applied sciences

Article Mechanical and Thermal Properties of Insulating Sustainable Mortars with mauritanicus and Pennisetum setaceum as Aggregates

Dionisio Badagliacco 1 , Carmelo Sanfilippo 1,*, Bartolomeo Megna 1,2 , Tommaso La Mantia 3 and Antonino Valenza 1

1 Department of Engineering, University of Palermo, 90128 Palermo, ; [email protected] (D.B.); [email protected] (B.M.); [email protected] (A.V.) 2 INSTM Research Unity of Palermo, 90128 Palermo, Italy 3 Department of Agricultural, Food and Forest Sciences, University of Palermo, 90128 Palermo, Italy; [email protected] * Correspondence: carmelo.sanfi[email protected]

Abstract: The use of natural fibers in cement composites is a widening research field as their application can enhance the mechanical and thermal behavior of cement mortars and limit their carbon footprint. In this paper, two different wild grasses, i.e., Ampelodesmos mauritanicus, also called diss, and Pennisetum setaceum, also known as crimson fountaingrass, are used as a source of natural aggregates for cement mortars. The main purpose is to assess the possibility of using  the more invasive crimson fountaingrass in place of diss in cement-based vegetable concrete. The  two fibers have been characterized by means of scanning electron microscopy (SEM), helium Citation: Badagliacco, D.; Sanfilippo, picnometry and thermogravimetric analysis. Moreover, the thermal conductivity of fiber panels has C.; Megna, B.; La Mantia, T.; Valenza, been measured. Mortars samples have been prepared using untreated, boiled and Polyethylene glycol A. Mechanical and Thermal 4000 (PEG) treated fibers. The mechanical characterization has been performed by means of three Properties of Insulating Sustainable point bending and compression tests. Thermal conductivity and porosity have been measured to Mortars with Ampelodesmos characterize physical modification induced by fibers’ treatments. The results showed better thermal mauritanicus and Pennisetum setaceum and mechanical properties of diss fiber composites than fountaingrass one and that fiber treatments Plants as Aggregates. Appl. Sci. 2021, lead to a reduction of the thermal insulation properties. 11, 5910. https://doi.org/10.3390/ app11135910 Keywords: natural fibers; reinforced mortar; thermal conductivity; sustainable mortar; mechanical

Academic Editor: Doo-Yeol Yoo characterization; surface treatments; diss; crimson fountaingrass

Received: 14 May 2021 Accepted: 22 June 2021 Published: 25 June 2021 1. Introduction In the last decades, a significant amount of relevant research has been carried out Publisher’s Note: MDPI stays neutral concerning the use of lignocellulosic materials in cementititous matrices for building with regard to jurisdictional claims in applications. For their high sustainability and good physical and mechanical properties, published maps and institutional affil- natural fibers find practical applications in composite materials for automotive parts and iations. building construction panels and furniture [1,2]. Residues of sisal, banana tree, common reed and eucalyptus have been used as reinforcement of cement leading to good mechanical performance of the composites [3–7]. The terms Agro-concrete or green concrete are defined as a mix between granulates from lignocellulosic plant (directly or indirectly from agriculture Copyright: © 2021 by the authors. or forestry origin), which represent the bulk of the volume, and a mineral binder [8]. Licensee MDPI, Basel, Switzerland. In this study, the characterization of eco-sustainable composite materials in the This article is an open access article field of green building, for application as insulating plasters has been performed. Diss distributed under the terms and (Ampelodesmos mauritanicus) and Crimson Fountaingrass (Pennisetum setaceum), have been conditions of the Creative Commons used as aggregates for cement mortars. Pennisetum setaceum is an invasive plant that is Attribution (CC BY) license (https:// gradually replacing the Ampelodesmos mauritanicus in Italy [9]. Alien invasive plants creativecommons.org/licenses/by/ represent one of the main threats to biological diversity on a world-wide scale, and 4.0/).

Appl. Sci. 2021, 11, 5910. https://doi.org/10.3390/app11135910 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 14

