Development of a sustainable and innovant hygrothermal bio-composite featuring the enhanced mechanical properties

Journal of Cleaner Production 229 (2019) 128e143

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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro

Development of a sustainable and innovant hygrothermal bio-composite featuring the enhanced mechanical properties Muhammad Riaz Ahmad a, b, Bing Chen a, b, *, M. Aminul Haque a, b, Syed Farasat Ali Shah a, b a b

State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai, 200240, PR China Department of Civil Engineering, Shanghai Jiao Tong University, Shanghai, 200240, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 February 2019 Received in revised form 30 March 2019 Accepted 1 May 2019 Available online 3 May 2019

In this study, machine controlled high compaction force was used to produce an innovant, sustainable and energy efficient bio-composite based on magnesium phosphate cement (MPC) as binder and corn stalk as bio-aggregate. Characterization of corn stalk was investigated by water absorption, image processing, scanning electron microscopy (SEM), x-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analysis. Mechanical, hygrothermal, aging and microstructural properties of biocomposites were examined. Compressive strength of bio-composites compacted by the machine was considerably higher than the manually compacted bio-composites. Capillary absorption and water absorption of bio-composites were lowered due to high compaction force. Increase in the temperature slightly affected the thermal properties of bio-composites. All the bio-composites were rated from good to excellent hygric regulators based on their moisture buffer performance. Influence of high compaction on the thermal, hygric and moisture buffer properties of bio-composites was discernible. Development of molds was detected on the specimens subjected to higher relative humidity level. Finally, formulated energy-efficient and sustainable bio-composites were classified into structural and insulation grade concrete. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Corn stalk Moisture buffer capacity Thermal properties Bio-composites Aging properties

1. Introduction Ordinary Portland cement (OPC) is the most common binder associated with the construction activities around the globe. According to a report (statista, 2013), the production of cement will rise from 3.27 billion metric tons in 2010 to 4.83 billion metric tons in 2030 and same amount of CO2 will be emitted into the environment. The cement industry is one of the major sources of CO2 emission (5e7% of total emission) and hence endangers the environment stability (Kene et al., 2011; Rao et al., 2014). Moreover, buildings constructed using the traditional materials consume approximately 50% of global energy consumption during their life cycle (SBCI, 2007). Hence, there is direct relationship between type of materials used in construction of buildings, energy consumption and environment stability. In this context, it is crucial to develop the sustainable and energy efficient materials which can be

* Corresponding author. State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai, 200240, PR China. E-mail address: [email protected] (B. Chen). https://doi.org/10.1016/j.jclepro.2019.05.002 0959-6526/© 2019 Elsevier Ltd. All rights reserved.

employed as replacement of traditional building materials. Supplementary cementitious materials (silica fume, fly ash, steel slag powder) and several other techniques (sewage sludge, upcycling CO2, lithium slag) have been utilized to improve the properties and to enhance the sustainability characteristics of cementitious materials including MPC (Ahmad et al., 2019; Huang et al., 2018; Jiang et al., 2019; Qin et al., 2018b; Tan et al., 2018b; Yang et al., 2019). In this study, fly ash was used as a pozzolanic material to enhance the sustainability characteristics of biocomposites. Hemp concrete (HC) or hemp-lime concrete (HLC) is an energy efficient, carbon-negative and low-embodied energy composite building material which is mainly constituted of hemp shiv (woody particles of hemp) as bio-aggregate, lime or ordinary Portland cement (OPC) as binder and water. HLC can absorb 14e35 kg of CO2 over the span of 100 years when compared with cellular concrete which releases nearly 52.3 kg CO2. Moreover, the production energy for HLC is 50% lower in comparison to equivalent cellular concrete (Boutin et al., 2006). Apart from hemp shiv as bioaggregate, other plant aggregates like sugarcane (Manohar et al.,

M.R. Ahmad et al. / Journal of Cleaner Production 229 (2019) 128e143

2002), diss (Sellami et al., 2013), cork (Silva et al., 2005), straw bales (Ashour et al., 2011), date palm (Haba et al., 2017), raw rice husk (Qin et al., 2018a), black tea (Huang et al., 2019) and rape straw (Rahim et al., 2016) have been also exploited to produce bioaggregate concrete. These types of lightweight bio-aggregate concretes are non-structural materials and primarily used for only plastering and insulation of walls, roofs and floors. The thermal, hygric and acoustic properties of plant aggregate concretes are outstandingly admirable as compared to conventional concrete thanks to their highly porous structure and low density (80e120 kg/m3) of plant particles. Bio-concrete demonstrates high water absorption properties (Ahmad et al., 2018) and its thermal conductivity differs from 0.06 to 0.2 W/m.K for the densities from 200 to 1100 kg/m3 (Ahmad et al., 2018; Amziane and Arnaud, 2013;
re
zo, 2005). However, compressive strength of bio-aggregate Ce concrete is considerably lower as compared to traditional building materials. Low compressive strength (<2 MPa) and low young modulus (
20 MPa) of bio-aggregate concrete make them unsuitable to be employed as loading bearing material (Bütschi et al., 2004; CONSTRUIRE, 2007; Eires et al., 2006; Elfordy et al., n.d.) as higher mechanical properties and rigidity are needed for a building material to be used for structural application (de Bruijn et al., 2009). Mechanical properties of bio-aggregate concrete depend upon the choice of binders. OPC, lime and gypsum are frequently used binders to formulate the bio-aggregate concrete. Several studies have discussed the incompatibility of cement with the plant aggregate and wood, and resulting bio-concretes demonstrate poor physical, mechanical and durability properties (short term and long term properties), that is largely associated with the extractive compounds of plants or wood like sugar, tannins or starch
lou et al., 2015; Miller and Moslemi, 2007). Moreover, high (Dique alkalinity of cement can degrade the polysaccharides and hemicelluloses of cell-wall into saccharides acids which can inhibit the hydration of cement together with the deterioration of other products (malic acid or glycolic pyruvic acid) (Bilba et al., 2003; Wilding et al., 1984). It has been explained that adsorption of extractive molecules onto the cement hydration products or unhydrated cement grains can poison the nucleation and hence hindering the growth of these hydrates (CeSeH or CeH) (Bishop and Barron, 2006). Similarly, precipitation or chelation of calcium ions can reduce its concentration into the solution and prevent the formation of CeSeH gel. Hence, chemical interaction between the lignocellulosic particles of plant aggregate and mineral binders can either prolong the hardening or inhibit it completely which can change the quality and quantity of hydration products in short term. Whereas, hardening mechanism of mineral binders can be vigorously influenced due to presence of plant aggregate particles in the long term. Bio-concrete based on these binders and aggregate lack the adeqaute cohesion which can be attributed to the
lou et al., 2015; Na et al., 2014; powdering of mineral binder (Dique Sedan et al., 2008). Performance of the building and hygrothermal comfort level is mainly evaluated by indoor relative humidity (Kalamees and Vinha, 2003). Higher level of indoor relative humidity effects the building occupants, degrade the structure of buildings, and hence shorten its life (Kong et al., 2019; Pereira et al., 2018; Shehadi, 2018). Studies have shown that the hygroscopic properties of bio-composites can contribute to regulate the indoor humidity level and improve the comfort levels of residents. Meanwhile, they can also cut down the level of energy consumption (Le et al., 2010; Rode et al., 2007; Shea et al., 2012). Previously, more attention is given on the moisture properties of bio-composites under the equilibrium and steady state conditions of temperature and relative humidity. However, study of hygroscopic properties under the dynamic conditions is more realistic. The release and storage of dynamic moisture content

