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Pakistani bentonite in mortars and concrete as low cost construction material
Applied Clay Science 45 (2009) 220–226
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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Pakistani bentonite in mortars and concrete as low cost construction material J. Mirza a,⁎, M. Riaz b, A. Naseer b, F. Rehman b, A.N. Khan b, Q. Ali b a b
Department of Robotics and Civil, Research Institute of Hydro-Quebec, Varennes, Quebec, Canada J3X 1S1 Department of Civil Engineering, NWFP University of Engineering and Technology, Peshawar, Pakistan
a r t i c l e
i n f o
Article history: Received 16 September 2008 Received in revised form 28 May 2009 Accepted 8 June 2009 Available online 13 June 2009 Keywords: Ordinary Portland cement Bentonite Mortar Concrete Mechanical properties
a b s t r a c t A Pakistani bentonite, in “as is” (21 °C) and “heated” (150 °C, 250 °C, 500 °C, 750 °C and 950 °C) conditions, was incorporated in mortar cubes, concrete cylinders and concrete beams as a partial substitute for Ordinary Portland cement (OPC) and studied in detail. Results showed that OPC mortars and concrete containing 20% “as is” and 25% “heated to 150 °C” bentonite could be used as low-cost construction materials. They will also reduce energy consumption, preserve natural resources and solve environmental problems related to cement production as well as augment the durability and life cycle of the concrete structures. The X-ray diffraction patterns showed that bentonite possessed both crystalline and amorphous phases. The strength activity indices (SAI) after 7 and 28 days were higher than 75% for “as is” and “heated” bentonite, except for the 950 °C samples, which was below the ASTM C618 specified limit of 75%. The maximum SAI was shown by “150 °C heated” bentonite. The compressive strength data also showed similar results for OPC mortar cubes and concrete cylinders containing “150 °C heated” bentonite. When compared with the control mixture, the compressive strength values were the same as for mortar containing 25% bentonite as replacement of OPC. However, these values decreased in concrete initially and started to gain strength remarkably after 28 days. Resistance to sulphate attack and water absorption tests on mortar cubes soaked in 2% magnesium sulphate and 5% sodium sulphate solutions demonstrated consistent improvement as the bentonite content in them was increased. The modulus of rupture of all concrete beams decreased as the OPC substitution level by bentonite increased from 20% to 40%. Bond strength of OPC mortar containing 20% “as is” and 25% “heated to 150 °C” bentonite in brick prisms was almost the same as that of control mixture. © 2009 Elsevier B.V. All rights reserved.
1. Introduction More than half of Pakistan's population lives below the poverty line and the increasing cost of daily usage items is a great burden to a majority. Besides other items, the cost of cement has also increased by more than 150% in a short span of ten years. Low-cost pozzolans (natural and industrial) must, therefore, be explored to benefit the construction industry, as well as the poor people of Pakistan. Natural pozzolans have been used in building construction for centuries. The use of volcanic ash and heated clay dates back to 2000 BC and earlier in some cultures. Many of Roman, Greek, Indian, and Egyptian pozzolan concrete structures still exist today, attesting to the durability of these materials. The North American experience with natural pozzolans dates back to the early 20th century public works projects, such as dams, where they were used to control temperature rise in mass concrete and to act as cementing materials (Kosmatka et al., 2002). It is generally accepted that the use of natural pozzolans in cement, or concrete systems results in many beneficial properties such as low heat of hydration, high ultimate strength, low permeability, high sulphate resistance, and low alkali–silica activity (ACI, 1994). In ad⁎ Corresponding author. Tel.: +1 450 652 8308; fax: +1 450 652 1316. E-mail address: [email protected] (J. Mirza). 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.06.011
dition, it is well established that the use of pozzolanic and cementitious materials can ensure sustainability of the cement and concrete industries. This importance is not only related to the energy efficiency and environmental aspects of the cement industry, but also to the durability and life cycle performance and costs of the concrete structures (Mehta, 1998). Bentonite, a natural pozzolan, is commonly divided into sodium (high-swelling), calcium (low-swelling) and intermediate (moderateswelling) type, containing both sodium and calcium ions. Typically high swelling bentonites have a frothy texture caused by its alternate swelling and drying, whereas low-swelling types have a cracked appearance. Therefore, the properties of mortars and concrete will vary accordingly depending upon the type of bentonite used in them. Bentonite occurs in many different areas of North West Frontier Province (NWFP) of Pakistan. One of the bentonites, originating from the Karak district and spread over an area of about 18 km2, was studied in this research program. Its thickness varies generally from 6 m to 24 m, however, thicknesses of about 39 m have also been found in some places. It is estimated that about 36 million tons of deposits of bentonite exist in the Karak district (Ahmad and Siddiqi, 1995). It can be dark bluish grey, greenish grey and brownish green in colour. Karak bentonite is rich in clay minerals comprising illite–smectite (with or without illite/muscovite) and kaolinite (Saleemi and Ahmed, 2000).
J. Mirza et al. / Applied Clay Science 45 (2009) 220–226
Several preliminary studies were undertaken to evaluate the industrial wastes in Pakistan, such as blast furnace slag, fly ash, rice husk ash, etc. However, the data for the natural pozzolans such as volcanic ash, bentonite, meta-kaolin, and other clays, etc., is at best scarce, or non-existent. This research program attempts to evaluate the performance of the Karak bentonite, as partial replacement for Ordinary Portland cement (OPC) in mortar and concrete mixtures. The main objectives were to determine the pozzolanic reactivity and to evaluate the durability characteristics of optimized substitution levels of bentonite in OPC mortar and concrete. Incorporating them in mortar and concrete as a substitute for cement will certainly reduce the cost, energy consumption, greenhouse gas emissions (GHG) and would also minimize depletion of natural resources. 2. Mechanism of pozzolanic reaction A “pozzolan is a siliceous or siliceous and aluminous material which in itself possesses little or no cementing property but will in a finely divided form and in the presence of moisture chemically reacts with calcium hydroxide (CH) at ordinary temperatures to form compounds possessing cementitious properties (ASTM, 2007a).” It is well known that OPC when mixed with water forms a binding hydrated cement paste (hcp) of calcium silicate hydrates (CSH) and liberates calcium hydroxide (CH). This reaction is generally quite rapid. However, when a pozzolan is present, its silica component reacts with the liberated CH in hcp and in the presence of water forms CSHs. This reaction is generally slow; resulting in a slow rate of heat liberation and strength development. Also, the reaction is limeconsuming, instead of lime-producing, which has an important bearing on the durability of hcp in acidic environments. Moreover, pore size distribution studies of hydrated pozzolanic cements have shown that the reaction products are very efficient in filling up the larger capillary pores, thus improving the strength and impermeability of the system (Mehta and Monteiro, 1993). In addition to reactive silica, pozzolans also contribute reactive alumina, which in the presence of CH and sulphate ions present in the system, form cementitious products such as teteracalcium aluminate hydrate (C4AH13), tricalcium aluminate hydrate (C3AH6), hydrated gehlenite (C2AH6), and CSH (Neville, 1981). 3. Experiment 3.1. Materials Locally available OPC conforming to the ASTM Standard C150-07 (ASTM, 2007b) was used (Table 1). Table 1 Physical properties and chemical composition of OPC and Karak bentonite. OPC Physical properties – % retained #325 mesh – Specific gravity (g/cm3) – Strength activity index (%) – 7 day – 28 day Chemical composition (%) SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3 Loss on ignition SiO2 + Al2O3 + Fe2O3
Bentonite “as is”
ASTM C618
17 2.63
34 max.
– –
96 82
75 min. 75 min.
20.10 6.12 3.20 63.34 2.43 – – 3.43 – –
49.44 19.7 6.20 7.45 1.61 0.87 0.69 – 13.74 75.34
– 3.15
Class N requirements (%)
5 max.