Appl. Sci. 2021, 11, 5910 2 of 13 gradually replacing the Ampelodesmos Mauritanicus in Italy [9]. Alien invasive plants rep- resent one of the main threats to biological diversity on a world-wide scale, and hard effort is required every year to ward off, control and eradicate them [10]. In , Pennisetum hardSetaceum effort is isone required of the most every aggressive year to ward and off,quick control invaders and of eradicate coastal and them hilly [10 ].areas. In Sicily, Dur- Pennisetuming the 70 years setaceum followingis one ofits the first most introduction, aggressive it and has quickestablished invaders and of spread coastal over and many hilly areas.coastal During areas up the to 70 600 years m.a.s.l. following and especially its first introduction, on south-facing it has slopes established within andthe th spreadermo- overMediterranean many coastal belt areas [11]. up It has to 600 also m.a.s.l. invaded and H. especially hirta thermo on south-facing-xeric slopes and within even thethe thermo-Mediterraneanmore mesophilous Ampelodesmos belt [11]. ItMauritanicus has also invaded grasslands.H. hirta It thermo-xericis likely to continue grasslands to spread and eveninto many the more other mesophilous favorable localities,Ampelodesmos with significant mauritanicus andgrasslands. long-lastin Itg isenvironmental likely to continue con- tosequences spread into [12]. many On the other basis favorable of these localities,considerations, with significantthis research and was long-lasting aimed to assess environ- the mentalpossibility consequences of using crimson [12]. On fountaingrass the basis of these in place considerations, of diss, which this in research previous was works aimed has to assessalready the been possibility used as ofan usingaggregate crimson for green fountaingrass concrete [ in8,13 place], in oforder diss, to which promote in previousthe erad- worksication has of alreadythis invasive been usedplant as with an aggregatefundamental for greenbenefits concrete for the [8 ,biodiversity13], in order of to the promote terri- thetory. eradication of this invasive plant with fundamental benefits for the biodiversity of the territory. 2. Materials and Methods 2. Materials and Methods 2.1.2.1. RawRaw MaterialsMaterials 2.1.1.2.1.1. DissDiss ((AmpelodesmosAmpelodesmos mauritanicus Mauritanicus)) DissDiss (Figure(Figure1 )1 is) is a largea large grass grass widespread widespread growing growing in thein the Mediterranean Mediterranean North North Africa Af- andrica in and the in dry the regions dry regions of of to Greece , to Balkans, Spain, Turkey Balkans, and Turkey Asia Minoris. and Asia In ,Minoris. it isIn establishedFrance, it is in established the departments in the ofdepartments Var, Southern of ,Var, Southern and Herault. Corsica, It isand a perpetual Herault. plantIt is a ofper thepetual plant family, of the which Poaceae lives family, in arid which and sandy lives soils,in arid often and in sandy neat associations, soils, often typicalin neat ofassociations, the Mediterranean typical of grasslands. the Mediterranean In Sicily, Ampelodesmosgrasslands. In Sicily, grasslands Ampelodesmos are commonly grasslands found fromare commonly 0 to 1200 mfound a.s.l., from chiefly 0 to within 1200 them a.s.l., meso chiefly Mediterranean within the and meso thermo-mediterranean Mediterranean and bioclimaticthermo-mediterranean belts [11]. It bioclimatic is composed belts of [ strong11]. It leaves,is composed up to of 1 mstrong long leaves, and about up to 7 mm1 m wide,long and very about rough, 7 mm with wide, tortuous very edges.rough, Thewith long tortuous andtough edges. leaves The long can and be sharp tough for leaves the skincan be if scrubbedsharp for betweenthe skin if fingers. scrubbed This between plant was fingers. previously This plant used was in thepreviously construction used ofin oldthe dwellingsconstruction because of old ofdwellings its mechanical because and of physicalits mechanical properties and physical [14]. Its fibrousproperties feature, [14]. withIts fibrous a thorny feature surface, with may a conferthorny high surface adhesion may confer to the cementhigh adhesion paste and to hencethe cement promising paste propertiesand hence topromising green concrete properties [15]. to Ingreen the literature,concrete [15 it]. is In reported the literature a Tensile, it is Strength reported of a 3 100Tensile MPa, Strength Density of of 100 850 MPa, g/cm Density, Modulus of 850 of Elasticityg/cm3, Modulus 2.17 GPa of Elasticity and a Water 2.17 Absorption GPa and a CoefficientWater Absorption at saturation Coefficient of 112% at [saturation16,17]. of 112% [16,17].

FigureFigure 1.1. DissDiss ((AmpelodesmosAmpelodesmos mauritanicusMauritanicus)) plant. plant.

2.1.2.2.1.2. CrimsonCrimson FountaingrassFountaingrass ((PennisetumPennisetum setaceum Setaceum)) PennisetumPennisetum setaceumSetaceum (Figure(Figure2 )2) is is a a plant plant of of the the Poaceae Poaceae family. family. The The species species is is native native toto NorthNorth Africa,Africa, thethe MiddleMiddle EastEast andand thethe ArabianArabian peninsula.peninsula. IntroducedIntroduced byby humanshumans inin manymany otherother geographicgeographic areasareas asas aa decorativedecorative plant,plant, itit hashas demonstrateddemonstrated toto bebe aa highlyhighly invasiveinvasive species.species. ItIt isis aa permanentpermanent herbaceousherbaceous plant,plant, withwith aa bushybushy growth,growth, upup toto 1.201.20 mm high. The ears are dark pink and progressively lighten with maturation, thus creating a variety of shades of color on the same plant. Pennisetum plant was introduced in Sicily as a decorative plant during the 1940s and just twenty years later it began to regenerate Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 14

Appl. Sci. 2021, 11, 5910 high. The ears are dark pink and progressively lighten with maturation, thus creating3 of 13 a variety of shades of color on the same plant. Pennisetum plant was introduced in Sicily as a decorative plant during the 1940s and just twenty years later it began to regenerate nat- naturally,urally, spreading spreading into into surrounding surrounding areas areas [18]. [Currently,18]. Currently, it has it successfully has successfully invaded invaded many manycoastal coastal and hilly and areas hilly up areas to 50 up0 m to a.s.l., 500 m mainly a.s.l., mainlyon south on-facing south-facing slopes within slopes the within thermo the- thermo-MediterraneanMediterranean bioclimatic bioclimatic belt [11]. belt [11].

Figure 2. Crimson Funtaingrass (Pennisetum setaceumSetaceum)) plant.plant.

2.1.3. Binder (i.pro(i.pro Plastocem)Plastocem) The binder used in this study is i.pro Plastocem a Hydraulic Binder according to The binder used in this study is i.pro Plastocem a Hydraulic Binder according to the the UNI 10892-1 standard. The compression strength at 28 days is higher or equal than UNI 10892-1 standard. The compression strength at 28 days is higher or equal than 3.0 3.0 MPa, it belongs to the 3.0 class and is defined as “UNI 10892 LIC” 3.0. It is used for MPa, it belongs to the 3.0 class and is defined as “UNI 10892 LIC” 3.0. It is used for the the production of mortars for internal and external plasters. PLASTOCEM is also suitable production of mortars for internal and external plasters. PLASTOCEM is also suitable for for the construction of mortars for masonry and substrates for floors. It is composed of the construction of mortars for masonry and substrates for floors. It is composed of a white a white cement and a calcareous filler as reported in the literature [19]. The result of the cement and a calcareous filler as reported in the literature [19]. The result of the STA (Fig- STA (Figure3) on the binder shows the presence of gypsum and calcium hydroxide, but ure 3) on the binder shows the presence of gypsum and calcium hydroxide, but no exo- no exothermic peaks associated with the presence of organic substances are observed. Appl. Sci. 2021, 11, x FOR PEER REVIEWthermic peaks associated with the presence of organic substances are observed. T4he of con- 14 The content of calcium carbonate, linked to the final weight drop (30%), is around 68%. tent of calcium carbonate, linked to the final weight drop (30%), is around 68%. Therefore, Therefore, it is possible to state that it is a predominantly cement-based binder. it is possible to state that it is a predominantly cement-based binder.

