Elemental Micro-characteristic, Thermogravimetric, and FTIR study of Green Sand(Poly Lactic Acid) for replacement of Fine Aggregate in Concrete Mix

Poly lactic acid (PLA) has made inroads for commercial market segment with lot many unique characteristics such as tenacity, low flame rate, moisture regain percentage, loss of ignition percentage, heat of combustion, UV resistance, Elastic recovery and higher melting point allowed it to be the fastest moving material in today's commercial market. An attempt has been made to test the feasibility and biocompatibility aspect of PLA with cement mix. Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 23 September 2020 doi:10.20944/preprints202009.0528.v1 © 2020 by the author(s). Distributed under a Creative Commons CC BY license. The basic strength and physical test results were carried out and published in an article, to the continuation of the work, micro-structural study was conducted to evaluate the elemental characteristics. Thermo gravimetric analysis revealed that PLA either in granular form or filament will hold good for the inclusion into construction applications, provided degradation aspects are to be looked out for improvisation. From DSC it was found that PLA in filament form is the best inclusion material for construction application, however the tenacity of fibers has to checked, as currently available filaments in market does not have high tenacity value. From EDX reports, 30% inclusion of PLA as replacement for fine aggregate has constituent members as Calcium carbonate(CaCO3), Silica(SiO2) and Wollastonite (CaK) resulted in best composition among the rest. FESEM images revealed that, proper gradation in size, rough surface of PLA granular form or filament form will definitely enhance the mechanical/physical or even chemical behavior of PLA.


Introduction
As per plastic insight [1], by 2020 global poly lactic acid(PLA) market will reach a milestone of more than US$ 5billion. Currently the usage has cater to many diversified domains such as Medical, Food packaging, Textile, Agricultural, Consumer electronics, personal care etc.
As in India, still the usage is limited to 1% of global utilization in 2015 [2]. Solid waste generation in India has opened a Pandora of opportunity boxes to recycle the dry solid waste with best outputs. Today, material characterization has become part of research, for identifying the critical parameters such as micro/nano structural study with the help of SEM/FESEM/TEM, loss of volatile components viz. moisture, solvent and monomers, decomposition, noticing the residue composition using TGA, Enthalpy, melting point and specific heat capacity by DSC, to arrive at an infrared spectrum of absorption or emission of all three phases such as a solid, liquid or gaseous material using FTIR. Current generation TGA DSC has wide range of measurement techniques, can be performed dynamically using a linear temperature ramp or isothermally [3]. Temperature ramps are used to investigate the temperature dependents and process such as loss of moisture, composition and chemical reaction. Isothermal measurements are mainly used to determine the oxidation induction time of material or to study the release or absorption of moisture. The atmosphere often switched from inert to oxidative to burn the carbon black or determine the ash or filler content. Measurements under reduced pressure or vacuum are employed to separate overlapping effect of vaporization and decomposition. Simultaneously measured DSC signal records exothermic and endothermic events such as the Glass Transition Temperature(GTT), melting, crystallization, chemical reaction and phase transitions.

Thermo gravimetric analysis (TGA)
Thermo gravimetric analysis (TGA) measures loss of mass of a sample when it is subjected to heat, cool or constant temperature. It represents the mass loss curve, in which abscissa illustrates temperature in degree Celsius and ordinate with weight percentage. The weight loss of coupon subjected to TGA is established via Five major steps. In first step, loss of volatile components such as moisture, monomers and solvents. Second step focuses on decomposition of specimen, in third step atmosphere switched from Nitrogen to Oxygen.
Fourth step is to combustion of carbon in endothermic or exothermic condition. Finally, inert inorganic residue of ash fillers. Aluminium crucibles were used to hold the specimens and working temperature of upto 600°C. The characterization was applied to PLA in granular form, PLA in filament form and PLA in wafer form derived from degradation of matrix in concrete mortar.

PLA granules:
PLA granules were procured from Nature works, USA. The figure 1 depicts the shape and aesthetic appeal of granules. The granules are embedded in concrete mix at a proportion of 0%, 10%, 30% and 50% volume fraction of PLA as a replacement for fine aggregate. Decomposition of PLA carbon and its allied members at 59.05°C in step 2. The third step atmosphere switched from Nitrogen to Oxygen, to cater for combustion of carbon in next step. Finally resulting into inert inorganic residue of ash fillers or other impurities. Evaluation of a pyrolysis weight loss step yields a polymer content of 98.8%, as a result 1.2% weight loss was achieved for granular form. PLA in its filament(wired) form was extruded to use for 3D printing applications. When the same is subjected to TGA resulted with comparatively less amount of carbon black percentage(approximately 1%), in figure 3 shows that minimal amount of impurity.
Temperature ramps are used to measure temperature dependent process such as loss of moisture, composition and chemical reaction. PLA in wired form weighing about 5.013 mg was heated at a heating rate of 10°C/min from 20°C to 200°C. The glass transition point Tg and melting point Tm was measured.