129

in hygroscopic materials is strongly linked to the exchange of moisture on their surface area and flow of moisture in material. The dynamic moisture properties of bio-composites are evaluated by their moisture buffer performance under the controlled environment protocols. Hence, there are compatibility issues of plant aggregates with the traditional binders, which lead to formulate a bio-composites with the low mechanical properties and formulated biocomposites cannot be utilized as structural materials. Moreover, higher number of studies address only thermal and hygric properties of bio-aggregate concrete and there is lack of research on improvement in the mechanical properties of plant aggregate concrete. Bio-composites possess higher amount of meso, macro and micro air-voids due to its highly porous structure. Air-voids of the bio-aggregates can be reduced or eliminated by the compaction process as the density of raw bio-aggregate is extremely low to give high strength bio-composites as compared to concrete. Such a process of compaction can increase the compressive strength of bio-composites to great extent. Therefore, the main aim of this research was to formulate the first of its kind high strength and high-performance bio-composite, which can not be only utilized as hygric and thermal regulator but can also be used for structural applications. In this research, combination of magnesia (MgO) and phosphate salt (NH4H2PO4) were used to produce a magnesium phosphate cement (MPC) binder with the characteristics of having almost neutral pH and high early strength as compared to traditional binders, quick setting time and excellent compatibility with the plant aggregates. However, decrease in air volume can compromise the thermal properties of these bio-composites. The high amount of industrial by-product fly ash (50% by weight of binder) was added, and hence sustainable and low embodied energy bio-composites was produced. Corn stalk was chosen as the bio-aggregate due to its abundant availability throughout the China. As per report of Foreign Agriculture Service (USDA, 2018), China produces 257 million metric tons of corn which is almost 21% of worldwide corn production. The bio-concrete based on MPC is designed to formulate pre-cast elements to be employed both for structural and insulation purposes using the manual compaction or high compaction process. Finally, the influence of compaction and binder to corn stalk content on the mechanical, thermal, hygric and aging properties of bio-composites were analyzed. 2. Experimental program 2.1. Raw materials Ammonium dihydrogen phosphate (NH4H2PO4, ADP), dead burned magnesia (MgO), fly ash (FA), crushed corn stalk (CS), sodium triphosphate (STP) and borax (Na2B4O7$10H2O) were used as raw materials. The dead burned magnesia with an average particle diameter of 23.3 mm and purity of 91.7% was supplied from Taishan Refractory Plant of Shanghai. The surface area of MgO was 625 m2/ kg. FA was obtained from the Shanghai Wujing Power plant with the average particle size of 3 mm and conformed to ASTM C618. STP and borax were used as retarders to delay the setting time of MPC (Tan et al., 2018a) and were transported from the Fine Chemical Plant of Wujiang, Jiangsu. Corn stalk was obtained after harvesting of corn crop from the Jiangsu province. Chemical composition of MgO, ADP, FA, STP and borax are given in Table 1. The physical and thermal properties of corn stalk are shown in Table 2. Chemical composition of corn stalk is given in Table 3. The contents of DM, C, Ash and EE were tested according to (Latimer and International., 2019). The contents of NDF, ADF and ADL were tested according to Van Soest et al. (1991). The contents of NDICP and ADICP were tested according to

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Table 1 Chemical composition of magnesia, fly ash, ADP and borax. Raw material

Mass fraction of the sample (%) MgO

Magnesia 91.7 Fly Ash 1.8 ADP (NH4H2PO4) Borax (Na2B4O7.10H2O) STP (Na5P3O10)

Al2O3

SiO2

P2O3

CaO

1.3 25.8

4.0 54.9

0.11 e

1.4 1.3 e 8.70 6.90 0.3 Industrial grade, >98% ADP Industrial grade, >97% borax Industrial grade, >97% STP

Table 2 Physical and thermal properties of corn stalk. Property name

Unit

Value

Thermal conductivity Volumetric heat capacity Thermal diffusivity Bulk Density dry density Mean diameter (image analysis) Avg diameter (sieve analysis) Natural moisture content water absorption capacity (1 h) water absorption capacity (24 h)

(Wm-1K1) (kJm-3K1) (mm2s-1) (Kgm-3) (Kgm-3) (mm) (mm) (%) (%) (%)

0.041 386 0.234 135 121.6 12.5 12 11 156.3 312

Table 3 Chemical composition of corn stalk (dry mass basis). Component

%

Dry mass (DM) Crude protein Ether extract (EE) Ash Neutral detergent fiber (NDF) Acid detergent fiber (ADF) Acid detergent lignin (ADL) Neutral detergent insoluble protein (NDICP) Acid detergent insoluble protein (ADICP)

93.41 6.38 2.06 7.18 68.93 38.81 5.12 5.29 4.8

Fe2O3

Na2O

K2O

SO3

FeO

TiO2

LOI

e 0.1

e 0.6

e e

e e

e 0.2

bio-composites with the corn stalk to binder percentages of 100, 50 and 33.3% respectively. Similarly, B1, B2 and B3 show biocomposites compacted by the compression machine with the corn stalk to binder percentages of 100, 50 and 33.3% respectively. The schematic diagram of compaction assembly and process of compaction is explained through Fig. 1. Same mixing process was adopted for the all bio-composite mixtures. First, MgO, ADP and FA were introduced in the mixing pan along with the borax and STP. A vertical axis planetary mixer was used for the mixing purpose which can rotate as high as 950 RPM. First, dry mixing was carried out at the low speed. After that, water was added into the dry materials to obtain the flowable slurry and mixture was rotated at the high speed. While carrying out the mixing process at high speed, corn stalk was added at the same time. Amount of water was adjusted according to percentage of corn stalk as given in Table 4. For the group A, all specimens were manually compacted by a wooden rod. In the manual compaction method, all specimens were compacted lightly by the wooden stake in the two layers (15 blows each time) so that level of compaction force is same for all the specimens. For the group B, fresh bio-composite were compacted through a compaction molds by adjusting the force on platens using the compression machine (Model no: DY-2008C, fully automatic concrete bending tester) as shown in Fig. 1. The compaction force of 15 kN was maintained on the samples for 20e30 min till the hardening of bio-composites. All specimens were demolded after 2 h of casting.