FigureFigure 3.3. STASTA resultresult ofof thethe Hydraulic Binder (i.pro Plastocem) Plastocem)..

2.2. Pretreatment of the Aggregates The fibers used for their physical and chemical characterization and for the prepara- tion of the mortars were obtained, for both the plants, as follows (Figure 4): - The stems were obtained from the raw plants and dried in oven at a temperature of 70 °C for 72 h until constant mass, in order to minimize the influence of the moisture content and hence to standardize the physical characteristics of the fibers. The drying temperature was set to 70 °C since it allows a relatively rapid drying of the fibers, which is preferable for the industrial development of the process, without modifying the chemical structure as evidenced by the STA results (Figure 3); - After the drying process, part of the stems were subjected to a thermal treatment by boiling for 4 h with distilled water in order to remove the sugars and extractives and then repeatedly washed with distilled water to clean their surface until the water became clean. This treatment was performed because the dissolution of sugars from vegetable fibers during the setting of the cement can act as a retarding agent for the cement paste [13]. The extracts of vegetable matters, in fact, consist mainly of hemi- cellulose polysaccharides that are supposed to delay the setting of the cement paste [20,21]. After that, the treated fibers were dried in oven at a temperature of 70 °C until constant mass; - The boiling and drying treatment is followed by a surface treatment. This treatment consists of the immersion in a solution of distilled water and Polyethylene Glycol 4000 (PEG) at 40% by weight for two hours, followed by 24 h during which the fibers were left to drip before further drying in the oven until they reached a constant mass; - Both treated and untreated stems were cut with a knife mill Retsch SM100 (Haan, Germany) operating at 1500 rpm using a mesh of 2 mm; - After the cutting process, the fibers were water-saturated in a container at 25 °C and humidity close to 100% R.H. before using them for the preparation of the mortars. This process makes it possible to obtain a better workability of the mortars without increasing the w/b ratio. Appl. Sci. 2021, 11, 5910 4 of 13

2.2. Pretreatment of the Aggregates The fibers used for their physical and chemical characterization and for the preparation of the mortars were obtained, for both the plants, as follows (Figure4): - The stems were obtained from the raw plants and dried in oven at a temperature of 70 ◦C for 72 h until constant mass, in order to minimize the influence of the moisture content and hence to standardize the physical characteristics of the fibers. The drying temperature was set to 70 ◦C since it allows a relatively rapid drying of the fibers, which is preferable for the industrial development of the process, without modifying the chemical structure as evidenced by the STA results (Figure3); - After the drying process, part of the stems were subjected to a thermal treatment by boiling for 4 h with distilled water in order to remove the sugars and extractives and then repeatedly washed with distilled water to clean their surface until the water became clean. This treatment was performed because the dissolution of sugars from vegetable fibers during the setting of the cement can act as a retarding agent for the cement paste [13]. The extracts of vegetable matters, in fact, consist mainly of hemicellulose polysaccharides that are supposed to delay the setting of the cement paste [20,21]. After that, the treated fibers were dried in oven at a temperature of 70 ◦C until constant mass; - The boiling and drying treatment is followed by a surface treatment. This treatment consists of the immersion in a solution of distilled water and Polyethylene Glycol 4000 (PEG) at 40% by weight for two hours, followed by 24 h during which the fibers were left to drip before further drying in the oven until they reached a constant mass; - Both treated and untreated stems were cut with a knife mill Retsch SM100 (Haan, Germany) operating at 1500 rpm using a mesh of 2 mm; - After the cutting process, the fibers were water-saturated in a container at 25 ◦C and humidity close to 100% R.H. before using them for the preparation of the mortars. Appl. Sci. 2021, 11, x FOR PEER REVIEW This process makes it possible to obtain a better workability of the mortars without5 of 14

increasing the w/b ratio.

Figure 4. Procedure for the preparation of the fibersfibers (example(example for boiled Diss fibers):fibers): (a)) drying;drying; ((b)) boiling; (cc)) washing;washing; ((dd)) drying;drying; ((ee)) cutting;cutting; ((ff)) waterwater saturation.saturation.

2.3. Characterization of the Fibers ForFor both the typetype ofof plants,plants, untreateduntreated andand boiled boiled fibersfibers havehave been been physicallyphysically andand chemically characterized,characterized, inin orderorder to to assess assess the the effect effect of of the the thermal thermal treatment treatment on on the the abso- ab- lutesolute density density (helium (helium picnometer picnometer analysis), analysis), morphology morphology (SEM) (SEM) and and elemental elemental composition composi- (SEMtion (SEM and STA). and STA). Furthermore, Furthermore, heat conductivity heat conductivity (heat (heatflow meter flow meter test) of test) the of untreated the un- fiberstreated have fibers been have evaluated been evaluated in order in to order estimate to estimate the insulating the insulating properties properties of both the of typeboth ofthe plants. type of plants.

