- Open Access
Effect of Metakaolin Content on the Properties of High Strength Concrete
© The Author(s) 2013
- Received: 15 November 2012
- Accepted: 30 May 2013
- Published: 10 September 2013
This study presents the effect of incorporating metakaolin (MK) on the mechanical and durability properties of high strength concrete for a constant water/binder ratio of 0.3. MK mixtures with cement replacement of 5, 10 and 15 % were designed for target strength and slump of 90 MPa and 100 ± 25 mm. From the results, it was observed that 10 % replacement level was the optimum level in terms of compressive strength. Beyond 10 % replacement levels, the strength was decreased but remained higher than the control mixture. Compressive strength of 106 MPa was achieved at 10 % replacement. Splitting tensile strength and elastic modulus values have also followed the same trend. In durability tests MK concretes have exhibited high resistance compared to control and the resistance increases as the MK percentage increases. This investigation has shown that the local MK has the potential to produce high strength and high performance concretes.
- elastic modulus
The quest for the development of high strength and high performance concretes has increased considerably in recent times because of the demands from the construction industry. In the last three decades, supplementary cementitious materials such as fly ash, silica fume and ground granulated blast furnace slag have been judiciously utilized as cement replacement materials as these can significantly enhance the strength and durability characteristics of concrete in comparison with ordinary Portland cement (OPC) alone, provided there is adequate curing (Neville 1997). Hence, high-performance concretes can be produced at lower w/b ratios by incorporating these supplementary materials. Fly ash addition proves most economical among these choices, even though addition of fly ash may lead to slower concrete hardening. However, when high strength is desired, use of silica fume is more useful (Basu 2003). When designed at very low water/binder ratio, the presence of silica fume explains the mechanical performance of high strength concrete. Silica fume provides a very good particle packing and, because of its strong pozzolanic property increases the resistance of the concrete to aggressive environments also (Abdul and Wong 2005). Silica fume, though initially considered as an industrial waste, has now become a world class product for which there is a constant demand in the construction industry. However, this product is rather expensive. In India, most of the good quality silica fume is imported and the cost is 9–10 times the cost of OPC.
Metakaolin (MK) or calcined kaolin, other type of pozzolan, produced by calcination has the capability to replace silica fume as an alternative material. In India MK can be produced in large quantities, as it is a processed product of kaolin mineral which has wide spread proven reserves available in the country (Basu et al. 2000; Tiwari and Bandyopadhyay 2003). At present the market price of MK in the country is about 3–4 times that of cement. Therefore the use of metakaoiln proves economical over that of silica fume. Previously, researchers have shown a lot of interest in MK as it has been found to possess both pozzolanic and microfiller characteristics (Poon et al. 2001; Wild and Khatib 1997; Wild et al. 1996). It has also been used successfully for the development of high strength self compacting concrete using mathematical modeling (Dvorkin et al. 2012). However, limited test data are available regarding the performance of the commercially available MK and Indian cements in the case of high strength concrete in the country (Basu 2003; Basu et al. 2000, Pal et al. 2001, Patil and Kumbhar 2012). The objective of this study was to investigate the effect of using local calcined kaolin or MK obtained commercially as pozzolan on the development of high strength and permeability/durability characteristics of concrete designed for a very low w/b ratio of 0.3. In addition, the optimum replacements with respect to strength and durability were determined by varying the amount of MK as partial cement replacement.
An experimental program was designed to produce a high strength concrete by adding several combinations of MK. The materials used and the experimental procedures are described in the following sections.
The cement used in all mixture was normal OPC (53 grade) conforming to IS: 12269 (BIS 1987). Commercially available MK was used as mineral additive. Their chemical composition is specified in Table 1. The X-ray diffraction (XRD) pattern of the MK used in this study is shown in Fig. 1.Table 1
Characteristics of cement and metakaolin.
Ferric oxide (Fe2O3)
Calcium oxide (CaO)
Magnesium oxide (MgO)
Sodium oxide (Na2O)
Potassium oxide (K2O)
Sulphuric anhydride (SO3)
Loss on ignition (LOI)
Good quality aggregates have been procured for this investigation. Crushed granite with nominal grain size of 20 mm and well-graded river sand of maximum size 4.75 mm were used as coarse and fine aggregates, respectively. The specific gravities of aggregates were determined experimentally. The coarse aggregates with 20, 12.5 mm fractions had specific gravities of 2.91 and 2.80, whereas the fine aggregate had specific gravity of 2.73, respectively.
Commercially available poly carboxylate ether (PCE)-based super-plasticizer (SP) was used in all the concrete mixtures.
2.2 Mixture Proportions
Details of the mix proportions in kg/m3.
2.3 Mixing and Casting Details
All the materials were mixed using a pan mixer with a maximum capacity of 80 l. The materials were fed into the mixer in the order of coarse aggregate, cement, MK and sand. The materials were mixed dry for 1.5 min. Subsequently three-quarters of the water was added, followed by the SP and the remaining water while mixing continued for a further 5 min in order to obtain a homogenous mixture. Upon discharging from the mixer, the slump test was conducted on the fresh properties for each mixture. The fresh concrete was placed into the steel cube moulds and compacted on a vibrating table. Finally, surface finishing was done carefully to obtain a uniform smooth surface.
2.4 Specimens and Curing
Three 100 × 100 × 100 mm cubes for the compressive strength.
Three 100 × 200 mm cylinders for the splitting tensile test.
Three 150 × 300 mm cylinders for the modulus of elasticity test.
