Keywords

1 Introduction

Engineered cementitious composites (ECCs) micromechanically designed by adding microfibers show significant improvements in the ductility, tensile strength, and toughness of cement-based materials. Nevertheless, they have no noticeable influence on compressive and flexural strength [1]. Microfibers such as polymer fibers, due to their relatively small surface areas, can have limited interfacial strength. Thus, they can pose problems in reinforced cementitious composites by entrapping air voids and degrading the strength [2]. In this regard, reinforcement of the ECC at the nanoscale might be an alternative option.

Research of cementitious composites reinforced with nanomaterials is rapidly expanding and recently the growing push for greener construction materials has shifted the attention to plant-based nanomaterials such as Nano cellulose (NC). NC is a nanofiber derived from cellulose, which is the primary component of plant cell walls. Being an abundant natural ingredient, it is one of the most sustainable raw materials [3]. In addition to its environmental benefits, including renewability, biodegradability, low environmental impact, and low health risks associated with production, it also has a low production cost. NC also has outstanding mechanical and physical benefits such as high modulus and strength, high specific and reactive surface, high aspect ratio and hydrophilic and hygroscopic properties, all of which are favorable for use as a reinforcement for cementitious composites [4].

NC-based composites have been successfully used in areas such as the medical, food, paper, and electrochemical industries [5]. However, their application in the construction industry is limited. Although improvement of the mechanical characteristics of NC-based composites has been established in the literature, research into their use in cementitious composites, notably ECC, has been minimal and none for hybrid ECC.

In this study, a novel NC-reinforced hybrid polyethylene (PE)-steel ECC was developed and the effect of NC on its compressive strength was investigated. A series of uniaxial compression tests were carried out, incorporating different dosages of NC to study the effect on the compressive properties. Compressive strength of the mixes was compared with a reference mix without NC, and the optimum quantity of NC that modifies the performance of ECC was determined.

2 Specimen Preparation

The raw materials used included cement, sand, water, fly ash, silica fume, superplasticizer, steel fibers, PE fibers, and NC. General Purpose Portland cement that conformed to AS3972, fly ash of ASTM class F (SG 2–2.5) and high-grade silica fume were used as the binding materials, while sand with a mean grain size of 225 µm was used as fine aggregate. PE fibers (1.5% by volume) and steel fibers (0.5% by volume) were used as high modulus and low modulus fibers, respectively, and cellulose nanofibrils (CNF) were used as the NC. Additionally, polycarboxylate-based high-range water-reducing admixture Rheobuild 10000N7 was used to attain good workability with consistent rheological properties for uniform fiber dispersion.

The NC was purchased from Cellulose Lab in Canada. It was derived from bleached softwood pulp and prepared by subjecting the pulp to intensive mechanical treatment by a high-pressure homogenizer. NC was provided as a slurry in aqueous gel form with 3.0 wt%.

Six NC-reinforced ECC mixes were prepared by adding NC dosages of 0.1%, 0.2%, 0.25%, 0.3% and 0.4% to the hybrid ECC. ECC with 0% NC was used as the reference mix. The hybrid ECC used in this study was a 0.5% steel and 1.5% PE ECC that was developed by the authors in a previous study [6]. Compressive tests were carried out for all seven mixes including the reference mix.

To make the samples, the dry ingredients including cement, fly ash, silica fume and sand) were mixed for about 2 min. The fibers were then slowly added, and the mixing was continued for a few more minutes until all the fibers were evenly distributed. Water and superplasticizer were combined and gradually added to the dry mix while the mixing continued. CNF was diluted in water using a hand blender and added to the mix and the mixing was continued for 3 min. Once the mix was liquefied and in a consistent and uniform state, it was poured into molds and vibrated for 30 s. Following this, the molds were cling film-wrapped and kept at room temperature for 24 h until demolding. The specimens were then wrapped in plastic sheets and placed in an oven at 23 ± 1 °C and relative humidity of 50% until the age of testing.

3 Results of Uniaxial Compression Test

Uniaxial compression tests were carried out in accordance with AS 1012. The tests were conducted at 28 days after casting the 50-mm cubic specimens. An INSTRON 5500R machine with 1000 KN capacity was used at a loading rate of 20 MPa/min. Three specimens from each mix were tested.

The results of the compression tests are presented in Fig. 1. All the mixes with NC showed an improvement in compressive strength compared with the reference mix without any NC. NC concentrations of 0.1%, 0.2%, 0.25%, 0.3% and 0.4% improved the compressive strength by 29.1% 45.3%, 29.4%. 15.8% and 5.8%, respectively. A threshold was reached at 0.2%, where a maximum compressive strength of 68.4 MPa was achieved. Beyond this threshold concentration, the compressive strength started to decrease.

Fig. 1
A vertical bar graph of compressive strength in megapascals for 0, 0.1, 0.2, 0.25, 0.3, and 0.4 percentages N C. The compressive strengths are as follows. 0% N C, 37.4. 0.1% N C, 52.8. 0.2% N C, 68.4. 0.25% N C, 53.0. 0.3% N C, 44.3. 0.4% N C, 39.7.

Compressive strength at 28 days with different dosages of nanocellulose (NC)

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Increased interaction between the nanofibers due to a densified matrix can be attributed to the improvement in strength, while the reduction in strength at concentrations above 0.2% could be attributed to fiber agglomeration arising from difficulty of dispersion.

An internal curing effect of NC that stems from the hydrophilic and hygroscopic nature of NC enhances the degree of hydration (DOH) of the matrix [7,8,9]. When the DOH is enhanced, porosity is reduced, and strength is increased. Moreover, the high aspect ratio and the high specific surface area increase the cellulose hydroxyl groups available for hydrogen bonding in the cementitious matrix [10]. Consequently, strong bonding is promoted, resulting in enhanced fiber–matrix interaction and a densified matrix.

On the other hand, the -OH groups at high density on the surface of cellulose fibers try to bond with adjacent –OH groups by hydrogen bonds causing agglomeration or entanglement of the fibers [11]. Due to the larger surface area, this effect is more significant in nanofibers. Therefore, when NC is applied in excessive quantities, dispersion becomes difficult and the fibers agglomerate, causing high porosity and the fibers acting as stress concentrators, resulting in a reduction in strength.

4 Conclusions

Compressive strength increased as the NC concentration increased until a threshold was reached at 0.2%, where the maximum compressive strength of 68.4 MPa was achieved. Beyond this threshold concentration, the compressive strength started to decrease. All the mixes with NC achieved enhancements of compressive strength compared with the reference mix, with the mix containing 0.2% NC achieving the highest improvement of 45.3%. The decline in strength at high doses of NC was attributed to fiber agglomeration. Microscale studies will be conducted in the future for a more insightful investigation.