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Cao et al., (2015) studied the effects of the addition of CNC to cementitious composites. Seven percentages of CNC ranging from 0% to 1.5% by the volume of cement were added to cementitious composites. An increase of approximately 30% was noticed for flexural strength for cementitious composite with a 0.2% volume of CNC with respect to cement. The rate of hydration of cementitious composite increased with the use of CNC, which was observed through isothermal calorimetry (IC) and thermogravimetric analysis (TAG). In addition, this increased hydration could be explained by two mechanisms. The first mechanism is steric stabilization which disperses cement particles similarly to that of many water reducing agents (WRA). The second mechanism is short circuit diffusion. It seems that this mechanism increases hydration rate by increasing the transportation of water from outside the hydration products to the dehydrated cement cores. Figure 2.33 shows a comparison between BSE-images of plain cementitious composite and 1.5% CNC cementitious composite, one interesting feature observed by 1.5% CNC cementitious composite was that a ring formed around the dehydrated cement grains. The

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adhering of CNC onto cement particles led to the steric stabilization effect. As a result, a continuation of the hydration process took place as shown in figure 2.34, illustrating how CNC may increase the hydration of the cement grain that have CNC on their surfaces due to steric stabilization effect. The results show a clear increase of flexural strength of cement composite having 0.2% volume of CNC with respect to cement. However, CNC are short fibres thereby significantly limiting their ability both to work as crack bridging and to improve the degree of hydration, as they could only transfer water to limited distances. Furthermore, CNC are one dimensional fibre thus they can only work in one dimension, compared with GO’s two-dimensional function. Lastly, the addition of CNC in high percentages is problematic in that there could be a lack of uniform dispersion thereby requiring the use of more energy or chemicals to disperse them.

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Figure 2.33: BSE-SEM images of (a) reference and (b) 1.5% mixture at the age of 7 days. The 1.5% CNC mixture shows ring features surrounding the unhydrated cement

cores (Cao et al., 2015).

In 2016 Cao et al. published a paper in which they described the influence of CNC on the microstructure of the cement paste. They have used a novel centrifugation method to measure the concentrations of the adsorbed cellulose nanocrystals (a CNC) on the cement surface and the free cellulose nanocrystals (fCNC) which are mobile in water. The results showed that there is an increase in the degree of hydration and there is reduction in porosity.

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Figure 2.34: A schematic illustration of the proposed hydration products forming around the cement grain from the age of 0-48 h in the (a) plain cement and (b) cement with CNC on a portion of the cement particle showing short circuit diffusion (Cao et al., 2015).

In addition, the nanoindentation results showed that there is an improvement of reduced modulus of high-density C-S-H. The reason for the improvements might be the high elastic modulus of CNC, which range from 110 to 220 Gpa, which is higher than that of the interfacial region (high density C-S-H) that have elastic modulus about 40-110 Gpa. For this reason, the elastic modulus of high-density C-S-H can be increased by CNC and

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another possible reason for this improvement is that CNC additions could cause a modification to the C-S-H (e.g. its structure or chemistry etc). Despite, the success demonstrated with this method, it suffers limitations, for instance, the ability to crack bridging and the enhancement of the degree of hydration are limited because the CNC are short fibres. In addition, CNC work in one dimension only due to CNC are one dimensional fibre thus their effects are limited to one dimension.

Mònica Ardanuy et al. (2012) examined the addition of Nano fibrillated cellulose (NFC) in the cement mortar composites. The experiments were performed to examine the effects of the extent of fibrillation of the fibres and the reinforcement final size on the mechanical performance of cement composites. Two types of fibres were used in this research, the first type was conventional pulps obtained by subjecting sisal fibres to a soft mechanical treatment. The second type was NFC pulps obtained by the application of a high intensity refining process. These fibres were dispersed in cement mortars and the samples were cured for 28 days. Flexural tests were used to test the mechanical performance of the different composites. flexural properties had improved with the addition of NFC to cement mortar. From the results, there is a clear increase of flexural strength of cement composite having NFC with respect to the cement composite having conventional pulps. However, the cement composite with NFC showed brittle behaviour due to the NFC contributing low capability to bridge incipient cracks bridging. This was because of their small size. Additionally, NFC has a high aspect ratio which produces an enhancement of fibre-matrix interaction, resulting in an excessively strong fibre-matrix bonding which although includes a more effective stress transfer from the matrix to the NFC, consequently also an embrittlement of the composite. In addition, an optimisation of the refining time is required

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to obtain long NFC fibres to improve the ductility while retaining the flexural properties of cement composite.

Mejdoub et al. (2017) studied the effect of adding NFC to Portland cement pastes. The porosity, thermal properties, compressive strength and degree of cement hydration of cement paste were investigated. The amount of NFC as a partial replacement ranged from 0.0% to 0.5% by weight of cement. NFC was made from eucalyptus pulp by using a high- pressure homogenization method. The results revealed that, the compressive strength had increased by more than 50% with 0.3-wt.% of NFC. Furthermore, porosity was decreased with NFC addition and the greatest result was achieved with 0.3-wt.% of NFC. Additionally, the degree of hydration had increased with NFC content and this was confirmed by using X-ray diffraction and Fourier Transform Infrared Spectroscopy. It seems reasonable to think that the NFC encourage the hydration of cement by producing

more Ca(OH)2 and C-S-H gel. From the results, there is a clear increase in the compressive

strength of cement composite, however, it is difficult to define the performance of cement paste based on the results shown here. It seems that the increase of compressive strength was very high and did not match the increase of hydration and the changes of microstructure of cement pastes, hence the results are not consistent or reliable. Additionally, the NFC contributed low capability to bridge incipient cracks bridging because of their small size.

In Summary: as discussed above, addition CNC to cementitious composites has many advantages such as, a clear increase of flexural strength of cementitious composite having CNC and a clear increase of hydration rate of cementitious composite having CNC. However as discussed above, CNC are short fibres thereby their short length significantly

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limiting their ability to work as crack bridging and to improve the hydration process, since they could only transfer water to limited distance. Furthermore, CNC are one dimensional fibers thus they can only work in one dimensional direction compared with GO who has two dimensional. Lastly, the addition CNC in high concentration is problematic in that there could be a lack of uniform dispersion thereby requiring the uses of more energy or more chemicals to disperse them.