binder material for the preparation of cement mortar. The oxide composition of this cement is shown in Table 2. The local river sand was used for the preparation of cement mortar. This sand did not contain any organic substances which might affect cement hydration reaction. To evaluate grading zone and average particle size of sand, sieve analysis was performed. From the sieve analysis (Fig. 1), it was conﬁrmed that the used sand is in grading zone II with average particle size 0.3 mm. TD-4 grade jute ﬁbres were used as reinforcing agent. As received jute ﬁbres, being long enough, could not be used as reinforcing
Table 2 The oxide composition of Portland pozzolanic (Ambuja) cement.
Loss of ignition.
agent in cement. Therefore to use the jute ﬁbre as reinforcement in cement composite, the long jute ﬁbres were chopped into 5 mm length. The average diameter of used jute ﬁbre was 0.062 ± 0.014 mm. The treatment composition of jute with alkali and polymer latex is shown in Table 3. First the requisite amount of jute as mentioned in Table 3 was soaked with the 0.5% alkali solution following which the spent alkali solution was decanted out after 24 h of soaking. Next the respective amounts of Sika latex containing 41% solid (carboxylated styrene butadiene (SBR)) was diluted with 1000 ml of water and added to the alkali soaked wet jute. The cement mortar was prepared following the composition shown in Table 4. In the mix design the weight fraction of cement:sand:ﬁbre:water was kept 1:3:0.01:0.6. The total alkali and polymer treated jute (as shown in Table 3) were mixed with half part of the cement required to make the mortar. A mechanical mixer was used to make uniform slurry after 10–15 min mixing. The required amount of sand, rest of the cement and additional amount of water was mixed thoroughly with the slurry for another 10–20 min. The fresh mortar thus prepared was cast immediately in 110 mm (length (l)) Â 20 mm (breadth (b)) Â 20 mm (depth (d)) mould for ﬂexural specimen and 70.6 mm cubic mould for compressive specimen. The mortar samples were allowed to set in the moulds for 24 h at ambient temperature (30 ± 2 °C). The samples after setting were removed from the mould and water cured for 7, 28, 42, and 90 days. After curing, the mortar specimens were dried under ambient condition. For the characterization of polymer modiﬁed jute ﬁbre reinforced mortar, minimum six samples of each batch were tested. As shown in Table 4, nine different formulations (viz., 1–9) were used for the preparation of the mortar samples. In these mortar samples, the ratio of cement:sand:ﬁbre were kept constant, however, the solid polymer: water (weight to volume) ratio were varied. The solid polymer content in emulsion (deﬁned as weight of solid polymer in 100 ml water) was varied in between 0.0257% and 0.205% (w/v). In this experiment, for each formulation 6 samples were fabricated for each test. To evaluate the durability of the alkali polymer modiﬁed jute ﬁbres in alkaline cementitious medium, the combined alkali polymer modiﬁed jute reinforced cement paste was prepared with the mix design cement:alkali treated jute:water 1:0.01:0.6. In this test 100 mm long jute ﬁbres were used. Initially the jute ﬁbres were treated with 0.5% NaOH solution for 24 h followed by mixing with polymer
Fig. 1. (a) A comparative study of cumulative mass (%) passing of ﬁne aggregate (sand) through the equivalent spherical diameter sized sieve with the standard value of grading zone II, (b) retention of ﬁne aggregate on different equivalent spherical diameter sized sieve.
S. Chakraborty et al. / Construction and Building Materials 49 (2013) 214–222
Table 3 Treatment composition of jute with alkali and polymer latex. Ingredients
For modiﬁcation with alkali and polymer latex
Jute ﬁbre (2–5 mm long) (g) Aqueous sodium hydroxide (0.5%) (ml)a Water based Sika latex (41% solid content) (ml)
30 900 0.625 0.257b 1000
Water (ml) a b
30 900 1.250 0.513b 1000
30 900 2.500 1.025b 1000
30 900 5.000 2.050b 1000
After 24 h of soaking in alkali spent alkali solution was decanted. Weight of polymer of Sika latex in respective formulations on dry basis.
