FASE DE EVALUACIÓN

There have been many renowned studies into the enhancement of flexural strength of cementitious composites using a range of additives including CNC, MWCNT and GOs. This study investigated the addition of BNP and the effects of this additive in comparison with traditional materials are outlined below. Figure 4.20 summarises the maximum enhancement of flexural strength of BNP used in this study compared to that of Cellulose Nanocrystals (CNC) by Cao et al. (2015, 2016). As shown, flexural strength was improved

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in most cases with significant amounts of BNP and CNC. The observed best performance enhancement included a 75% increase of flexural strength with 0.2 wt% of BNP, a 50% increase of flexural strength with 0.5-1.5vol% of CNC (Cao et al., 2016b) and a 30% increase with 0.2vol% CNC (Cao et al., 2015).

Figure 4.20: Maximum enhancement of flexural strength; comparison of BNP and CNC

In addition, the maximum improvement of flexural strength using MWCNT alone compared with BNP in this research are summarised in figure 4.21. In all cases the flexural strength was enhanced with additions of BNP and MWCNT. The best performance enhancement included a 75% increase of flexural strength with 0.2-wt% of BNP; a 50% increase with 0.1wt% of MWCNT (Tyson et al., 2011), a 35.5% increase with 0.2wt% of MWCNT (Luo et al., 2009), a 35% increase with 0.08wt% of MWCNT (Metaxa et al.,

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2012b), a 38% increase with 0.2wt% of MWCNT (Xu et al., 2015), a 25% increase with 0.5wt% of MWCNT, a 10% increase with 0.042wt% of MWCNT (Li et al., 2005), a 34% increase with 0.5wt% of MWCNT (Musso et al., 2009), a 40% increase with 0.08wt% of long MWCNT (Konsta-Gdoutos et al., 2010), a 25% increase with 0.25wt% of MWCNT (Chan and Andrawes, 2010) and a 50% increase of flexural strength with 0.048wt% of Carbon Nanofibres (CNFs) (Metaxa et al., 2012a).

Figure 4.21: Maximum enhancement of flexural strength; comparison of BNP and MWCNT

Figure 4.22 summarises the maximum improvement of flexural strength using BNP compared with those studies using GO. Here and as seen previously, the flexural strength was improved with additions of both BNP and GO. The best performance enhancement included a 75% increase of flexural strength with 0.2wt% of BNP, a 67.1% increase with

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0.04wt% of GO (Lv et al., 2014b), a 42.1% increase with 0.05wt% of GO (Lv et al., 2014a), a 60.7% increase with 0.03wt% of GO (Lv et al., 2013), a 31.63% increase with 0.01wt%

of GO (Zhang H, 2015), a 14.2% increase with 0.04wt% of GO (Li et al., 2017), a 16.7%

increase with 0.04wt% of GO (Zhou et al., 2017), a 59% increase with 0.05wt% of GO (Muhit B. 2015), and a 30.37% increase of flexural strength with 0.022wt% of GO (Zhao et al., 2017).

Figure 4.22: Maximum enhancement of flexural strength; comparison of BNP and GO

In this study, using BNP greatly enhanced the flexural strength in all cases when compared to previous studies using CNC, MWCNT and GOs. It seems that BNP are very small and adsorbed on the surface of the cement particles and they improve the rate of hydration through two mechanisms: (i) the first mechanism is steric stabilization, which is the same

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mechanism observed in many water reducing agents (WRA) to disperse cement particles during cement mixing. This results in a finer and more uniform distribution of cement; (ii) the second mechanism is short circuit diffusions which is the mechanism of water molecules diffusing along the BNP networks in the hydration products (Cao, et al., 2015). When the BNP adhere on the surface of hydrated products, the BNP create paths (i.e., channels) for water molecules to more easily diffuse through the hydrated shell and reach the inner unhydrated cement core.

4.4 Conclusions

Experimental tests have been carried out to investigate the performance of cementitious composites modified with BNP. The experimental programme has been described and results obtained from experimental tests have been presented and discussed. The TGA/DTA provides insight into the chemical reaction mechanisms in cementitious materials during heating. It can be observed that the mass loss of the cement paste composites decreases with increasing BNP concentration. This is due to the increase of the high density C-S-H content and the creation of new intercalated BNP/C-S-H

nanocomposites with higher density. Additionally, the amount of Ca(OH)2 increases with

increasing BNP concentration at 7 and 28 days. This could be attributed to the addition of BNP sheets accelerating the hydration of cement resulting in the production of higher

Ca(OH)2 content at 7 and 28 days. The XRD further confirms the TGA findings. This

means the addition of BNP promotes the hydration of cements thereby increasing the amount of the hydration products. This could be attributed to the highly hydrophilic BNP sheets which tend to store water molecules on their surface thus acting like water reservoirs which may release the free water for further hydration. Additionally, the results clearly

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show that the DOH increases with increasing BNP content. The microstructure of the plain

cement paste consists mainly of Ca(OH)2 crystals with a low C-S-H content. The addition

of 0.2-wt% BNP leads to higher C-S-H content, fewer Ca(OH)2 crystals and denser

microstructure. The TEM investigations suggest a good compatibility between the BNP sheets and the hydration products. Based on the mechanical properties results, for all three curing ages, the increase in the flexural strength of the cement pastes reaches its maximum at an optimum content of 0.20-wt% BNP. This increase, however, diminishes at higher BNP concentrations and this is more pronounced at 0.6-wt% BNP. This could be attributed

to the synergetic effect of restacking of BNP sheets and the high content of Ca(OH2)

crystals.

As the flexural strength is a vital structural property, the maximum increase of flexural strength for our cementitious composites modified with BNP have been compared with previous composites and the results showed that the maximum increase of flexural strength of our composite was significantly greater than the rest of the previous work.

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