Licitra et al. (1996). 2.3. Testing program 2.2. Preparation of bio-composites Bio-composites were categorized into the two groups (A is manually compacted (Low compaction), and B is compacted by the compression machine up to a constant compaction force of 15 kN (High compaction)). The mix proportion for both groups is given in Table 4. MgO, ADP and FA were used as binder. The ratio of MgO and ADP was 3:1 in the MPC, whereas fly ash was added as 50% of the MPC. The corn stalk to binder percentage was kept 100, 50 and 33.3% for each type of group (or binder to corn stalk ratio was 1, 2 and 3). Notations A1, A2 and A3 show manually compacted

2.3.1. Characterization of corn stalk Several properties of corn stalk were determined through the different experimental techniques. The bulk density, dry density, natural moisture content, water absorption capacity and size of corn stalk are provided in Table 2. Water absorption curve of corn stalk was determined by immersing the dry sample of corn stalk into the water and measuring the gain in weight due to immersion at predetermined interval up to 24 h. Particle size distribution of corn stalk was conducted through the sieve analysis and image processing analysis as described in the following literature (Amziane and Collet, 2017). Characterization of corn stalk was

Table 4 Formulation of corn stalk bio-composites (% by weight). Compaction level

Low Low Low High High High

Mix ID

A1 A2 A3 B1 B2 B3

MPC

FA

Corn Stalk

Water Content

Retarders

Dry density

MgO:ADP

FA/MPC

Corn Stalk/Binder

WB/Binder

WCS/CS

Borax/MgO

STP/MgO

(%)

(%)

(%)

(%)

(%)

(%)

3:1 3:1 3:1 3:1 3:1 3:1

50 50 50 50 50 50

100 50 33.3 100 50 33.3

15 15 15 15 15 15

40 33 26 40 33 26

5 5 5 5 5 5

5 5 5 5 5 5

kg/m3

552 903 1196 927 1192 1500

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131

Fig. 1. High compaction setup by compression machine.

carried with the help of SEM, XRD and FTIR analysis techniques. Corn stalk was first dried in an oven to the constant weight at temperature of 50 C. For the XRD and FTIR analysis, corn stalk samples were grinded to fine powder. D8 ADVANCE multifunctional X-ray diffractometer and Nicolet 6700 infrared spectrometer were used for the XRD and FTIR analysis respectively. The powder sample for the XRD analysis was scanned from 2q ¼ 5e100 at the scanning step of 0.02 . The corn stalk sample for the FTIR analysis was prepared according to potassium bromide pellet method (Wang et al., 2007) and scanning was conducted from 400 to 4000 cm-1 region at a interval of 2 cm-1. For the SEM analysis, gold coating of corn stalk samples was first carried out through the digital ion coater (SPT-20 digital ion coater) and samples were examined for the microstructure through the SEM COXEM EM-30 Plus. SEM analysis was carried out at the voltage of 15 kV. 2.3.2. Characterization of bio-composites 2.3.2.1. Compressive strength. The formulated bio-composites were tested for the mechanical, hygric, thermal and microscopic properties. Compressive strength test on samples size of 70  70  70 mm3 was conducted through the compression testing machine at the age of 28 days and displacement rate of 0.03 mm/s according to the BS EN 12390-3 (2009). The step-wise compression of bio-composite (A1) is shown in Fig. 2, showing that biocomposites can sustain the high deformations with the continuous increasing loading. Compressive strength of each mixture was calculated by taking the average of three specimens. The effect of compaction on the pore structure was studied by slicing the samples into two halves and observing their structure by ZEISS Stemi 508 - Greenough Stereo Microscope.

Fig. 2. Deformation of corn bio-composite under the compressive strength test (displacement rate 0.03 s/mm).

2.3.2.2. Hygroscopic properties. Hygric properties of bio-composites were investigated through the capillary saturation method as per (BS EN, 1925:1999, 1999) and free saturation method as per (BS EN 12087:2013, 2013). The samples were first dried in an oven at fixed temperature of 70 C to constant weight. For the capillary water uptake test, specimens were supported on the rectangular steel rods (wrapped in the water-resistant aluminum foil) in such a manner that 5±1 mm bottom of specimens was immersed into the water. The experiment was continued up to 7 days and water uptake was measured at the predetermined intervals. The results of test were reported by calculating the capillarity water absorption coefficient, Cw (kg.m-2.h-1/2). For the free saturation test, samples were completely immersed into the water in such a way that difference between the level of water and top of samples was at least 50 mm. The samples were removed from the water tank at specified intervals and inclined at 45 for 2 min to free the excess water. Water absorption of samples was calculated as the percentage of water absorbed and expressed by coefficient a. The relative humidity (RH) and temperature of laboratory room was maintained at 50 ± 3% and 23 ± 2 C respectively for the capillary saturation and free saturation tests. Drying kinetics of specimens was studied by drying the water immersed samples in an oven at 70 C (Haba et al., 2017). The drying weight of samples was measured at the regular interval until the change in weight between the two successive measurements is same. The loss in mass of bio-composites due to immersion in the water was calculated by calculating the weight difference before immersion in water and after drying in oven. Three specimens were used against each mixture to perform the free saturation test and capillary water uptake test. 2.3.2.3. Thermal properties. Thermal conductivity, thermal diffusivity and volumetric specific heat capacity of bio-composites were measured to evaluate the thermal properties. Thermal properties of bio-composites were determined with the KD2 Pro thermal properties analyzer based on the transient line heat source method. The dual needle sensor SH-1 with the needles spacing of 6 mm, length 3 cm and diameter 1.3 mm was used. The two parallel holes for the sensor were drilled through the pilot pins. The SH-1 dual needle sensor can measure the thermal properties of concrete within the wide range. Thermal properties of bio-composites were measured perpendicular to the main direction of corn stalk particles. Different temperature levels (20, 25, 30, 35, 40 and 45 C) were chosen to study the influence of temperature on the thermal properties of bio-composites. All samples were of cubic size with the dimensions 70  70  70 mm3. Two sets of reading were used to take the average of each thermal property of all bio-composites. 2.3.2.4. Moisture buffer value (MBV). The practically determined moisture buffer value (MBV) of materials evaluate their moisture