Table 4 Composition of jute reinforced cement mortar. Components
Cement (kg) Sand (kg) Raw/0.5% alkali treated jute Weight of polymer (dry basis) Water (ml) (for polymer emulsion) Additional water (ml) (for mortar mixing) a b c d
Formulation No 1
3 9 – – – 1800
3 9 30a – – 1800
3 9 30b 0.257c 1000d 800
3 9 30b 0.513c 1000d 800
3 9 30b 1.025c 1000d 800
3 9 30b 2.050c 1000d 800
3 9 30b 0.513c 1000d 740
3 9 30b 1.025c 1000d 680
3 9 30b 2.050c 1000d 620
Weight of water soaked raw jute (g). Weight of alkali treated jute (g). Weight of polymer of Sika latex in respective formulations on dry basis. Added water for making polymer emulsion (ml).
emulsion (0.0513% polymer content in emulsion) for 10 min. The combined alkali polymer modiﬁed jute ﬁbres were then dispersed in cement slurry to prepare jute cement paste. After waiting for 24 h, the semi-hardened and hydrated jute cement paste was water cured up to 360 days. During curing, at least twenty-ﬁve single strand jute ﬁbres were isolated in regular intervals (cured for 7, 28, 42, 90, 180 and 360 days), washed sequentially in water and acetone, and oven dried at 105 °C for 24 h. Finally, the tensile strength of these ﬁbres was measured using a universal testing machine.
2.2. Physical properties and microstructure of jute ﬁbre and ﬁbre reinforced mortar Flow behaviour of the freshly prepared cement mortar (which indicates its workability) is estimated by a ﬂow table test in accordance with IS 1727 standard . The bulk density (both wet and dry), water absorption, and apparent porosity of the water cured mortar samples were estimated according to ASTM C 948  standard. Fourier transformed infra-red spectroscopy (FTIR) measurements were performed on jute ﬁbre as well as mortar samples using a spectrometer (Nexus 870, Thermo Nicolet Corp. USA). Oven dried (at 85 °C for 1 h) jute ﬁbre as well as powdered mortar samples were mixed with KBR to make pellets for FTIR measurements. The FTIR spectra were recorded in the wave number range between 4000–400 cmÀ1 after averaging 32 scans. The structural characteristics of raw as well as alkali modiﬁed jute ﬁbres and the water cured mortar samples were investigated using an X-ray diffractometer (Ultima III, Rigaku Inc. Japan). Cu Ka radiation (40 kV, 30 mA) was used to record the X-ray diffractograms of these samples in the rage of 2h between 10° and 60° maintaining at a scanning rate of 1ominÀ1.
The micrographs of the jute ﬁbre and fractured surface of the mortar specimens were recorded using a scanning electron microscope (Vega-LSV, TESCAN, Czech Republic). A thin gold coating was applied on the surface of the samples to avoid charging. 2.3. Mechanical properties of jute ﬁbre and jute ﬁbre reinforced mortar The compressive strength measurements were carried out using a 1000 kN hydraulic universal testing machine (AIM: 31402, S. No. 091020). Mortar cubes (volume = 3.52 Â 105 mm3) samples were tested (without any preload) using a loading rate 13 kN minÀ1 in compliance with the IS 516 standard . The compressive strength or cold crushing strength (CCS in MPa) was calculated measuring the fracture load (F in N) and area of the face of the cube (A in mm2) using the following relation.
CCS ¼ F=A
The ﬂexural tests were performed using a universal testing machine (Hounsﬁeld H10KS). A three point bending conﬁguration was used to determine the modulus of rupture (MOR). Rectangular water cured mortar specimen (110 mm (l) Â 20 mm (b) Â 20 mm (d)) was used as sample. During the ﬂexural tests, the span length (L) = 60 mm and constant loading rate 1.2 mm minÀ1 were maintained as per IS 4332  speciﬁcation. The MOR is determined using the following relation 2
MOR ¼ 3P Á L=ð2 Á b Á d Þ
Fig. 3. Variation of the density (wet and dry), water absorption and apparent porosity of jute modiﬁed mortar samples with the polymer content in emulsion (%).
S. Chakraborty et al. / Construction and Building Materials 49 (2013) 214–222 Table 5 The ﬂow table value of the control and polymer modiﬁed jute cement mortar. Formulation No.