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The fine aggregate used was natural silica river sand from Nizam Pur with a fineness modulus of 2.3. The coarse aggregate used was crushed limestone from Basai quarry in NWFP, with a maximum size of 19 mm and a bulk specific gravity 2.66. Karak bentonite was collected and studied in “as is” (21 °C) and “heated” (150°, 250°, 500°, 750° and 950 °C) conditions. The bentonite was ground to powder, heated for 3 h and after cooling to room temperature, sieved through #325 sieve (45 mm). The bentonite had high calcium content (Table 1). 3.2. Specimen preparation Mortar cubes, 50 mm ×50 mm ×50 mm in size, were prepared in accordance with ASTM Standard C109 (ASTM, 2007c). The control mixture consisted of cement-to-sand ratio of 1:2.75 and water-to-cement ratio of 0.485. In all other specimens water-to-binder (cement+ bentonite) ratio (W/B) was also kept constant at 0.485. The cement in the mortar was replaced by bentonite in proportions of 20%, 25%, 30%, 40%, 50% and 100% by mass, for the compressive strength test. These mortars were also tested for strength activity indices (SAI) in “as is” and in “heated” conditions, resistance to sulphate attack and water absorption. The resistance to sulphate attack was based on the relative loss of compressive strength when the OPC mortar cubes containing bentonite were immersed in 2% magnesium sulphate (MgSO4) and 5% sodium sulphate (Na2SO4) solutions for periods up to 28 days. The cubes were cured for 7 days in potable water prior to immersion in the sulphate solutions to ensure that the cubes gained sufficient strength before sulphate attack. The solutions were changed every week to keep its concentration stable. For the water absorption test, the cubes were first kept in an oven at 105 °C for 24 h and weighed. They were then immersed in water for 24 h and weighed again. The masonry bond strength of OPC mortars brick prisms (22.9 cm × 22.9 cm × 45.8 cm), were cast on a smooth platform with binder-to-sand ratio of 1:5, both for the control as well as the mortars containing bentonite. The OPC in the test mortar was replaced by 20, 25 and 40% of bentonite. The W/B was maintained constant at 0.9, for all the mortar specimens. Concrete cylinders, 150 mm × 300 in size (cement:sand:coarse aggregates ratio = 1:2:4, commonly used in Pakistan) were prepared using a constant W/B of 0.55. The cement in the concrete was replaced by bentonite in proportions of 20 and 30% (“as is”) and 20%, 25%, 30%, 40% and 50% (heated to 150 °C) by mass. The concrete cylinders were tested after 7, 14, 28 and 56 days. The effect of bentonite on the workability of concrete was determined by comparing slump values of concretes at constant W/B, but with different percentages of bentonites as OPC replacement. Concrete beams, 150 mm × 150 mm × 750 mm in size, were also cast to determine the modulus of rupture, or flexural strength of concrete after 28 days. Mixture design was the same as for concrete cylinders. In addition, four reinforced beams, 2.10 m long were also cast both for control concrete and test beams containing 25% of bentonite as a replacement of OPC. The beam cross section was 150 mm×300 mm. Minimum reinforcement was used in all beams, consisting of two 12.5 mm diameter bars in tension and another two in the compression zone. Normal 9.5 mm diameter steel bars were used as stirrups throughout the beam at a spacing factor of 150 mm on centres. The beams were cured for 28 days before testing for deflection and load carrying capacity. All of the mortar and concrete specimens were kept in potable water to cure at an ambient laboratory temperature of 21 ± 1 °C and relative humidity of 100%, until the testing day. 3.3. Tests
10 max. 70 min.
The tests performed were: X-ray diffraction (XRD), strength activity index (ASTM C618, 2008), compressive strength of OPC and
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bentonite mortars and concretes (ASTM C109, 2007c), resistance to sulphate attack, water absorption (ASTM C642) and masonry bond strength tests on mortar specimens, modulus of rupture or flexural strength (ASTM C78-1994) on concrete beams. All specimens were prepared and tested according to their respective ASTM standards, wherever applicable (Section 3.2). 4. Results and discussion 4.1. XRD analysis According to XRD data (Fig. 1) the “as is” as well as the “heated” bentonites both contained some crystalline minerals and amorphous phases, as indicated by somewhat raised background of the diffraction patterns. All of them showed two major reflections, one for quartz and other for illite. Kaolinite, chlorite, feldspar and muscovite were also present in minor quantities. 4.2. Strength activity index As prescribed in ASTM Standard C618 (ASTM, 2008), the strength activity index (SAI) is defined as: SAI = ð A = BÞ × 100k where A = average compressive strength of the cement mortar cubes containing bentonite, and B = average compressive strength of cement mortar cubes without bentonite. According to the ASTM Standard C618, SAI should be a minimum of 75% of the control mixture at both 7 and 28 days. Fig. 2 shows the SA1 of OPC mortar and those incorporating “as is” (21 °C) and “heated to 150 °C, 250 °C, 500 °C, 750 °C and 950 °C” bentonites as a replacement of OPC. Test data showed that: – All of the OPC mortars containing “as is” and “heated” bentonites conformed to the ASTM Standard C618 specifications, except for “950 °C heated” bentonite, which was below the specified limit of 75%. – SAI of OPC mortars containing “150 °C heated” bentonite was slightly higher than the control mixture, mortars containing “as is” and all other heated bentonites, after 7 and 28 days. It was significantly low for mortar containing “950 °C heated” bentonite. Therefore, only “150 °C heated” bentonite was selected for all other tests aside from the mortars containing “as is” bentonite.
Fig. 1. XRD patterns of “as is” and “heated” bentonites.
Fig. 2. Strength activity indices of OPC mortars containing “as is” and “heated” bentonites after 7 and 28 days.
– The reactivity rates of the OPC mortar containing “150 °C heated” bentonite was the same, both after 7 and 28 days. This indicated that between 7 and 28 days, the OPC hydration and the pozzolanic hydration reactions contributed to similar strength development rate of the mortars, with or without the Karak bentonite. On the other hand, the rate of reactivity of the cement mortar containing “as is” bentonite was higher after 7 days than the 28 days reactivity, showing a decrease in the reactivity rate. These tests should have been carried out for longer periods of time to confirm the published studies ( Berry, 1980; Mehta, 1981; Hansen, 1990) which showed that the compressive strength increases considerably after 28 days. It was also reported (Marsh and Day, 1988) that from the time of casting until approximately 14 days, pozzolans had no significant effect on strength. However, because of its higher SAI, Karak bentonite (“as is” and “150 °C heated”) could be used in the field which would produce very economical OPC mortar and concrete mixtures. 4.3. Tests on mortars 4.3.1. Compressive strength 4.3.1.1. “As i s” bentonite. – After ages 7, 14 and 28 days, the compressive strength of all OPC mortar cubes incorporating “as is” bentonite was lower than the control mixture, i.e., one containing 100% OPC (Fig. 3, Table 2).
Fig. 3. Compressive strength of OPC mortars containing “as is” bentonite after 7, 14 and 28 days.
