Zhidong Zhang

Last Name

Zhang

First Name

Zhidong

ORCID

0000-0003-1701-9181

Organisational unit

09593 - Angst, Ueli / Angst, Ueli

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Publications 1 - 10 of 85

Modelling Effect of Coarse Aggregates on Oxygen Transport and Corrosion Products Precipitation in Reinforced Concrete
Zhang, Zhidong; Angst, Ueli (2021)
4th International Rilem Conference on Microstructure Related Durability of Cementitious Composites: Microdurability 2020
The structure of concrete, in particular the microstructure of the steel-concrete interface (SCI), can significantly affect corrosion of rebars. To support corrosion, oxygen needs to diffuse from the ambient environment to the steel surface. Meanwhile, corrosion products transport through the SCI and precipitate in concrete. Transport properties of concrete play an important role in these two processes. However, the effect of the heterogeneous structure of concrete especially coarse aggregates on oxygen transport and corrosion products precipitation in reinforced concrete is rarely studied in the literature. This study employed a numerical model to investigate such effect. Simulation domains in 2D were created with different aggregate contents and sizes. The model included oxygen diffusion, ions diffusion and migration, corrosion products oxidation and precipitation. The simulation results showed that the presence of aggregates significantly reduces oxygen diffusion. It becomes more pronounced for high aggregate contents which lead to more corrosion products formed at the interface. Furthermore, results showed that the interfacial transition zone (ITZ) around aggregates can enhance oxygen diffusion.
Multi-scale structure of in-situ polymerized cementitious composites with improved flowability, strength, deformability and anti-permeability
Xu, Chengji; Dai, Yuqing; Peng, Yu; et al. (2022)
Composites Part B: Engineering
n-situ polymerization of polymers with cement can greatly improve the engineering performances of cementitious composites, the structural mechanisms, however, remain still not fully established. Herein in-situ polymerization modified cementitious composites (iPMCC) with sodium acrylate (SA) in different dosages (0%, 4% and 8%) were fabricated. Flowability, strength, deformability, and water sorptivity of the iPMCCs were tested, and the multi-scale structures in terms of pore size distribution, microscopic morphology, chemical characteristics, and solid structure of calcium-silicate (-aluminate)-hydrates were characterized. Results demonstrate that, compared with neat cement paste, the iPMCC with 4% SA showed the improvement of flowability by 35.1%, the increases of flexural and compressive strength by 109.0% and 11.8%, the rises of flexural and compressive deformation energy by 458.8% and 161.0%, and the decrease of water sorptivity by 19.1%. The in-situ SA polymerizations lead to substantial pore refinement, but macro defects in the iPMCC with 8% SA. Cross-linked sodium polyacrylate networks formed in the cement matrix and interacted with the cement hydrates, accounting for the improved strength, deformability, and water resistance. Cement hydration depression by polymers was evidently supported by the NMR data. The findings would deepen the understandings of multi-scale structure of cementitious composites with polymers, enabling development of highly ductile and durable construction materials.
Microstructure and moisture transport in carbonated cement-based materials incorporating cellulose nanofibrils
Zhang, Zhidong; Angst, Ueli (2022)
Cement and Concrete Research
Carbonation of cement-based materials may lead to durability problems, so that it is necessary to find a way to reduce the effect of carbonation on concrete durability. This study investigated the effects of cellulose nanofibrils (CNFs) on the durability of carbonated cement-based materials. Two dosages of CNFs were used to prepare cement pastes and mortars. Before and after accelerated carbonation (4 % CO2 concentration and 57 % relative humidity), their microstructure, mineralogical composition, moisture retention capacity, drying kinetics, and water absorption were measured. Results show that the contents of hydration products (CH, C-S-H, and ettringite) slightly decrease with the increase of CNFs dosage. After carbonation, the carbonation ratios of hydration products decrease with CNFs dosage so that more hydration products remain in the carbonated materials. The calcite content was found to increase with the dosage of CNFs which may help the transformation of other metastable calcium carbonates to calcite. The coarsening effect of carbonation on pore structure was clearly observed in measured pore size distribution for all materials, while our results show that this effect is weakened by CNFs. Moisture transport is clearly accelerated by carbonation, but the acceleration rate is diminished with the increasing dosage of CNFs, suggesting that CNFs are able to reduce the microstructural damage by carbonation.
Accelerating CaCO3 Precipitation of Recycled Concrete Aggregates through Enzyme Carbonic Anhydrase
Chen, Xiulin; Zhang, Zhidong; Giovanoli, Diego; et al. (2024)
1st RILEM International Conference on Mineral Carbonation for Cement and Concrete
Microstructure refinement of calcium-sulfate-aluminate and portland cement (CSA-PC) hybrids with accelerated CO₂ curing (ACC)
Lan, Yan; Dai, Yuqing; Zeng, Qiang; et al. (2024)
Journal of Materials Research and Technology
Calcium-sulfate-aluminate and Portland cement (CSA-PC) hybrids with balanced engineering properties would be a preferable solution to mitigate the increasingly raised CO2 emissions by PC. Meanwhile, CSA hydrates are rather active to carbonation, therefore, it is urgent to clarify how accelerated CO2 curing (ACC) impacts the microstructure and engineering properties of CSA-PC hybrids. Herein, an ACC scheme with the CO2 gas pressure of 0.5 MPa and duration of 24 h was designed to treat hybrid CSA-PC pastes and mortars. Mechanical properties and capillary absorption of the ACC-treated CSA-PC hybrids were evaluated with profound analysis of microstructure and mineral characteristics by SEM, MIP, Nitrogen adsorption, XRD and TG/DTG. Results show that the ACC treatment can systematically increase compressive strength by up to 65.2 % and depress capillary absorption rate by up to 72.2 %, respectively, for the optimal CSA-PC mixes. Rapid carbonation of the cement clinkers and hydrates, e.g., calcium hydroxide, ettringite, and ye'elimite, results in precipitation of CaCO3 that refines the pore structure and improves the material compactness. 25 % replacement of PC with CSA shows the highest strength gain to CO2 uptake ratio (SGCUR) of 1.05. The findings deepen the mechanistic understandings in microstructure refinement of ACC-treated CSA-PC hybrids with lower CO2 emissions.
