Microscopic and Chemical Characterization of Elephant Grass and Corn Leaves and Their Ashes

Many agrowastes are being used for energy production by combustion in thermal power plants. This process generates huge amounts of ashes, which have a potential pozzolanic activity for blending with Portland cement or hydrated lime. Elephant grass (Pennisetum purpureum) and corn straw (Zea mays) are two types of biomass whose ashes could be valorized in construction materials. The aim of the present research was to analyze the chemical composition of leaves and the corresponding ashes in terms of inorganic elements characterization (silicon, potassium, calcium, sodium, chloride,..). Microscopic studies (FESEM) on the dried leaves and the characterization of the ashes obtained at different calcination temperatures have been carried out. Energy dispersive X-ray spectroscopy (EDX) has been applied for quantifying the percentage of the main chemical elements. Elephant grass ashes (EGA) maintained the spodogram after calcinations at 450 and 650oC. Silica (SiO2) was the main oxide in EGA (55.7%); also CaO, MgO and K2O were in high percentage and chloride content was very low. In corn leaf ashes (CLA) the spodogram was maintained at 350 and 450oC. Silica was also the main oxide (62.6%). The amount of K2O was also high (26.8%) and chloride content was significant (2.4%) which must taken into account for the use of CLA in concrete. Both ashes could be interesting for OPC-based blended cements because their potential pozzolanic properties. Introduction Supplementary cementing materials (SCMs) in the production of cements and concretes are widely studied by many researchers [1]. The key of their use is due, in many cases, to the presence of amorphous silica and alumina in the compositions of SCMs. These SCMs are usually obtained as byproducts or wastes from different human activities: industrial, agricultural, energetic and decontamination processes. Silica (SiO2) and alumina (Al2O3) amorphous phases are usually present and they reacts towards hydrated lime Ca(OH)2 by means of the pozzolanic reaction: this is the reason because this type of SCMs are called pozzolans. This pozzolanic reaction yields hydraulic cementing products, similar to those found in the hydration of ordinary Portland cement (OPC). The use of these SCMs in concrete lets to reduce the OPC consumption and additionally avoid the disposition of wastes in landfills. From the technical point of view, the use of these pozzolans in OPC-based binders has advantages such as lower heat of hydration, higher compressive strength and lower permeability/porosity of the capillary network. These advantages makes that these pozzolanic materials could play an important role in the performance of concrete, specifically in its durability. Global warming, attributed to the CO2 increase in the atmosphere, is becoming the more relevant worldwide issue [2]. Portland cement fabrication is responsible of 5% of CO2 worldwide emissions and consequently, another advantage in the use of SCMs from wastes is considered in environmental concerns: the reduction of the greenhouse gaseous (GHG) emissions. Agricultural wastes can be converted into interesting power sources by combustion. This process converts biomass in water vapor, carbon dioxide and ashes, among others. CO2 emission has not net environmental impact because is not from fossil origin, this carbon released during combustion was previously captured by plants in a recent period. One of the products produced during combustion is the inorganic residue, the ash. This ash must be collected from the boiler (bottom ash) or from the mechanical/electrostatic precipitators (fly ash). In both cases, the chemical nature of the ash could be interesting for blending with OPC. In many cases, ashes are rich in amorphous silica, which is a main reagent for producing the pozzolanic reaction. Different types of biomass ashes [3, 4] are being studied for their reusing in OPC binders. The most well-known ash is that produced by combustion of rice husks. Rice Husk Ash (RHA) has been extensively analyzed when mixed with OPC. Also Sugar Cane Bagasse Ash (SCBA) and Palm Oil Fuel Ash (POFA) have been tested in the last decade. However, there is an increasing interest on assessing the behavior of new ashes from biomass [5]: wood, sugarcane straw, bamboo leaves, coffee husks and cashew nut rinds [6], among others. Few studies have been also carried out by using ashes from maize (corn) and elephant grass plants. Maize (Zea mais) is grown throughout the world, and it is one of the most important cereals cultivated both for human and animal consumption (for grain and forage) and in the last years for bioethanol production. More than 875 million tons of grains were worldwide produced in 2012 [7]. Corn cob is a waste product obtained from maize and it can be used as biomass for combustion. Corn Cob Ashes (CCA) have been previously studied [8-10]. Adesanya and Raheem [8] used CCA by blending with OPC (in 0-25% by mass of CCA). SiO2 percentage ranged in 65-68%, and Al2O3 ranged in 6-9%. This means that the sum of both oxides reached 70%, minimum required according to ASTM C-618 for pozzolans. Replacement of OPC by CCA produces an increasing in the initial and final setting times of pastes. Compressive strength values at early curing ages for mortars containing CCA were lower than those found for control mortar [9]; however, an important strength gain was observed for longer curing period (120 days), and 8% replacement of OPC by CCA was the optimum for yielding the