J. Mirza et al. / Applied Clay Science 45 (2009) 220–226 Table 2 Compressive strength of mortars containing “as is” bentonites as a % of control mixture. Cement % 100 80 75 70 60 50 0
Bentonite % 0 20 25 30 40 50 100
Compressive strength (% of control) 7 day
14 day
28 day
100 96 94 90 82 60 15
100 93 91 87 67 50 9
100 82 82 72 58 42 7
– The compressive strength of OPC mortars containing 20% and 25% bentonite as a replacement of OPC showed similar values after 7, 14 and 28 days. It started decreasing progressively for OPC mortars containing 30% and higher proportions of bentonite. – When 100% bentonite was used in the mortar, i.e., without OPC, its compressive strength was 2.4 and 1.6 MPa after 7 and 28 days, respectively. This showed that the Karak bentonite is mostly pozzolanic with some cementing properties. – After 7 days, the strength development gain in OPC mortars containing “as is” bentonite was higher compared with the 14 day strength, which in turn, was higher than the 28 day strength. The strength gain ratio thus decreased as the amount of OPC substitution by bentonite increased. This tendency showed a lower strength development rate as the curing period increased. This could perhaps be due to high loss on ignition content in bentonite (Table 1). – Twenty-five percent substitution of OPC by bentonite showed strength which was 82% of the control mixture (19.7 MPa) after 28 days. Later age studies are definitely warranted to determine whether one should use this bentonite mortar as a low-cost construction material. 4.3.1.2. “150 °C heated” bentonite. – After 7 and 28 days, when compared to the control mixture, the compressive strength increased for OPC mortars containing 20% of “150 °C heated” bentonite, but remained slightly lower for the mortar with 25% bentonite. Even these values were well over 20 MPa at 30% bentonite substitution level after 28 days. The compressive strength decreased sharply when 50% or higher percentage of OPC was replaced by bentonite, suggesting that up to 30% OPC can be replaced with “150 °C heated” bentonite without any significance decrease in strength for low-cost construction work (Fig. 4, Table 3). – Compared to mortars containing “as is” bentonite, OPC mortars containing “150 °C heated” bentonite was more beneficial from the
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Table 3 Compressive strength of mortars containing “150 °C heated” bentonites as a % of control mixture. Cement (%)
Bentonite (%)
100 80 75 70 60 50 40 30 20
0 20 25 30 40 50 60 70 80
Compressive strength (% of control) 7 day
28 day
100 101 98 86 74 44 31 26 19
100 104 98 84 75 58 41 33 20
mortar strength point of view. Its heat treatment cost could certainly be compensated by the compressive strength gain. – After 28 days, the strength development gain was generally higher than after 7 days for OPC mortars incorporating “150 °C heated” bentonite. This data also indicated that during the first 7 days of hydration, pozzolanic reaction contributed to the strength development of OPC mortar. In addition, these results also showed its advantage over the OPC mortar containing “as is” bentonite. – The strength gain ratio generally increased as the level of bentonite substitution in the OPC mortars was increased. This showed again that when the Karak bentonite mortar is used as a low-cost construction material, it should be “heated to 150 °C” to achieve a higher strength. 4.3.2. Resistance to sulphate attack – The compressive strength loss was much higher in the control mixture compare to the mortars containing 25% bentonite as replacement of cement in 2% MgSO4 and 5% Na2SO4 solutions (Table 4). – The sulfate resistance of OPC containing bentonite increased considerably compared with the control mortar. Sulfate resistance of the former was 31% higher than for the latter when immersed in 5% Na2SO4 solution for 70 days, while in 2% MgSO4 solution, it was 32% higher than the control mixture. This higher sulphate resistance could be due to lower CH concentration in the mortar samples, which is highly prone to sulphate attack. – The effect of 2% MgSO4 and 5% Na2SO4 solutions on compressive strength of OPC mortars containing 25% bentonite was slightly lower than the ones cured in water. However, this effect remained the same for both sulphate solutions. This indicated that due to relatively lower effect of the sulphate solutions, the mortars containing bentonites could be used in areas where there is a strong possibility of sulphate attack. Other studies (Barger et al., 1997) also demonstrated that the heated clay provided sulphate resistance greater than the high sulphate resistant Type 50 cement. The substitution of about 30–40% of pozzolan for cement in concrete can result in a considerable increase in the resistance to 2% MgSO4 and 5% Na2SO4 solutions, but it is less effective against 5% MgSO4 solution (Lea, 1971). The sulphate resistance of the cement mortar improved with an increase in the pozzolan content. The cement containing 20% to 30% pozzolan were considered satisfactory with regard to chemical durability (Mehta, 1981). 4.3.3. Water absorption Water absorption was determined as:
Fig. 4. Compressive strength of OPC mortars containing “150 °C heated” bentonite after 7 and 28 days.
Water absorptionðkÞ = ðsaturated weight − oven dry weightÞ ðoven dry weightÞ × 100k
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Table 4 Effect of sulphate solutions on compressive strength of OPC mortars incorporating bentonite. Cement (%) Bentonite (%) Compressive strength (MPa)
100 75
0 25
Compressive strength loss (%)
Water 2% MgSO4 5% Na2SO4 2% MgSO4 cured solution cured cured
5% Na2SO4 solution
27.36 29.27
42 11
15.70 26.17
15.96 25.91
43 11
Table 5 Water absorption of OPC mortars incorporating bentonite. Cement (%) 100 75
Bentonite (%) 0 25
Table 6 Compressive strength of concrete containing “as is” bentonites as a % of control mixture. Cement (%)
Bentonite (%)
100 80 70
0 20 30
Compressive strength (% of control) 7 day
14 day
28 day
56 day
100 55 43
100 61 46
100 69 50
100 74 60
4.4.2. Compressive strength 4.4.2.1. “As is” bentonite.
Water absorption (%) 28 day
56 day
10 9
9 7
– After 28 days, water absorption was almost the same for OPC mortar as well as that containing 25% bentonite as a replacement of OPC (Table 5). – After 56 days, water absorption was slightly lower for the mortar containing 25% bentonite than for the control mixture. This shows that the water absorption decreased with the length of the curing period; thus decreasing the permeability which can considerably improve their durability characteristics. This decrease was due to the fact that the chemical reactions between the natural pozzolans and CH of hydrated cement paste (hcp) can fill the micropores in the cement matrix and can help improve the durability of mixtures significantly by changing the framework of the matrix (Shannag, 2000; Sabir et al., 2001; Pan et al., 2003).
4.4. Tests on concrete
– The compressive strength of the concrete containing “as is” bentonite decreased as its substitution level in OPC increased, after 7, 14, 28 and 56 days. A similar tendency was also observed in the OPC mortars (Section 4.3.1.1). However, the compressive strength increased as the curing period increased from 7 to 56 days at all bentonite substitution levels (Fig. 5, Table 6). – The compressive strength of concrete cylinders containing 20% bentonite was 74% (18.3 MPa) compared with the 100% (24.8 MPa) value of the control mixture, after 56 day. These values were still satisfactory and could perhaps be increased as described in the literature (Mehta, 1981; Neville, 1981; Mehta and Monteiro, 1993). These literature studies showed that the strength gain for pozzolan-containing concrete was generally slow at early ages. It could, therefore, be concluded from this data that the Karak bentonite could be used for up to 20% OPC replacement for strength and economic point of view. However, before using it in the field, it must be evaluated further for its durability properties over a longer period of time. – The compressive strength values decreased substantially (50%) as the OPC substitution level by bentonite increased to 30%. It is reported (Lea, 1971) that 30% substitution of pozzolana reduced the strength by about 35–40% at 28 days, 20% at 90 days, 10% at 180 days and very little at one year. At smaller percentage substitution, such as 10%, the strength after 28 days was almost the same as that of control mixture.
4.4.1. Effect of bentonite on workability The slump values, a measure of the mixture workability, decreased as the OPC substitution by bentonite increased. This reduction in slump may be due to the relatively high fineness and low density of the concrete mixtures. Therefore, for the same W/B, concrete made with cement containing bentonite is less workable than the control concrete mixture. The use of superplasticizers is certainly warranted to increase the workability.
4.4.2.2. “150 °C heated” bentonite.
Fig. 5. Compressive strength of OPC concrete containing “as is” bentonite after 7, 14, 28 and 56 days.
Fig. 6. Compressive strength of OPC concrete containing “150 °C heated” bentonite after 7, 14, 28 and 56 days.
– The compressive strength of concrete, containing “150 °C heated” bentonite decreased as the substitution level increased, after 7, 14, 28 and 56 days. A similar tendency was also observed in the concrete containing “as is” bentonite (Section 4.4.2.1). However, its compressive strength increased as the curing period increased from 7 to 56 days at all substitution levels of OPC by bentonite (Fig. 6, Table 7).
J. Mirza et al. / Applied Clay Science 45 (2009) 220–226
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Table 7 Compressive strength of concrete containing “150 °C heated” bentonites as a % of control mixture. Cement (%) 100 80 75 70 60 50
Bentonite (%) 0 20 25 30 40 50
Compressive strength (% of control) 7 day
14 day
28 day
56 day
100 68 64 43 50 34
100 67 64 46 46 38
100 74 70 50 49 36
100 79 78 60 57 40
– The compressive strength of concrete containing 20% and 25% bentonite by mass of cement, was 79% and 78%, respectively, of the control mixture strengths after 56 days. These values could perhaps be increased as described in the literature (Mehta, 1981; Neville, 1981; Mehta and Monteiro, 1993), which showed that the strength gain for the pozzolans is generally slow at early ages. Thus the Karak bentonite could be used to replace up to 25% cement for strength and economic considerations. – The compressive strength values decreased substantially as the bentonite substitution levels increased from 30% to 50% at all ages of 7, 14, 28 and 56 days.