2.3.1. Helium Picnometry The real densities of untreated and boiled fibers were evaluated through helium pic- nometry analysis in a Picnomatic ATC Thermo Fisher Scientific (Bohemia, NY, USA) pic- nometer. The values were recorded as the average of at least 8 good measures (maximum standard deviation of 0.05%) in a maximum of 15 test cycles.

2.3.2. Scanning Electron Microscopy (SEM) Untreated and boiled fibers have been observed in the low vacuum mode in a scan- ning electron microscope in order to study the morphology of the two species of plants and their modification induced by the thermal treatment. The images have been captured both in SE and BSE mode by setting a high voltage of 30 kV a spot size of 4 nm and a nominal working distance of 10 mm. Quantitative EDX analysis of the ashes was per- formed in order to study the elemental composition. In particular, the amount of silicon in the fibers has been evaluated.

2.3.3. Simultaneous Thermal Analysis (STA) Simultaneous thermal analysis was performed using the Netzsch STA 449 Jupiter F1 (Weimar, Germany) instrument. The tests were performed in the 30–1100 °C range, with 10 °C /min heating rate, 20 mL/min nitrogen flux and 40 mL/min air flux.

2.3.4. Heat Flow Meter Test For the thermal conductivity characterization of the fibers, a LaserComp FOX314 TA- instruments (New Castle, DE, USA) heat flow meter was used according to the standard ASTM C518. From polystyrene panels of size 300 × 300 × 40 mm a square cavity of dimen- sions 150 × 150 × 40 mm was obtained. It was filled with dried fibers, and the panels were wrapped with paper and weighed in order to evaluate the apparent density of the pile of fibers before performing the thermal test. The values of the heat conductivity of the panels were evaluated once reached the steady state of the thermal flux between the upper and lower plates set to 25 °C and 5 °C , respectively. Appl. Sci. 2021, 11, 5910 5 of 13

2.3.1. Helium Picnometry The real densities of untreated and boiled fibers were evaluated through helium picnometry analysis in a Picnomatic ATC Thermo Fisher Scientific (Bohemia, NY, USA) picnometer. The values were recorded as the average of at least 8 good measures (maximum standard deviation of 0.05%) in a maximum of 15 test cycles.

2.3.2. Scanning Electron Microscopy (SEM) Untreated and boiled fibers have been observed in the low vacuum mode in a scanning electron microscope in order to study the morphology of the two species of plants and their modification induced by the thermal treatment. The images have been captured both in SE and BSE mode by setting a high voltage of 30 kV a spot size of 4 nm and a nominal working distance of 10 mm. Quantitative EDX analysis of the ashes was performed in order to study the elemental composition. In particular, the amount of silicon in the fibers has been evaluated.

2.3.3. Simultaneous Thermal Analysis (STA) Simultaneous thermal analysis was performed using the Netzsch STA 449 Jupiter F1 (Weimar, Germany) instrument. The tests were performed in the 30–1100 ◦C range, with 10 ◦C/min heating rate, 20 mL/min nitrogen flux and 40 mL/min air flux.

2.3.4. Heat Flow Meter Test For the thermal conductivity characterization of the fibers, a LaserComp FOX314 TA- instruments (New Castle, DE, USA) heat flow meter was used according to the standard ASTM C518. From polystyrene panels of size 300 × 300 × 40 mm a square cavity of dimensions 150 × 150 × 40 mm was obtained. It was filled with dried fibers, and the panels were wrapped with paper and weighed in order to evaluate the apparent density of the pile of fibers before performing the thermal test. The values of the heat conductivity of the panels were evaluated once reached the steady state of the thermal flux between the upper and lower plates set to 25 ◦C and 5 ◦C, respectively.

2.4. Preparation of the Mortars Four different types of mortars and the cement paste, used as reference, were prepared. These are defined as follows: • CEM: Cement paste; • CEM_30AM: Mortar constituted of binder and 30% by total volume of diss fibers; • CEM_30BAM: Mortar constituted of binder and 30% by total volume of boiled diss fibers; • CEM_30TAM: Mortar constituted of binder and 30% by total volume of PEG treated diss fibers; • CEM_30PS: Mortar constituted of binder and 30% by total volume of crimson foun- taingrass fibers; • CEM_30BPS: Mortar constituted of binder and 30% by total volume of boiled crimson fountaingrass fibers; • CEM_30TPS: Mortar constituted of binder and 30% by total volume of PEG treated crimson fountaingrass fibers In order to evaluate the proper amount of water for each mixture, the test for the determination of the consistence of fresh mortar (by flow table) has been carried out on a Controls 63-L0037-E flow table according to the standard UNI EN1015-3. It was thus calculated the water/binder ratio for every type of mortar, which led to the same workability corresponding to flow values of the mixture between 13.5 and 14 cm. For CEM_30AM, CEM_30BAM and CEM_30TAM mortars, the water/binder ratio used was equal to 0.38 while for those CEM_30PS, CEM_30BPS and CEM_30TPS it was 0.40. For the Cement paste it was 0.28. Appl. Sci. 2021, 11, 5910 6 of 13

Three samples for each type of mortar have been prepared by using prismatic steel molds of dimensions 40 × 40 × 160 mm according to the EN UNI 1015-11 standard in order to perform the mechanical characterization through the three-point bending and compression tests. Disks of 5 cm of diameter have been prepared for the thermal characterization, by using cylindrical PVC tubes as molds. The setting and hardening of the mortars occurred for 7 days within the molds and for 21 days after demolding in laboratory conditions at 21 ◦C and 60% of R.H.