best strength development. Corn Stover Ashes (CSA; from leaves and canes) were also used for OPC blending [10]: the produced ashes contained high proportion of K2O (25.41%) and SiO2 (28.42%) and showed low pozzolanic reactivity. When these ashes were washed with hydrochloric acid, the potassium content strongly diminished (4% of K2O) and their reactivity was significantly enhanced, yielding higher compressive strength than control mortar. Elephant grass (Pennisetum purpureum) is cultivated for feeding ruminants. It grows rapidly (40 ton/ha/year) and is considered as an efficient carbon dioxide sink. Also, it can be used in thermoelectric power plants as biomass source. In this process, huge amount of elephant Grass Ashes (EGA) is produced. The chemical composition [11] of these ashes showed the high percentage of silica (56-68%) and alumina (22-23%). K2O content was also important, although the percentage depends on the biomass treatment prior to burning (7.4% without treatment, 3.5% with hot water, and 2.0% with hydrochloric acid). EGA presented similar pozzolanic activity to other biomass ashes, such as SCBA and RHA. This reactivity let to prepare 20% OPC replaced mortars by using these EGA, maintaining compressive strength after 28 days of curing. In other study [12], it was found that different cultivars yielded different ashes: Napier variety produced higher rich silica ashes than Cameroon variety (80 vs 50% in SiO2). Both ashes obtained at 700oC showed high pozzolanic activity in EGA/Ca(OH)2 systems: more than 80% of calcium hydroxide was fixed in 1:1 mixtures after 7 days of curing at 22oC. Plants absorb silicon from the soil as silicic acid and also deposited as amorphous hydrated silica or opal (SiO2.nH2O) in the cell wall, inside the cells (intracellular spaces or cell lumen) and in the intercellular spaces of different plant organs such as leaves, pods, stems, inflorescences (i.e. herringbones) and epidermal appendages (trichomes) [13]. Different siliceous structures have been found in plants. There are special siliceous cells that are called phytoliths or silico-phytoliths [14]. The aim of this research is to study in depth, from the microscopic point of view, the ashes obtained from Elephant grass leaves and corn (maize) leaves, in order to analyse the silica content and the presence of other compounds, and also the presence of silica bodies (phytoliths). Both leaves could be used as biomass for power plants and the corresponding ashes could be potential pozzolanic materials for blending with OPC. Experimental section Leaf samples of elephant grass (Pennisetum purpureum, EGL) were collected in Pirassununga (São Paulo, Brazil); and corn leaves (Zea mais, CL) were collected in Ilha Solteira (São Paulo, Brazil). To correctly identify structures rich in silica (phytoliths) and their location within the plant, studies on both collected material (dried) and ash resulting from combustion in a muffle furnace at selected temperatures were performed. Leaves were previously washed with deionized water. Preparation of samples: a) Dried material studies: small samples of leaves were dried at 105°C for 24 h in a laboratory oven (Memmert UN model); b) Calcined material (ash) studies: The dried samples were calcined for 1 hour at the selected temperature (depending on the leaf, 350, 450 and 650oC) in a muffle furnace (Carbolite RHF model 1500). The obtained ashes were: Elephant Grass Ash (EGA) and Corn Leaf Ash (CLA). Microscopy equipment: Field emission scanning electron microscopy (FESEM) was used for microscopy and microanalysis studies by using a ZEISS ULTRA 55 microscope. Samples for taking images were studied at 20kV and covered with gold (working distance 11-11.5 mm); and at 1-3 kV and not covered (working distance 3.4-4.9 mm). Samples for chemical analysis (EDX) were not covered and studied at 15kV. The EDX analysis was carried out by analysing a 115 m x 85 m area: from the 5 tests, mean values and their standard deviations were calculated. Results and Discussion Microscopic characterization of dried Elephant Grass Leaves (EGL). FESEM studies were carried out on EGL samples dried at 105oC. Selected micrographs are shown in Figure 1. They show the lower leaf surface (abaxial surface) and the upper leaf surface (adaxial surface). In the abaxial surface, many structures (Figure 1a) have been found: hairs, stomas and phytoliths. In Figure 1b, these structures are showed in detail. Phytoliths showed different shapes: bilobated, trialobated and tetralobated. Adaxial surface was similar, as can be seen in Figure 1c. (a) (b) (c) Figure 1. FESEM micrographs of abaxial (a and b) and adaxial (c) surfaces of Elephant Grass Leaves (EGL) dried at 105oC. Microscopic characterization of Elephant Grass leaf Ashes (EGA). FESEM studies were carried out on EGA samples obtained at 450oC and 650oC. Selected micrographs of EGA ashes obtained at 450oC are shown in Figure 2. Figure 2a shows a general view of the spodogram obtained when a large part of organic matter has been removed by calcination. It can be observed that the structure remained, due to the presence of important percentage of inorganic matter. In figure 2b, a detailed zone of the spodogram is observed: hairs and phytolits showed similar shape than that observed in Figure 1, meaning that these structures are rich in inorganic elements. These phytoliths are arranged in a similar way than found in sugarcane leaves [15,16], that is, they are aligned and arranged so that the major axis thereof is parallel to the major axis of the epidermal cells (Figures 2c and 2d).