3.1. Compressive Strength
Fig.1 shows the variation in the compressive strength of ordinary and RHA cement mortars at various contents of nano-SiO2. It can be seen that the compressive strength of cement mortars with nano-SiO2 are all higher than that of plain cement mortar. Results indicate that the optimal content of nano-SiO2 for reinforcing concrete/mortar purpose should be about 7%. It is clear that increasing the amount of the nano-SiO2 from 7% to 9% doesn't have considerable effect on compressive strength. Moreover, larger amounts of nano-SiO2
actually reduce the strength of composites instead of improving it. It was found that when the content of nanoparticles is large, they are more difficult to disperse uniformly. Therefore, they create weak zone in form of voids; consequently the homogeneous hydrated microstructure cannot be formed, and low strength will be expected. From the results, it is clear that nano- SiO2 is more effective in reinforcement of cement mortar than that of RHA. Also, it can be observed that the nano-SiO2 improved the compressive strength of RHA mortars and incorporating nano-SiO2 particles by RHA in mortar can lead to further improvements in compressive strength and likely other properties of the cement mortar.
Figure 1. Compressive strength of the ordinary and RHA cement mortars at different content of nano- SiO2.
3.2. Flexural Strength
The flexural strength of ten mortar mixtures at different ages is shown in Fig.2. It can be seen that the flexural strengths of the specimens with substitution of cement by nano-SiO2 are all higher than that of plain mortar with the same water-to-binder ratio. The greatest increase among all ages is observed for the batch A4 with the 5% nano-SiO2. At 5% nano-SiO2, the flexural strength at 28 days was 7.5 MP, whereas it decreased to 6.2 MP with 9% nano-SiO2. It indicates that high amounts of nano-SiO2 (especially in excess of 7%) have negative effect on flexural strength. It is clear that the nano-SiO2 particles are more effective in developing flexural strength than that of RHA, and incorporating nano-SiO2 in cement mortars containing RHA can further increase the flexural strength. Two fundamental mechanisms can be deduced for strength enhancement by nano-SiO2. The first strengthening mechanism is the filler effect. The micro-filling effect of nano-SiO2 is one of the important factors for the development of dense concrete/mortar with very high strength, because it has been found that the small amounts of air content significantly decrease the strength of the mortar [16]. It has been reported that the size ratio between filler and the aggregates is one of the main parameters that strongly affects the strengthening caused by filling effect. Thanks to the high size ratio between nano-SiO2 and aggregates, the filling effect of nano-SiO2 particles is more obvious. Furthermore, the microstructure of the transition zone between aggregates and cement paste strongly influences the strength and durability of concrete [17]. Absence of
nano-SiO2 particles reduces the wall effect in the transition zone between the paste and the aggregates and strengthens this weaker zone due to the higher bond between those two phases. This mechanism also leads to an improvement in microstructure and properties of the mortars/concretes [18]. The second strengthening mechanism is the pozzolanic activity. Two major products of cement hydration are calcium silicate hydrate (CSH) and calcium hydroxide (CH), respectively. Calcium silicate hydrate, which is produced by hydration of C3S and C2S, plays a vital role in mechanical characteristics of cement paste, whereas calcium hydrate, which is also formed by hydration of cement, has not any cementing property. It contains about 20-25% of the volume of the hydration products. Calcium hydrates due to their morphology are relatively weak and brittle. Cracks can easily propagate through regions populated by them, especially at the aggregate /cement paste interface [16]. Nano-SiO2
particles react with Calcium Hydrates formed during hydration of cement rapidly and produce calcium silicate hydrate with cementitious properties, which is beneficial for enhancement of strength in concrete/mortar.
Figure 2. Flexural strength of the ordinary and RHA cement mortars at different content of nano-SiO2.
