Effects of Accelerated Carbonation Testing and by-Product Allocation on the CO2-Sequestration-to-Emission Ratios of Fly Ash-Based Binder Systems

Philip Van Den Heede,Nele De Belie

doi:10.3390/app11062781

Abstract

Carbonation of cementitious binders implies gradual capture of CO2 and significant compensation for the abundant cement-related CO2 emissions. Therefore, one should always look at the CO2-sequestration-to-emission ratio (CO2SP/EM). Here, this was done for High-Volume Fly Ash (HVFA) mortar (versus two commercial cement mortars). Regarding their CO2 sequestration potential, effects of accelerated testing (at 1–10% CO2) on as such estimated natural carbonation degrees and rates were studied. Production related CO2 emissions were evaluated using life cycle assessment with no/economic allocation for fly ash. Natural carbonation rates estimated from accelerated tests significantly underestimate actual natural carbonation rates (with 29–59% for HVFA mortar) while corresponding carbonation degrees are significantly overestimated (67–74% as opposed to the actual 58% for HVFA mortar). It is advised to stick with the more time-consuming natural tests. Even then, CO2SP/EM values can vary considerably depending on whether economic allocation coefficients (Ce) were considered. This approach imposes significant portions of the CO2 emissions of coal-fired electricity production onto fly ash originating from Germany, China, UK, US and Canada. Ce values of ≥0.50% lower the potential CO2SP/EM values up to a point that it seems no longer environmentally worthwhile to aim at high-volume replacement of Portland cement/clinker by fly ash.

Highlights

Recent reports indicate that CO2 emissions should be reduced as much as needed to keep global temperature increase relative to pre-industrial levels below 1.5 ◦ C by 2100
An experimental natural carbonation rate accelerated carbonation experiment at 10% CO2 (Anat) of 15.29 mm/ years does not really come as a surprise for this mix design. When comparing this value with the natural carbonation rates estimated from the slightly/highly accelerated carbonation tests using Equation (1), it is immediately clear that this approach results in a significant underestimation of the actual natural carbonation rate Anat
When the estimation is based on the output of a highly accelerated carbonation experiment at 10% CO2 (Anat : 6.35 mm/ years), this implies an underestimation of the actual value with no less than 59%

Summary

Introduction

Recent reports indicate that CO2 emissions should be reduced as much as needed to keep global temperature increase relative to pre-industrial levels below 1.5 ◦ C by 2100. The cement industry, accountable for no less than 8% of the global anthropogenic CO2 emissions [2], is being encouraged strongly to cut down on its carbon footprint This can be done by shifting more to binders with considerable portions of carbon-intensive Portland clinker replaced by supplementary cementitious materials (SCMs). The reduced CO2 buffering capacity of these binders may diminish the considerable CO2 sequestration potential [3] The latter property covers the CO2 uptake that inevitably occurs during the use and end-of-life phase of cement-bound building materials and the structures made thereof. According to Xi et al [4], the estimated global carbon uptake between 1930 and 2013 through cement carbonation during service life and after demolition and secondary use of concrete waste represents a large and growing net sink of CO2 , growing from 0.10 GtCarbon yr−1 in 1998 to 0.25 GtCarbon yr−1 in 2013

Objectives

Methods

Results

Conclusion