Alkali-activated materials(AAM) are known to be environmentally friendly alternatives to cement-based materials because they can potentially reduce greenhouse gas emissions and reutilize industrial by-products. However, the application of AAM is still limited by the lack of mixture design regulation. Unlike cement, the very different chemical composition of the precusors and alkaline activators may result in a very fluctuating strength. In order to study the factors influencing the strength of slag and fly ash-based alkali-activated materials (BFS/FAAAM), and clarify their reaction mechanism, this paper reviews current knowledge about the mechanical properties and the reaction mechanisms of BFS/FA-AAM. The control factors of strength are BFS/binder ratio, Na2O/binder ratio, and SiO2/Na2O ratio. The ion concentrations, determined by these control factors, play a decisive role in the development of strength. Generally, the strength is proportional to the BFS/binder ratio. The best strength could be obtained at the optimum values of Na2O/binder ratio, and SiO2/Na2O ratio. The optimum values of the SiO2/binder ratio of BFS-AAM and FA-AAM are between 5.5%-8% and between 7-10%, respectively. The optimal values of the SiO2/Na2O ratio of BFS-AAM and FA-AAM are between 0.85-1.4 and between 0.6-1, respectively. For BFS/FA-AAM, the optimum ratio is still unknown. Further study is needed to investigate the effect of control factors on the mechanical properties of BFS/FA-AAM.
Micromechanical interface properties in a concrete repair system determine the performance and reliability of a repaired structure. In order to characterize these properties, nanoindentation technique was applied. Three different repair material mixtures, based on Portland cement or partial replacement of Portland cement with blast furnace slag, were tested. Hardness and elastic modulus values obtained from the nanoindentation were used directly as input for simulated direct tension test. This way, the fracture behaviour of original microstructure, "mimicked" by Delft lattice model, is analysed. Backscattered electron (BSE) image analysis is utilized to estimate the average size of the interface zone which is distinguished as locally more porous area. This, together with simulation results, is further used for calculation of the interface stiffness. Simulation results enable prediction and better insight into fracture propagation and micromechanical response of the tested zone. They also indicate the ratio between interface and bulk material fracture properties when different types of repair materials are used. Load displacement diagrams of interface, old and new material can serve as an input for numerical modelling and understanding of fracture behaviour of the repair systems at the higher scale. Potentially, this approach may develop as a tool for verifying and engineering interface properties such that desired performance of the repaired structure is achieved.
It is well known that the interfacial transition zone (ITZ) of concrete between aggregate and bulk paste often acts as a negative effect with respect to overall properties like strength and durability. To reduce the negative influence of ITZ, it was usually considered to add various pozzolan or mineral additions into concrete, such as granulated blast furnace slag (GBFS). These additions could incur both physical packing and affect the chemical hydration reaction. In this paper, the physical packing effect in ternary blended cementitious system was studied in more detail. The maximum packing density can be calculated by employing linear packing model (LPM), based on the discretization of particle size distribution (PSD). With various mix proportions of raw materials, the overall distribution of packing density was displayed. In order to show how the initial ITZ property was influenced by packing density, numerical modeling by means of random sequential addition (RSA) has also been conducted.
This work presents an experimental investigation on the effect of Napier grass ash (NGA) on the early age autogenous shrinkage of NGA blended cement paste with low water/binder ratio. Further burnt process was conducted on the as received raw NGA to decrease the carbon content, and comparison study was performed. Comprehensive tests were performed on the sealed samples to evaluate the behavior of autogenous shrinkage and self-desiccation in NGA blended cement pastes with low water/binder ratio 0.25. The autogenous shrinkage was conducted according to the standard method ASTM C1698. Setting time was determined by the Vicat needle apparatus according to the standard method ASTM C191. Internal RH measurement was performed to reveal the self-desiccation process of the sealed cement pastes at very early age. Considering the relationship between the self-desiccation and autogenous shrinkage of cementitious materials, a new ‘time zero’ was proposed. The effect of replacement of cement by NGA on the autogenous shrinkage of low water/binder ratio cement pastes was revealed by comparison study between the autogenous shrinkage measured from final setting time as defined in standard method ASTM C1698 and the new ‘time zero’.
