To reduce environmental pollution induced by the production of Portland cement and sequestrate greenhouse gas, a novel approach was developed to manufacture nano-calcium carbonate (nano-CaCO3) suspension by upcycling carbon dioxide. The influence of this nano-CaCO3 suspension on basic performances of Portland cement paste was experimentally evaluated and related mechanisms were demonstrated by isothermal heat conduction calorimeter (TAM Air), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetry-differential thermal analysis (TG-DTA), mercury intrusion porosimeter (MIP), scanning electron microscope (SEM) and transmission electron microscope (TEM) measurements. Experimental results showed that the manufactured CaCO3 presented spherical and cubic shapes with size of 20 to 50 nm. This CO2 upcycling method can improve compressive strength of cement paste by 5.8~9.9% at ages of 3 to 56 days and significantly reduce the initial and final setting times. The introduction of CO2 in form of nano-CaCO3 accelerated the early age hydration of Portland cement and refined the pore structure. Around 0.4~2.4 kg of CO2 can be recycled by every ton of Portland cement while the usage efficiency 22 of cement was evidently improved. Therefore, both capture and solidification of carbon dioxide and 23 carbon footprint reduction of cement industry can be simultaneously achieved by this technology.

Introduction

It is well known that Portland cements (PC) is one of the most widely used materials for construction of buildings and other infrastructures (Biernacki et al., 2017˗Schneider et al., 2011; Shi et 29 al., 2011). The reported worldwide production of Portland cements reached as high as 4.6 billion tons in 2016, of which more than 60% was contributed by China (CEMBUREAU, 2016). The PC production is a typical CO2 release and energy consumption process, being responsible for nearly 9% of anthropogenic CO2 emission and accounts for approximately 3% of the global energy use (Shi et al., 2011). Generally speaking, around one ton of CO2 is discharged by the production of every ton PC (Hemalatha and Ramaswamy, 2017) and such CO2 emission is attributable to two aspects. On the one hand, CO2 is a by-product from the calcination of limestone to create reactive calcium silicates (Shi et al., 2011; Vance et al., 2015); on the other hand, the combustion of fossil fuel required for clinkering raw materials (clay and limestone) at 1450 ć also releases a great deal of CO2 (Vance et al., 2015). Therefore, the carbon impact of Portland cement industry has attracted increasing attentions and many efforts have been carried out to address a lower-carbon or greener production of cements (Bourtsalas et al., 2018; Benhelal et al., 2018; Carvalho et al., 2017). The cement concrete industry worked with the International Energy Agency to outline the ambitious effort to decrease industrial emissions to 50% below levels of 2006 till 2050 (Monkman and Macdonald, 2017). It is in accordance with the “blue map scenario” (International Energy Agency, 2008). There are four suggested approaches for achieving this target (Monkman and Macdonald, 2017): to improve the energy efficiency of cement kilns, to increase the usage of low-carbon supplementary 46 cementitious materials (SCMs) to replace clinker, to utilize alternative raw materials and/or fuels for manufacture of Portland clinker and to capture or sequestrate CO2.