Carbonation degree of autoclaved aerated concrete
Fumiaki Matsushita*,Yoshimichi Aono,Sumio Shibata
Sumitomo Metal Mining Siporex Co.,Ltd,Rearch and Development Center,Mie Branch,1117-11,Ege,Seki-cho,Suzuka-gun,Mie,519-1106,Japan
Received18May1999;accepted5September2000
Abstract听众英语
Neutralization depth has been the criterion in the carbonation or neutralization of ordinary concrete.On the other hand,autoclaved aerated concrete(AAC)has been investigated in terms of the carbonation degree.However,various definitions of carbonation degree of AAC have been propod and applied.We have studied the definition of carbonation degree of AAC in order to investigate various rearch works under the same criterion.In this paper,we propo the carbonation degree(D c)of AAC,D c(%)=(CÀC o)/(C maxÀC o)Â100where C,C o and C max are the amount of carbon dioxide in the sample,in the non-carbonated sample and the theoretical amount of carbon dioxide needed to combine with the total calcium oxide in the sample to form calcium carbonate,respectively.In addition,we propo the heating headmaster
weight loss from600°C to800°C in TG-DTA analys as the method to determine the amount of carbon dioxide for carbonation degree of AAC.D2000 Elvier Science Ltd.All rights rerved.
Keywords:Autoclaved aerated concrete;Carbonation;Degradation;Durability;Aging
1.Introduction
Carbonation is one of the main factors of the aging deterioration of autoclaved aerated concrete(AAC),where tobermorite-11AÊand well-crystallized C-S-H,the main structural minerals of AAC,react with carbon dioxide under the existence of moisture and finally decompod to silica gels and calcium carbonate.
For the ordinary concrete,carbonation is the chemical reactions of low-crystallized C-S-H and calcium hydro-xide with atmospheric carbon dioxide,leading to the neutralization that deprives concrete of the rust resis-tance for reinforcement steel bars.Therefore,the criter-ion of the neutralization depth has been favorably adopted.Many rearchers have investigated their works by this criterion.
On the other hand,the main structural minerals of AAC are tobermorite-11AÊand well-crystallized C-
S-H,which are originally neutral becau of the sufficient curing in an autoclave.Conquently,embedded reinforcement steel bars in AAC are coated with rust-resistant materials becau AAC has no passivating effect on them.Carbona-tion of AAC leads to deterioration such as changes of minerals and microstructures,the degradation of proper-ties,cracking and the increa of drying shrinkage.For the reasons,many studies have been made on carbona-tion of AAC and deterioration[1±7].However,various definitions of carbonation degree of AAC have been propod and applied making studies difficult to compare with each other.We have studied the definition of carbo-nation degree of AAC in order to investigate by a unified criterion in the same way as neutralization depth in concrete study.
2.Definition of carbonation degree of AAC
2.1.Carbonation of AAC
The raw materials of AAC,such as quartz sand, slag,cement and lime,are usually blended in a lower Ca/(Si+Al)ratio than0.83,which is the theoretical Ca/(Si+Al)ratio of tobermorite-11AÊ.The foamed green body of AAC is cured sufficiently under the saturated vapor pressure at approximately180°C in the auto-clave.Accordingly,the main structural minerals of AAC are tobermorite-11AÊand well-crystallized C-S-H,
*Corresponding author.Tel.:+81-5959-6-1131;fax:+81-5959-6-
1987.
双线竹芋E-mail address:fumiaki_matsushita@jp(F.Matsushita).
Cement and Concrete Rearch30(2000)1741±1745
0008-8846/00/$±e front matter D2000Elvier Science Ltd.All rights rerved.