Reacted layer on PLA granules:
As depicted in Figure 4, PLA in its diaphragm or wafer form does subjected to chemical reaction with concrete mortar for 28 days of curing. When the coupon tested at 7 days time limit, there was no sign of degradation in its outer peels of PLA granule but full cured sample yielded with white layer in the form of wafer. The wafer, as shown in figure 5 reveals that, entire process of moisture desorption and decomposition carried out in two steps. The percentage of mass loss observed was 25% yielding as expected 75% before it flattens with respect to abscissa measure with temperature at 200°C . Among the three cases, it is interesting to know further like what parameters caused the weight loss of PLA and any chemical reaction with cement mortar or water soaking (submerging in water) to convert an hydrophilic to hydrophobic are the areas of interest. To know and analyze on these issues the

Differential scanning calorimetry (DSC)
DSC is a thermo analytical method. It is used to study the behavior of material as a function of temperature or time. Melting point, crystallization behavior and chemical reaction are just some of the many properties or processes that can be measured by DSC [3]. It measures the energy when subjected to heat, cool or held at isothermal condition. The respective samples may undergo one or more phase changes during heating or cooling. These changes are called "thermal transitions" of a polymer. Examples of the thermal transitions are glass transitions, crystallization, and melting of a polymer.
In the polymer field it is important to know the temperature range in which the polymers stable and in range which polymer decomposition occurs. DSC provides an information on purity of the sample if the stoichiometry of reaction is known. Results of isothermal and temperature ramped case can be used to determine the reaction kinetics. Other applications include the investigations of desorption or adsorption processes, evaporation behavior or influence of reactive gas. DSC applicable for evolved gas analysis of hyphenated techniques such as Mass spectrometry, FTIR.
Generally polymers behave in a significantly different manner when the temperature drops below Tg (more brittle) or goes up to higher temperatures than Tg (more rubbery). Hence to select a material for a specific application it is essential to know the behavior of composites under applied heat flow. DSC measurements were conducted using a calibrated instrument available with a cooling attachment, in inert nitrogen atmospheric condition. Each samples were cut into tiny pieces (about 4 -10 mg) and sealed in hermetic pans and lids. To build a reasonable reliability of the results, for each material three pieces of sample from different region were evaluated by DSC. The data were collected by repeated heating-cooling-heating cycles. The heating and cooling rate was 10 °C/min.

PLA granules
In this analysis, weight of sample taken was about 7.725 mg and the samples were heated up to 190 °C and held in a molten state for 5 min, followed by cooling down to 20 °C.

Reacted layer on PLA granules
From figure 8 it can be inferred that concentration of PLA as gets linked with cement an hydrolysis process delivers an early degradation of coupons. Higher the percentage of inclusion of PLA combination yields with less defensive and is characterized by decreased degradation temperature. This decrease in temperature result due to low bond strength and early decay of material [5].
In this analysis, weight of sample taken was about 4.999 mg and the samples were heated up to 190 °C and held in a molten state for 5 min, followed by cooling down to 20°C [6].

PLA Granules
The FTIR spectroscopy deals with the study of functional clusters such as PLA granules at various weight percentage inclusions to diagnose the effect of PLA with concrete and any chemical changes happening in the surrounding region of bonding. From figure 9 it can be inferred that, a broad peak of 3437 cm -1 was observed indicating a hydroxyl group with pristine sample subjected to testing with lesser lignin and hemicellulose content than wired filament form. The effect when compared with Kenaf fiber with treated 3324cm -1 and untreated 3318cm -1 [7]. Further the characteristics were compared with PLA, PLA/Kenaf (Untreated), PLA/Kenaf (Treated) and PLA/Kenaf (Treated) /EJO samples [7]. Bio composite coupon interpreting a combination of PLA and fibre mixing peak. Considering the fact that concrete will undergo hydration and calcinations process the expected result will definitely affect for combination of PLA and concrete coupon. However as for all three samples besides PLA spectrum, they had approximately similar wavelength as compared to PLA's except the difference in the peak intensity in the C=O stretching and C−O stretching for PLA.