4.5. Modulus of rupture or flexural strength The modulus of rupture decreased as the OPC substitution levels was increased by “as is” and “150 °C heated” bentonite. However, these values were similar for concrete beams containing 20% “as is” and 25% “heated to 150 °C” bentonites (Table 8).
4.6. Load carrying capacity of reinforced beam – The control and bentonite containing concrete beams demonstrated third-point loads of 5.4 and 3.8 tonnes at cracking of the beams compared with design capacities of 4.1 and 4.0 tonnes, respectively. This crack originated from central bottom portion at a deflection of 3 mm. The crack width increased with the load increments. Flexural cracks were followed by web shear cracks. Finally the control and bentonite containing concrete beams failed at 10 and 8 tonnes, respectively (Fig. 7). – Load versus deflection curves for the two beams were plotted in Fig. 7 for the third point loading. The control and bentonite concrete beams demonstrated yield of steel reinforcement at loads of 7.8 and 6.0 tonnes, respectively. The large deflection of the concrete beam containing bentonite also (Table 9) depicted the high ductility ratio, which is very useful property to dissipate energy in case of earthquakes. With high ductility ratio the area under curve increases which provides capacity to dissipate energy during an earthquake, which could have serious consequences for human lives.
Fig. 7. Load deflection curve for reinforced cement concrete beams with and without bentonite.
4.7. Masonry bond strength The masonry bond strength decreased slightly as the proportion of the OPC replacement by bentonite increased (Figs. 8 and 9, Table 10). The difference in the cracking strength was almost the same for OPC mortars containing 20% “as is” and 25% “heated to 150 °C” bentonites (Fig. 8). At 40% replacement of OPC by bentonite, the cracking strength dropped significantly to 85% of the control specimen. The crushing strength (Fig. 9), showed a similar trend after the masonry was weakened by the bond cracking. The ultimate failure was in the form of complete crushing of the mortar. The brick prism strength data led to the conclusion that 25% of the OPC could be replaced by bentonite in masonry work without an appreciable loss in strength: this will result in considerable cost savings. 5. Economic feasibility The cost of 1 tonne of bentonite in Pakistan Rupees (Rs.) is 1100 (~ $18.00/tonne), whereas the cement costs about Rs. 4500 per tonne (~ $75.00/tonne). Incorporation of bentonite as partial replacement of cement leads to considerable savings. It also fulfils the principles of sustainable development. These savings can be considerably higher, when the concrete containing bentonite is used in hydraulic structures, such as dams, where hundred of thousands to more than a million tonnes of cement are normally used, depending on their length, width and height. About 11% of the total cost (includes construction costs which can vary with the infrastructure system) could be saved by replacing 25% of the cement by bentonite. As the cement costs are normally about 30% of the total cost of the concrete in a dam project, the net saving will be 3.3% (30% × 11% = 3.3%) of the total project cost. Similarly, the use of bentonite in primary school buildings in villages and towns will result in a net saving of 5% of the total building cost. This saving can be increased considerably by replacing 40% of the cement by bentonite, because in normal brick
Table 8 Modulus of rupture of concrete incorporating bentonite. Mixture (%) Cement
Bentonite
Modulus of rupture (MPa)
100 80 75 60
0 20 “as is” 25 “heated to 150 °C” 40 “heated to 150 °C”
3.25 2.63 2.60 2.44
% control
Table 9 Ductility ratio of concrete beams.
100 81 80 75
Beam type
Ultimate load deflection Deflection at yield Ductility ratioa
100% OPC (control) 37.00 75% OPC + 25% bentonite 56.83 a
4.87 5.66
Ratio of the deflection at ultimate load to the deflection at yield.
7.61 10.05
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J. Mirza et al. / Applied Clay Science 45 (2009) 220–226 Table 10 Cracking and crushing strengths of bentonite in brick prisms. OPC (%) Bentonite (%) Cracking strength % control Crushing strength % control (MPa) (MPa) 100 80 75 60
0 20 “as is” 25 “heated to 150 °C” 40 “heated to 150 °C”
2.19 2.17 2.15
100 99 98
4.76 4.61 4.58
100 97 96
1.86
85
4.17
88
masonry only 20 to 30% of the ultimate strength of the masonry is actually utilized.
of the control beams. This could guarantee good energy dissipation during earthquakes. 8. The bond strength of OPC mortars in brick masonry was similar to that of the mortars containing 20% “as is” and 25% “heated to 150 °C” bentonite. This further suggested that these could be used as low-cost construction materials.
6. Conclusions
Acknowledgments
1. The XRD patterns of “as is” and “heated” bentonites possessed both crystalline and amorphous phases. 2. SAI of OPC mortars containing “150 °C heated” bentonite was slightly higher than the mortars containing “as is” bentonite as well as the control mixture after 7 and 28 days. 3. OPC mortars containing 25% “as is” and 30% “heated to 150 °C” bentonites showed compressive strength values of 19.6 MPa and 20.9 MPa, respectively, after 28 days, demonstrating the effectiveness of these mortars as a low cost construction material. 4. After 7 days, the strength development gain in OPC mortars incorporating bentonite was higher compare to 14 day strength, which in turn, was higher than the 28 days strength development. This suggested a decrease in the strength gain ratio as the bentonite substitution ratio increased in the OPC mortar. This tendency showed a lower strength development rate as the curing period increased. 5. The compressive strength of concrete containing “as is” and “150 °C heated” bentonites decreased as their substitution level in the cement increased, after 7, 14, 28 and 56 days. However, the OPC concrete containing 20% “as is” and 25% “heated to 150 °C” bentonites, showed similar compressive strength values after 28 days. These could, therefore, be used as low-cost construction materials. 6. Tests on mortar cubes soaked in 2% MgSO4 and 5% Na2SO4 solutions demonstrated a consistent improvement in sulphate attack resistance as the bentonite content in them was increased. 7. The modulus of rupture of concrete beams containing bentonite was lower than the control concrete beams. This decreased further as the percentage substitution of bentonite is increased, but the ductility ratio of the bentonite concrete beams was larger than that
The authors gratefully acknowledge the financial support from the NWFP University of Engineering and Technology, Peshawar. Gratitude is also extended to Industrial Estate Peshawar Laboratory and PCSIR Laboratory, Peshawar, for providing services for specimen grinding, heat treatment and chemical analyses.
Fig. 8. Cracking strength of OPC concrete brick prisms containing bentonite as percentage of control mixture beams with and without bentonite.
Fig. 9. Crushing strength of OPC concrete brick prisms containing bentonite as percentage of control mixture.
