A structural investigation relating to the pozzolanic activity of rice husk ashes

Abstract

Various factors determine the applicability of rice husk ash (RHA) as a pozzolanic material. The amount and accessibility of reactive sites is thought to be a key factor. A structural study of RHA samples in relation to their reactivity has been performed; Silica in RHA formed by burning rice husk in a laboratory furnace under continuous supply of air have been characterized as a function of incineration temperature, time and cooling regime. The characterization methods included chemical analyses, conductivity measurements, microscopic analysis, X-ray diffraction (XRD) and 29 Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR). In line with earlier observations, the analyses show that the highest amounts of amorphous silica occur in samples burnt in the range of 500°C-700°C. The 29 Si NMR data allow direct identification of the reactive silanol sites in the RHA samples. De-convolution of the NMR spectra clearly shows that the quickly cooled RHA resulting from burning rice husk for 12 h at 500°C has the highest amount of silanol groups. This sample also induced the largest drop in conductivity when added to a saturated calcium hydroxide solution giving an indication of its reactivity towards lime. Therefore, this RHA is the favorable sample to be used as pozzolanic cement additive.

Comparison of high temperature chars of wheat straw and rice husk with respect to chemistry, morphology and reactivity

“…In the present study, a lower relative area of Q 4 sites than in investigations of Nair et al [17], except for RHA-12Q sample oxidized for 12 h, was related to the differences in a rice husk preparation. In their study, the rice husk was oxidized at 500, 700 and 900 C in a fixed bed furnace from 15 min to 24 h, and the rice husk ash was collected immediately or after it was naturally cooled down inside the oven, whereas in the present study the rice husk char was immediately cooled down after fast pyrolysis in a temperature range of 1000e1500 C. The fast quenching of rice husk chars in the entrained flow reactor favored the formation of amorphous silicates which is similar to the structure of silica glasses prepared at cooling rates of 1 kK s…”

“…It seemed that no significant changes appeared in the 29 Si NMR spectra and thereby in the rice husk silicate structure at fast heating conditions. The increasing heat treatment temperature caused the relative amount of Q 3 species of original rice husk to decrease and Q 4 sites of original rice husk to increase, leading to a denser network with less surface sites and smaller reactivity towards oxygen [17]. The site oxygen reactivity in the silicon network follows the trend…”

“…Nair et al [17] showed that 1 H - 29 Si CP/MAS solid-state NMR spectroscopy is more accurate than X-ray diffraction for the detection and quantification of the amorphous and crystalline Si bearing compounds. Hamdan et al [18] studied rice husk ash, oxidized above 700 C, and observed a broad Q 4 resonance without any silanol groups.…”

Valorisation of rice husks using a TORBED® combustion process

“…Cement is currently selling in the US for an average of $105.5/T [27]. RHA has been shown to improve the durability of concrete and has been used for high performance cement [12]. It is reasonable to suggest that this being the case, a price similar to concrete may be obtained.…”

“…Reported optimal temperatures for limiting the crystallisation of silica within the literature vary widely: < 725°C [10]; limited crystallisation at 700°C [11]; and < 900°C [12]. Crystal lisation is not merely a function of temperature.…”