2.5. Characterization of the Mortars 2.5.1. Three-Point Bending Test At least three specimens of each type of mortar were tested according to the standard EN 1015-11 to evaluate the flexural strength at 28 days. Before performing the tests, the actual dimensions (length, thickness and width) of each sample have been recorded with the aid of a centesimal caliper. The reported measurement was the average of three different measures. Three point bending test has been performed in displacement control mode in a Zwick/Roell Z005 testing machine equipped with a 5 kN capacity load cell. The crosshead speed was 0.5 mm/min in order to induce the rupture of the specimens within 1 ÷ 2 min. The preload was 20 N, while the span length was set to 100 mm. The flexural strength was calculated using the following Equation (1)

3P L σ = max (1) b 2bh2

where Pmax is the maximum flexural load, L is the span length, b and h are respectively the width and thickness of the specimen.

2.5.2. Compressive Test For each mortar mix, six specimens were tested, and the results reported as average and standard deviation of the maximum compression strength obtained from the valid tests (at least 5 for each type). The tests were carried out in force control on the halves of the samples previously tested in bending, in a Universal Electromechanical Machine MP Strumenti Tools WANCE UTM 502 equipped with a 50 kN capacity load cell. The loading speed was equal to 200 N/s for CEM mortars, 100 N/s for CEM_30AM, CEM_30BAM and CEM_30TAM mortars and 50 N/s for CEM_30PS, CEM_30BPS and CEM_30TPS in order to induce the rupture of all the specimens within 30 s and 90 s according to the standard EN 1015-11. A compression jig assembly was used in order to compensate the lack of parallelism between the loaded surfaces of the specimen during the compression test. The compression strength values for each sample were obtained by dividing the maximum load by the resistant cross section of the specimen (40 × 40 mm2).

2.5.3. Heat Flow Meter Test Disks between 2 cm and 2.5 cm of thickness were cut from the cylinders after 28 days of hardening by means of an IMER COMBI 250 V (Poggibonsi, Italy) water-cooled saw. Before the execution of tests, the samples were dried in oven at 60 ◦C for 24 h until constant mass, in order to eliminate the water absorbed by the samples after the cutting process and to prevent any discrepancy of results caused by variable moisture contents. Then the specimens have been polished using a PRESI-Mecapol-2B (Eybens Auvergne, France) polishing machine in order to obtain a uniform thermal flow and low contact thermal resistances. Minimum three samples for each type of mortar were tested in a LaserComp FOX 50 TA-instruments (New Castle, DE, USA) heat flow meter according to ASTM C518 and ISO 8301. The values of thermal conductivity were evaluated through the following Equation (2): sq  W  λ = (2) ∆T mK Appl. Sci. 2021, 11, 5910 7 of 13

where s is the sample thickness, q is the heat flux at the steady state and ∆T is the difference between the upper and lower plates temperature respectively set to 25 ◦C and 15 ◦C. The results were reported as average and standard deviations.

2.5.4. Helium Picnometry After performing the mechanical tests, fragments from the crushed samples have been analysed by means of helium picnometry in a Picnomatic ATC Thermo Fisher Scientific (Bohemia, NY, USA) picnometer in order to assess the porosity of the mortars. The values of real density were calculated as the average of at least 8 good measures (maximum standard deviation of 0.05%) in a maximum of 15 test cycles. The porosity of the mortars was obtained by using the following Equation (3):   ρapp Porosity [%] = 100 1 − (3) ρreal

where: ρapp is the apparent density, measured as the mass by volume ratio of the prismatic samples before the flexural test, and ρreal is the real density.

3. Results and Discussion The results of helium picnometry analysis on untreated and boiled fibers (Figure5) Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 14 showed that the thermal treatment is supposed to modify the chemical composition of the diss fibers probably due to the removal of sugar and extractives as also reported by other authors in the literature [8,13].

Figure 5. Density results of untreated and treated diss and crimson fountaingrass fibers. Figure 5. Density results of untreated and treated diss and crimson fountaingrass fibers. SEM image analysis of the two types of plants (Figure6) evidenced the different surfaceSEM morphology image analysis between of the diss two and type crimsons of plants fountaingrass (Figure 6) fibers. evidenced In fact, the the different former sur- are characterizedface morphology by the between presence diss of and spines, crimson which fountaingrass may improve fibers the adhesion. In fact, betweenthe former fibers are andcharacterized cementitious by the matrix presence due to of the spines, better which mechanical may improve interlocking, the adhesion while the between latter present fibers aand smooth cementitious surface. matrix It is also due important to the better noting mechanical that the thermal interlocking, treatment while does the notlatter affect present the morphologya smooth surface. of the It fibers is also as important can be demonstrated noting that bythe observing thermal treatment the Figure doe7 thats not shows affect thethe permanencemorphology of of the the spines fibers onas can the dissbe demonstrated fibers’ surface by after observing the boiling the treatment.Figure 7 that Quantitative shows the EDXpermanence analysis of (Figure the spines8) on theon ashesthe diss of untreatedfibers’ surface diss andafter crimson the boiling fountaingrass treatment. evidenced Quantita- thetive higher EDX analysis content ( ofFigure silicon 8) inon the the former ashes (approximatelyof untreated diss 22% and by crimson weight) fountaingrass compared to theevidenced latter (approximately the higher content 9%). of This silicon result in the may former indicate (approximately that diss fibers 22% are by characterized weight) com- bypared either to the better latter mechanical (approximately properties 9%). or This higher result reactivity may indicate with thethat cementitious diss fibers are matrix. char- Theacterized latter by can either be due better to mechanical the SiO2 content properties in the or fibershigher may reactivity react with with the the cementitious portlandite presentmatrix. The in the latter matrix can be to producedue to the further SiO2 content calcium in silicatethe fibers hydrates may react which with are the responsible portlandite forpresent the mechanical in the matrix properties to produce of the further cement. calcium The STA silicate results hydrates confirmed which those are obtainedresponsible by for the mechanical properties of the cement. The STA results confirmed those obtained by EDX since the amount of uncombusted was higher for the diss rather than the crimson fountaingrass fibers (Figure 9).