FT ¼ ðAbsorbed energy during flexural testÞ=ðArea of the broken sectionÞ
where the numerator is the area under the load deﬂection curve (shaded region in Fig. 2). Flexural modulous (F.M) is estimated using the following relation 3
F Á M ¼ m Á L3 =ð4b Á d Þ
where ‘m’ is the slope of the load–deﬂection curve during elastic deformation (usually in the deﬂection regime between 0.05 and 2.0 mm) and L is support span length. Toughness indices (TI) are deﬁned as ratio of the whole area under the ﬂexural load–deﬂection curve and the area under the deﬂection of maximum load. This is also termed as peak load toughness indices . Finally, the difference between the total absorbed energy during ﬂexural test and absorbed energy up to peak load is known as post cracking resistance energy. This is estimated as the difference between the whole area under the ﬂexural load–deﬂection curve and area under the deﬂection of maximum load. The tensile strength of the jute ﬁbres, isolated from jute reinforced cement paste after curing for speciﬁed period in the range of 7–360 days, were measured using a 10 kN universal tensile testing machine (H10KS, Hounsﬁeld, Salfords, UK). A gauge length of 25 mm was employed with a crosshead speed of 2 mm/min in accordance with ASTM D3822-01 (ASTM, 2001) . For this test each single ﬁbre was mounted within a cardboard frame (with a rectangular opening of 15 mm in width and 30 mm in height) using adhesive. The frame was placed within the jaws of universal testing machine (UTM) equipped with a 100 N load cell. At least twenty-ﬁve single ﬁbres each randomly drawn from cement paste were tested.
3. Result 3.1. Physical, mechanical and microstructure analysis of combined alkali and polymer modiﬁed jute ﬁbre reinforced mortar
Fig. 4. Variation of ﬂexural modulus (FM), compressive and ﬂexural strength of the control and ﬁbre-reinforced mortar samples (cured for 28 days) with the polymer content in emulsion (%).
Fig. 5. SEM micrographs of the fractured surface of the (a) control, (b) 0.0257%, (c) 0.0513%, and (d) 0.02050% polymer modiﬁed jute reinforced mortar specimens.
where P is the fracture load, ‘L’ is the support span length, ‘b’ is the breadth and ‘d’ is the depth of the mortar samples. From the recorded load–deﬂection curve (Fig. 2); ﬂexural modulus (FM), ﬂexural toughness (FT), toughness index (TI), and post cracking resistance energy (PCRE) are estimated as described below: Toughness is the energy absorption capacity of the composite which deﬁnes its ability to resist fracture under static, dynamic or impact load. The ﬂexural toughness (FT) is determined using the following relation
Fig. 3 shows the variation of the density (wet and dry), water absorption and apparent porosity of jute modiﬁed mortar samples as a function of the polymer content in emulsion (%) used for cement hydration. As shown in the ﬁgure, the densities are increased, whereas the water absorption as well as apparent porosity is reduced with the polymer content. Usually in most of the studies on polymer modiﬁed cement composites, the weight ratio of polymer: cement is kept more than 5% [25–28]. It is reported that higher polymer: cement affects the cement hydration, however, coherent polymer ﬁlm retards the propagation of tiny cracks in cement mortar forming an interpenetrating structure with the modiﬁed cement mortar with lower rigidity. Therefore optimum polymer: cement ratio improves the mechanical properties of cement mortar. Unlike all these reports, in the present work, very small amount of SBR based latex is used to make the cement mortar. As shown in Table 5, the ﬂow table value of the jute-modiﬁed cement mortar is systematically increased with the polymer content in emulsion used for cement hydration. The role of the polymer modiﬁcation in controlling the physical and mechanical properties of jute ﬁbre reinforced concretes are discussed later. Fig. 4 shows the variation of ﬂexural modulus (FM), compressive strength (CCS) and modulus of rupture (MOR) of the control and ﬁbre-reinforced mortar samples (cured for 28 days) with the polymer content in emulsion (%). Interestingly, both compressive strength and MOR are improved up to the 0.0513% polymer content. Thus with polymer modiﬁcation, the CCS and MOR of the control mortar has increased from 28 MPa and 7.0 MPa to 35 and 9.0 MPa respectively. With further increase of polymer content up to 0.2050%, the CCS and MOR of the jute reinforced polymer modiﬁed mortars are decreased however, still remains comparable to/better than the control sample. In contrast to this, the ﬂexural modulus values are decreased with the increase of the polymer content (%). As expected, the CCS and MOR values increase with curing days, however, the trend of their variation with the polymer content in emulsion remains similar to that presented in Fig. 4 for mortar samples cured for 28 days. Thus for 90 days cured mortar samples the CCS and MOR values are measured to be 37.9 MPa and 12.8 MPa respectively. As presented in Table 5, the ﬂow table values were systematically increased with the increase of the polymer content in emulsion (%). For the polymer content 0.0513%, the
S. Chakraborty et al. / Construction and Building Materials 49 (2013) 214–222
Fig. 6. The force-extension curves of control, raw jute reinforced and polymer modiﬁed jute reinforced mortar specimens with polymer contents varying from 0.0257% to 0.2050% (six samples of formulation Nos. 1–6 as indicated in Table 4).
