The XRD pattern of samples, as illustrated in Figure 1, shows that calcite is the main mineral (signals at 2θ=23.1, 29.5, 39.5, 47.6, 48.6°). The signals at 2θ=20.8, 26.7, 39.4° correspond to quartz which came from soil.
The FTIR spectra of the bare sticky-rice and SLM are presented in Figure 2. The bands at 875, and 710 cm -1 were ascribed to the deformation vibration of CO32- and O-C-O of calcite. The infrared absorption bands at 1421cm -1 of calcite are shifted to 1448 cm -1. The reason that the asymmetric stretching vibration of ν (O-C) of CaCO3 was shifted to higher wavenumber could be that the organic materials in mortars caused the crystal deformation of CaCO3. The bands in the 1000–1150 cm -1 spectral region were ascribed to the stretching vibration of C-OH of polysaccharide which was the degradation product of sticky-rice.
In order to confirm whether the lime had transformed into calcite completely, the pH of the SLM was calculated. The pH of the SLM was 11.73 meaning that the Ca(OH)2 was not transformed into CaCO3 completely. This result could indicate that the unreacted Ca(OH)2 could be wrapped by organic materials or else it should have been transformed into calcite in the air after such a long period of time. At the same time, the residual Ca(OH)2 maintained an alkaline environment which restrained the growth of the bacteria and conserved the SLM for such a long period of time.
The I2-starch reaction has proven to be a powerful tool to show whether amylopectin, the main component of sticky-rice, existed or not. It is a very sensitive chemical reaction even though the amount of the iodine was only 4×10-5mol/l. The I2-starch experiment yielded a purple solution when the SLM was reacted with the iodine. The result of the I2-starch reaction supported the hypothesis that there was still sticky-rice in the samples after such a long period of time under the burial condition. More details are discussed in the following sections.
Scanning electron microscopy (SEM) was employed to explore the microstructure of the SLM. Figure 3 shows the SEM micrographs of SLM. It can be seen from Figure 3 that the particles are small, and of irregular shape, which further suggests that the lime and/or calcite might be wrapped by sticky-rice.
We were not able to find out the exact initial ratio of each component in SLM, but TG could be a very useful instrument to investigate the composition of the mortar. Figure 4 shows the results of derivative thermogravimetry (DTG) as a function of temperature for the pure sticky-rice and SLM. It was found that the maximal decomposition temperature for the pure sticky-rice was at 257°C. It can be seen clearly that the decomposition of SLM involved three stages, and the maximal decomposition temperatures were found at 249°C, 395°C and 708°C respectively. The three stages seemed to correspond to the degradation of the organic component, the decomposition of Ca(OH)2, and the decomposition of the CaCO3, respectively. The maximal decomposition temperature for organic component in SLM was slightly lower than that of the pure sticky-rice, which could be attributed to the effects of the inorganic components. The organic component and CaCO3 constituted 2.7% and 78% of SLM mass according to the weight loss data, respectively.
In order to further confirm the decomposition patterns, the FTIR spectra corresponding to the three decomposition stages were obtained by TG-FTIR. Figure 5 shows the change in absorbance for bands at 2359 cm -1of the FTIR spectra for the gas generated when heating the SLM. The band at 2359 cm -1 indicates the presence of CO2 gas streams released from the samples. When samples were heated during the first stage, the slowly change in characteristics of the band at 2359 cm -1 suggests the released substance containing CO2 gas, matching the decomposition of sticky-rice and its degradation products. When the SLM was further heated, at 43 min, CO2 (2359 cm -1) was disappeared, matching the second stage of the decomposition of Ca(OH)2. The intensity of absorption band at 2359 cm -1 increased quickly after the CO2 signal started to increase at 53 min, and the maximum absorbance was at 64 min. The results of the bands at 2359 cm -1 obtained by TG-FTIR supported the evidence that the SLM comprised organic, Ca(OH)2 and a large amount of CaCO3.