References ACI, 1994. Use of natural pozzolans in concrete. American Concrete Institute, ACI Committee 232 Report. ACI Materials Journal 91 (4), 410–426. Ahmad, Z., Siddiqi, R.A., 1995. Minerals and Rocks for Industry, pp. 202–245. ASTM, 1994. Standard Test Method for Flexural Strength of Concrete (Using Simple Beam with Third Point Loading. C78-94. ASTM International, West Conshohocken, Pa. ASTM, 2007a. Standard Terminology Relating to Concrete and Concrete Aggregates. C125-07. ASTM International, West Conshohocken, Pa. ASTM, 2007b. Standard Specification for Portland Cement. C150-07. ASTM International, West Conshohocken, Pa. ASTM, 2007c. Standard Test Method for Compressive Strength of Hydraulic Cement Mortars (Using 2-in. or [50-mm] Cube Specimens). ASTM C109/C109 M-07. ASTM International, West Conshohocken, Pa. ASTM, 2008. Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete. C618-08a. ASTM International, West Conshohocken, Pa. Barger, G.S., Luhkarila, M.R., Martin, D.L., Lane, S.B., Hansen, E.R., Ross, M.W., Thompson, J.L., 1997. Evaluation of a blended cement and a mineral admixture containing heat heated clay natural pozzolan for high-performance concrete. Proceeding of the 6th International Purdue Conference on Concrete Pavement Design and Materials for High Performance. Purdue University, West. Lafayette, Indiana, p. 21. Berry, E.E., 1980. Strength development of some blended cement mortars. Cement and Concrete Research 10, 1–11. Hansen, T.C., 1990. Long-term strength of high fly ash concrete. Cement and Concrete Research 20, 193–196. Kosmatka, S.H., Kerkhoff, B., Panarese, W.C., MacLeod, N.F., McGrath, R.J., 2002. Design and control of concrete mixture, 7th edition, pp. 60–61. Lea, F.M., 1971. The Chemistry of Cement and Concrete, 3rd edition. Chemical Publishing Company Ltd, New York, pp. 358–397. Marsh, B.K., Day, R.L., 1988. Pozzolanic and cementitious reaction of fly ash in blended cement pastes. Cement and Concrete Research 18 (2), 301–310. Mehta, P.K., 1981. Studies on blended Portland cement containing Santorin earth. Cement and Concrete Research 11, 507–518. Mehta, P.K., 1998. Role of pozzolanic and cementitious materials in sustainable development of the concrete industry, ACI SP-178, American Concrete International, Farmington Hills, Mich., pp. 1–25. Mehta, P.K., Monteiro, J.M., 1993. Concrete – Structure, Properties and Materials, 3rd edition. Prentice Hall, Englewood Cliffs, New Jersey, pp. 207–208. Neville, A.M., 1981. Properties of Concrete, 3rd edition. English Language Book Society, pp. 77–81. Pan, S., Tseng, D., Lee, C.C., Lee, C., 2003. Influence of the fineness on strength of sewage sludge ash on the mortar properties. Cement and Concrete Research 33, 1749–1754. Sabir, B., Wild, S., Bai, J., 2001. Metakaolin and heat heated clays as pozzolan for concrete. Review 23, 441–454. Saleemi, A.A., Ahmed, Z., 2000. Mineral and chemical composition of Karak Mud Stone, Kohat Plateau, Pakistan: implications for smectite–illitization and provenance. Sedimentary Geology 130, 229–247. Shannag, M., 2000. High strength concrete containing natural pozzolan and silica fume. Cement Concrete Composites 22, 399–406.
Contents lists available at ScienceDirect
Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Pakistani bentonite in mortars and concrete as low cost construction material J. Mirza a,⁎, M. Riaz b, A. Naseer b, F. Rehman b, A.N. Khan b, Q. Ali b a b
Department of Robotics and Civil, Research Institute of Hydro-Quebec, Varennes, Quebec, Canada J3X 1S1 Department of Civil Engineering, NWFP University of Engineering and Technology, Peshawar, Pakistan
a r t i c l e
i n f o
Article history: Received 16 September 2008 Received in revised form 28 May 2009 Accepted 8 June 2009 Available online 13 June 2009 Keywords: Ordinary Portland cement Bentonite Mortar Concrete Mechanical properties
a b s t r a c t A Pakistani bentonite, in “as is” (21 °C) and “heated” (150 °C, 250 °C, 500 °C, 750 °C and 950 °C) conditions, was incorporated in mortar cubes, concrete cylinders and concrete beams as a partial substitute for Ordinary Portland cement (OPC) and studied in detail. Results showed that OPC mortars and concrete containing 20% “as is” and 25% “heated to 150 °C” bentonite could be used as low-cost construction materials. They will also reduce energy consumption, preserve natural resources and solve environmental problems related to cement production as well as augment the durability and life cycle of the concrete structures. The X-ray diffraction patterns showed that bentonite possessed both crystalline and amorphous phases. The strength activity indices (SAI) after 7 and 28 days were higher than 75% for “as is” and “heated” bentonite, except for the 950 °C samples, which was below the ASTM C618 specified limit of 75%. The maximum SAI was shown by “150 °C heated” bentonite. The compressive strength data also showed similar results for OPC mortar cubes and concrete cylinders containing “150 °C heated” bentonite. When compared with the control mixture, the compressive strength values were the same as for mortar containing 25% bentonite as replacement of OPC. However, these values decreased in concrete initially and started to gain strength remarkably after 28 days. Resistance to sulphate attack and water absorption tests on mortar cubes soaked in 2% magnesium sulphate and 5% sodium sulphate solutions demonstrated consistent improvement as the bentonite content in them was increased. The modulus of rupture of all concrete beams decreased as the OPC substitution level by bentonite increased from 20% to 40%. Bond strength of OPC mortar containing 20% “as is” and 25% “heated to 150 °C” bentonite in brick prisms was almost the same as that of control mixture. © 2009 Elsevier B.V. All rights reserved.
1. Introduction More than half of Pakistan's population lives below the poverty line and the increasing cost of daily usage items is a great burden to a majority. Besides other items, the cost of cement has also increased by more than 150% in a short span of ten years. Low-cost pozzolans (natural and industrial) must, therefore, be explored to benefit the construction industry, as well as the poor people of Pakistan. Natural pozzolans have been used in building construction for centuries. The use of volcanic ash and heated clay dates back to 2000 BC and earlier in some cultures. Many of Roman, Greek, Indian, and Egyptian pozzolan concrete structures still exist today, attesting to the durability of these materials. The North American experience with natural pozzolans dates back to the early 20th century public works projects, such as dams, where they were used to control temperature rise in mass concrete and to act as cementing materials (Kosmatka et al., 2002). It is generally accepted that the use of natural pozzolans in cement, or concrete systems results in many beneficial properties such as low heat of hydration, high ultimate strength, low permeability, high sulphate resistance, and low alkali–silica activity (ACI, 1994). In ad⁎ Corresponding author. Tel.: +1 450 652 8308; fax: +1 450 652 1316. E-mail address: [email protected] (J. Mirza). 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.06.011
dition, it is well established that the use of pozzolanic and cementitious materials can ensure sustainability of the cement and concrete industries. This importance is not only related to the energy efficiency and environmental aspects of the cement industry, but also to the durability and life cycle performance and costs of the concrete structures (Mehta, 1998). Bentonite, a natural pozzolan, is commonly divided into sodium (high-swelling), calcium (low-swelling) and intermediate (moderateswelling) type, containing both sodium and calcium ions. Typically high swelling bentonites have a frothy texture caused by its alternate swelling and drying, whereas low-swelling types have a cracked appearance. Therefore, the properties of mortars and concrete will vary accordingly depending upon the type of bentonite used in them. Bentonite occurs in many different areas of North West Frontier Province (NWFP) of Pakistan. One of the bentonites, originating from the Karak district and spread over an area of about 18 km2, was studied in this research program. Its thickness varies generally from 6 m to 24 m, however, thicknesses of about 39 m have also been found in some places. It is estimated that about 36 million tons of deposits of bentonite exist in the Karak district (Ahmad and Siddiqi, 1995). It can be dark bluish grey, greenish grey and brownish green in colour. Karak bentonite is rich in clay minerals comprising illite–smectite (with or without illite/muscovite) and kaolinite (Saleemi and Ahmed, 2000).
J. Mirza et al. / Applied Clay Science 45 (2009) 220–226
Several preliminary studies were undertaken to evaluate the industrial wastes in Pakistan, such as blast furnace slag, fly ash, rice husk ash, etc. However, the data for the natural pozzolans such as volcanic ash, bentonite, meta-kaolin, and other clays, etc., is at best scarce, or non-existent. This research program attempts to evaluate the performance of the Karak bentonite, as partial replacement for Ordinary Portland cement (OPC) in mortar and concrete mixtures. The main objectives were to determine the pozzolanic reactivity and to evaluate the durability characteristics of optimized substitution levels of bentonite in OPC mortar and concrete. Incorporating them in mortar and concrete as a substitute for cement will certainly reduce the cost, energy consumption, greenhouse gas emissions (GHG) and would also minimize depletion of natural resources. 2. Mechanism of pozzolanic reaction A “pozzolan is a siliceous or siliceous and aluminous material which in itself possesses little or no cementing property but will in a finely divided form and in the presence of moisture chemically reacts with calcium hydroxide (CH) at ordinary temperatures to form compounds possessing cementitious properties (ASTM, 2007a).” It is well known that OPC when mixed with water forms a binding hydrated cement paste (hcp) of calcium silicate hydrates (CSH) and liberates calcium hydroxide (CH). This reaction is generally quite rapid. However, when a pozzolan is present, its silica component reacts with the liberated CH in hcp and in the presence of water forms CSHs. This reaction is generally slow; resulting in a slow rate of heat liberation and strength development. Also, the reaction is limeconsuming, instead of lime-producing, which has an important bearing on the durability of hcp in acidic environments. Moreover, pore size distribution studies of hydrated pozzolanic cements have shown that the reaction products are very efficient in filling up the larger capillary pores, thus improving the strength and impermeability of the system (Mehta and Monteiro, 1993). In addition to reactive silica, pozzolans also contribute reactive alumina, which in the presence of CH and sulphate ions present in the system, form cementitious products such as teteracalcium aluminate hydrate (C4AH13), tricalcium aluminate hydrate (C3AH6), hydrated gehlenite (C2AH6), and CSH (Neville, 1981). 3. Experiment 3.1. Materials Locally available OPC conforming to the ASTM Standard C150-07 (ASTM, 2007b) was used (Table 1). Table 1 Physical properties and chemical composition of OPC and Karak bentonite. OPC Physical properties – % retained #325 mesh – Specific gravity (g/cm3) – Strength activity index (%) – 7 day – 28 day Chemical composition (%) SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3 Loss on ignition SiO2 + Al2O3 + Fe2O3
Bentonite “as is”
ASTM C618
17 2.63
34 max.