water content required for cement hydration was reduced from 60% to 58% to yield the ﬂow table value (156 ± 8 mm) similar to that of control mortar (155 ± 5 mm). As a result, the CCS and MOR values of the mortar samples were found to increase further up to 36 MPa and 9.3 MPa respectively after 28 days curing. Interestingly, when the curing time is increased for 90 days, the CCS and MOR values of these samples are increased to 38.4 MPa and 14 MPa respectively. The above results illustrate that by tuning the processing methodologies, the mechanical properties of polymer modiﬁed jute reinforced cement mortars may be ﬁne tuned depending on the application needs. Fig. 5 shows the SEM micrographs of the fractured surface of the (a) control, (b) 0.0257%, (c) 0.0513%, and (d) 0.2050% polymer modiﬁed jute reinforced mortar specimens. As observed clearly in Fig. 5(c) and (d), with the increase of the polymer contents the porosity of the mortar matrix is markedly reduced. The rubber like SBR contains both rigid styrene and ﬂexible butadiene chains  which help to form coherent polymer ﬁlm and probably an interpenetrating structure with the mortar matrix. Unlike the other literature reports, in this work we have found that a comparatively diluted polymer emulsion is sufﬁcient to yield a coherent polymer ﬁlm. As presented earlier in Fig. 4, since the ﬂexural modulus is reduced with the increase in the polymer content, therefore it advantageous to keep the polymer content low enough to form a coherent ﬁlm and interpenetrating structure with the mortar matrix. Fig. 6 shows the force-extension curves of control, raw jute reinforced and polymer modiﬁed jute reinforced mortar specimens with polymer contents varying from 0.0257% to 0.2050% (6 samples of formulation Nos. 1–6 as indicated in Table 4). It is observed
Fig. 8. Variation of the ﬂexural toughness (FT) and toughness index (TI) with the polymer content in emulsion (%).
Fig. 9. Variation of the post cracking resistance energy (PCRE) with the polymer content in emulsion (%).
from the ﬁgure that maximum load is carried by 0.0513% polymer modiﬁed jute ﬁbre reinforced cement mortar sample. Very small extension is observed for control sample as compared to that of the mortar sample prepared by polymer modiﬁed jute ﬁbre reinforcement. This is due to the occurrence of sudden failure for control sample (Fig. 7(a)), however higher extension for polymer modiﬁed jute reinforced cement mortar is due to the gradual failure as envisaged from the Fig. 7(b). The mortar specimens were also characterized in terms of their ﬂexural toughness and toughness index characteristics. As explained previously (Fig. 2) the fracture toughness and toughness index were calculated from this load extension curve. As envisaged from Fig. 6, initially, the load
Fig. 7. Failure mode in bending test of the control and polymer modiﬁed jute reinforced cement mortar samples.
