Resistance of class C fly ash belite cement to simulated sodium sulphate radioactive liquid waste attack

Abstract

The resistance of class C fly ash belite cement (FABC-2-W) to concentrated sodium sulphate salts associated with low level wastes (LLW) and medium level wastes (MLW) is discussed. This study was carried out according to the Koch and Steinegger methodology by testing the flexural strength of mortars immersed in simulated radioactive liquid waste rich in sulphate (48,000 ppm) and demineralised water (used as a reference), at 20 degrees C and 40 degrees C over a period of 180 days. The reaction mechanisms of sulphate ion with the mortar was carried out through a microstructure study, which included the use of Scanning electron microscopy (SEM), porosity and pore-size distribution and X-ray diffraction (XRD). The results showed that the FABC mortar was stable against simulated sulphate radioactive liquid waste (SSRLW) attack at the two chosen temperatures. The enhancement of mechanical properties was a result of the formation of non-expansive ettringite inside the pores and an alkaline activation of the hydraulic activity of cement promoted by the ingress of sulphate. Accordingly, the microstructure was strongly refined.

Study of alkaline hydrothermal activation of belite cements by thermal analysis

“…Among the methods used for activating belite cements are those based on current cement technology: (i) the use of the most reactive low-temperature a' L polymorph (orthorhombic symmetry), which is stabilised during clinker production by rapid quenching of the clinker and the introduction of impurity ions (alkalis, Fe, Al sulphate, and so on) into the crystal structure during formation [3][4][5][6], (ii) the use of lowtemperature processes, which increase the surface area of C2S and therefore its reactivity [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23], and (iii) the addition of nano-materials during hydration [24][25][26].…”

“…The precursors are then dehydrated by heating without grinding at temperatures ranging from 500 to 900 °C [7][8][9]; with this process, it is possible to achieve BET-N 2 surface areas between 7 and 13 m 2 g _1 . Another low-temperature process is based on the formation of high-surface area precursors by hydrothermally treating the appropriate raw materials [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26].…”

“…Using the aforementioned procedure, we obtained belite cements (referred to as fly ash belite cement (FABC)), in which fly ash from coal combustion (class F [11][12][13][14] or class C [15][16][17][18][19][20][21][22][23][24][25][26]) was used as the raw material. The common technical characteristics of the cements are as follows: (i) the a'L-Ca2Si04 polymorph is as the main component, (ii) it contains elements, such as Fe, Al, Mg, Na and K, that increase its reactivity, (iii) it does not need to be ground, (iv) it can be used directly as cement without additional CaSOzt, since this is contained in the fly ash, and (v) it has excellent durability against the very concentrated salts that accompany low level radioactive waste (LLW) [14,[19][20][21][22].…”

“…The common technical characteristics of the cements are as follows: (i) the a'L-Ca2Si04 polymorph is as the main component, (ii) it contains elements, such as Fe, Al, Mg, Na and K, that increase its reactivity, (iii) it does not need to be ground, (iv) it can be used directly as cement without additional CaSOzt, since this is contained in the fly ash, and (v) it has excellent durability against the very concentrated salts that accompany low level radioactive waste (LLW) [14,[19][20][21][22].…”

Aplicación integral en construcción de materiales eco-eficientes con incorporación de cenizas de biomasa

“…Es importante señalar, que estos resultados mineralógicos manifiestan una alta cristalinidad de CB, ya que su difractograma no presenta un halo amorfo significativo como ocurre en general en el caso de las cenizas volantes [69][70][71].…”