### Recycled Glass Powder as a Supplementary Cemertitious Material  
In principle, glass is 100% recyclable, and every tonne of glass re-melted saves ~580 kg of carbon dioxide emissions ([PERSON], 2016), as well as decreasing the energy needed to make glass and reducing reliance on virgin raw materials. Recycling rates across Europe vary; for example, in the British Isles, 84% of glass is recycled in Ireland ([PERSON], 2016) and 74% is recycled in the UK ([PERSON], 2016). Waste glass cullet is commonly recycled, but its reuse in producing new glass is limited by its composition. Recycled cullet not recovered for glass container manufacturing (due to unsuitable composition, sizing, contamination, or market conditions) is often disposed of in landfill, which attracts Landfill Tax, adding to the cost of disposal. Container glass is uneconomic to decolourize, which means coloured glass is unlikely to be recycled by the glass industry, making its use as an SCM particularly attractive. The recycling of container glass cullet contributes to national _\"circular economy\"_ aspirations, while at the same time eliminates all costs associated with landfill. The incorporation of glass in cements is not a new idea and has been the subject of study for some decades [17; 18; 19], and of course natural glasses have been used in cements since antiquity. There are two important factors to consider in using glass as an SCM, due to the potential occurrence of alkali-silica reactions (ASRs) in concrete. Alkalis in concrete mixes can be a cause for concern. High alkali loads in concrete can trigger deleterious expansion if reactive silicate aggregates are present. The first factor is the alkali metal content and the second is particle size. Alkali metals are introduced as metal carbonate flux, used to lower the melting temperature of the glass and for container glass and comprise around 13% Na\({}_{2}\)O by mass of the whole [20]. The alkali load in concrete is proportional to the binder content and the alkali level of the binder, expressed as equivalent sodium oxide content, where:  
\[\%\text{Na}_{2}\text{O equivalent}=\text{Na}_{2}\text{O}+0.658\%\text{K}_{2} \text{O}\ \text{(by mass)} \tag{1}\]  
A suitable specification for recycled glass cullet suitable for _\"flux/binders\"_ can be found in BS PAS 101 _\"Recovered container glass Specification for quality and guidance for good practice in collection\"_ See Table A1 of ref [21]. Practically in cements, the alkali content of the binder is limited as a precaution against alkali silica reaction (ASR) at 0.6% (Equation (1)). In most countries, alkalis are limited to the equivalent Na\({}_{2}\)O content per cubic meter of concrete at 1.8 to 4.5 kg/m\({}^{3}\), depending on the reactivity classification of the aggregate used and on how variability in the binder alkali level is reported by the cement manufacturer. Alternatively, a _'low alkali'_ binder, defined as one that does not exceed 0.6% Na\({}_{2}\)Oeq\({}_{2}\) may be used if aggregate reactivity is high or unknown. The second consideration is the particle size of the glass. The suppression of alkali silica reaction is also addressed by fine grinding of the glass (to a few microns), as has been demonstrated elsewhere [19], and it has been found that the addition of glass powder can enhance ASR resistance if the particle size is below 300 microns [20]. This introduces another dimension to using glass powder in blended cement, that of grinding energy. For a comminution process to grind a material to increasingly smaller sizes, commensurate amounts of energy are required. This is not a linear relationship, however, but is governed by the defect density of the material to be processed. In the case of glasses, comminution to a few microns is relatively straightforward, but further reduction is increasingly energy intensive.