S. Chakraborty et al. / Construction and Building Materials 49 (2013) 214–222 Table 6 Tensile strength retention (%) of raw and alkali polymer (0.0513%) modiﬁed jute ﬁbres exposed in alkaline cementitious environment for 7–360 days. Exposure time (days)
0 7 28 42 90 180 360
Tensile strength retention (%) of raw and combined alkali polymer modiﬁed jute in cement environment Raw jute
100.0 98.5 92.8 91.9 82.1 77.6 73.7
100.0 99.8 99.1 98.7 96.7 94.3 93.2
of the polymer content. Comparing the results presented in Table 1, it is encouraging to note that as compared to other jute ﬁbre reinforcement report, substantial improvement is achieved in CCS, MOR and FT values reported in the present work. Fig. 9 presents the variation of the post cracking resistance energy (PCRE) with the polymer content in emulsion. The PCRE is substantially improved up to 0.0513% polymer content. With further increase of the polymer content, the PCRE is reduced, however, up to 0.2050% polymer content the estimated PCRE is found to be far improved as compared to the control mortar sample. The results presented above are summarized as follows: Addition of diluted SBR based latex in alkali modiﬁed jute-ﬁbre reinforced mortar is found to systematically increase the ﬂow-table value and density, while reducing the water absorption and apparent porosity of the mortar. Using optimal polymer content in emulsion (0.0513%) substantial improvement in CCS and MOR values has readily been achieved. The ﬂexural toughness is also markedly increased when 0.0513% polymer modiﬁer is used. Irrespective of the polymer contents the ﬂexural modulus is decreased with the increase in the polymer content in emulsion (%). We have observed that the toughness index as well as the post cracking resistance energies is substantially improved in polymer modiﬁed jute reinforced mortars. 3.2. Durability study of jute ﬁbre in cement medium
Fig. 10. The X-ray diffraction patterns of (i) control mortar and (ii) 0.0513% polymer modiﬁed jute reinforced mortar samples cured for 28 days. The letters referring to the XRD peaks are (a) Alite, (b) Belite, (c) Calcite, (g) Genite, (p) Portlandite, and (q) Quartz.
The durability of raw as well as combined alkali polymer (0.0513%) modiﬁed jute ﬁbre (in alkaline cement paste) was also investigated by estimating the value of tensile strength retention of respective ﬁbres. Table 6 shows the tensile strength retention (%) of raw and combined alkali polymer (0.0513%) modiﬁed jute ﬁbres as a function of time for alkaline cementitious environment. As shown in Table 6, in cement environment, combined alkali polymer (0.0513%) modiﬁed jute ﬁbres have better tensile strength retention (%) as compared to their raw jute counterpart. After 360 days exposure in cement paste, almost 93% of tensile strength was retained in combined alkali polymer modiﬁed jute ﬁbre as compared to 74% retention for raw jute ﬁbre. The main reason of natural ﬁbre degradation in alkaline matrix is attributed to be due to Ca2+ ﬁxation (known as mineralization) on ﬁbre surface . When the ﬁbre surface is coated with polymer latex, the Ca2+ ﬁxation seems to be minimized to retard the degradation. 4. Discussion
Fig. 11. Experimental, ﬁtted and the deconvoluted XRD peaks in the 2h range between 15° and 40° of control mortar sample.
carrying capacity increases up to 0.0513% polymer modiﬁed jute ﬁbre reinforced cement sample (formulation code of the sample is 4). With further increase of polymer content load carrying capacity decreases gradually. Therefore, optimal polymer content (0.0513%) shows maximum load carrying capacity. Fig. 8 shows the variation of the ﬂexural toughness (FT) and toughness index (TI) with the polymer content in emulsion (%). As compared to the control specimen, the ﬂexural toughness is substantially increased up to 0.0513% polymer content. With further increase of the polymer content the FT values are decreased, however, the values still remain better than the control sample. On the other hand, the toughness index increases with the increase
In the preceding section we have reported an overall improvement of the physical characteristics and mechanical properties of polymer modiﬁed alkali treated jute ﬁbre reinforced cement mortar samples. The durability of raw as well as combined alkali polymer (0.0513%) modiﬁed jute ﬁbre (in alkaline cement paste) has also been reported. In order to ﬁnd a plausible mechanism controlling these improvements we have performed X-ray diffraction and FTIR analyses of the modiﬁed jute ﬁbres and cement mortar samples. Fig. 10 shows the X-ray diffraction patterns of (i) control mortar and (ii) 0.0513% polymer modiﬁed jute reinforced mortar samples cured for 28 days. It is known that the major constituents of Portland pozzolana cement are alite (a) [tricalcium silicate C3S (Ca3SiO5)], belite (b) [dicalcium silicate C2S (Ca2SiO4)], tricalcium aluminate [C3A (Ca3Al2O6)], and tetracalcium alumino ferrite [C4AF (Ca4AlnFe2ÀnO7,)]. As compared to ordinary Portland cement (OPC), the Portland pozzolana cement contains substantial amount of quartz (q) and minute quantity of gypsum (g) as well. The hydration of alite and belite phases produces Ca(OH)2 (p) (portlandite) and amorphous calcium–silica–hydrate (C–S–H) . All the
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Fig. 12. (a) The FTIR spectra of (i) alkali treated jute ﬁbre and (ii) polymer (0.0513%) modiﬁed jute ﬁbre, (b) the FTIR spectra of (i) mortar (control) and (ii) 0.0513% polymer modiﬁed jute reinforced mortar specimen cured for 28 days, (c) the experimental, ﬁtted and deconvoluted modes for the mortar (control) specimen.
