Animal fibers as water reservoirs for internal curing of mortars and their limits caused by fiber clustering

https://doi.org/10.1016/j.conbuildmat.2020.120918Get rights and content

Highlights

  • Microhardness of the paste increases near the fibers.

  • The size of unhydrated cement particles reduces as fiber dosage increases.

  • Fibers cannot supply all the necessary internal curing water based on chemical shrinkage.

  • 2 kg of fibers per m3 of mortar is the optimal dosage.

Abstract

We present a bottom-up experimental research to address evidence of internal curing of mortars using randomly distributed pig-hair as water reservoirs. Plain and reinforced mortars with pig hair ranging from 0 to 8 kg of fibers per cubic meter of mortar were prepared. The microstructures of plain and reinforced mortars were scanned using electron microscopy and the microhardnesses were measured within the bulk cement paste and cement paste near pig fibers. Electrical resistivity, surface absorption, and residual compressive strength of mortars after freeze-thaw cycles were used to test the effects of internal curing caused by pig hair. Natural fibers used to reinforce mortars increase their toughness and provide part of the necessary water for internal curing, yet internal curing originated by the addition of natural fibers is not proportional to fiber dosage; where the potential to form fiber clusters increases as fiber dosage increases. Results show that there is an optimum fiber dosage that maximizes internal curing caused by these fibers. This study contributes to the research on reinforced mortars with natural fibers to provide sustainable solutions for construction materials.

Introduction

Natural fibers improve impact strength, toughness and reduce cracks widths of high strength mortars [1], while at the same time exchange water within mortar microstructure that could change the Degree Of Hydration (DOH) of cement paste. In recent years, the effectiveness of different types of natural fibers on damage control and mechanical performance of reinforced construction materials has been studied [2], [3], [4], [5], [6]. One of the reasons why the use of natural fibers for material reinforcement has increased is the need to provide ways to reduce food-industry waste [1], [5]. For example, waste management from pork-based food products has many challenges including the disposal of its hair worldwide [7], [8]. On the other hand, mortar and concrete represent the most consumed engineered material worldwide [9]. Although in terms of energy required and CO2 emissions cementitious materials are one of the most efficient engineering materials available nowadays [10], their global volume production is what makes them an environmental concern [11]. Barcelo et al. indicate that cement manufacturing alone represents 5–7% of the global anthropogenic CO2 emissions in 2000[12].

Compared to regular aggregates used in concrete, natural fibers generally have high water absorption (e.g. bamboo: 40%, coconut:180%) [13]. Specifically, Araya et al. [1] determined that pig hair has 95% absorption. Therefore, as pig hair absorbs a significant amount of water during mixing, it is possible that these fibers could transfer water to the cement paste, contributing to the hydration process of cement paste or in other words, IC.

A state of the art review by Bentz and Weiss [14] says that IC using LightWeight Aggregates (LWAs) has been recognized since the 1950 s. Before that, LWAs were used to manufacture lightweight concrete and their use unintentionally aided IC, although this concept was unknown at the time [14]. The effect of IC on the microstructure of mortars was addressed by Bentz and Stutzman [15] by analyzing microstructures of mortars at an age of 120 days using saturated LWAs and regular aggregates separately. They showed that the former presented lower density pore structure and less content and size of unreacted cement with respect to the latter. The reported changes in microstructure, and suggested that IC helps sustain a higher degree of saturation within the hydrating paste [15]. As explained by Bentz and Stutzman [15], this saturation has a direct impact on the unreacted cement fraction and capillary porosity content. Trtik et al. described the solution exchange between dry LWAs added into a low w/c system for IC [16];  they showed that a dry LWA inserted into the paste absorbs some pore solution as late as 4 h after mixing, while the material is still fresh. Trtik et al. [16] showed that part of the absorbed water by the LWA was released back into the cement paste at later stages, mostly within the first 24 h, also contributing to the IC process.

In the same way as dry LWAs, dry natural fibers added to low w/c mortars should also absorb and release water and, therefore, this phenomenon requires analysis. The use of natural fibers, specifically pig hair, represents an opportunity of using these waste-based fibers not only as a mechanism to control crack development but also as a water reservoir for IC. The review by Bentz and Weiss [14] indicates that until 2011 only one work about using saturated wood fibers as water reservoirs for IC was available in the literature. Another recent work by Liu et al. shows that there are other materials under study, such as superfine powders, that absorb water by physical capillary forces that could provide IC water to mortars [17]. A recently published work about concrete mixtures with a fixed volume of vegetal fibers and different cementitious mixtures does not show clear evidence of natural fibers impacting on the hydration of the paste by IC [18]. Testing the effect of natural fibers on IC of cementitious mixtures requires to test different fiber dosages as they could cause fiber clustering at some level. If the fiber dosage is controlled, the effect of IC caused by natural fibers could be addressed, for example at the microscopic level using microhardness testing on the cement paste.

The use of animal fibers recovered from industrial waste as reinforcement of mortars, is an example of a circular economy system [1], [5]. The valorization of natural fibers mitigates the problem of transporting and disposing of this material in landfills with a direct impact on the economic and environmental costs of worldwide pork-based food production [7], [8]. In a previous work, we characterized tensile strength and absorption of pig hair [1], [19]. We also addressed the effect of reinforcing high strength mortars with pig hair on mechanical strength and crack control. Results showed that mortars with pig hair presented similar abrasion resistance, crack control, and impact strength than mortars with commercial polypropylene fibers [20], [21], [22]. The use of natural fibers in mortars raises the question of its role in the internal water exchange with the cement paste.