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3. Results and discussions

capacity of 326% at 24 h. SEM, XRD and FTIR analysis of corn stalk particles are shown in Fig. 4. SEM morphology of corn stalk (Fig. 4a and b) exhibited that it consists of hexagonal and honeycombing cellular shaped structure. The internal structure of corn stalk is highly porous and contains lot of vessels with average diameter in the range of 80e95 mm. The pore size of corn stalk was significantly higher than the hemp shiv's pore size of 20e25 mm (Pantawee et al., 2017). The crystallinity of corn stalk particles was determined from the XRD pattern (Fig. 4c). The Segal empirical formula (Segal et al., 1959) was used to determine the cellulosic crystallinity index (CI ¼ (I002-Iam)/I002) of corn stalk. Whereas, I002 is the maximum diffraction intensity of lattice peak at 2q angle between 22 and 23 (which is representative of both amorphous and crystalline materials) and Iam is the minimum diffraction intensity at 2q angle between 18 and 19 and representative of amorphous material (Roncero et al., 2005). Maximum and minimum diffraction intensities are at 2q ¼ 22.06 (which shows cellulose crystallographic plan) and at 2q ¼ 18.06 respectively. Hence, the calculated CI of corn stalk was 40.27%. The infrared spectrum of powdered corn stalk sample is plotted in Fig. 4d. The functional groups and their infrared absorptions for the corn stalk powder are shown in Table 5 (Zhao et al., 2013). Five components (cellulose, hemicellulose, pectin, wax and lignin at different peaks of 2930, 1740, 1630, 1520, 1380, 1250, 1160, 1050 and 667 cm-1) were found in the corn stalk sample when comparing the results of FTIR plot with infrared absorption of corn stalk provided in Table 5. Crystallinity index (CI) can also be calculated from the FTIR plot by taking the ratio of peaks at 1368 cm-1 and 662 cm-1, 1423 cm-1 and 896 cm-1, and 1368 cm-1 and 2887 cm-1 (Dai and Fan, 2010). The CI value for all these peaks was same and equal to 57.5%. Stevulova et al. also determined the CI of hemp hurd by the FTIR and XRD analysis and reported that CI values determined by FTIR were higher than the XRD analysis (Stevulova et al., 2014). Moreover, FTIR analysis was preferred to be used for the evaluation of cellulose crystallinity in lignocellulosic material. The cellulosic CI of corn stalk employed in this study was slightly higher than the hemp hurd and hemp fiber (Dai and Fan, 2010; Stevulova et al., 2014). The concept of crystalline cellulose is very important as only its surface part can be accessible and it can considerably effect the durability of cellulose, its reaction with the aggressive agents, its sorption properties and sensitivity to the microorganisms (Amziane and Collet, 2017).

3.1. Characterization of corn stalk

3.2. Effect of compaction on density and compressive strength

Corn stalk used in the presented study is shown in Fig. 3a. Particle size distribution of corn stalk was carried out using the sieve analysis and image processing analysis. The result of sieve analysis (Fig. 3b) showed that size of corn stalk particles was mostly in the range of 5 mme20 mm. The mean diameter (d50) of the corn stalk was 12 mm. The particle size distribution of corn stalk by image analysis is shown in Fig. 3c and d. The mean length (major axis) and mean width (minor axis) of corn stalk particles was 17 mm and 8 mm respectively. The mean diameter calculated from the sieve analysis (12 mm) and image analysis (12.5 mm) were close to each other. The thermal and physical properties of corn stalk are given in Table 2. Thermal conductivity of corn stalk was 0.041 Wm-1K1, which indicates its importance to be utilized as thermal insulation material. Water absorption behavior of corn stalk was determined by immersing in water for 24 h and result is reported in Fig. 3e. It can be seen that absorption rate of corn stalk is very fast in its initial stage of immersion due to its associated with its low density and high porous structure. Water absorption rate of corn stalk after 1 min and 60 min immersion in water was 156% and 275% respectively which was 48% and 84% of total water absorption

3.2.1. Density The images of bio-composites for group A and group B observed by the stereo microscope are shown in Fig. 5. It can be visually observed that proportion of corn stalk in the bio-composites (A1, A2 and A3) was reduced gradually by reducing the corn talk to binder percentage from 100% (A1) to 33.3% (A3). Moreover, biocomposite with the higher percentage of corn stalk (A1) had higher open porosity as compared to the bio-composites containing lower percentages of corn stalk (A2 and A3), which was associated with the lower amount of binder in the A1 than the A2 and A3. The bio-composite B1 has denser structure as compared to A1 due to the high compaction force level applied on the B1. Similarly, biocomposites B2 and B3 also exhibited denser structure as compared to A2 and A3. The results of compaction force on the density of bio-composites is shown in Fig. 6a. For the group A, the density of bio-composites A1, A2 and A3 was 552, 902 and 1196 kg/m3 respectively. For the group B, the density of bio-composites B1, B2 and B3 was 927, 1192 and 1500 kg/m3 respectively. Hence, it can be deduced that high compaction force can significantly densify the structure of bio-

buffering capacity under the dynamic conditions and is determined according to the method addressed by the NORDTEST project (Rode et al., 2005). The MBV relates the moisture uptake or release per surface area under the cyclic variation of relative humidity on the daily basis. The MBV value of a material can be determined from formula 1.

MBV ¼



Dm

A RHhigh  RHlow



(1)

where, MBV is moisture buffer value (g/m2.%RH), Dm is the uptake or release of moisture (kg), A is the surface area and RHhigh and RHlow are high and low relative humidity levels (%) For the MBV test, specimens of size 70  70  35 mm3 were prepared and sealed on the five sides. After that, specimens were stabilized in desiccator at 50% RH and 23 C temperature for 48 h. After stabilization, all samples were exposed to the RH variations of 8 h at 75% RH level and 16 h at 33% RH level at the constant temperature of 23 C. The test continues until the mass change is same during the last three cycles (
5%). This scheme replicates the daily cyclic variation of RH in offices and the bedroom where the major load comes roughly in 8 h. RH conditions were generated by different saturated salt solution in the desiccators. Temperature and relative humidity inside the desiccators were monitored periodically by the relative humidity sensor (HTC-2). The saturated solution of NaCl and MgCl2 were used to maintain the RH of 33% and 75% respectively. MBV values was calculated by taking the average of two specimens. 2.3.2.5. Mold development protocols. The mold development of the bio-composites was observed by exposing them into the different aging protocols for different temperature and relative humidity as suggested in past literature (Delannoy et al., 2018; Marceau et al., 2017). In this context, two type of aging protocols are studied in this work to observe the effect of RH and temperature on the mold development. The first was used as reference where samples were exposed to 23 ± 2 C and 50% RH. In the second aging protocols, samples were stored in desiccators at 30 ± 2 C and 98% RH to observe the development of molds.

M.R. Ahmad et al. / Journal of Cleaner Production 229 (2019) 128e143

(a)

(b)

100

Percentage passing (%)

133

80

60

40

20

0 0

5

10

15

20

25

Sieve size (mm)

15

60

10

40

5

20

0

0 1-3

4-5

6-7

8-9 10-11 12-13 14-5 16-17 18-19 20-21

Frequency (%)

80

Cummulative frequency (%)

Frequency (%)

20

--

25

100

(d)

100

20

80

15

60

10

40

5

20

0

0

--

1-8

9-11

width of corn stalk (mm)

12-14 15-17 18-20 21-23 24-26 27-29 30-32 33-35

Cummulative frequency (%)

(c)

25

--

Length of corn stalk (mm)

(e)

350

Water absorption (%)

300 250 200 150 100 50 0 0

5

10

15

20

25

30

35

40

Time (min1/2) Fig. 3. Particle size distribution (aed) and water absorption (e) of corn stalk.

composite and improve their mechanical properties. 3.2.2. Compressive strength The compressive strength of bio-composites compacted by wooden rod and compression machine is shown in Fig. 6b. The compressive strength of bio-composites A1, A2 and A3 was 2.96, 4.69 and 8.17 MPa respectively, which shows that bio-concrete (A3) containing higher binder content demonstrated higher strength values. Obviously, strength of plant aggregate (corn stalk has higher

porous structure and lower density as compared to binder) is considerably lower as compared to binder, and hence compressive strength of bio-composite increases with the decrease in corn stalk content. Decrease in the corn stalk to binder ratio from 100 to 33.3% increase the compressive strength of bio-composite by 2.76 times. The effect of high compaction on strength can be observed by comparing the strength of group A and group B for the same corn stalk to binder ratio. The compressive strength of bio-composites B1, B2 and B3 was 4.71, 6.96 and 12 MPa, which was higher than

134

M.R. Ahmad et al. / Journal of Cleaner Production 229 (2019) 128e143

1800

(c)

80

(d)

1600

Transmittance (%)

1400

Counts

1200 1000 800 600 400

70

60

50

40

200 0 10

20

30

40

50

60

2

30 500

1000

1500

2000

2500

3000

3500

-1

Wavenumber (cm )

Fig. 4. SEM, XRD and FTIR analysis of corn stalk (a) longitudinal x-section (b) transverse x-section (c) XRD diffraction diagram (d) FTIR analysis diagram.