Table 7 Assignment of the FTIR modes of hydrated cement mortar. Peak position (cmÀ1)
OAH stretching of Ca(OH)2 Symmetric and asymmetric stretching (m1 and m3) of the OAH vibrator of the water molecules The asymmetric stretching of HAC bond present in the organic compound m2 Deformation mode of the molecular water HAOAH absorbed
2928 1644 1477 and 1420 970
m3 of COÀ2 3 The m3 stretching of SiAO bond of calcium silicate hydrate (CASAH). This mode accounts for the polymerization of the SiO4À 4 units present in C3S and C2S during hydration
diffraction peaks corresponding to these phases in the cured mortar specimens are indexed in Fig. 10. As noted in Fig. 10, the characteristic peak that corresponds to portlandite phase (p) appears at 2h = 18°. The diffraction peak of the major reactant alite (a) is identiﬁed at 2h = 29.4°. Estimation of the ratio of the integrated area of the portlandite (p) and allite (a) peak ratio could therefore be treated as the index of the degree of hydration. The XRD pattern of the cured mortar specimen was ﬁtted using a commercial software (Peakﬁt 4.1, Jandel Scientiﬁc) and Fig. 11 shows the ﬁtted as well as deconvoluted XRD peaks in the 2h range 15–40°. Similar ﬁtting was also performed of the XRD pattern of 0.0513% polymer modiﬁed jute reinforced cement mortar sample (not shown). We have found that the integrated peak area ratio of the peaks corresponding to the portlandite (p) and alite (a) phase (Ap/Aa) of the control
sample (0.169) is reduced to 0.140 in polymer modiﬁed jute reinforced mortar sample. This is indicative to two possibilities: ﬁrst, as compared to the control sample, either the formation of portlandite is less in the polymer modiﬁed mortar specimen or the hydrated product (portlandite) (in the polymer modiﬁed jute reinforced mortar specimen) is consumed elsewhere. To better understand this phenomenon we have performed FTIR analyses of the alkali modiﬁed jute ﬁbre and the polymer modiﬁed jute ﬁbres. Fig. 12 (a) compares the FTIR spectra of alkali treated jute ﬁbre with the one after polymer modiﬁcation. As shown in the ﬁgure, the absorption band 3559 cmÀ1 is assigned to be due to OAH stretching. The mode at 2921 cmÀ1 is assigned to be due to CAH stretching vibration. The absorption band at 1739 cmÀ1 is absent in alkali modiﬁed ﬁbre and appears only in polymer modiﬁed ﬁbre (encircled with dotted mark). The mode is assigned to be due to the C@O stretching of ester linkage . The appearance of this band is indicative to some kind of interaction between the polymer additive and alkali modiﬁed jute ﬁbres. In Fig. 12(b) we have compared the FTIR spectra of (i) control mortar and (ii) 0.0513% polymer modiﬁed jute reinforced mortar specimen cured for 28 days. All the absorption bands are indexed and the assigned modes along with their wave numbers are tabulated in Table 7 . As indicated in Table 7, the absorption mode at 3638 cmÀ1 is indexed to be due to OAH stretching of the portandite (Ca(OH)2) phase. The mode 2928 cmÀ1 is due to asymmetric stretching of HAC bond from the organic moieties present in the mortar sample. Considering the mode 2928 cmÀ1 as an internal standard , the change in the intensity of the OAH stretching mode of portandite phase (both in control and polymer modiﬁed mortar specimens) is estimated by ﬁtting these modes using commercial software. The typical ﬁtting and the deconvoluted modes for the control specimen is shown in Fig. 12(c). The ratio of the integrated area of OAH
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Fig. 13. Schematic of the plausible mechanism for the interfacial bonding between the alkali modiﬁed jute ﬁbre and cement matrix (see text for details).