The novelty of this work is to study the potential use of pig hair in mortars as a water reservoir for IC. The methodology proposed is based on measurements and data collection from experimental tests that allowed us addressing the possibility that natural fibers could influence the DOH of the cement paste of mortars. For the sake of robustness, more than one experimental technique was used to validate or reject our expectations. We implemented a “bottom-up” approach so the information extracted from the description of the constituents of mortar at the microscopic level could be correlated with the macroscopic response. For the latter, a characterization was performed based on Scanning Electron Microscope (SEM) images and microhardness measurements of the bulk cement paste and the cement paste near the fibers. Macroscopically, mortars were exposed to freeze–thaw (FT) cycles and the residual strength estimated from experiments. Water absorption and surface resistivity measurements of mortars complent the results of this work.

Section snippets

Mortar: properties of constituents, mixture proportions, and preparation of specimens

For all mortar mixtures in this study, Portland-pozzolan cement (Type IP), following the ASTM C595 standard [23] (Table 1) and two types of fine aggregates, meeting the standard ASTM C33 [24] (Table 1), were used. The workability of mortar mixtures was adjusted using polymer-based plasticizer by cement weight (Table 1). Clean pig hair (Fig. 1a) was obtained to reinforce mortar mixtures. Water absorption of pig hair (Table 1) was determined considering a sample of 100 g of fibers and measuring

Experimental hardness of the bulk paste using different fiber dosages

The bulk paste micro indentations performed in this study were 73, 68, 52, and 97 measurements for mortars with 8, 4, 2, and 0 kg/m3 of pig hair, respectively. Consequently, as the latter number of evaluations was large, Analysis Of Variance (ANOVA) was used to assess the statistical significance of adding pig hair to the hardness of bulk paste for different fiber dosages. Initially, this study evaluated a null hypothesis (H0), that the mean performance of all mortar mixtures was equal (i.e.

Comments on the role of natural fibers on mortar protection to FT cycles

The use of LWAs as water reservoirs for IC has shown improvements on FT resistance of mortar and concrete [34] as it provides additional space to accommodate freeze water. Similarly, pig hair creates new spaces as it reduces its volume as water from the mixture runs out to the surrounding paste. Eventually, these new spaces might contribute to accommodate frozen water (Fig. 11). It has been previously suggested that LWAs might contribute to the entrained air volume [34]. Regarding this

Conclusions and remaining challenges about the effects of natural fibers on IC of mortars

We conclude that mortar reinforced with natural fibers cannot be analyzed as a typical reinforced material where there is only a mechanical relationship between matrix and fibers. If their role on DOH and porosity formation is better understood, natural fibers cannot only contribute to the damage control of mortars, the same as synthetic fibers, but can also be used as a reservoir for IC. The use of pig hair could complement the incorporation of LWAs for IC of mortars to mitigate side effects

CRediT authorship contribution statement

F.C. Antico: Conceptualization, Methodology, Writing - original draft, Writing - review & editing. P. Rojas: Conceptualization, Resources, Funding acquisition. F. Briones: Data curation. G. Araya-Letelier: Formal analysis, Software.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Authors thank the students: José Concha Riedel, Arline Ferrante, Maximiliano Cervantes, Bárbara Quezada, Cristobal Vargas Ríos, Juan Pablo Miranda, Gian Piero Canevari Maturana, Vicente Fernandez Zapico; the technical staff: Wladimir Vergara Tapia, María Virraroel Muñoz, Paula Llanos and Richard Aguirre, for their assistance with part of the testing execution; Ian Scott for his reviews; and the anonymous reviewer for the detailed comments and suggestions that helped to improve this work. FCA

References (44)

  • S. Igarashi et al.

    Microhardness testing of cementitious materials

    Adv. Cem. Based Mater.

    (1996)
  • D.P. Bentz et al.

    Protected paste volume in concrete: Extension to internal curing using saturated lightweight fine aggregate

    Cem. Concr. Res.

    (1999)
  • G. Araya-Letelier et al.

    Experimental mechanical-damage assessment of earthen mixes reinforced with micro polypropylene fibers

    Constr. Build. Mater.

    (2019)
  • Z. Wu et al.

    Effect of nano-SiO2 particles and curing time on development of fiber-matrix bond properties and microstructure of ultra-high strength concrete

    Cem. Concr. Res.

    (2017)
  • J.J. Beaudoin et al.

    A study of mechanical properties of autoclaved calcium silicate systems

    Cem. Concr. Res.

    (1975)
  • A. Çavdar

    Investigation of freeze–thaw effects on mechanical properties of fiber reinforced cement mortars

    Compos. B Eng.

    (2014)
  • J. Nam et al.

    Frost resistance of polyvinyl alcohol fiber and polypropylene fiber reinforced cementitious composites under freeze thaw cycling

    Compos. B Eng.

    (2016)
  • X. Ma et al.

    A review on the use of LWA as an internal curing agent of high performance cement-based materials

    Constr. Build. Mater.

    (2019)
  • X. Sun et al.

    Investigation of internal curing effects on microstructure and permeability of interface transition zones in cement mortar with SEM imaging, transport simulation and hydration modeling techniques

    Constr. Build. Mater.

    (2015)
  • J. Giamalva

    Pork and swine industry and trade summary

    United States International Trade Commission

    (2014)
  • Iberwaste, “Disposal and valorization of Iberian pig wastes from slaughterhouses,” LIFE11 ENV/ES/000562,...
  • J.M. Allwood et al.

    Steel, aluminium and carbon: alternative strategies for meeting the 2050 carbon emission targets

    R’09 Conference

    (2009)
  • Cited by (0)

    View full text