Effect of hydrothermal treatment temperature on morphology of obtained calcium silicate from rice husk ash

This research report on the effect of hydrothermal treatment temperature on morphology of obtained calcium silicate from Vietnam Rice Husk Ash (VRHA). VRHA is collected at the Mekong Delta River, burn to obtain the active silica which can be used for further step. The obtained silica is hydrothermal treated in the present of Ca-source so that the Ca/Si molar ratio of 1.0 for different treatment temperature such as 110, 130, 150 and 180oC. XRD and SEM of samples before and after hydrothermal treatment confirm the present of nano-Calcium Silicate Hydrate (CSH) such as Tobermorite and Xonotlite. These CSH can be used as inorganic light weight thermal insulator application. The obtained calcium silicate after hydrothermal treatment at high temperature have leave-like crystal, and these crystal interlock together.


INTRODUCTION
Vietnam is an agriculture producing country, in which produce lot of rice and its by product is rice husk ash (RHA).The RHA is consider as waste of agriculture industry, and treated by burn in the open air.This process cause air pollution, thus attracted researcher to find alternative method to reduce the impact of rice husk ash to environment [1][2][3].The research group in Department of Ceramic Materials aim to reuse rice husk ash as source of Silica (SiO 2 ) [4][5].Our research group successful to synthesize the rice hush ash using hydrothermal treatment method [6][7][8].However, the effect of hydrothermal treatment method on crystal morphology is still unknown.In this research, the effect of hydrothermal treatment temperature on the morphology of obtained calcium silicate was reported.VRHA was characterized using X-ray Diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR).The chemical composition of RHA was analyzed using X-Ray Fluorescent (XRF) method.In addition, CaO was used from the commercial without further purified (Xilong Chemical, China).The phase composition of commercialized CaO was comfirmed using XRD.The mixture of RHA and CaO was mixed with the Ca/P molar ratio of 1.0 with the moisture of 10% (weight percent) then pressing at 30 MPa to form the compacted disk with diameter of 9mm.The compacted disk was hydrothermal treated at 110 o C, 130 o C, 150 o C and 180 o C for 24 hours to obtain CSH mineral.

Phase analysis:
The powder Xray Diffraction (XRD) patterns of disk samples were recorded with a vertically mounted diffractometer system (Bruker-AXS: D8 ADVANCE, Germany) using Ni filtered CuKa generated at 15 kV.

Inspection the morphology of samples using Scanning Electron Microscope (SEM):
The surface of samples was observed using a scanning electron microscope (SEM) (JSM 5400LV, JEOL Co. Ltd., Japan) under an accelerating voltage of 20 kV after being coated with gold.

Chemical bonding of sample:
The samples were mixed with KBr with the ratio 1: 200 and analyzed using Fourier Transform Infrared (FTIR) method with the waveband vary from 400 -4000 cm -1 .

Chemical composition of sample:
The sample is energied using X-ray Fluoresence (XRF) (MESA-50, Horiba, Japan) and measure the secondary beam to analyze the chemical composition of sample.

Bulk density:
The bulk density of sample is measured using the weight and volume ratio of sample (n=6), and is expressed by mean ± standard deviation.ANOVA analysis to use for statistical analysis.

RESULS AND DISCUSSION
The chemical composition of RHA is given in Table 1.This data indicates that dominant oxide in RHA is silica (>92%) Therefore RHA can be used as source of qualified silica for the next experiment in order to turn RHA from trash to treasured materials.
The phase analysis of RHA is also given in Figure 1, indicating that RHA is composed of low degree of crystallinity crystolbalite according to PDF card #39-1425.

TAÏ P CHÍ PHAÙ T TRIEÅ N KH&CN, TAÄ P 19, SOÁ K6-2016 Trang 71
In addition, the FTIR spectrum of RHA is given in Figure 2, indicated that the main chemical bonding of RHA is O-Si-O, and these data go well with XRD data given in Figure 1 The phase analysis of CaO is given in Figure 3, indicating that commercialized CaO is pure and can be used for further reaction.The bulk density of sample before and after hydrothermal treatment is also given in Figure 6.
The bulk density decrease with the increase of hydrothermal treatment temperature, and go well with the increase the number of pores in Figure 5d and 5e.

By using hydrothermal
husk was burn at 500 o C at the heating rate of 10 o C/min (Naberthem 1400, Nabertherm, Germany), then soaking at 2 hour for complete burning.The phase composition of obtained SCIENCE & TECHNOLOGY DEVELOPMENT, Vol 19, No.K6-2016 Trang 70 The phase analyses of compacted disk before and after hydrothermal treatment at 110 o C, 130 o C, 150 o C and 180 o C for 24 hours.Before hydrothermal treatment, the phase composition of sample is low degree of crystobalite and Ca(OH) 2 .The present of Ca(OH) 2 is formed by hydration of CaO and water during the mixing process.After hydrothermal treatment, the new phase of Calcium Silicate Hydrate (CSH), Tobermorite (T) and Xonotlite (X) sre observed.

Figure 4 .
Figure 4. XRD patterns of compacted disk before and after hydrothermal treatment at various temperature: (a) before; (b) 110 o C; (c) 130 o C; (d) 150 o C and (e) 180 o C The morphology of sample before and after hydrothermal treatment at 110 o C, 130 o C, 150 o C and 180 o C for 24 hours also is given at Figure 5.

Figure 5 .
Figure 5. SEM images of sample before and after hydrothermal treatment at different temperature: (a) before; (b) 110 o C; (c) 130 o C; (d) 150 o C and (e) 180 o C.

Figure 6 .
Figure 6.Bulk density of sample before and after hydrothermal treatment at different temperatures.

Table 1 .
Chemical composition of RHA (weight percentage)