Table 5 Main infrared absorption and their functional groups. Wavenumber (cm-1)

Functional groups

Infrared absorption

3399 2924 1736 1623 1515 1425 1380 1246 1202 1155 1048, 1019, 995 896 662

Cellulose/Hemicellulose Cellulose/Hemicellulose Pectin/Waxes Water Lignin Cellulose Cellulose/Hemicellulose Lignin Cellulose/Hemicellulose Cellulose/Hemicellulose Cellulose/Hemicellulose Cellulose Cellulose

OH and NeH stretching CeH and CeH2 stretching C¼O stretching OH bending of absorbed water C¼C aromatic symmetrical stretching HCH and OCH in-plane bending vibration In-the-plane CH bending C¼O and G ring stretching CeOeC symmetric stretching CeOeC asymmetrical stretching CeC, CeOH, CeH ring and side group vibrations COC, CCO and CCH deformation and stretching CeOH out-of-plane bending

A1, A2 and A3 by 59%, 48% and 47% respectively. Previously, the strength of hemp based bio-composites varied from the 0.2 MPae2 MPa for the density from 460 to 830 kg/m3 and hemp to binder ratio of 13e50% (Arnaud and Gourlay, 2012; Kioy, 2013; Magniont, 2010; Nguyen et al., 2010; Walker et al., 2014; Walker and Pavía, 2014). By comparing the strength of these studies with the present study, compressive strength of all corn stalk MPC composites was significantly improved as compared to hemp lime

and hemp concrete even at higher dosage of corn stalk particles. Hence, there was a significant increase in the compressive strength with the increase in compaction force for the same binder to corn stalk ratio, which was associated with the denser structure of highly compacted bio-composites and bio-compatibility of MPC with the plant aggregates.

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manually compacted bio-composites (A1, A2 and A3). High residual strength of bio-composites with high corn stalk ratio (A1, A2, B1 and B2) was associated with the gradual compression of pores and specimen. 3.4. Capillary water absorption

Fig. 5. Image of formulated bio-composites and their slices by microscope.

3.3. Stress-strain relationship of bio-composites The stress-strain relationship of corn stalk bio-composites is plotted in Fig. 7a and Fig. 7b. Post peak behavior of curve was significantly improved for the higher corn stalk ratio (A1 and A2) as compared to A3. The strain of all bio-composites was recorded up to 15%. Fig. 7a shows that residual strength of A3 was only 39% at the 15% strain, which was increase to 92% for the A2. Whereas, composite with the highest corn stalk to binder content (A1, corn stalk to binder percentage of 100%) keep gaining the strength even at higher strain percentage. The continuous compression of A1 sample is shown in Fig. 2. The stress-strain curves for the highly compacted specimens (B1, B2 and B3) also followed the same trend as

Hygroscopic nature of bio-composite building materials plays an important role to control the relative humidity inside the buildings. The capillary water uptake test was continued up to the 28 days and results are shown in Fig. 8a and Fig. 8b. The water uptake for the A1, A2 and A3 bio-composites at the end of experiment was 31.76, 24.12 and 19.88 kg/m2. Hence, water uptake was decreased with the increase in binder content or decrease in corn stalk content, which is understandable and can be related with the increasing density of bio-composites. The same trend of decrease in water uptake of bio-composites B1, B2 and B3 was observed, where water uptake was 25.32, 20.45 and 13.32 kg/m2 respectively. For same binder to corn stalk ratio, the effect of high compaction on the water uptake was evident. By increasing the compaction of biocomposite, the water uptake was reduced by 25.4, 18 and 49% for the corn stalk to binder percentages of 100, 50 and 33.3% respectively. Moreover, rate of water uptake was in the range of 45e50% for the all bio-composites irrespective of type of compaction. Water absorption coefficient (Cw) of bio-composites derived from the slope of capillary water uptake curves are given in Table 6. The water absorption coefficients of bio-composites A1, A2 and A3 were 3.90, 3.15 and 2.65 kgm-2h-1/2 respectively. The increase in compaction of specimens lowered the water uptake coefficients of bio-composites. The values of Cw observed for corn stalk composites in this study was also in consistent with the literature of hemp ~o concrete; in the range of 0.6e4.5 kgm-2h-1/2 (del Valle-Zermen et al., 2016) and 2.65 to 3.37 kgm-2h-1/2 (Walker and Pavía, 2014) for the different hemp to shiv ratio. Primarily, bio-composites composed of macro pores (due to imperfect arrangement of plant aggregates), mesopores (pores inside the plant aggregate and binder) and micropores (inside the binder). These pores are interconnected and support the mass and heat transfer and moisture storage on the surface and inside the specimen (Collet et al., 2008). 3.5. Free saturation and drying kinetics The results of free saturation for the corn stalk bio-composites 14

(a)

1400

(b)

LC HC

Compressive strength, fc' (MPa)

1600

Density (kg/m3)

1200 1000 800 600 400 200 0

A1

B1

A2 B2 Mixture ID

A3

B3

12

LC HC

10 8 6 4 2 0

A1

B1

A2 B2 Mixture ID

Fig. 6. Effect of compaction on density (a) and strength of bio-composites (b).

A3

B3

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M.R. Ahmad et al. / Journal of Cleaner Production 229 (2019) 128e143

A3 A2 A1

(a) 8

(b)

12

B3 B2 B1

6

Stress (MPa)

Stress (MPa)

10

4

8

6

4

2 2

0

0 0

5

10

0

15

5

10

15

Strain (%)

Strain (%)

Fig. 7. Stress-strain relationship for bio-composites: (a) Low compaction; (b) High compaction.

35

A1 A2 A3

Capillarity water uptake (kg/m

2

25

Capillarity water uptake (kg/m2

(a) 30 25 20 15 10 5

B1 B2 B3

(b)

20

15

10

5

0

0 -5 0

5

10

15

20

25

0

30

5

10

15

20

25

30

Time (h1/2)

Time (h1/2)

Fig. 8. Capillary water absorption of bio-composites: (a) Low compaction; (b) High compaction.