Figure 6. Comparison between untreated diss (left) and crimson fountaingrass fibers (right). Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 14

Figure 5. Density results of untreated and treated diss and crimson fountaingrass fibers.

SEM image analysis of the two types of plants (Figure 6) evidenced the different sur- face morphology between diss and crimson fountaingrass fibers. In fact, the former are characterized by the presence of spines, which may improve the adhesion between fibers and cementitious matrix due to the better mechanical interlocking, while the latter present a smooth surface. It is also important noting that the thermal treatment does not affect the morphology of the fibers as can be demonstrated by observing the Figure 7 that shows the permanence of the spines on the diss fibers’ surface after the boiling treatment. Quantita- tive EDX analysis (Figure 8) on the ashes of untreated diss and crimson fountaingrass evidenced the higher content of silicon in the former (approximately 22% by weight) com- pared to the latter (approximately 9%). This result may indicate that diss fibers are char- acterized by either better mechanical properties or higher reactivity with the cementitious Appl. Sci. 2021, 11, 5910 8 of 13 matrix. The latter can be due to the SiO2 content in the fibers may react with the portlandite present in the matrix to produce further calcium silicate hydrates which are responsible for the mechanical properties of the cement. The STA results confirmed those obtained by EDX since the amount of uncombusted was higher for the diss rather than the crimson fountaingrass fibersfibers (Figure(Figure9 9).).

Appl.Appl. Sci. Sci.2021 20, 1121, x11 FOR, x FOR PEER PEER REVIEW REVIEW 9 of9 14of 14

Figure 6. Comparison between untreated diss (leftleft)) andand crimsoncrimson fountaingrassfountaingrass fibersfibers ((rightright).).

FigureFigure 7. Comparison 7. Comparison between between untreated untreated (left ((leftleft) and)) andand boiled boiledboiled (right ((rightright) diss)) dissdiss fibers fibers.fibers. .

FigureFigure 8. Comparison 8. Comparison of quantitative of quantitative elemental elemental composition compositioncomposition between betweenbetween diss dissdiss (left ((leftleft) and)) andand crimson crimsoncrimson fountaingrass fountaingrassfountaingrass ashes ashesashes (right ((rightright). ).).

Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 14

Figure 7. Comparison between untreated (left) and boiled (right) diss fibers.

Appl. Sci. 2021, 11, 5910 9 of 13 Figure 8. Comparison of quantitative elemental composition between diss (left) and crimson fountaingrass ashes (right).

Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 14

Figure 9. STA of diss (above) and crimson fountaingrass (below) fibers. Figure 9. STA of diss (above) and crimson fountaingrass (below) fibers. The results of the heat flow meter test of the fibers shown in Table1 evidenced goodThe insulating results of properties the heat flow of both meter the test types of ofthe plants fiberswith show valuesn in Table of thermal 1 evidenced conductivity good insulatingequal to 0.059properties W/mK of andboth 0.054the type W/mK,s of plants respectively with values for diss of thermal and crimson conductivity fountaingrass equal toaggregates. 0.059 W/mK These andresults 0.054 W are/mK in accordance, respectively to thefor higherdiss and apparent crimson density fountaingrass of the pile aggre- of the gateAmpelodesmoss. These results mauritanicus are in accordancefibers compared to the to higher the Pennisetum apparent setaceumdensity onesof the (Table pile 1of). the Ampelodesmos Mauritanicus fibers compared to the Pennisetum Setaceum ones (Table 1). Table 1. Thermal conductivity and Apparent density of the aggregates. Table 1. Thermal conductivity and Apparent density of the aggregates. Thermal Apparent Type of Thermal Con- AmountAmount of of 3 Apparent Den- Conductivity λ VolumeVolume (cm ) Density of the Aggregate Aggregate (g) 3 Type of Aggregate (W/mK)ductivity λ Aggregate 3 sityPile of (g/cm the Pile) (cm ) 3 Ampelodesmos (W/mK) (g) (g/cm ) Ampelodesmos Mau- 0.059 280.4 900 0.312 mauritanicus 0.059 280.4 900 0.312 ritanicus Pennisetum 0.054 240.5 900 0.267 Pennisetumsetaceum Setaceum 0.054 240.5 900 0.267

The results of the mechanical tests (Figures 10 and 11) on the mortars showed that The results of the mechanical tests (Figures 10 and 11) on the mortars showed that diss aggregate leads to better performances both in terms of flexural and compression diss aggregate leads to better performances both in terms of flexural and compression strength compared to crimson fountaingrass one. This can be due to either the better mor- strength compared to crimson fountaingrass one. This can be due to either the better phology or the chemical composition of the diss compared to the crimson fountaingrass morphology or the chemical composition of the diss compared to the crimson fountain- fibers as verified by the results of the fibers’ characterization. The thermal treatment sig- grass fibers as verified by the results of the fibers’ characterization. The thermal treat- nificantly improves the mechanical properties of the mortars additivated with Ampe- lodesmos Mauritanicus aggregates. This can be probably attributed to the chemical modifi- cation of the diss fibers (as confirmed by the increase in the real density) which leads to better compatibility between natural fibers and matrix, allowing the correct setting and hardening of the cement as also evidenced by other works reported in the literature [8,13]. The surface treatment of the fibers slightly improves the mechanical properties of mortars additivated with Pennisetum Setaceum fibres. Appl. Sci. 2021, 11, 5910 10 of 13

ment significantly improves the mechanical properties of the mortars additivated with Ampelodesmos mauritanicus aggregates. This can be probably attributed to the chemical modification of the diss fibers (as confirmed by the increase in the real density) which leads Appl. Sci. 2021, 11, x FOR PEER REVIEWto better compatibility between natural fibers and matrix, allowing the correct setting11 of 14 and

Appl. Sci. 2021, 11, x FOR PEER REVIEWhardening of the cement as also evidenced by other works reported in the literature11 [ 8of,13 14]. The surface treatment of the fibers slightly improves the mechanical properties of mortars additivated with Pennisetum setaceum fibres.