stretching mode and asymmetric HAC mode (AOAH/AHAC) are estimated from these ﬁt. The ratio is estimated to be 0.39 for control and 0.37 for 0.0513% polymer modiﬁed jute reinforced mortar samples. In line to the XRD analyses presented above, the FTIR analyses also indicate that the formation of portandite is retarded in polymer modiﬁed specimens. Through these analyses it is clearly demonstrated that the polymer coating are chemically interacted with alkali modiﬁed jute ﬁbre and retards the formation of the major cement hydration product. Viewing in light of the above analyses we are making an attempt to understand the improvement of both physical and mechanical properties in polymer modiﬁed alkali treated jute reinforced mortar samples. Alkali treatment modiﬁes the ﬁbre composition by removing the amorphous constituents of jute ﬁbres (viz. hemicelluloses, wax etc.) and thereby increasing its crystallinity . The polymer latex modiﬁes the surface of the ﬁbre as well as the mortar matrix to impart homogeneous distribution of the ﬁbre as well as strong interfacial bonding to the mortar matrix. The coherent polymer ﬁlm and its interpenetrating structure with the mortar matrix improve the density reducing the apparent porosity of the modiﬁed mortar. The strong interfacial bonding between the uniformly dispersed jute ﬁbre and mortar matrix retard the crack propagation during fracture. The main constituent of jute ﬁbre is cellulose which contains large number of inter and intra molecular hydroxyl groups. During the alkali (NaOH) treatment, some of these hydroxyl groups in the jute ﬁbre react with Na+ ion to form base exchanged cellulose ﬁbres with OÀNa+ groups (see Fig. 13) . The carboxylated styrene butadiene rubber (SBR) based polymer latex contains carboxylic acid groups . The base exchanged cellulose ﬁbre reacts with the ÀCOOH group of the carboxylated SBR to form an ester linkage forming NaOH as by-product. The formation of such ester linkage has clearly been identiﬁed in FTIR analyses (Fig. 12(a)) To form interfacial bonding to the mortar matrix, some of these ÀCOOH groups of the carboxylated SBR also reacts with the hydrated cement (Ca(OH)2) forming H2O as a byproduct. These reactions are also shown schematically in Fig. 13. As some part of the hydration product (viz. portlandite) is consumed in such reaction, as indicated both in XRD and FTIR analyses (see Fig. 10 and Fig. 12b) the amount of the hydration product is found to be less in the polymer modiﬁed mortar samples. Since a much diluted polymer emulsion is used in the present study, it
seems to be unlikely that the polymer modiﬁcation itself would retard the cement hydration. 5. Conclusions Jute as a natural ﬁbre is used as a reinforcing agent to improve the physical and mechanical properties of cement mortar. The mix design of the mortar was kept, cement:sand:Fibre:water::1:3:0.01:0.6. The chopped jute-ﬁbre (2–5 mm in length) was pre-treated by immersing in 0.5% dilute sodium hydroxide solution overnight prior to disperse in mortar matrix. In this investigation the solid polymer content in emulsion (deﬁned as weight of solid polymer in 100 ml water) was varied in between 0.0257% and 0.205% (w/v). A novel processing methodology was developed to homogeneously disperse alkali and polymer modiﬁed jute ﬁbre into the mortar matrix. The combined alkali and polymer treatment yield mortar where the workability is found to increase systematically from 155 ± 5 mm (control mortar) to 167 ± 8 mm (0.2050% polymer modiﬁed mortar). The density of the mortar is increased from 2092 kg/m3 to 2136 kg/m3 with a concomitant reduction of both water absorption and apparent porosity. Optimal polymer content in emulsion (0.0513%) is found to increase the compressive strength (CCS), modulus of rupture and ﬂexural toughness 25%, 28%, 387% respectively as compared to control mortar without any jute reinforcement. Though the toughness index as well as the post cracking resistance energies are substantially improved with the increase in polymer strength, the ﬂexural modulus is found to decrease as compared to control mortar specimen. Based on XRD and FTIR analyses we have identiﬁed that the alkali treatment and polymer modiﬁcation help the reinforcing jute ﬁbre to form strong interfacial bond with mortar matrix. A plausible mechanism for such bond formation has been proposed to explain the observed improvements in physical characteristics and the mechanical properties of the mortar. Acknowledgement Part of this research work was supported by a research grant from the National Jute Board, Govt. of India. One of the authors
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Mr. S.P. Kundu gratefully acknowledges CSIR for providing ﬁnancial support in the form of an individual junior research fellowship.
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