Table 6 Water absorption properties of corn stalk bio-composites. Bio-composite

Water absorption coefficient (kgm-2h-1/2)

R2

Water absorption (%)

Loss in mass (%)

A1 A2 A3 B1 B2 B3

3.90 3.15 2.65 3.18 2.45 1.60

0.9743 0.9847 0.9871 0.9721 0.9577 0.9765

108.2 54.8 31.4 62.8 37.2 18.4

10.21 4.22 1.2 7.23 3.22 0.24

are plotted in Fig. 9a and Fig. 9b. Highly compacted bio-composites (B group) showed considerably lower amount of water absorption as compared to manually compacted bio-composites (A group), which can be attributed to denser structure of group B biocomposites. The water absorption values of A1, A2 and A3 mixtures were 104.2, 54.5 and 31.4% respectively, which were 72, 46 and 71% higher than the B1, B2 and B3 mixtures respectively. The rate of water absorption was higher for the A group and was in the range 60e80% during the first 6 h. Whereas, water absorption rate for the group B was in the range of 47e59% during the first 6 h. Previous researchers also investigated the water absorption properties of bio-composites based on the plant aggregate and found

the similar trend of water absorption curves (Benmansour et al., 2014; Chikhi et al., 2013; Haba et al., 2017). It is noticeable that immersion of bio-composites in the water for longer duration could dissolve the binder and hence, weaken the cohesion between the plant aggregate and binder. The color of yellowish water after completion of experiments showed that some of plant aggregate were detached from the binder or dissolved into the water. The loss in mass of specimens due to immersion in water is given in Table 6. The drying kinetics for the both type of group is shown in Fig. 10a and Fig. 10b. According to a research (Haba et al., 2017), the drying phenomenon of bio-composites consists of two phases. (1) moisture diffusion from inside of samples to the surface, and (2)

M.R. Ahmad et al. / Journal of Cleaner Production 229 (2019) 128e143 120

80

(a)

A1 A2 A3

(b)

water absorption (%)

100

water absorption (%)

137

80

60

40

B1 B2 B3

60

40

20

20

0

0

0

10

20

30

40

50

0

10

Time (hours)

20

30

40

50

Time (hours)

Fig. 9. Water absorption behavior of bio-aggregate composites (a) Light compaction (b) High compaction.

120

70

100

B1 B2 B3

60 50

80

I

II

Water content (%)

Water content (%)

(a)

A1 A2 A3

(a)

III

60

40

20

40

I

II

III

30 20 10 0

0 0

50

100

150

200

250

300

Time (hours)

350

0

50

100

150

200

250

300

350

Time (hours)

Fig. 10. Drying kinetics of corn stalk composites: (a) Low compaction; (b) High compaction.

moisture evaporation from the sample's surface. The samples were exposed to the 70 C and progressive decrease in the mass of water was measured until all the samples attained constant mass. In this study, behavior of drying kinetics curves of bio-composites was divided into the three different phases; (I) linear and sharp loss in mass of water associated with the water evaporation from the surface and inside of specimen due to continuous supply of water toward the specimen surface. This phase was dominant for the first 10 h of drying, (II) during the second phase, the rate of loss in water was decreased and drying kinetics curved exhibited the non-linear behavior. This phase was continued up to the 150 h of drying and non-linear behavior of curve was associated with the water supply reduction from the inside of sample, (III) during the last phase, linear and slow vapor diffusion of residual 1e3% of water occurred, and it was continued up to 300 h till the constant mass of specimens. The behavior of this part of curve was due to interrupted supply of liquid water. Hence, it can be concluded that drying behavior of bio-composites depends on their initial moisture contents and boundary conditions. Drying process can become difficult and long due to transformation of moisture and transition of phases (from liquid water transport to vapor transport) (Haba et al., 2017). 3.6. Thermal properties of bio-composites 3.6.1. Thermal conductivity Energy usage level of the building is primarily affected by the thermal properties of insulation materials. Thermal conductivity is one of the most important thermal property and inherent

characteristics of any materials, and largely influenced by the internal structure of material. It is the potential of any material to conduct the heat under the temperature variations and in steady state conditions. Thermal diffusivity of any material can be calculated from its volumetric heat capacity and thermal conductivity. Results of thermal conductivity for bio-composites at different temperature are plotted in Fig. 11a and Fig. 11b. The influence of binder content, compaction and temperature on the thermal conductivity is evaluated. For the same temperature, thermal conductivity of bio-composites was increased with the increase in binder content (or decrease in corn stalk content), as l value of A1, A2 and A3 was0.161, 0.281 and 0.375 Wm-1K1 at the temperature value of 45 C. This was due to higher thermal conductivity of binder (l 1.2e1.5 Wm-1K1) than the corn stalk (l 0.041 Wm-1K1). Group B also followed the same trend of decrease in l with the decrease in binder content. There were slight variations in thermal conductivity with the increase in temperature and l value was increased with the increase in temperature; for example, l value of bio-composite A1 was 0.137, 0.144, 0.151, 0.156, 0.157 and 0.161 Wm-1K1 at the temperature 20, 25, 30, 35, 40 and 45 C respectively, keeping all the other factors same. These results are in agreement with the previous literature that increase in the temperature tends to increase the thermal conductivity of bio-composites (Tandiroglu, 2010; Vololonirina et al., 2014). and (Haba et al., 2017) also examined the effect of temperature on thermal properties of perlite aggregate concrete, wood-based construction materials and date palm concrete and showed that l was increased with the increase in

138

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(a)

A2

A3 W.m-1K-1)

A1

0.35 0.30

Thermal conductivity,

Thermal conductivity,

W.m-1K-1)

0.40

0.25 0.20 0.15 0.10 0.05

(b)

B1

B2

B3

0.4

0.3

0.2

0.1

0.0

0.00 20

25

30

35

40

20

45

25

30

35

40

45

Temperature ( C)

Temperature ( C)

Fig. 11. Variation in thermal conductivity of bio-composites with the change in temperature (a) Low compaction (b) High compaction.

temperature. Both groups exhibited the same behavior with the increase in temperature and l value of all bio-composites was slightly increased. Moreover, results of the thermal conductivity in present study are in accordance with the previous studies and values of l were of same order (0.1e0.14 Wm-1K1) for the densities from 565 to 700 kg/m3 (Nguyen et al., 2010; Walker et al., 2014; Walker and Pavía, 2014). The effect of high compaction was more obvious on thermal conductivity of corn stalk bio-composites. For same temperature (20 C) and corn stalk to binder percentage of 100%, the thermal conductivity of A1 and B1 was 0.137 Wm-1K1 and 0.227 Wm-1K1 respectively, which shows that l value was increased by 65% for the high compaction level. The increase in the thermal conductivity can be related to the internal void structure or porosity of biocomposites. High compaction process decreases the open and internal porosity of material (increases the density of biocomposites) and resulted bio-composites showed higher value of thermal conductivities. Therefore, as a result of high compaction, l values of group B bio-composites were higher than the group A for the same corn stalk to binder percentage and constant temperature.