FigureFigure 10. 10.ThreeThree-point-point bending bending test test results results.. Figure 10. Three-point bending test results.

Figure 11. Compression test results. Figure 11. Compression test results. FigureOverall, 11. Compression the results test of results the heat. flow meter tests reported in Figure 12, evidenced the significantOverall, t contributionhe results of the of both heat theflow aggregates meter tests in reported reducing in the Figure thermal 12, evidenced conductivity the of significantthe mortarsOverall, contribution in the comparison results of bothof the with the heat theaggregates flow reference meter in one.reducing tests This reported the result thermal in is Figure in accordance conductivity 12, evidenced with of the what the mortarssignificantwas obtained in comparison contribution by Saghrouni with of theboth et reference al. the who aggregates evidenced one. This in reducingaresult decrease is in the inaccordance thermal the thermal conductivity with conductivity what was of the in obtainedmortarsfunction by in of Saghrounicomparison the fiber percentage et with al. who the forreference evidenceJuncus one.d Maritimus a decreaseThis resultfibrous in is the in mortar accordancethermal composites conductivity with what [4]. Other inwas functionobtainedworks of ascribe the by fiberSaghrouni the percentage reduction et al. of forwho the Juncus thermalevidence Maritimus conductivityd a decrease fibrous to inmortar the the decrease thermal composites in conductivity the [ density4]. Other and in worksfunctioncorrespondent ascribe of the the increasefiberreduction percentage in theof the water forthermal absorptionJuncus conductivity Maritimus in function fibrous to the of thedecrease mortar fiber composites percentage in the density [[64,].7 , and22Other,23 ]. correspondentworksThermal ascribe conductivity increasethe reduction ofin CEM_30PSthe of waterthe thermal mortar absorption conductivity is slightly in function lowerto the thandecrease of the CEM_30AM fiber in the percentage density probably and [6,7,22,23]correspondentdue to the. Thermal lower increase thermalconductivity in conductivity the of water CEM_30P absorptionof theS mortar crimson in is functionslightlyfountaingrass lower of the than fibers. fiber CEM_30AM The percentage thermal probably[6,7,22,23]treatment due. ofThermal to thethe disslower conductivity fibers thermal leads conductivityof toCEM_30P significantlyS of mortar the worse crimson is slightly thermal fountaingrass lower insulating than fibersCEM_30AM properties,. The thermalprobablycompared treatment due to theto ofthe untreated the lower diss thermal ones.fibers Itleads canconductivity beto justifiedsignificantly of bythe the crimsonworse slightly thermal fountaingrass higher insulating apparent fibers prop- density. The ertiesthermal(Figure, compared 13treatment) and to the the of lower theuntreated diss porosity fibers ones (Figure leads. It can to14 be )significantly of justified the mortars by worse the additivated slightly thermal higher with insulating boiledapparent prop- fibers densityertiescompared ,(Figure compared to those13) toand additivated the the untreated lower with porosity ones untreated. It can(Figure onesbe justified 14) due of to the theby themortars better slightly compatibility additivated higher apparent between with boileddensitythe mortarfibers (Figure compared and the13) thermaland to thosethe treated lower additivated aggregate.porosity with (Figure The untreated thermal 14) of treatment onesthe mortars due doesto theadditivated not better significantly com- with patibilityboiledaffect the fibersbetween thermal compared the properties mortar to those and of Pennisetum theadditivated thermal setaceum treatedwith untreatedmortars. aggregate It ones has. The to due bethermal consideredto the treatmentbetter that com- the doespatibilityhigher not significantly values between of thermal the affect mortar conductivitythe thermal and the properties thermal obtained treated byof Pennisetum both aggregate the types Setaceum. The of mortars thermal mortars. additivated treatment It has to doesbe considered not significantly that the affect higher the values thermal of thermal properties conductivity of Pennisetum obtained Setaceum by both mortars. the types It has of to mortars be considered additivated that the with higher PEG values treated of aggregatesthermal conductivity can be attributable obtained by to both the the higher types amountof mortars of residual addit ivatedwater inside with PEGthe test treated specimens aggregates due to canthe presence be attributable of PEG to4000 the which higher mayamount act as ofa sealresidualant of water the surface inside theof the test aggregates. specimens Thedue majorto the presenceamount of of entrapped PEG 4000 whichwa- ter,may fillin actg theas a porosity sealant ofof thethe surfacemortars, of can the leadaggregates. to an overestimation The major amount of their of thermalentrapped con- wa- ductivity.ter, fillin g the porosity of the mortars, can lead to an overestimation of their thermal con- ductivity. Appl. Sci. 2021, 11, 5910 11 of 13

with PEG treated aggregates can be attributable to the higher amount of residual water inside the test specimens due to the presence of PEG 4000 which may act as a sealant of the Appl.Appl.Appl. Sci.Sci. Sci. 2020 20212121,, 1111, 11,, xx, xFORFOR FOR PEERPEER PEER REVIEWREVIEW REVIEWsurface of the aggregates. The major amount of entrapped water, filling the porosity121212 ofofof of the1414 14

mortars, can lead to an overestimation of their thermal conductivity.

FigureFigureFigureFigure 12.12. 12.12. HeatHeat Heat flowflow flowflow metermeter metermeter testtest testtest results.results. results.results.