The development of the elastic properties of a hardening cement paste results from the microstructural evolution due to cement hydration. The elastic behaviour of cement paste can be predicted by a combination of the hydration model and micromechanical analysis, which originates from a microstructural representative volume where the elastic behaviour of different hydrating cement products can be recognised. In this paper, the formation of the microstructural volume is simulated with the computational code HYMOSTRUC3D for cement hydration. The obtained microstructure is an input for a micromechanical modelling. A 3D regular lattice model is proposed to predict the elastic modulus of the microstructure, considering a water-to-cement (w/c) ratio within the range [0.30–0.50]. In addition, the Finite Element Method (FEM) is used to compare and validate the results from the lattice model. Predictions from these two modelling approaches are then compared to the experimental results provided by the EMM-ARM (Elasticity Modulus Measurement through Ambient Response Method) testing technique, the latter allowing measurement of the elastic modulus of hydrating cement pastes. Finally, the above-referred numerical models are used to evaluate the influence of the following features: the particle size distribution of the cement grains, the microstructure discretisation refinement and the elastic modulus of the C-S-H cement hydration product.
Various Ce-Fe-Co alloys were prepared in DMF.Electroreduction behaviors of Ce(Ⅲ),Fe(Ⅱ) and Co(Ⅱ) on Cu electrodes were studied using electrochemical workstation.Their chemical composition,structure and property were analyzed by EDX,SEM,XRD and VSM analyses.The results demonstrated that(1) Ce(Ⅲ)was not reduced to deposit on the surface of electrode,but deposited by deposition of Co and Fe;(2) Deposition potential strongly affected chemical composition of alloys,but the effect of bath composition and deposition time was weak;(3) Magnetic property of alloys was increased by adding Ce;(4) The spherical crystal formed with the increase of annealing temperature,resulting in decrease of magnetic property.
Ultra-high performance concrete(UHPC) becomes one of promising concretes in the last decade due to its excellent per-formance such as ultra high strength,low permeability and very good durability.However,like high performance concrete,the UHPC is also subjected to high autogenous shrinkage due to a great amount of cement and silica fume used.The reduction of the autogenous shrinkage is an important task in the field of cement and concrete research.In this work,the rice husk ash(RHA) as an agriculture waste was used to mitigate the autogenous shrinkage of UHPC.The results show that RHA with a high content of amorphous SiO2 and the special porous structure.When 20% RHA with particle size of 5.6 μm was used,the shrinkage of UHPC was eliminated after curing for 15 d.RHA used in UHPC can reduce the cost,improve the early age properties of the UHPC and increase the environ-mental benefits because RHA has high content of atmorphous SiO2 and special structure.
The pressure to reach sustainability favours the development of ternary composite cement. The synergistic effect on mechanical behaviour at 28 days between limestone powder (LP) and pozzolanic additives, i.e. fly ash (FA) and blast furnace slag (BFS), has been documented. In order to better understand the synergistic effect, this article investigated the effect of LP on the microstructure of PC-FA and PC-BFScementitious system. The mineralogy and pore structure were determined after 28 days of curing at 20°C and 95% relative humidity. The mineralogy in pastes was identified by means of X-Ray diffraction (XRD) and thermogravimetry (TG). The pore structure was evaluated by Mercury intrusion porosimetry (MIP). The results showed that neither monosulfoaluminate nor ettringite was found in any of the XRD patterns. Instead, carboaluminate was observed. Hemicarboaluminate produced in FA-PC or BFS-PC system transformed into monocarboaluminate with the addition of LP. The porosity was enlarged compared with LP-free paste system. It seems that both the physical and chemical effect of LP contribute to the synergistic effect on mechanical behaviour of cementitious system hydrated up to 28 days.