3.6.2. Volumetric specific heat capacity Volumetric specific heat capacity (or thermal inertia) is the ability of materials to store the internal energy while undergoing through the temperature variations but without any phase transition. It is one of the major factors that controls the energy consumption of building and thermal comfort. It plays an important role for materials to be used as thermal mass. The results of volumetric specific heat (Cp) for the group A and group B are shown in

(a)

A1

A2

3.6.3. Thermal diffusivity The results of thermal diffusivity are plotted in Fig. 13a and Fig. 13b for the group A and group B respectively. Thermal diffusivity of bio-composites can be determined from the correlation of density, volumetric specific heat and thermal conductivity. Hence, the change in thermal diffusivity of bio-composite is plotted accordingly. Thermal inertia of building materials imparts main role to regulate the internal relative humidity, improve the thermal comfort and to save the heating expenses of buildings. Generally, construction materials with the good thermal inertia properties must have low thermal properties (thermal conductivity and thermal

A3

Volumetric specific heat, Cp (MJ/m3.K)

Volumetric specific heat, Cp (MJ/m3.K)

2.5

Fig. 12a and Fig. 12b. For the both groups, value of Cp was increased with the increase in binder content (or decrease in corn stalk content) keeping the temperature same (e.g. Cp value of A1, A2 and A3 was 1.03, 1.22 and 2.26 MJm-3K1 at the temperature 20 C). There was only slight effect of temperature on the specific heat and Cp value was slightly decrease with the increase in temperature. However, high compaction of specimens considerably changed the values of Cp. Keeping the same temperature (20 C) and corn stalk to binder percentage (100%), the value of Cp for A1 and B1 was 0.943 and 1.63 MJm-3K1. Hence, it can be deduced that increase in compaction of bio-composite increased their volumetric specific heat. This increase in value of Cp was associated with the increase in density of bio-composites (higher is binder content, higher will be density; higher the compaction level, higher will be the density). Previous studies have also shown that concrete with the higher density normally possesses the higher value of volumetric specific heat (Zhou and Brooks, 2019).

2.0

1.5

1.0

0.5

0.0 20

25

30

35

Temperature ( C)

40

45

3.5

(b)

B1

B2

B3

3.0 2.5 2.0 1.5 1.0 0.5 0.0 20

25

30

35

40

45

Temperature ( C)

Fig. 12. Variation in volumetric specific heat of bio-composites with the change in temperature (a) Low compaction (b) High compaction.

M.R. Ahmad et al. / Journal of Cleaner Production 229 (2019) 128e143 0.18

A1

A2

(b)

A3

mm2/s

0.25

Thermal diffusivity,

B1

B2

B3

0.16

0.20

0.15

0.10

0.05

mm2/s

(a)

0.14

Thermal diffusivity,

0.30

139

0.10

0.12

0.08 0.06 0.04 0.02 0.00

0.00 20

25

30

35

40

20

45

25

30

35

40

45

Temperature ( C)

Temperature ( C)

Fig. 13. Variation in thermal diffusivity of bio-composites with the change in temperature (a) Low compaction (b) High compaction.

manually compacted bio-composites. MBV value of the corn stalk composites is compared with the MBV of traditional building materials reported by Rode et al. (2005) in Fig. 15. It can be observed that moisture buffer performance of bio-composites can be divided into five classes (negligible MBV<0.2; limited 0.2 < MBV<0.5; moderate 0.5 < MBV<1; good 1 < MBV<2 and excellent MBV>2). From the classification, brick and concrete fall into the “limited” MBV category, whereas plaster can be classified into “moderate” MBV materials. Cellular concrete, A2, A3, B2 and B3 share the same category of “good” MBV materials. A1 and B1 bio-composites showed the highest moisture buffer capacity as compared to these traditional building materials and performance of both biocomposites was classified into the “excellent” MBV materials. Comparing the values of corn stalk concrete with the other vegetal concrete or plaster, the MBV value obtained for the corn stalk concrete (A1 and B1) were higher than the MBV of hemp concrete (1.99e2.15 g/m2%RH) (Collet et al., 2013; Collet and Pretot, 2012; Le, 2010) and hemp plaster (1.23e1.64 g/m2%RH) (Mazhoud et al., 2016). This may be due to the higher percentage of corn stalk (corn stalk to binder weight percentage ¼ 100) in present study than the hemp concrete and hemp plaster. The higher content of plant aggregate can induce the lower density and higher permeability in the corn stalk composites and hence, excellent hygric

diffusivity) and high thermal inertia. Materials (e.g. steel and aluminum have high thermal conductivity, high thermal diffusivity and high thermal inertia) with the high thermal properties can store large amount of heat during the day but also release it quickly in the night. An efficient thermal mass material accumulates heat energy during the day and liberate is very slowly during the night (Demirboǧ;a and Gül, 2003; Howlader et al., 2012). The biocomposites produced in this study are also in line with the requirement of efficient thermal mass materials (lower thermal properties and high thermal inertia). 3.7. Moisture buffer capacity The results of moisture buffer capacity for group A and group B are shown in Fig. 14a and Fig. 14b. In case of both groups, specimens achieved steady state after third or four cycle. The final MBV at steady state are given in Table 7. The MBV of A1, A2 and A3 are 3.15, 1.79 and 1.36 g/m2%RH respectively, which shows that MBV value was decreased by increasing the binder content (or decreasing the corn stalk content). The effect of compaction on the moisture buffer capacity is also evident. For the corn talk to binder percentage of 100, 50 and 33.%, MBV of highly compacted bio-composites was decreased by 20, 11 and 22% respectively in comparison with

4.5

A1

(a)

4.0

A2

3.0

A3

B2

B3

2.6

3.5

2.4

3.0

MBV (g/m2.H)

MBV (g/m2.H)

B1

(b)

2.8

2.5 2.0

2.2 2.0 1.8

1.5

1.6

1.0

1.4 1.2

0.5

1.0

0

1

2

3

4

5

Cycles

6

7

8

9

0

1

2

3

4

5

Cycles

Fig. 14. Moisture buffer capacity of bio-composites (a) Low compaction (b) High compaction.

6

7

8

9

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M.R. Ahmad et al. / Journal of Cleaner Production 229 (2019) 128e143 Table 7 MBV of corn stalk bio-composites. Bio-composite ID

MBV (g/m2%RH)

l le ce

d

Ex

oo G

eg N

nt

3.15 1.79 1.36 2.62 1.60 1.11

lig Li ible m ite d M od er at e

A1 A2 A3 B1 B2 B3

A1 A2 A3 B1 B2 B3 Cellular concrete Plaster Fig. 16. Development of molds/microorganisms on the bio-composites.

Brick Concrete 0

0.5

1

1.5

2

2.5

3

3.5

Fig. 15. Comparison of MBV of corn stalk composites with the traditional building materials.

performance. 3.8. Development of molds/microorganisms Formation of molds on the bio-composites observed by microscope is shown in Fig. 16. Several types of microorganisms were observed. The molds observed in Fig. 16a and b are hyphae and spores. Hyphae are composed of feathery and tiny strands, and can develop the shape of black spots or filament. Hyphae can grow both on the surface and inside the plants in the form of clusters. These hyphae on further growth can be visible by the naked eyes and known as mycellium. The seed like spores were also observed and attached to the hyphae. It can be seen from Fig. 16a and b that size of spores can vary and they can drift though the air. The spores can support the formation of hyphae if they have an ambinet temperature and high relative humidity for growth as also observed in Fig. 16a and b (Delannoy et al., 2018; Marceau et al., 2017). Eliminating the molds can be quite difficult but their growth can be controlled by reduction of moisture inside the homes. The mold shown in Fig. 16c and d appreas to be penicillium. These molds also grow in the higher relative humidity environment. Fig. 16e and f shows that some other type of microorganism with the white color and round to elongated shape were also found on the biocomposites. These molds or microorganism can attack the cell wall of plants and reduce its thickness. They can also effect the cellulose and hemicellulose and cause debonding of cells form each other (Hill and Papadopoulos, 2001; Schwarze et al., 2003). Moreover, molds were only observed on the bio-composites placed in the higher RH of 96%.

