‫ە‬Research Trends and Results

CO2 Uptake through Recycling ofConcrete RubblesKANDA Taro, Researcher
SONE Shinri, HeadRoad Environment Division, Environment Department(Key words) Construction byproducts, concrete rubble (demolished and crushed concrete), recycled concrete
1. Introduction
 The recycling level of concrete rubbles following
demolition of civil engineering structures has been
kept at about 98 percent in Japan, since the“Construction Material Recycling Law” came intoeffect. Almost all the concrete rubbles are recycled asaggregates. Recently, some researchers report that
CO2 uptake to concrete structures during their life
cycle is not negligible. The process of CO2 uptake is
known as carbonation (or neutralization), the reactionof Ca ions in cement ( pH is high) with carbonate ionsin the atmosphere. The uptake rate probably becomes
higher in a recycling process because the surface areaof the cement is increased by the demolition andcrushing processes.
The amount of the CO2 uptake should be subtracted
from the amount of CO2 emissions due to the energy
consumption etc. in the calculation of the net CO2emission during the recycling process. In the presentpaper, we introduce our nation-wide investigation todetermine the amount of the CO2 uptake throughrecycling of concrete rubbles.2. Mechanism of CO2 uptakeLimestone is a main raw material of cement, which isdecomposed into CaO and CO2 in the calcination process.
During the hydrate process, CaO in cement pastetransmutes into Ca(OH)2 which absorbs the atmosphericCO2. The CO2 behavior through life cycle of concrete
structure can be written as follows. In addition toCa(OH)2, calcium silicate hydrate (C-S-H) also absorbs
CO2.
(i) Cement manufacturing: CaCO3ĺCaO+CO2(ii) Hydration (concrete mixing): CaO+H2OĺCa(OH)2(iii) Use (service)-: Ca(OH)2+CO2ĺCaCO3+H2O

3. Overview of nation-wide investigation
 We collected 46 specimens of concrete rubbles
around the nation. Each specimen was divided into
two parts. The amount of CO2 was measured
immediately for one part (denoting uptake duringservice period of concrete structures), and after 28-day
exposure to the environment for the other. Figure 1shows the result. The amount of CO2 in theimmediately analyzed specimens was 14kg- CO2/t onaverage. That increased to 23 kg- CO2/t after 28-day
exposure. The amount of CO2 uptake throughrecycling was about 9kg- CO2/t on average (minimum:3, maximum 17), calculated as the increment between
28-day exposures. The smaller the grain size was, themore CO2 the specimen absorbed. This amount iscomparable with the amount emitted to produce therecycled concrete aggregates.
4. Future work
 We will determine the amount of CO2 uptake whichshould be considered in the calculation of the net CO2emission due to manufacturing of recycled concrete
aggregates. We will identify the influential factors on
the CO2 uptake to propose easy and effective
techniques for increasing the amount of CO2 uptake.
References:
1) Ministry of Land, Infrastructure, Transport andTourism:
http://www.mlit.go.jp/common/000121183.pdf
2) Y. Kuroda, T. Kikuchi: Concrete Research andTechnology (in Japanese), Vol. 20(1), Jan. 2009
Grain size⢏ᚄ'(D)

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Amount of CO2 uptake (kg-CO2/t)
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aggregate, recycling, CO2 uptake 㻞㻜PD[㻤㻚㻥㻢㻚㻤㻝㻟㻚㻤㻡㻜

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recycling㻣㻚㻜“Immediately””after 28-day౪⏝  ౪⏝෌㈨※໬exposure”

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Figure 1. Amount of CO2 in Concrete Rubbles (tentative)

3) T. Kanda, S. Sone, H. Kishida: Meeting of The
Institute of Life Cycle Assessment (in Japanese),
vol. 6, 2011‫ە‬Research Trends and Results