PLA in wired form
PLA wired(filament) form does depict similar peak regions as of granules with slight variation in wavelength. The region shown in figure 10 with hydroxyl group is varied with ±3 cm -1 . Table 2 indicate the variation of bonding with type and characteristic peak position study.

Reacted layer on PLA granules
This particular model, subjected to a combination of PLA and concrete hydration or calcinations process due to which the peak of hydroxyl group got shifted as compared to its pristine sample tested for PLA. As the figure 11 shows, symmetric bonding -C=O-shifted

Energy-dispersive X-ray Spectroscopy
EDS allows measure/study elemental composition of the specimen. The EDX system may also be able control the SEM scanning system in order to collect elemental distribution maps or elemental line profiles.
In the present study, the samples used for FESEM and EDS are of M20 grade 28 days cured concrete. These tests were conducted on conventional M20 grade concrete and on the concrete where fine aggregate was replaced by PLA granules partially. Replacement levels were 10%, 30% and 50% volume of fine aggregate

Conventional concrete / Concrete without PLA granules
Energy-dispersive spectra obtained from the quantitative elemental analysis on conventional concrete are shown in below figure. behaviour of PLA with concrete and reduction in the amount of river sand in concrete on its replacement. The small crystals may be calcium sulfoaluminate, as indicated by the small Sulphur peak in the EDX spectrum [8].
From the table 4, it can be concluded that as the percentage of inclusion of PLA raised from 0 to 10%, calcium carbonate and silica content have raised steeply along with Wollastonite (CaK). From 10 to 30% increase in PLA resulted in further raise of all these elements. Which depicts that concrete mix of cement and coarse aggregate behaves in brittle form and PLA as virgin material behaves brittle in nature, resulting in better mechanical strengths and compared to control concrete mix 30% inclusion results were far more convincing than earlier.

Micro-structural study with Scanning Electron Microscopy(SEM)
The entire micro structural study with FESEM plus EDS was carried out at CMTI, Tumkur road, Bangalore. The FESEM images were generated with higher spatial resolution, ultraprecision and accuracy using Carl Zeiss, Sigma 300 VP series model. The detailed study, results and discussion were made in future topics.

Control concrete mix with 0%PLA inclusion
Control concrete mix samples with zero percent PLA inclusion were subjected to microstructural study, reveal that uniform agglomeration of cement, sand, fine aggregate and coarse aggregate. Figure 17(c) depicts micro cracks observed at fine aggregate removed location. Fig 17. (a-d) Fracture morphology of control concrete mix with 0% inclusion of PLA Fine aggregates having density of 2.5gm/cc, when casting the control concrete it was found that with PLA inclusion percentages from 0 to 50%, the granules are subjected to movement as they were light in weight(density 1.25gm/cc). This resulted in a top layer becoming more brittle and subjected to higher load bearing capacity. It was clear from figure 18(e-h) that perfect homogeneous blend was arrived with control mix concrete, as fine aggregates were varied in size from 4.75mm down size to till 300μm. Which resulted in air pocket covering at region were coarse aggregate to cement paste bonding occurred and as cement always go for volume shrinkage it did maintained the bonding even after 28 days of curing. shows the air pocket inside the PLA granular structure, even being hydrophobic as parent material, when mixed with cement and coarse aggregate, casted and kept for curing at 28 days it did went on to degrade. (f-g) illustrates the bead like structure, these flakes have non uniform lengths showing peaks and valleys in their sizes, providing additional space for inclusion of water and air particles to penetrate into the member and probably allow it for early degradation. aggregate mixture, less bonding with PLA granules and finally, outcome shows lowered mechanical strength .   Fig 23. (a-d) Fracture morphology of control concrete mix with 50% inclusion of PLA From figure 24(e-h), lot more issues related to weak regions in the entire matrix resulting in early failure, and as 50% yielded with higher degradation rate than its lower inclusion rates.
The strengths dipped to all time low rate for flexural and impact in comparison to controlled concrete and its successive cases. In case of compressive strength, maximum load will be bear by PLA granules and are really hard to break into pieces. The combination suites not so well for split tension test, as it is subjected to loading, results in early debonding of granules of PLA to cement paste. When external load goes beyond elastic limit of constituent materials in a matrix, results in micro crack and in turn crack deflection will be attained, slowly debonding initiates and rupture of the coupons will be observed. chemical behavior of PLA. On the other hand degradation is another critical issue with existing biomaterials, So with help of physical vapor deposition (PVD) method a hydrophobic and biocompatible chemical can be coated on the outer periphery of filament or granular form.