2.6.1 Introduction
In the last decade, nanoclay based on montmorillonite clay is gaining global interest to use in polymer nanocomposites. In construction and building materials, nanoclay is still a new filler to be used in cement based composites. The new calcined nanoclay could be more acceptable in use in concrete than using nanoclay itself (Shebl et al., 2009). The calcined nanoclay can be prepared by the calcination of nanoclay in order to transform it from semi-crystalline to amorphous state (calcined nanoclay) with high pozzolanic reactivity (Morsy et al., 2010). In this case, calcined nanoclay could be a good competitor to nano- silica or nano-SiO2 and it can be potential candidate as new pozzolanic materials (Al-
91 Historically, metakaolin (MK) is a popular type of calcined clay minerals and classified as one of supplementary cementitious materials. It has been used extensively in cement mortar and concrete due to its high pozzolanic activity (Sabir et al., 2001, Barbhuiya et al., 2015). It can be prepared by heating kaolin clay within the temperature range of 700– 850°C (Fernandez et al., 2011, Tironi et al., 2013). Several studies have been carried out on the use of MK in concrete. It has been reported that the replacement of cement by 5– 20% MK results in significant increase in compressive strength for high-performance concretes and mortars at 28 days (Siddique and Klaus, 2009, Juenger and Siddique, 2015). On the other hand, montmorillonite clay is a very common clay mineral but its use in the cement and concrete industry is limited (Fernandez et al., 2011, Garg and Skibsted, 2014). However, calcined montmorillonite clay can be alternative of comment pozzolanic materials (such as metakaolin) for cement and concrete due to its fairly good pozzolanic activity. Calcined montmorillonite clay is produced by heating montmorillonite clay within the temperature range of 800–930°C (He et al., 1996, Garg and Skibsted, 2014). It was found that for the cement mortars with 30 wt% of Portland cement replaced by 830°C calcined montmorillonite at 28 days, compressive strength was 25% higher than control mortar (He et al., 1996). Calcined montmorillonite clay could be potential candidate as a new supplementary cementitious material.
2.6.2 Microstructure and mechanical properties of Cement-Clay Nanocomposites
Recently, the calcined nanoclay is very new nanomaterials that can be employed in concrete. However, there are very few studies in literature that present the effect of calcined nanoclay on the microstructure and mechanical properties of cement composites. In polymer matrix there were few studies about calcined nanoclay (Abdel Gawad et al., 2010, Botana et al., 2010, Filippi et al., 2011).
Morsy et al. (2010) studied effect of nano-metakaolin (NMK) on the microstructure and mechanical properties of cement mortar at 28 days. NMK was prepared by heating the nano-kaolin at 750°C for two hours. Ordinary Portland cement (OPC) was partially replaced by nano-metakaolin of 2, 4, 6 and 8 % by weight of OPC. Cement mortar was casted using cement/sand ratio of 1:2 and water/binder ratio was 0.5. As shown in Figure
92 2.53, the compressive strength of NMK nanocomposite was found to increase with the increase in NMK content from 2 to 8%. The similar trend was found for tensile strength in samples shown in Figure 2.54 where tensile strength increases with an increase in nanoclay contents. The authors reported that improved strength was due to filler effect of NMK to fill the voids in matrix as well as pozzolanic effect that increased CSH gel. The SEM micrographs for control and 10 wt% NMK mortar to support their argument are shown in Figure 2.55a-b. It was found that the microstructure of control mortar (Figure 2.55 a) displayed more CH crystals, Ettringite and pores whereas the microstructure of nanocomposite containing 10 wt% NMK (Figure 2.55 b) had more CSH gel with less pores and CH crystals. Thus indicating that microstructure of nanocomposite was denser and compact that control matrix.
Figure 2.53: Compressive strengths of control mortar and NMK nanocomposites (Morsy
93 Figure 2.54: Tensile strengths of control mortar and NMK nanocomposites (Morsy et al., 2010).
Figure 2.55: SEM micrographs of: (a) control mortar and (b) NMK nanocomposites (Morsy et al., 2010).
94 Al-Mishhadani et al. (2013) investigated effect of calcined nanoclay (nano-metakaolin) on the compressive and splitting tensile strength of concrete. Nano-metakaolin (NMK) was
prepared by calcining the nanoclay (nanokaolin) at 750°C for two hours. Portland cement
(PC) was partially replaced by NMK of 3, 5 and 10 % by weight of PC. Concrete was cast using water/cement ratio of 0.53 and the samples were tested at 7, 28, 60 and 90 days. Control concrete and concrete nanocomposites were termed as C45, C45+3%NMK, C45+5%NMK and C45+10%NMK respectively. Figure 2.56 shows the results of compressive strengths of control and NMK nanocomposites. It was observed that compressive strength was enhanced atall NMK percentages and at all ages due to micro filling action, better pore refinement and pozzolanic reaction. In addition, as shown in Figure 2.57, splitting tensile strength of C45+5%NMK and C45+10%NMK concrete nanocomposites also increased at all ages (7, 28, 60 and 90 days) when NMK content increased. However, the splitting tensile strength of C45+3%NMK did not show any improvement at all ages.
Figure 2.56: Compressive strengths of control and NMK nanocomposites (Al-Mishhadani
95 Figure 2.57: Splitting tensile strengths of control and NMK nanocomposites (Al- Mishhadani et al., 2013)
Shebl et al. (2009) investigated the effect of nanoclay (inactivated nano-silicate (NS)) and calcined nanoclay (activated nano-silicate) on indirect tensile strength of cement paste at 7 days. Activated nano-silicate was prepared by calcining the nanoclay (Cloisite 30B) at 850°C for two hours. Portland cement (PC) was partially replaced by aactivated nano- silicate of 2, 5 and 10 % by weight of PC and similar replacements for inactivated nano- silicate (NS) which are 2, 5 and 10 wt%. It was found that the indirect tensile strength of nanocomposites containing activated NS were higher than nanocomposites containing inactivated NS as shown in Figure 2.58. The authors reported that this improvement was due to the high pozzolanic reactivity of activated NS (Shebl et al., 2009) than inactivated NS that could lead to consumption of more CH to produce more CSH gel as well as the efficient filler effect. Furthermore, the optimum content of either activated NS or inactivated NS was 2 wt%. For example the indirect tensile strength of cement nanocomposite containing 2 wt% activated NS was increased by 50% compared to nanocomposite containing 2 wt% inactivated NS. However, the team concluded that
96 further addition of activated NS or inactivated NS beyond the optimum content led to a decrease in the indirect tensile strength due to the poor dispersion and agglomerations of the high NS contents which create more voids in the matrix.
Figure 2.58: Indirect tensile strength of cement paste, nanocomposites containing activated NS and nanocomposites containing inactivated NS (Shebl et al., 2009).