3.3. Water Absorption
The absorption characteristics indirectly represent the porosity through an understanding of the permeable pore volume and its connectivity [19]. In order to investigate the effect of nano-SiO2 particles on cement mortar permeability, water absorption test was carried out on mixes A-1 (plain cement mortar), A-5 (cement mortar incorporated 7% nano-SiO2), A-7 (cement mortar with 20% RHA replacement), A9 (cement mortar with 20%RHA replacement incorporated 3% nano- SiO2).The final absorption of mixes above are presented in Table 4. It can be seen that mixture A-5(cement mortar incorporated 7% nano-SiO2) showed the lowest absorption between all the mixtures, which shows that nano-SiO2 is more effective in reduction of permeability than that of RHA. Integrating nano-SiO2 into RHA mortar reduced the water absorption from 5.42% to 4.45%. Results showed that the presence of nano-SiO2
particles in cement mortar could decrease the water absorption and likely permeability of cement mortar. This impermeability increase can be attributed to two concomitant phenomena: 1. Nano-SiO2 particles generate a large number of nucleation sites for hydration products and induce a more homogenous distribution of CSH and hence less pore structure. 2.
Nano-SiO2 particles block the passages connecting capillary pores and water channels in cement paste [20].
Table 4. Water absorption values of different mixes
Batch No Absorption (%)
A-1 6.12
A-5 4.23
A-7 5.421
A-9 4.458
3.4. Shrinkage
Prismatic specimens with 50×50×200 mm dimensions were prepared. The specimens were cured in the laboratory environment. The average temperature in the laboratory was 27±3 °C, and the relative humidity was 70%. The first measurement was taken using a length comparator with a precision of 2µm after 24 h of mixing, while the rest of measurements were taken at different ages of 3, 7, 14, 21, 28, 35, 42 days. The shrinkage behavior of mortars containing nano-SiO2 is presented in Fig.3. From the results, it can be seen that drying shrinkage of mortars with nano-SiO2 is apparently higher than that of control mortar and increases with increasing nano-SiO2 content. Fig.4 shows the influence of nano-SiO2 on shrinkage behavior of the RHA mortar. Results showed that RHA mortar experienced higher shrinkage than that of ordinary cement mortar. An increase was observed in RHA mortar containing nano-SiO2 in comparison with the RHA mortar. The increase in the drying shrinkage of mortar containing nano-SiO2 might be due mainly to refinement of pore size and increase of mesopores, which is directly related with the shrinkage due to self-desiccation.
Moreover, it has been found that Nano-SiO2, due to its high specific surface, serves additional nucleation sites for hydration products whereby chemical reactions are accelerated. Therefore, the degree of hydration increases as the amount of nano-SiO2 increases and the autogenous shrinkage related to chemical shrinkage also increases [21].
3.5. Microstructure
Cement paste characteristics, for instance, strength and permeability, significantly depended on its nanostructure features in particular nanoporosity [22]. In recent years, the electron microscopy has demonstrated to be a very valuable method for determination of microstructure. Numerous studies on the influence of nano-SiO2 on microstructure of plain cement mortar have been carried out. The results showed that nano-SiO2 particles formed very dense and compact texture of hydrate products and decreased the size of big crystals such as Ca(OH)2. In this paper in order to study the microstructure of RHA mortar, with and without nano-SiO2, a XL30-type scanning electron microscope produced by Philips Company was used. The microstructure of RHA mortar with 3% replacement of nano-SiO2 and without nano-SiO2 at curing age of seven days are presented in Figures 5 and 6, respectively. Results showed that nano-SiO2 particles improved the microstructure of RHA mortar on dense and compact form and generated more homogenous distribution of hydrated products.
Figure 3. Shrinkage of mortars containing nano-SiO2 versus time.
Figure 4. Shrinkage of RHA mortar with and without nano-SiO2 versus time.
Figure 5. SEM micrograph of RHA mortar.
Figure 6. SEM micrograph of RHA mortar with 3% nano-SiO2.
Conclusion
Noticeable increase was observed in compressive and flexural strength of ordinary cement mortars upon adding nano-SiO2. Compressive and flexural strength of RHA cement mortars improved with the incorporation of nano-SiO2. Integrating nano-SiO2 with cement mortar containing RHA improved the microstructure of products on dense and compact form.
Nano-SiO2 had significant impact on drying shrinkage of mortars. The mortar samples with nano-SiO2 experienced higher drying shrinkage. This effect was more prominent for larger amounts of nano-SiO2. According to the results, there was a significant improvement in water
absorption of mortars with intigerating nano-SiO2. Nano-SiO2 particles decreased the water absorption of cement composite by pore filling and pozzolanic effects. Also, it was observed that nano-SiO2 particles were more effective in reduction of permeability than that of RHA.
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Editors: A. K. Haghi and G. E. Zaikov © 2013 Nova Science Publishers, Inc.