Figure 13. Apparent density of the proposed mortars. FigureFigureFigure 13.13. 13. ApparentApparent Apparent densitydensity density ofof of thethe the proposedproposed proposed mortars.mortars. mortars.

FigureFigureFigureFigure 14.14. 14.14. HeliumHelium Helium picnometrypicnometry picnometrypicnometry resultsresults results.results.. .

InInInIn FigureFigure Figure 1313 1313 areare areare reportedreported reported reported thethe the the resultsresults results results ofof of thethe of the the apparentapparent apparent apparent densitydensity density density ofof of thethe the of proposedproposed the proposed proposed mortars.mortars. mortars. mor- AAtars.Ass sexpectedexpected expected As expected,,, thethe, the apparentapparent apparent the apparent densitiesdensities densities densities ofof of thethe the CEM_30PSCEM_30PS CEM_30PS of the CEM_30PS mortarsmortars mortars areare are mortars slightlyslightly slightly are lowerlower lower slightly comparedcompared compared lower totocomparedto thethe the correspondentcorrespondent correspondent to the correspondent CEM_30AMCEM_30AM CEM_30AM CEM_30AM onesones ones duedue due toto to the onesthe the lowerlower lower due toapparentapparent apparent the lower densitydensity density apparent ofof of thethe the density PenniPenni Pennise-se- ofse- tumtumthetum SetaceumSetaceumPennisetum Setaceum aggregatesaggregates aggregates setaceum withwith aggregateswith respectrespect respect withtoto to thethe therespect AmpolodesmosAmpolodesmos Ampolodesmos to the AmpolodesmosMauritanicusMauritanicus Mauritanicus onesones ones mauritanicus (Table(Table (Table 1).1). 1). onesTheThe The thermalthermal(thermalTable1 treatment).treatment treatment The thermal ofof of thethe the treatment aggregatesaggregates aggregates of the doesdoes does aggregates notnot not significantlysignificantly significantly does not significantlyaffectaffect affect thethe the apparentapparent apparent affect the densitydensity density apparent ofof of thethethe mortarsmortars mortars whilewhile while thethe the chemicalchemical chemical treatmenttreatment treatment leadsleads leads toto to aa a moderatemoderate moderate increaseincrease increase inin in tt hishisthis physicalphysical physical propertypropertyproperty duedue due toto to thethe the PEGPEG PEG impregnationimpregnation impregnation ofof of thethe the aggregatesaggregates aggregates whichwhich which causescauses causes thethe the reductionreduction reduction ofof of theirtheirtheir internalinternal internal voids.voids. voids. InIn In FigureFigure Figure 1414 14,, the,the the valuesvalues values ofof of porosityporosity porosity ofof of thethe the mortarsmortars mortars areare are reported.reported. reported. ItIt It isis is evidentevidentevident thatthat that thethe the boilingboiling boiling treatmenttreatment treatment leadsleads leads toto to aa a reductionreduction reduction ofof of thethe the pp porosityorosityorosity forfor for thethe the CEMCEM CEM_30BAM_30BAM_30BAM mortarsmortarsmortars comparedcompared compared toto to thethe the CEM_30AMCEM_30AM CEM_30AM onesones ones thatthat that furtherfurther further demonstratesdemonstrates demonstrates thethe the effectivenesseffectiveness effectiveness ofof of Appl. Sci. 2021, 11, 5910 12 of 13

density of the mortars while the chemical treatment leads to a moderate increase in this physical property due to the PEG impregnation of the aggregates which causes the reduc- tion of their internal voids. In Figure 14, the values of porosity of the mortars are reported. It is evident that the boiling treatment leads to a reduction of the porosity for the CEM_30BAM mortars compared to the CEM_30AM ones that further demonstrates the effectiveness of the thermal treatment to improve the compatibility between Ampelodesmos mauritanicus and the cement matrix as also confirmed by the results of mechanical and thermal tests. On the other hand, the opposite result was recorded for the CEM_30BPS mortars that evidenced a significant increase in the porosity that justifies the decay of the mechanical properties due to incompatibility between the thermal treated aggregate and the matrix. Fi- nally, the boiling and subsequent PEG treatment led to a slight improvement of the physical and mechanical properties of the mortars additivated with Pennisetum setaceum aggregate, while it is pejorative for those additivated with Ampelodesmos mauritanicus aggregate due to the worse fiber-matrix interface.

4. Conclusions In this work two species of plants, i.e., diss (Ampelodesmos mauritanicus) and crimson fountaingrass (Pennisetum setaceum) as aggregates of green concretes have been compared for application as insulating plasters. In particular, the possibility of using the more invasive crimson fountaingrass in place of diss, has been investigated. The characterization of the fibers demonstrated that both from chemical and morphological point of view, diss fibers are preferable to crimson fountaingrass as also confirmed by the mechanical results. Although boiling treatment of diss fibers significantly improves flexural and compression strength of the mortar, the effect on the insulating properties is extremely detrimental and may limit their use as aggregates for insulating plasters. In future works further physical and chemical treatments on Pennisetum setaceum will be investigated in order to improve the compatibility with the cement matrix and encourage their use in place of diss for green concretes.

Author Contributions: Conceptualization, B.M., T.L.M. and A.V.; methodology, B.M. and D.B.; validation, B.M., D.B. and A.V.; formal analysis, B.M. and D.B.; investigation, B.M., D.B. and C.S.; resources, B.M. and A.V.; data curation, B.M., D.B and C.S.; writing—original draft preparation, D.B.; writing—review and editing, D.B. and B.M; supervision, A.V.; project administration, A.V. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Data Availability Statement: All data is contained within the article. Conflicts of Interest: The authors declare no conflict of interest.

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