班组年度工作总结PII:S0008-8846(00)00424-
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and almost all the calcium must exist in them.Tober-morite-11AÊand well-crystallized C-S-H react chemically with carbon dioxide under the existence of moisture and are decompod into silica gels and calcium carbonate as below,
5CaOÁ6SiO2Á5H2O tobermoriteÀ11 A 5CO2
35CaCO3 6SiO2 5H2O Y
x CaOÁy SiO2Áz H2O wellÀcrystallized CÀSÀH
x CO2
3x CaCO3 y SiO2 z H2O X
The morphologies of tobermorite-11AÊand well-crystal-lized C-S-H are platy.The morphology of silic
a gels decompod from tobermorite-11AÊand well-crystallized C-S-H are also platy,becau calcium ions come out from them and combine with carbon dioxide to form calcium carbonate.Carbonation conditions,such as relative humid-ity,carbon dioxide concentration and curing time,affect the ratio of the polymorphs of calcium carbonate that form.[1] The content of tobermorite-11AÊand well-crystallized C-S-H and the chemical compositions are not fixed becau of the difference in composition of raw materials and the curing conditions in the autoclave by manufacturers.Conquently, carbonation degree must be applied for the various AAC in the same way.
2.2.Preceding rearch
Sauman[1]studied the relations between compressive strength and relative humidity,carbon dioxide concentration and curing time for accelerated carbonation.Sun et al.[2] propod the ratio of compressive strength before and after carbonation as a criterion of carbonation.Compressive strength is,however,very strongly affected by bulk density and the shapes of samples.
Hanecka et al.[3]indicated the increa of bulk density as a criterion of carbonation.However,they could not describe the carbonation degree from the increa of bulk density becau the amount of adsorbed water may have changed during carbonation.
Asano et al.[4]ud the XRD peak intensities as a criterion for accelerated carbonation of laboratory-synthe-sized tobermorite-11AÊ.However,well-crystallized C-S-H can be hardly detected by XRD.
Dapkus and Stankevisius[5]and Iwasaki and Tada[6] propod the carbonation degree as Eq.(1)at nearly the same time,where C,C o and C max are the amounts of carbon dioxide in the sample,in the non-carbonated sample and at the maximum carbonation,respectively.
CÀC o a C maxÀC o Â100 1
Dapkus and Stankevisius indicated an approximation of 2%for C o and16%for C max.Iwasaki and Tada analyzed the amount of carbon dioxide by chemical analysis using hydrochloric acid,where C o was that for non-carbonated AAC and C max was that for fully carbonated AAC in accelerated conditions.
Sun et al.[2]and Ochiai[7]propod the carbonation degree as Eq.(2)regardless of C o in Eq.(1).
C a C maxÂ100 2
Sun et al.analyzed the amount of carbon dioxide,C,by measuring the infrared absorption peak at1430cmÀ1 indicating calcium carbonate,where C max was that for fully carbonated AAC in acceler
ated conditions.Ochiai analyzed the amount of carbon dioxide,C,by chemical analysis using hydrochloric acid and calculated C max by the product of calcium oxide content,which was analyzed by XRF.
2.3.This study
As we mentioned above,almost all the calcium must exist in tobermorite-11AÊand well-crystallized C-S-H.The maximum amount of carbon dioxide that combines with the calcium has not been evident,though Dapkus and Stankevisius[5]indicated an approximation of16%for it and Iwasaki and Tada[6]analyzed it by chemical analysis using hydrochloric acid for fully carbonated AAC in accelerated conditions.In addition,chemical compositions of AAC made all over the world are different becau of the variety of mix proportions. Conquently,it is preci and appropriate to define the carbonation degree of AAC by the molar ratio of calcium carbonate to the total amount of calcium oxide to com-pare with various rearch works.From a measurement point of view,it is better to be transformed to the weight/ molar ratio of the amount of carbon dioxide in calcium carbonate to the amount of carbon dioxide where the total amount of calcium oxide combine with,in other words, theoretical maximum amount of carbon dioxide.However, in order to limit the changes during carbonation,the amount of carbon dioxide in non-carbonated samples must be subtracted.