– –
96 82
75 min. 75 min.
20.10 6.12 3.20 63.34 2.43 – – 3.43 – –
49.44 19.7 6.20 7.45 1.61 0.87 0.69 – 13.74 75.34
– 3.15
Class N requirements (%)
5 max.
221
The fine aggregate used was natural silica river sand from Nizam Pur with a fineness modulus of 2.3. The coarse aggregate used was crushed limestone from Basai quarry in NWFP, with a maximum size of 19 mm and a bulk specific gravity 2.66. Karak bentonite was collected and studied in “as is” (21 °C) and “heated” (150°, 250°, 500°, 750° and 950 °C) conditions. The bentonite was ground to powder, heated for 3 h and after cooling to room temperature, sieved through #325 sieve (45 mm). The bentonite had high calcium content (Table 1). 3.2. Specimen preparation Mortar cubes, 50 mm ×50 mm ×50 mm in size, were prepared in accordance with ASTM Standard C109 (ASTM, 2007c). The control mixture consisted of cement-to-sand ratio of 1:2.75 and water-to-cement ratio of 0.485. In all other specimens water-to-binder (cement+ bentonite) ratio (W/B) was also kept constant at 0.485. The cement in the mortar was replaced by bentonite in proportions of 20%, 25%, 30%, 40%, 50% and 100% by mass, for the compressive strength test. These mortars were also tested for strength activity indices (SAI) in “as is” and in “heated” conditions, resistance to sulphate attack and water absorption. The resistance to sulphate attack was based on the relative loss of compressive strength when the OPC mortar cubes containing bentonite were immersed in 2% magnesium sulphate (MgSO4) and 5% sodium sulphate (Na2SO4) solutions for periods up to 28 days. The cubes were cured for 7 days in potable water prior to immersion in the sulphate solutions to ensure that the cubes gained sufficient strength before sulphate attack. The solutions were changed every week to keep its concentration stable. For the water absorption test, the cubes were first kept in an oven at 105 °C for 24 h and weighed. They were then immersed in water for 24 h and weighed again. The masonry bond strength of OPC mortars brick prisms (22.9 cm × 22.9 cm × 45.8 cm), were cast on a smooth platform with binder-to-sand ratio of 1:5, both for the control as well as the mortars containing bentonite. The OPC in the test mortar was replaced by 20, 25 and 40% of bentonite. The W/B was maintained constant at 0.9, for all the mortar specimens. Concrete cylinders, 150 mm × 300 in size (cement:sand:coarse aggregates ratio = 1:2:4, commonly used in Pakistan) were prepared using a constant W/B of 0.55. The cement in the concrete was replaced by bentonite in proportions of 20 and 30% (“as is”) and 20%, 25%, 30%, 40% and 50% (heated to 150 °C) by mass. The concrete cylinders were tested after 7, 14, 28 and 56 days. The effect of bentonite on the workability of concrete was determined by comparing slump values of concretes at constant W/B, but with different percentages of bentonites as OPC replacement. Concrete beams, 150 mm × 150 mm × 750 mm in size, were also cast to determine the modulus of rupture, or flexural strength of concrete after 28 days. Mixture design was the same as for concrete cylinders. In addition, four reinforced beams, 2.10 m long were also cast both for control concrete and test beams containing 25% of bentonite as a replacement of OPC. The beam cross section was 150 mm×300 mm. Minimum reinforcement was used in all beams, consisting of two 12.5 mm diameter bars in tension and another two in the compression zone. Normal 9.5 mm diameter steel bars were used as stirrups throughout the beam at a spacing factor of 150 mm on centres. The beams were cured for 28 days before testing for deflection and load carrying capacity. All of the mortar and concrete specimens were kept in potable water to cure at an ambient laboratory temperature of 21 ± 1 °C and relative humidity of 100%, until the testing day. 3.3. Tests
10 max. 70 min.
The tests performed were: X-ray diffraction (XRD), strength activity index (ASTM C618, 2008), compressive strength of OPC and
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bentonite mortars and concretes (ASTM C109, 2007c), resistance to sulphate attack, water absorption (ASTM C642) and masonry bond strength tests on mortar specimens, modulus of rupture or flexural strength (ASTM C78-1994) on concrete beams. All specimens were prepared and tested according to their respective ASTM standards, wherever applicable (Section 3.2). 4. Results and discussion 4.1. XRD analysis According to XRD data (Fig. 1) the “as is” as well as the “heated” bentonites both contained some crystalline minerals and amorphous phases, as indicated by somewhat raised background of the diffraction patterns. All of them showed two major reflections, one for quartz and other for illite. Kaolinite, chlorite, feldspar and muscovite were also present in minor quantities. 4.2. Strength activity index As prescribed in ASTM Standard C618 (ASTM, 2008), the strength activity index (SAI) is defined as: SAI = ð A = BÞ × 100k where A = average compressive strength of the cement mortar cubes containing bentonite, and B = average compressive strength of cement mortar cubes without bentonite. According to the ASTM Standard C618, SAI should be a minimum of 75% of the control mixture at both 7 and 28 days. Fig. 2 shows the SA1 of OPC mortar and those incorporating “as is” (21 °C) and “heated to 150 °C, 250 °C, 500 °C, 750 °C and 950 °C” bentonites as a replacement of OPC. Test data showed that: – All of the OPC mortars containing “as is” and “heated” bentonites conformed to the ASTM Standard C618 specifications, except for “950 °C heated” bentonite, which was below the specified limit of 75%. – SAI of OPC mortars containing “150 °C heated” bentonite was slightly higher than the control mixture, mortars containing “as is” and all other heated bentonites, after 7 and 28 days. It was significantly low for mortar containing “950 °C heated” bentonite. Therefore, only “150 °C heated” bentonite was selected for all other tests aside from the mortars containing “as is” bentonite.
Fig. 1. XRD patterns of “as is” and “heated” bentonites.
Fig. 2. Strength activity indices of OPC mortars containing “as is” and “heated” bentonites after 7 and 28 days.
– The reactivity rates of the OPC mortar containing “150 °C heated” bentonite was the same, both after 7 and 28 days. This indicated that between 7 and 28 days, the OPC hydration and the pozzolanic hydration reactions contributed to similar strength development rate of the mortars, with or without the Karak bentonite. On the other hand, the rate of reactivity of the cement mortar containing “as is” bentonite was higher after 7 days than the 28 days reactivity, showing a decrease in the reactivity rate. These tests should have been carried out for longer periods of time to confirm the published studies ( Berry, 1980; Mehta, 1981; Hansen, 1990) which showed that the compressive strength increases considerably after 28 days. It was also reported (Marsh and Day, 1988) that from the time of casting until approximately 14 days, pozzolans had no significant effect on strength. However, because of its higher SAI, Karak bentonite (“as is” and “150 °C heated”) could be used in the field which would produce very economical OPC mortar and concrete mixtures. 4.3. Tests on mortars 4.3.1. Compressive strength 4.3.1.1. “As i s” bentonite. – After ages 7, 14 and 28 days, the compressive strength of all OPC mortar cubes incorporating “as is” bentonite was lower than the control mixture, i.e., one containing 100% OPC (Fig. 3, Table 2).
