Sustainable Cements: Hybrid Alkaline Cements Overview

The ordinary Portland cement (PC) production contributes 2-3% of global energy use and 8% to 10% of world-wide carbon dioxide emissions (CO2). According to estimates in 2050, the CO2 emissions associated with the manufacture of PC, could increase between 240-260% compared to the 1990 emissions. Faced with this need, the Portland cement industry has been considering several options. One of them is to replace Portland clinker to the greatest extent possible, with low-carbon Supplementary Cementitious Materials (SCMs) (whether added with the cement or directly into the concrete mix, "Blended cement").The type and quantity of mineral additions is regulated by the standards of each country, although these values do not usually exceed 35%. Higher values are not allowed because the early mechanical strength decreases. However, applying alkaline activation technology can reach replacement levels of up to 60% or 70%, in the so-called "Hybrid Alkaline Cements" (HAC). The SCMs are basically natural pozzolans, such as blast furnace slag (BFS) or fly ash (FA). To increase the reactivity of mineral additions, especially at early ages, an alkaline activator is added into the mixture, normally salts of moderate alkalinity. This work shows a review of the theoretical knowledge of the hydration of hybrid cements, as well as their mechanical performance in mortars, concrete, and in industrial-scale applications.