A Dual-Permeability Approach to Study Anomalous Moisture Transport Properties of Cement-Based Materials
Zhang, Zhidong; Angst, Ueli (2020)
Transport in Porous Media
Anomalous moisture transport in cement-based materials is often reported in the literature, but the conventional single-porosity moisture transport models generally fail to provide accurate simulation results. Previous studies suggested that the anomalous moisture transport could be caused by different moisture transport velocity in large and small pores. Based on this concept, the present study proposes a continuous dual-permeability model for cement-based material. The proposed model includes the transport contribution of both liquid water and water vapor, which are governed by liquid advection and vapor diffusion, respectively. We explicitly consider that moisture transport in the large pore region is faster than the small pore region. The volumetric fraction of each region is determined when fitting the measured sorption isotherms by using a bimodal equation. The validation with experimental data shows that the dual-permeability model can well simulate both the “normal” and the anomalous moisture transport. The applicability of the proposed model implies that the “dual-porosity property” could be one of reasons that cause anomalous moisture transport in cementitious materials. In addition, results show that vapor diffusion can be neglected for moisture transport in both porosities at high relative humidity (RH), while at low RH, vapor diffusion must be considered.
Supercritical drying of cementitious materials
Zhang, Zhidong; Scherer, George W. (2017)
Cement and Concrete Research
Techniques to characterize the microstructure of hydrated cement require dried materials. However, the microstructure of hydrated products is significantly altered by high capillary forces during drying when using the conventional drying methods. To avoid drying stresses when preparing samples, we have employed supercritical drying (SCD) which has been used for decades to prepare aerogels that undergo no shrinkage during drying, but has rarely been used for cementitious materials. The pore solution is first replaced with isopropanol, and then with trifluoromethane (R23). The temperature and pressure are raised above the critical point, where no menisci or capillary pressure can exist; therefore, the dried samples are free of artifacts created by stresses. Images from scanning electron microscope show less compact morphology for supercritically dried samples than that dried by conventional methods, while BET surface areas of SCD samples are very close to samples dried by the isopropanol replacement method. This can be explained by the fact that isopropanol and supercritical fluid enter the micropores and block them. The nature of the chemical interactions of isopropanol and R23 with cement pastes are still not clear, but no reaction products were identified in the present study.
Rethinking concrete durability: Ensuring the longevity of low-carbon concretes through climate-informed corrosion modeling
Albert, Cristhiana; Schmid, Thilo; Zhang, Zhidong; et al. (2025)
Decarbonizing concrete is essential for meeting global climate targets. Many effective strategies to reduce greenhouse gas emissions result in lower concrete alkalinity, which has traditionally been viewed as a durability concern, namely promoting steel corrosion in concrete. This prevailing durability paradigm – firmly embedded in engineering mindsets and industry standards – hinders the practical implementation and thus the realization of the full potential of low-carbon concretes. To resolve this situation, we propose a conceptual framework that offers a perspective on ensuring the durability of reinforced concrete, while tolerating reduced concrete alkalinity. Our approach is inspired by empirical observations and recent scientific insights that consistently pinpointed the decisive role of moisture in concrete durability. We leverage advancements in moisture transport modeling in porous media and corrosion science to predict the time-evolution of the microclimate and corrosion kinetics at the steel-concrete interface. By drawing on time-resolved meteorological data from four examples in Europe, Asia and South America, we showcase the approach, underlining the critical role of local climate for predicting the durability performance of concrete. This work establishes the basis for developing test methods, standards, and predictive tools to promote eco-friendly concrete and evaluate how climate change impacts their durability for generations to come.
A framework for modelling corrosion-related degradation in reinforced concrete
Zhang, Zhidong; Angst, Ueli; Michel, Alexander (2018)
Life-Cycle of Civil Engineering Systems ~ Life Cycle Analysis and Assessment in Civil Engineering: Towards an Integrated Vision
Quantifying the anomalous water absorption behavior of cement mortar in view of its physical sensitivity to water
Ren, Fangzhou; Zhou, Chunsheng; Zeng, Qiang; et al. (2021)
Cement and Concrete Research
The one-dimensional water absorption into cement-based materials, possibly the simplest case of unsaturated water flow playing a fundamental role in predicting mass transport and durability, is still far from being completely understood due to several puzzling anomalies. Considering the water sensitivity of C-S-H gels (swell upon wetting and contract upon drying), a modified Richards equation (MRE) is proposed to quantify the anomalous water absorption into cement-based materials of changing porosity. Consistent experimental tests on a cement mortar and calculations indicate that the MRE with time-decaying water permeability can quantitatively capture the long-term absorption with two linear stages and a nonlinear transition stage between them. The water sensitivity is validated to be responsible for the anomalous deviation of water absorption from the classical square root of time law. It also partially contributes to the anomalous dependence of sorptivity on the temperature-dependent surface tension and viscosity of water and many organic liquids. © 2021 Elsevier Ltd.

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Zhidong Zhang

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