Fig. 3. Compressive strength of OPC mortars containing “as is” bentonite after 7, 14 and 28 days.
J. Mirza et al. / Applied Clay Science 45 (2009) 220–226 Table 2 Compressive strength of mortars containing “as is” bentonites as a % of control mixture. Cement % 100 80 75 70 60 50 0
Bentonite % 0 20 25 30 40 50 100
Compressive strength (% of control) 7 day
14 day
28 day
100 96 94 90 82 60 15
100 93 91 87 67 50 9
100 82 82 72 58 42 7
– The compressive strength of OPC mortars containing 20% and 25% bentonite as a replacement of OPC showed similar values after 7, 14 and 28 days. It started decreasing progressively for OPC mortars containing 30% and higher proportions of bentonite. – When 100% bentonite was used in the mortar, i.e., without OPC, its compressive strength was 2.4 and 1.6 MPa after 7 and 28 days, respectively. This showed that the Karak bentonite is mostly pozzolanic with some cementing properties. – After 7 days, the strength development gain in OPC mortars containing “as is” bentonite was higher compared with the 14 day strength, which in turn, was higher than the 28 day strength. The strength gain ratio thus decreased as the amount of OPC substitution by bentonite increased. This tendency showed a lower strength development rate as the curing period increased. This could perhaps be due to high loss on ignition content in bentonite (Table 1). – Twenty-five percent substitution of OPC by bentonite showed strength which was 82% of the control mixture (19.7 MPa) after 28 days. Later age studies are definitely warranted to determine whether one should use this bentonite mortar as a low-cost construction material. 4.3.1.2. “150 °C heated” bentonite. – After 7 and 28 days, when compared to the control mixture, the compressive strength increased for OPC mortars containing 20% of “150 °C heated” bentonite, but remained slightly lower for the mortar with 25% bentonite. Even these values were well over 20 MPa at 30% bentonite substitution level after 28 days. The compressive strength decreased sharply when 50% or higher percentage of OPC was replaced by bentonite, suggesting that up to 30% OPC can be replaced with “150 °C heated” bentonite without any significance decrease in strength for low-cost construction work (Fig. 4, Table 3). – Compared to mortars containing “as is” bentonite, OPC mortars containing “150 °C heated” bentonite was more beneficial from the
223
Table 3 Compressive strength of mortars containing “150 °C heated” bentonites as a % of control mixture. Cement (%)
Bentonite (%)
100 80 75 70 60 50 40 30 20
0 20 25 30 40 50 60 70 80
Compressive strength (% of control) 7 day
28 day
100 101 98 86 74 44 31 26 19
100 104 98 84 75 58 41 33 20
mortar strength point of view. Its heat treatment cost could certainly be compensated by the compressive strength gain. – After 28 days, the strength development gain was generally higher than after 7 days for OPC mortars incorporating “150 °C heated” bentonite. This data also indicated that during the first 7 days of hydration, pozzolanic reaction contributed to the strength development of OPC mortar. In addition, these results also showed its advantage over the OPC mortar containing “as is” bentonite. – The strength gain ratio generally increased as the level of bentonite substitution in the OPC mortars was increased. This showed again that when the Karak bentonite mortar is used as a low-cost construction material, it should be “heated to 150 °C” to achieve a higher strength. 4.3.2. Resistance to sulphate attack – The compressive strength loss was much higher in the control mixture compare to the mortars containing 25% bentonite as replacement of cement in 2% MgSO4 and 5% Na2SO4 solutions (Table 4). – The sulfate resistance of OPC containing bentonite increased considerably compared with the control mortar. Sulfate resistance of the former was 31% higher than for the latter when immersed in 5% Na2SO4 solution for 70 days, while in 2% MgSO4 solution, it was 32% higher than the control mixture. This higher sulphate resistance could be due to lower CH concentration in the mortar samples, which is highly prone to sulphate attack. – The effect of 2% MgSO4 and 5% Na2SO4 solutions on compressive strength of OPC mortars containing 25% bentonite was slightly lower than the ones cured in water. However, this effect remained the same for both sulphate solutions. This indicated that due to relatively lower effect of the sulphate solutions, the mortars containing bentonites could be used in areas where there is a strong possibility of sulphate attack. Other studies (Barger et al., 1997) also demonstrated that the heated clay provided sulphate resistance greater than the high sulphate resistant Type 50 cement. The substitution of about 30–40% of pozzolan for cement in concrete can result in a considerable increase in the resistance to 2% MgSO4 and 5% Na2SO4 solutions, but it is less effective against 5% MgSO4 solution (Lea, 1971). The sulphate resistance of the cement mortar improved with an increase in the pozzolan content. The cement containing 20% to 30% pozzolan were considered satisfactory with regard to chemical durability (Mehta, 1981). 4.3.3. Water absorption Water absorption was determined as:
Fig. 4. Compressive strength of OPC mortars containing “150 °C heated” bentonite after 7 and 28 days.
Water absorptionðkÞ = ðsaturated weight − oven dry weightÞ ðoven dry weightÞ × 100k
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Table 4 Effect of sulphate solutions on compressive strength of OPC mortars incorporating bentonite. Cement (%) Bentonite (%) Compressive strength (MPa)
100 75
0 25
Compressive strength loss (%)
Water 2% MgSO4 5% Na2SO4 2% MgSO4 cured solution cured cured
5% Na2SO4 solution
27.36 29.27
42 11
15.70 26.17
15.96 25.91
43 11
Table 5 Water absorption of OPC mortars incorporating bentonite. Cement (%) 100 75
Bentonite (%) 0 25
Table 6 Compressive strength of concrete containing “as is” bentonites as a % of control mixture. Cement (%)
Bentonite (%)
100 80 70
0 20 30
Compressive strength (% of control) 7 day
14 day
28 day
56 day
100 55 43
100 61 46
100 69 50
100 74 60
4.4.2. Compressive strength 4.4.2.1. “As is” bentonite.
Water absorption (%) 28 day
56 day
10 9
9 7
– After 28 days, water absorption was almost the same for OPC mortar as well as that containing 25% bentonite as a replacement of OPC (Table 5). – After 56 days, water absorption was slightly lower for the mortar containing 25% bentonite than for the control mixture. This shows that the water absorption decreased with the length of the curing period; thus decreasing the permeability which can considerably improve their durability characteristics. This decrease was due to the fact that the chemical reactions between the natural pozzolans and CH of hydrated cement paste (hcp) can fill the micropores in the cement matrix and can help improve the durability of mixtures significantly by changing the framework of the matrix (Shannag, 2000; Sabir et al., 2001; Pan et al., 2003).
4.4. Tests on concrete
– The compressive strength of the concrete containing “as is” bentonite decreased as its substitution level in OPC increased, after 7, 14, 28 and 56 days. A similar tendency was also observed in the OPC mortars (Section 4.3.1.1). However, the compressive strength increased as the curing period increased from 7 to 56 days at all bentonite substitution levels (Fig. 5, Table 6). – The compressive strength of concrete cylinders containing 20% bentonite was 74% (18.3 MPa) compared with the 100% (24.8 MPa) value of the control mixture, after 56 day. These values were still satisfactory and could perhaps be increased as described in the literature (Mehta, 1981; Neville, 1981; Mehta and Monteiro, 1993). These literature studies showed that the strength gain for pozzolan-containing concrete was generally slow at early ages. It could, therefore, be concluded from this data that the Karak bentonite could be used for up to 20% OPC replacement for strength and economic point of view. However, before using it in the field, it must be evaluated further for its durability properties over a longer period of time. – The compressive strength values decreased substantially (50%) as the OPC substitution level by bentonite increased to 30%. It is reported (Lea, 1971) that 30% substitution of pozzolana reduced the strength by about 35–40% at 28 days, 20% at 90 days, 10% at 180 days and very little at one year. At smaller percentage substitution, such as 10%, the strength after 28 days was almost the same as that of control mixture.
4.4.1. Effect of bentonite on workability The slump values, a measure of the mixture workability, decreased as the OPC substitution by bentonite increased. This reduction in slump may be due to the relatively high fineness and low density of the concrete mixtures. Therefore, for the same W/B, concrete made with cement containing bentonite is less workable than the control concrete mixture. The use of superplasticizers is certainly warranted to increase the workability.
