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Conventional supplementary cementitious materials (SCMs), such as blast furnace slags or fly ashes, have been used for many decades, and a large body of knowledge has been collected regarding their compositional make-up and their impacts on cement hydration and concrete properties. This accumulated empirical experience can provide a solid, confident base to go beyond the status quo and develop a new generation of low-clinker cements composed of new types and combinations of SCMs.The need for new sources of SCMs has never been greater, as supplies of traditional SCMs are becoming restricted, and the demand for SCMs to reduce CO2 emissions from concrete production is increasing. In this paper, recent research on emerging SCM sources is reviewed, along with new developments in characterizing and qualifying SCMs for use and improved knowledge of SCMs on long-term concrete performance and durability. Keywords(B) Characterization; (C) Durability; (D) Blended Cement; (D) Filler; (D) Pozzolan Material Chemistry Used as SCM (Mt/y)Total volume est. (Mt/y) Comments Blast furnace slag Ca-Si-Al 330 300-360 Nearly fully used, latent hydraulic Coal fly ash Si-Al 330-400 700-1100 Subject to limitations on carbon content, reactivity Natural pozzolans Si-Al 75 Large accessible reserves Large variety/variability, often high water demand Silica fume Si 0.5-1 1-2.5 Used in high-performance concrete Calcined clays Si-Al 3 Large accessible reserves Metakaolin performs best, often high water demand Limestone CaCO3 300 Large accessible reserves Cementitious contribution in combination with reactive aluminates Biomass ash Si 0 100-140 Competition with use as soil amendment, high water demand, (for some: high alkali contents) MSWI bottom ash Si-Al-Ca 0 30-60 Expansive and corrosive components, leaching issues Steel slag Ca-Si-Fe 0 (negligible) 170-250 Various types, can contain expansive components (CaO) or leachable heavy C L reactivity Copper slag Fe-Si 0 (some as filler) 30-40 Low reactivity, leaching of heavy metals, more research needed Other non-ferro slags Fe-(Si)-(Ca) 0 (some as filler) 5-15 Mt/y each (FeCr, Pb, Ni) Low reactivity, leaching of heavy metals, more research needed

This paper is the work of working group 3 of the RILEM Technical Committee on Hydration and Microstructure of Concrete with SCM (TC 238-SCM). The pore solution is an essential but often overlooked part of hydrated cements. The composition of the cement pore solution reflects the ongoing hydration processes and determines which solid phases are stable and may precipitate, and which phases are unstable and may dissolve. The study of the cement pore solution therefore contributes to the understanding of the mechanisms as well as of the kinetics of cement hydration. This paper reviews the impact of supplementary cementitious materials (SCMs) on the pore solution composition of blended cements. In a first part, the extraction and analysis methods of cement pore solutions are reviewed, leading to a set of practical guidelines and recommendations. In a second part, an extensive literature survey is used to document the effect of the addition of SCMs (blast furnace slag, fly ash and silica fume) on the pore solution. Finally, in a third part the collected literature data are compared to thermodynamic simulations. The performance and current limitations of thermodynamic modelling of blended cement hydration are demonstrated and discussed in view of future progress.

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Supplementary cementitious materials (SCMs) are widely used to partially replace portland clinker in blended cements. Reducing clinker contents further without compromising the development of early strength necessitates a better assessment and enhancement of the reactivity of the available SCMs. To this purpose, the reactivity of synthesized calcium aluminosilicate glasses covering a compositional range from blast-furnace slags (BFS) over fly ashes to silica fume was analyzed by dissolution experiments. Initial glass dissolution rates were measured at 20°C and pH 13, and with varying initial concentrations of aqueous Al, Ca, and Si. At pH 13, glass dissolution rates were observed to scale linearly with the glass Ca/(Al + Si) molar ratio. Ca-rich blast-furnace type glass dissolution was shown to be up to one order of magnitude faster than tectosilicate fly ash and silica fume type glass dissolution, supporting different pathways to dissolution. In solutions that are strongly undersaturated with respect to hydrous glass and hydration products, glass dissolution rates are independent of changes in solution undersaturation and aqueous Si activity. In contrast, dissolution rates decrease with aqueous Ca concentration for all glasses and with aqueous Al concentration for tectosilicate-type glasses. The insights gained are instrumental in finding ways to enhance SCM reactivity.

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Huge amounts of waste are being generated, and even though the incineration process reduces the mass and volume of waste to a large extent, massive amounts of residues still remain. On average, out of 1.3 billion tons of municipal solid wastes generated per year, around 130 and 2.1 million tons are incinerated in the world and in Belgium, respectively. Around 400 kT of bottom ash residues are generated in Flanders, out of which only 102 kT are utilized here, and the rest is exported or landfilled due to non-conformity to environmental regulations. Landfilling makes the valuable resources in the residues unavailable and results in more primary raw materials being used, increasing mining and related hazards. Identifying and employing the right pre-treatment technique for the highest value application is the key to attaining a circular economy. We reviewed the present pre-treatment and utilization scenarios in Belgium, and the advancements in research around the world for realization of maximum utilization are reported in this paper. Uses of the material in the cement industry as a binder and cement raw meal replacement are identified as possible effective utilization options for large quantities of bottom ash. Pre-treatment techniques that could facilitate this use are also discussed. With all the research evidence available, there is now a need for combined efforts from incineration and the cement industry for technical and economic optimization of the process flow.