Two 100 × 100 × 100 mm cubes for water absorption study.
Two 150 × 150 × 150 mm cubes for the GWT water permeability test.
Three 150 × 150 × 150 mm cubes for the water penetration depth test.
Two 100 × 200 mm cylinders for the rapid chloride penetrability test. Samples of 100 × 52 mm were prepared from these cylinders.
All the specimens were cast on mechanical vibration table. After casting, all the specimens were covered with plastic sheets and water saturated burlap, and left at room temperature for 24 h. The specimens were demolded after 24 h of casting and were then cured in water at approximately 27 °C until the testing day.
2.5 Experimental Procedures
The workability of the fresh concrete is measured by using the standard slump test apparatus.
The unconfined compressive strength was obtained, at a loading rate of 2.5 kN/s at the age of, 3, 7, 28 and 90 days on 3,000 kN machine. The average compressive strength of three specimens was considered for each age. The split tensile strength was also tested on the same machine at the age of 28 days.
The absorption test was carried out on two 100 mm cubes as per ASTM C 642 (ASTM 2006a) at 28 days of water curing. Saturated surface dry cubes were kept in a hot air oven at 100–110 °C till a constant weight was attained. These are then immersed in water and the weight gain was measured at regular intervals until a constant weight is reached. The absorption at 30 min (initial surface absorption) and final absorption (at a point when the difference between two consecutive weights at 12 h interval was almost negligible) is reported to assess the concrete quality. The final absorption in all cases is observed to be at 72 h.
The rapid chloride penetrability test was conducted in accordance with ASTM C 1202 (ASTM 2006b). These were also determined at 28 days. This test measures the ease with which concrete allows the charge to pass through and gives an indication of the concrete resistance to chloride-ion penetration. Three specimens of 100 mm in diameter and 52 mm in thickness conditioned according to the standard were subjected to 60-V potential for 6 h. The total charge that passed through the concrete specimens was determined and used to evaluate the chloride penetrability of each concrete mixture.
3.1 Fresh Properties
3.1.1 Plastic Density
The results of the plastic densities with respect to the corresponding MK percentages are given in Table 2. From this it can be seen that the plastic densities varied between 2,421 and 2,520 kg/m3. The slight reduction in the densities of MK concretes was due to the lower specific gravity of MK compared to cement alone.
3.1.2 SP Demand
As far as the workability is concerned, in fact all the concretes the control and the MK mixtures have obtained their design slumps as shown in Table 2. According to these results, concretes obtained had high slump values, highly cohesive and can be easily pumpable. No wide variations in the slump values for the mixtures containing increased amounts of MK were observed.
3.2 Mechanical Properties
3.2.1 Compressive Strength
Mechanical properties of the concretes investigated.
Compressive strength age (days) (MPa)
Splitting ten. str. (MPa)
Elastic modulus (GPa)
f sp /f ck (%)
3.2.2 Splitting Tensile Strength
3.2.3 Elastic Modulus
In addition, the predicted values according to the American Concrete Institute (ACI) model (E = 4.73√f ck ) and BIS model (E = 5 √f ck ) are also plotted in the same Fig. 8. The figure shows that the data points of MK mixtures lie slightly above the predicted modulus of ACI model but the BIS model overestimates the values obtained by actual testing.
3.3 Durability Studies
Durability properties of the concretes investigated.
Permeability (×10−12 m/s)
Water penetration depth (mm)
Chloride permeability (Coulombs)
3.3.1 Water Permeability
In another study, according to Bai et al. (2002), the decrease in sorptivity (indirect measure of permeability of concrete) is due to the influence of particle packing on the capillary pore structure wherein a wide distribution of MK particle sizes exists resulting in a denser packing than the mixtures with cement only, thus reducing the sorptivity. In their study, it was also reported that the relative sorptivity values clearly reflected the strength values whereby the lowest sorptivity values had the highest strength except when the replacement level was 40 % (Bai et al. 2002). This is quite contrary to the results obtained in the present investigation wherein the lowest value of permeability of MK15 did not exhibit the highest compressive strength for the w/b ratio studied. As already stated, the dilution effect, which is the result of the high cement replacement level, will inhibit strength gain rate. The water penetration depths results also followed a similar trend as shown in Table 4. There exists a correlation between the volume of water permeating and the water penetration depths. As the volume of water permeating is more obviously the water penetration also shows an increase in depths. From the above results it can be inferred that, the permeability of the MK mixtures decreased with increase in percentage replacement of MK irrespective of the strengths achieved for the water binder ratio studied.
3.3.3 Chloride Permeability
Using MK as a partial replacement for cement decreased the plastic density of the mixtures.
The results shows that by utilizing local MK and cement designed for a low water/binder ratio of 0.3, high strength and high performance concretes can be developed and compressive strengths of more than 100 MPa can be realized.
The optimum replacement level of OPC by MK was 10 %, which gave the highest compressive strength in comparison to that of other replacement levels; this was due to the dilution effect of partial cement replacement. These concretes also exhibited a 28-day splitting tensile strength of the order of 5.15 % of their compressive strength and showed relatively high values of modulus of elasticity. Splitting tensile strengths and elastic modulus results have also followed the same trend to that of compressive strength results showing the highest values at 10 % replacement.
As far as the durability properties are concerned, local MK found to reduce water permeability, absorption, and chloride permeability as the replacement percentage increases. This may be due to the filler effect of MK particles which has substantially reduced the permeability or porosity of the concrete.
This article is published under license to BioMed Central Ltd. Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.
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