4.4.2.2. “150 °C heated” bentonite.
Fig. 5. Compressive strength of OPC concrete containing “as is” bentonite after 7, 14, 28 and 56 days.
Fig. 6. Compressive strength of OPC concrete containing “150 °C heated” bentonite after 7, 14, 28 and 56 days.
– The compressive strength of concrete, containing “150 °C heated” bentonite decreased as the substitution level increased, after 7, 14, 28 and 56 days. A similar tendency was also observed in the concrete containing “as is” bentonite (Section 4.4.2.1). However, its compressive strength increased as the curing period increased from 7 to 56 days at all substitution levels of OPC by bentonite (Fig. 6, Table 7).
J. Mirza et al. / Applied Clay Science 45 (2009) 220–226
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Table 7 Compressive strength of concrete containing “150 °C heated” bentonites as a % of control mixture. Cement (%) 100 80 75 70 60 50
Bentonite (%) 0 20 25 30 40 50
Compressive strength (% of control) 7 day
14 day
28 day
56 day
100 68 64 43 50 34
100 67 64 46 46 38
100 74 70 50 49 36
100 79 78 60 57 40
– The compressive strength of concrete containing 20% and 25% bentonite by mass of cement, was 79% and 78%, respectively, of the control mixture strengths after 56 days. These values could perhaps be increased as described in the literature (Mehta, 1981; Neville, 1981; Mehta and Monteiro, 1993), which showed that the strength gain for the pozzolans is generally slow at early ages. Thus the Karak bentonite could be used to replace up to 25% cement for strength and economic considerations. – The compressive strength values decreased substantially as the bentonite substitution levels increased from 30% to 50% at all ages of 7, 14, 28 and 56 days.
4.5. Modulus of rupture or flexural strength The modulus of rupture decreased as the OPC substitution levels was increased by “as is” and “150 °C heated” bentonite. However, these values were similar for concrete beams containing 20% “as is” and 25% “heated to 150 °C” bentonites (Table 8).
4.6. Load carrying capacity of reinforced beam – The control and bentonite containing concrete beams demonstrated third-point loads of 5.4 and 3.8 tonnes at cracking of the beams compared with design capacities of 4.1 and 4.0 tonnes, respectively. This crack originated from central bottom portion at a deflection of 3 mm. The crack width increased with the load increments. Flexural cracks were followed by web shear cracks. Finally the control and bentonite containing concrete beams failed at 10 and 8 tonnes, respectively (Fig. 7). – Load versus deflection curves for the two beams were plotted in Fig. 7 for the third point loading. The control and bentonite concrete beams demonstrated yield of steel reinforcement at loads of 7.8 and 6.0 tonnes, respectively. The large deflection of the concrete beam containing bentonite also (Table 9) depicted the high ductility ratio, which is very useful property to dissipate energy in case of earthquakes. With high ductility ratio the area under curve increases which provides capacity to dissipate energy during an earthquake, which could have serious consequences for human lives.
Fig. 7. Load deflection curve for reinforced cement concrete beams with and without bentonite.
4.7. Masonry bond strength The masonry bond strength decreased slightly as the proportion of the OPC replacement by bentonite increased (Figs. 8 and 9, Table 10). The difference in the cracking strength was almost the same for OPC mortars containing 20% “as is” and 25% “heated to 150 °C” bentonites (Fig. 8). At 40% replacement of OPC by bentonite, the cracking strength dropped significantly to 85% of the control specimen. The crushing strength (Fig. 9), showed a similar trend after the masonry was weakened by the bond cracking. The ultimate failure was in the form of complete crushing of the mortar. The brick prism strength data led to the conclusion that 25% of the OPC could be replaced by bentonite in masonry work without an appreciable loss in strength: this will result in considerable cost savings. 5. Economic feasibility The cost of 1 tonne of bentonite in Pakistan Rupees (Rs.) is 1100 (~ $18.00/tonne), whereas the cement costs about Rs. 4500 per tonne (~ $75.00/tonne). Incorporation of bentonite as partial replacement of cement leads to considerable savings. It also fulfils the principles of sustainable development. These savings can be considerably higher, when the concrete containing bentonite is used in hydraulic structures, such as dams, where hundred of thousands to more than a million tonnes of cement are normally used, depending on their length, width and height. About 11% of the total cost (includes construction costs which can vary with the infrastructure system) could be saved by replacing 25% of the cement by bentonite. As the cement costs are normally about 30% of the total cost of the concrete in a dam project, the net saving will be 3.3% (30% × 11% = 3.3%) of the total project cost. Similarly, the use of bentonite in primary school buildings in villages and towns will result in a net saving of 5% of the total building cost. This saving can be increased considerably by replacing 40% of the cement by bentonite, because in normal brick
Table 8 Modulus of rupture of concrete incorporating bentonite. Mixture (%) Cement
Bentonite
Modulus of rupture (MPa)
100 80 75 60
0 20 “as is” 25 “heated to 150 °C” 40 “heated to 150 °C”
3.25 2.63 2.60 2.44
% control
Table 9 Ductility ratio of concrete beams.
100 81 80 75
Beam type
Ultimate load deflection Deflection at yield Ductility ratioa
100% OPC (control) 37.00 75% OPC + 25% bentonite 56.83 a
4.87 5.66
Ratio of the deflection at ultimate load to the deflection at yield.
7.61 10.05
226
J. Mirza et al. / Applied Clay Science 45 (2009) 220–226 Table 10 Cracking and crushing strengths of bentonite in brick prisms. OPC (%) Bentonite (%) Cracking strength % control Crushing strength % control (MPa) (MPa) 100 80 75 60
0 20 “as is” 25 “heated to 150 °C” 40 “heated to 150 °C”
2.19 2.17 2.15
100 99 98
4.76 4.61 4.58
100 97 96
1.86
85
4.17
88
masonry only 20 to 30% of the ultimate strength of the masonry is actually utilized.
of the control beams. This could guarantee good energy dissipation during earthquakes. 8. The bond strength of OPC mortars in brick masonry was similar to that of the mortars containing 20% “as is” and 25% “heated to 150 °C” bentonite. This further suggested that these could be used as low-cost construction materials.
6. Conclusions
Acknowledgments
1. The XRD patterns of “as is” and “heated” bentonites possessed both crystalline and amorphous phases. 2. SAI of OPC mortars containing “150 °C heated” bentonite was slightly higher than the mortars containing “as is” bentonite as well as the control mixture after 7 and 28 days. 3. OPC mortars containing 25% “as is” and 30% “heated to 150 °C” bentonites showed compressive strength values of 19.6 MPa and 20.9 MPa, respectively, after 28 days, demonstrating the effectiveness of these mortars as a low cost construction material. 4. After 7 days, the strength development gain in OPC mortars incorporating bentonite was higher compare to 14 day strength, which in turn, was higher than the 28 days strength development. This suggested a decrease in the strength gain ratio as the bentonite substitution ratio increased in the OPC mortar. This tendency showed a lower strength development rate as the curing period increased. 5. The compressive strength of concrete containing “as is” and “150 °C heated” bentonites decreased as their substitution level in the cement increased, after 7, 14, 28 and 56 days. However, the OPC concrete containing 20% “as is” and 25% “heated to 150 °C” bentonites, showed similar compressive strength values after 28 days. These could, therefore, be used as low-cost construction materials. 6. Tests on mortar cubes soaked in 2% MgSO4 and 5% Na2SO4 solutions demonstrated a consistent improvement in sulphate attack resistance as the bentonite content in them was increased. 7. The modulus of rupture of concrete beams containing bentonite was lower than the control concrete beams. This decreased further as the percentage substitution of bentonite is increased, but the ductility ratio of the bentonite concrete beams was larger than that
The authors gratefully acknowledge the financial support from the NWFP University of Engineering and Technology, Peshawar. Gratitude is also extended to Industrial Estate Peshawar Laboratory and PCSIR Laboratory, Peshawar, for providing services for specimen grinding, heat treatment and chemical analyses.
Fig. 8. Cracking strength of OPC concrete brick prisms containing bentonite as percentage of control mixture beams with and without bentonite.
Fig. 9. Crushing strength of OPC concrete brick prisms containing bentonite as percentage of control mixture.
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