Therefore,we propo the carbonation degree(D c)of AAC(Eq.(3)),
D c % CÀC o a C maxÀC o Â100 3
where C,C o and C max are the amount of carbon dioxide in the sample,in the non-carbonated sample and the theoretical amount of carbon dioxide needed to combine with the total calcium oxide in the sample to form calcium carbonate, respectively.The total amount of calcium oxide has to be
F.Matsushita et al./Cement and Concrete Rearch30(2000)1741±1745 1742
analyzed by a reliable method such as ICP,while it is necessary to determine the measurement method of the amount of carbon dioxide.
3.Measurement method of carbonation degree of AAC 3.1.Samples
AAC blocks made by Sumitomo Metal Mining,Siporex Yokohama factory in Japan were ud for analys.They were shaped in 40Â80Â10mm and subjected to carbonation process under the conditions of 20°C,90%relative humidity and 3vol.%carbon dioxide concentration for 0(non-carbonated),10,20and 50days,respectively.Each sample was dried at 105°C for 2h before analys.3.2.Experiment
There are veral measurement methods to analyze the amount of carbon dioxide such as TG-DTA,IR spectrum during heating,chemical analysis using hydrochloric acid and so on.The analyzed values may be different from
each other.We examined the first three methods to find suitable one.
`TG-DTA'was carried out under the conditions of nitrogen gas flow,rising temperature rate of 20°C/min and the temperature range from 20°C to 1000°C.`IR spectrum during heating'was carried out as IR spectrum with quantitative analys of carbon dioxide gas decom-pod by heating at 1000°C.`Chemical analysis by hydro-chloric acid'was carried out by neutralizing of solutions containing carbon dioxide gas decompod chemically by hydrochloric acid.
The amount of calcium oxide was analyzed by ICP.Crystal phas were characterized by XRD.Thermal decomposition of calcium carbonate in the 50-day carbo-nated sample,before and after calcining at 600°C and 800°C for 2h was characterized by IR spectrum.3.3.Results and discussion
XRD patterns are shown in Fig.1.As the curing days incread,the changes of crystal phas due to carbona-tion were obrved where calcium carbonate (calcite
and
Fig.1.XRD patterns for samples cured for 0,10,20and 50
days.
Fig.2.TG-DTA curves for non-carbonated
sample.
Fig.3.TG-DTA curves for sample cured for 50
days.
Fig.4.IR spectrum for 50-day carbonated sample before and after calcining at 600°C and 800°C.
F .Matsushita et al./Cement and Concrete Rearch 30(2000)1741±17451743
vaterite)incread and tobermorite-11AÊdecread and finally disappeared.
TG-DTA curves for the non-carbonated sample and the sample cured for50days are shown in Figs.2and3, respectively.DTA curves show endothermic peaks at 700°C for the non-carbonated sample,790°C for the sample cured for50days and100±200°C and570°C
for each samples.The heat decomposition temperature of pure calcium carbonate crystal is898°C and that of pure magnesium carbonate crystal,which is another possible carbonate salt in AAC decompod at a temperature below1000°C,is540°C.IR spectrum for the50-day carbonated sample before and after calcining at600°C and800°C for2h is shown in Fig.4.IR peak at1430 cmÀ1indicating calcium carbonate[2]was obrved before and after calcining at600°C,however,it disap-peared after calcining at800°C.Accordingly,the endo-thermic peaks around700±780°C in DTA curves may be attributed to the heat decomposition of calcium carbonate. Therefore,we can determine the amount of carbon dioxide in calcium carbonate as the heating weight loss from600°C to800°C to exclude magnesium carbonate and carbon dioxide adsorbed on the surface and dissolved in the adsorbed w
ater.
The analyzed amounts of carbon dioxide by three meth-ods are shown in Table1.The amount of carbon dioxide determined by`IR spectrum during heating'and`chemical analysis'were bigger than that of`TG-DTA'becau the first two methods include all free carbon dioxide.The amount of carbon dioxide in magnesium carbonate may change during carbonation.Especially,the carbon dioxide adsorbed on the surface and dissolved in the adsorbed water may also change during carbonation becau microstructure and specific surface area of carbonated AAC are changed [6].Conquently,we propo the heating weight loss from 600°C to800°C by TG-DTA analys as the method to determine the amount of carbon dioxide for carbonation degree of AAC.