3.9. Interaction of MPC binder and corn stalk SEM analysis of bio-composites was carried out to examine the adhesion properties of corn stalk and the MPC binder. It can be observed from the micrographs in Fig. 17 that corn stalk particles exhibited excellent bonding properties when mixed with the MPC binder, which confirmed the superiority and bio-compatibility of MPC binder as compared to traditional binders. Moreover, compaction force did not affect the bonding of MPC binder and corn stalk, and strong interlacing of corn stalk and MPC was noticed in all bio-composites. There was no clear gap present between the two materials in any bio-composite. The manual compaction did not change the interior structure of corn stalk particles, which helps to

Fig. 17. Interfacial transition zone between the corn stalk and MPC.

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Table 8 Classification of bio-composites according to RILEM functional classification. Compaction level

Low

High

Mix ID

A1 A2 A3 B1 B2 B3

Compressive strength (MPa)

2.96 4.69 8.17 4.71 6.95 12.03

Thermal conductivity (Wm-1K1)

Classification

20  C

25  C

30  C

35  C

40  C

45  C

0.137 0.235 0.338 0.227 0.292 0.406

0.144 0.251 0.356 0.233 0.295 0.415

0.151 0.256 0.363 0.23 0.297 0.421

0.156 0.256 0.367 0.249 0.321 0.438

0.157 0.261 0.375 0.25 0.322 0.441

0.161 0.281 0.375 0.262 0.331 0.447

improve the acoustic and thermal properties of bio-composites. However, high compaction force compressed the corn stalk particles into better packing density and hence, enhanced the mechanical properties of bio-composites. 3.10. Classification and sustainable aspects of bio-composites Functional classifications of formulated corn stalk biocomposites were determined as per RILEM functional classifica
contrainte, 1983). Concrete can be classified tion of concrete (Pre into the Class-II (Structural and insulation grade concrete) if compressive strength is in the range of 3e15 MPa and thermal conductivity is less than 0.75 Wm-1K1. Whereas, concrete with the compressive strength higher than 0.5 MPa and thermal conductivity less 0.3 Wm-1K1 can be categorized into Class-III concrete (Insulation grade concrete). The compressive strength and thermal conductivity of all bio-composites are given in Table 8. A1 can be classified as Class-III bio-composites, whereas A3 and B3 can be classified as Class-II bio-composites. The remaining bio-composites (A2, B1 and B2) share the Class-II and Class-III grade concrete and can be employed both as structural or insulation materials. Sustainability characteristics of any constructional materials are accessed by its renewable source, energy consumption and carbon input. The bio-composites formulated in this study are composed of magnesium phosphate cement (MPC) containing 50% fly ash (industrial by-product and can be obtained from the coal power plants) and corn stalk (agriculture by-product). The total amount of energy to produce one ton of MPC is estimated nearly 2186 MJ. Moreover, one-ton production of MPC binder prepared in this study causes only 0.32-ton emission of CO2 into the environment. On the other hand, one-ton production of OPC needs approximately 4800 MJ of energy and emits nearly one-ton CO2 into the environment. Hence, each ton of MPC binder not only save the 2614 MJ energy but also emits 0.68 ton less CO2 as compared to traditional Portland cement (for unit ton production) (Wagh, 2016). The corn stalk (plant aggregate) is by-product of corn crop and from renewable source. It is available in abundant worldwide. Moreover, composites based on the MPC binder and corn stalk showed excellent mechanical, thermal and hygric performance and solution of delayed setting time observed in the hemp concrete composites. Also, they can be employed as efficient thermal mass materials (due to their high thermal inertia, low thermal conductivity and thermal diffusivity). Hence, it can be deduced from the experimental findings that MPC based corn stalk composites can be exploited as alternative constructional materials due to their sustainable nature and low energy consumption during the production and building life cycle. 4. Conclusions This study mainly evaluated the effect of high compaction on the mechanical, thermal, hygroscopic and aging properties of corn stalk concrete mixtures having the density in the range of

Class-III Class-II & Class-III Class-II Class-II & Class-III Class-II & Class-III Class-II

550e1500 kg/m3. Following conclusion are drawn from the experimental investigations on corn stalk bio-composites. 1. Increase in the compaction significantly improved the density and mechanical properties of corn stalk bio-composites. Compressive strength of highly compacted composites was 59%, 48% and 47% higher than the manually compacted composites for the corn stalk to binder percentage of 100, 50 and 33.3% respectively. 2. Water absorption by capillarity and free saturation was decreased for the highly compacted bio-composites. The drying process of specimens was divided into three different phases due to interruption in supply of liquid water supply, evaporation of water and vapor diffusion phenomena. 3. Thermal properties of composites were slightly influenced due to the increase in temperature. Thermal conductivity was increased, and volumetric specific heat capacity was gradually decreased with the increase in temperature. Whereas, thermal conductivity and volumetric specific heat capacity was increased by increasing the compaction force. Thermal conductivity varied from 0.137 to 0.45 Wm-1K1 with the variation of density from 550 to 1500 kg/m3. Finally, bio-composites were classified into Class-II (structural and insulation grade concrete) and Class-III (Insulation grade concrete) concrete according to the RILEM functional classification. 4. All the bio-composites were categorized as good to excellent hygric regulators and their MBV values were higher than the 1. 5. Several types of molds (hyphae, spores and penicillium) were observed on the surface of bio-composites. The ambient temperature and higher relative humidity levels were necessary for the growth of molds. 6. The formulated bio-concrete required less energy input with the lower carbon footprint and exhibited superior mechanical, hygric and thermal properties as compared to other biocomposites and traditional building materials. Hence, a new sustainable material was proposed based on the MPC as binder and corn stalk as plant aggregate. The additional work in future regarding the MPC based bio-composite includes flexural performance, void content analysis, durability and permeability properties. Acknowledgement This research work was financially supported by the National Natural Science Foundation of China, Grant No. 51778363. References Ahmad, M.R., Chen, B., Yousefi Oderji, S., Mohsan, M., 2018. Development of a new bio-composite for building insulation and structural purpose using corn stalk and magnesium phosphate cement. Energy Build. 173, 719e733. https://doi.org/ 10.1016/j.enbuild.2018.06.007. Ahmad, M.R., Chen, B., Yu, J., 2019. A comprehensive study of basalt fiber reinforced magnesium phosphate cement incorporating ultrafine fly ash. Compos. B Eng.

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