Damage Law and Life Prediction of CO2 Curing Biochar Recycled Concrete under Salt Freezing Environment

doi:10.16552/j.cnki.issn1001-1625.2025.1257

Abstract

Abstract:

To enhance the durability of recycled concrete in cold saline-alkaline regions， this study focused on C30 recycled concrete with five biochar incorporation levels （0%，2.0%，4.0%，6.0%， and 8.0%，mass fraction）. Two curing methods，standard curing and CO₂ curing，were applied. Sulfate freeze-thaw cycle tests were conducted to measure macroscopic indicators including mass loss rate， relative dynamic elastic modulus， and compressive strength loss rate. Microscopic pore structure analysis was performed using nuclear magnetic resonance （NMR） technology. A damage model based on the Weibull distribution was established to predict service life of concrete under salt freezing environment. The results show that biochar significantly improves the salt-freeze resistance of recycled concrete， with an optimal incorporation rate being 4.0%. After 300 freeze-thaw cycles under standard curing， the mass loss rate， relative dynamic elastic modulus， and compressive strength loss rate of the 4.0% biochar group improve by 46.8%， 58.0%， and 22.7%， respectively， compared with the reference group. CO₂curing further enhances the performance， reducing the mass loss rate and compressive strength loss rate by 36.8% and 33.1%， respectively， compared with the standard-cured group at the same biochar content. The porous adsorption characteristics of biochar， combined with the densification effect of CO₂ curing through carbonation， synergistically optimize the pore structure and reduce the proportion of harmful pores. The damage model based on the Weibull distribution exhibits a good fit and predicte that the service life of the optimal combination （4.0% biochar with CO₂ curing） in North China could reach 9 288 d， which is 175.4% longer than that of the reference group. The study confirms that the synergistic effect of biochar and CO₂ curing produces a “1+1>2” enhancement effect on the salt-freeze resistance of recycled concrete， significantly extending its service life in cold saline-alkaline environments.

Key words: biochar, recycled concrete, CO₂ curing, sulfate freeze-thaw, Weibull probability distribution

CLC Number:

TU528

WANG Mengyao, ZHAO Yuanzhuo, DU Wenfeng, HE Zhongying. Damage Law and Life Prediction of CO₂ Curing Biochar Recycled Concrete under Salt Freezing Environment[J]. BULLETIN OF THE CHINESE CERAMIC SOCIETY, 2026, 45(7): 2299-2311.

Figures/Tables 18

Table 1 Main chemical composition of cement

Chemical composition	CaO	SiO₂	Al₂O₃	Fe₂O₃	MgO
Mass fraction/%	62.5	21.3	5.8	3.1	2.2

Table 2 Basic physical properties of recycled aggregate

Type

Bulk density/（kg·m^-3）

Apparent

density/（kg·m^-3）

Water absorption/%

Crushing index/%

Flaky and elongated particle mass fraction/%

Mortar mass fraction/%

Water-soluble sulfate ion mass fraction/%

Table 3 Main chemical composition of biochar

Chemical composition	C	SiO₂	Al₂O₃	CaO	K₂O
Mass fraction/%	78.5	12.3	3.2	2.1	1.8

Table 4 Basic characteristic indexes of biochar

Type	Specific surface area/（m²·g^-1）	Porosity/%	Carbon mass fraction/%	pH value	Moisture mass fraction/%
Biochar	426.5	58.2	82.3	9.1	2.1

Fig.1 SEM images of biochar

Table 5 Mix proportion of concrete with different biochar content

Sample	Mix proportion/（kg·m^-3）
Sample	Cement	Biochar	Recycled aggregate	Sand	Water	Water reducing agent
OC	380.0	0	1 180	620	180	1.9
BRC-2.0%	372.4	7.6	1 180	620	180	1.9
BRC-4.0%	364.8	15.2	1 180	620	180	1.9
BRC-6.0%	357.2	22.8	1 180	620	180	1.9
BRC-8.0%	349.6	30.4	1 180	620	180	1.9

Fig.2 CO2 curing device

Fig.3 Carbonation depth test

Fig.4 Sulfate freeze-thaw cycle test

Fig.5 Carbonation depth of concrete

Fig.6 Effect of biochar on mass loss rate of concrete under different curing methods

Fig.7 Effect of biochar on relative dynamic elastic modulus of concrete under different curing methods

Fig.8 Effect of biochar on compressive strength loss rate of concrete under different curing methods

Fig.9 Variation of transverse relaxation time of biochar concrete under different curing methods

Fig.10 Variation law of pore integral area of biochar concrete under different curing methods

Fig.11 Variation of pore integral area of biochar concrete under different curing methods

Fig.12 Linear regression analysis

Table 4 Prediction of service life of concrete with different biochar content groups in salt freezing

Sample	Curing method	Northeast life/d	North life/d	Northwest life/d
OC	Ordinary curing	2 232	3 192	2 280
OC	CO₂ curing	2 364	3 372	2 412
BRC-4.0%	Ordinary curing	4 200	6 000	4 296
BRC-4.0%	CO₂ curing	6 480	9 288	6 612

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Damage Law and Life Prediction of CO₂ Curing Biochar Recycled Concrete under Salt Freezing Environment

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