Table2shows the carbonation degrees of the samples with TG-DTA for the amount of carbon dioxide and ICP for the amount of calcium oxide.With the method we propod, it is possible to investigate carbonation of AAC more precily than others.
4.Application of carbonation degree
The carbonation degrees of brand-new AAC panels and in-rvice AAC panels ud for5±33years under nearly the same climate are shown in Fig. 5.The chemical compositions of them are shown in
Table3. The raw materials of AAC manufactured older than23 years were quartz sand,cement and slag,while that younger than23years were quartz sand,cement and lime.Although the chemical compositions were different, the carbonation degree can be successfully applied.The
Table1
The analyzed amount of carbon dioxide
Amount of carbon dioxide(wt.%)
Curing days TG-DTA IR spectrum
during heating
Chemical analysis
using hydrochloric acid
0(non-carbonated)0.91 2.64 1.78 10 4.937.70 6.98 209.3812.512.0 5011.815.815.9
Table2
假如生活欺骗了你普希金Carbonation degree by TG-DTA and ICP
Curing days Amount of
carbon dioxide
(wt.%)(TG-DTA)
Amount of
calcium oxide
(wt.%)(ICP)
Carbon
degree
(%)
0(non-carbonated)0.9124.90.0 10 4.93±21.6 209.38±45.4 5011.8±
58.4
Fig.5.Carbonation degree of in-rvice AAC panels.
Table3
The chemical compositions of brand-new and in-rvice AAC panels Chemical composition(wt.%)
ICP TG-DTA Years CaO SiO2Al2O3Fe2O3CO2 024.949.8 2.57 1.660.91 526.751.4 2.64 1.42 3.95 924.952.7 2.51 1.46 6.36 1328.847.0 2.76 1.527.53 1529.746.6 2.41 1.688.34 1724.850.7 2.21 1.288.86 2227.945.3 3.22 1.5712.08 2327.432.68.750.7612.21 2524.236.010.20 2.0510.49 2625.738.09.34 1.6712.09 3125.437.97.07 1.7112.07 3324.939.39.36 1.5611.50
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carbonation degree that we propod can work well for the various AAC.
5.Conclusions
We propo the carbonation degree(D c)of AAC, D c(%)=(CÀC o)/(C maxÀC o)Â100where C,C o and C max are the amount of carbon dioxide in the sample,in the non-carbonated sample and the theoretical amount of carbon dioxide needed to combine with the total calcium oxide in the sample to form calcium carbonate,respec-tively.In addition,we propo the heating weight loss from600°C to800°C in TG-DTA analys as the method to determine the amount of carbon dioxide for carbonati
on degree of AAC.With the method that we propod,it is possible to investigate carbonation of AAC more precily than others.References
[1]Z.Sauman,Effect of CO2on porous concrete,Cem Concr Res2(1972)
541±549.
[2]G.Sun,D.Tang,Y.Zhao,The carbonation of autoclaved aerated con-
crete and its hydration products,Proc Int Symp Cem Concr,Beijing,3, 1985,pp.331±342.
[3]K.Hanecka,O.Koronthalyova,P.Matiasovsky,The carbonation of
autoclaved aerated concrete,Cem Concr Res27(4)(1997)589±599.
[4]S.Asano,Y.Kamatani,Y.Inoue,On the carbonation of calcium silicate
compounds,Yogyo Kyokaishi79(9)(1971)303±311.
suggestion
[5]G.Dapkus,V.Stankevicius,Cellular concrete carbonation,Build Res
Pract13(3)(1985)184±187.
[6]M.Iwasaki,S.Tada,Carbonation of aerated concrete,Proc Int Symp
Cem Concr,Beijing,3,1985,pp.414±423.
腌咸鱼[7]T.Ochiai,Carbonation of autoclaved light-weight concretes,Gypsum
Lime242(1993)22±31.
F.Matsushita et al./Cement and Concrete Rearch30(2000)1741±17451745
