Abstract
The UK nuclear industry has historically used a unique specification of cement powder that differs from construction industry requirements. However, the slags that complied with this specification have become unavailable. The material now used to meet the requirements of the specification is a blend of standard construction industry ground granulated blast-furnace slag (GGBS), which has high fineness, with Calumite which is a coarser slag powder. Both materials have very similar chemical compositions, and the main reason for blending is to control the particle size distribution (PSD) to replicate the performance of the previous supply. The effect of changing the PSD on the performance properties of the cement paste was investigated. Isothermal conduction calorimetry at elevated temperatures was carried out to monitor the heat of hydration; it was found that the peak heat and total heat evolution increased with an increase in GGBS content. It was also found that Calumite contributes very little to the hydration reaction and thus behaves similarly to an inert filler. As the GGBS content was decreased, the fluidity of the pastes increased up to a certain point, but decreased again for systems dominated by very coarse particles, indicating that there is an optimum balance between the finer and coarser slag particles within this cementing system.
Highlights
Supplementary cementitious materials such as blast-furnace slag (BFS) and pulverised-fuel ash have been used as a partial replacement for Portland cement (PC) in UK and international nuclear waste encapsulation for many years (Angus et al, 2010; Sharp et al, 2003)
The samples were initially measured at 25°C as reported previously (Sanderson et al, 2017) and further testing was carried out at 35°C and 60°C to analyse the heat output of the cement pastes at elevated temperatures that may be more representative of the conditions inside a large waste container undergoing cementation
The results show that the peak heat and the total heat evolution both increased with an increase in ground granulated blast-furnace slag (GGBS) content
Summary
Supplementary cementitious materials such as blast-furnace slag (BFS) and pulverised-fuel ash have been used as a partial replacement for Portland cement (PC) in UK and international nuclear waste encapsulation for many years (Angus et al, 2010; Sharp et al, 2003). Encapsulation in a cementitious wasteform is the current preferred method of disposal of intermediate level waste in the UK (Sharp et al, 2003). The cement powders used for encapsulation in the UK must conform to tight specification requirements, both as a dry material and when combined in a grout. The current formulation used in the baseline tests is a 3:1 BFS/PC powder ratio with a 0·35 water/solids (w/s) mass ratio (Angus et al, 2010; Cann and Carruthers, 2012). There are many advantages to having such a high degree of PC replacement in the grout, such as a reduced amount of heat generation, improvements in workability and increased long-term durability of the wasteform produced (Rashad and Sadek, 2017; Raut et al, 2015; Ulubeyli and Artir, 2015)
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