The industrial area produces lots of solid waste materials with CO2 emission. One of the most effective ways to solve these problems is the utilization of these waste materials. The production ...process of cements from its raw materials produces a lot of CO2. The most effective way to decrease CO2 emission of cement industry is the substitution of a proportion of cement with supplementary cementing materials. Cement blended with metakaolin (MK) is also required as a countermeasure to reduce the amount of CO2 generation. Metakaolin (MK), Al2Si2O7, is a highly amorphous dehydration product of kaolinite, Al2(OH)4Si2O5. The aim of our research was to investigate the effect of up to 20wt% substitutions of OPC by MK on the hydration characteristics of MK-blended cement pastes. The physico-chemical properties of the hardened cement pastes were studied up to 90days of hydration. The hydration products of some selected samples were investigated using XRD, DTA and DTG techniques. The results indicated that substitution of up to 20wt% OPC by MK as pozzolanic materials resulted in an increase in the standard water of consistency, acceleration of the initial setting times, high compressive strength values at earlier ages and improvement of the mechanical and durability properties.
In this paper, we present a new multi-physics computational framework that enables us to capture and investigate complex fracture behavior in cement-based materials at early-age. The present model ...consists of coupling the most important chemo-thermo-mechanical processes to describe temperature evolution, variation of hydration degree, and mechanical behavior. The changes of material properties are expressed as a function of the hydration degree, to capture the age effects. Fracture analysis of these processes is then accommodated by a versatile phase field model in the framework of smeared crack models, addressing the influence of cracks on hydration and thermal transfer. We additionally describe a stable and robust numerical algorithm, which aims to solve coupled problems by using a staggered scheme. The developed approach is applied to study the fracture phenomena for both homogeneous and heterogeneous concrete structures. Especially, in the second case, all microstructural heterogeneities of sand and cement matrix are explicitly accounted. Nucleation, initiation, and propagation of complex crack network are simulated in an efficient way demonstrating the potential of the proposed approach to assess the early-age defects in concrete structures and materials.
Carbon nanotubes (CNTs) and graphene oxide (GO) are the most studied carbon-based materials for the modification of cementitious materials. Prior studies have shown that CNTs (due to high bending ...stiffness) can improve flexural strength more efficiently than GO, whereas GO (due to active surface groups) performs better in improving the compressive strength of cementitious composites. This laboratory study investigates the role of unzipped CNTs (UCNTs) in cement pates. All three types of UCNTs have a similar C/O atomic ratio as GO, without any wrinkles to be observed under transmission electron microscope. The UNCTs admixed at 0.1% by weight of cement improved the compressive and flexural strengths of a cement paste (w/c of 0.38) by 22% and 51%, respectively, greatly outperforming CNTs and GO, respectively. Such outstanding reinforcement efficiency of UCNTs resulted from their abundant surface chemistry (similar to GO) as well as one-dimensionality and bending stiffness (similar to pristine CNTs).
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Supplementary cementitious materials (SCMs) are key components of sustainable, low carbon cements. To maximize their use in blended cements, the impact of SCMs on cement hydration needs to be ...understood and accurately captured by models. A central element in such models is the reactivity of the SCM, which is tedious to measure. Establishing relationships between SCM properties and their intrinsic reactivity is therefore highly important. Moreover, mechanisms enhancing or limiting SCM reactivity in blended cements need to be well-understood. This work reviews recent progress in the description and understanding of the reactivity of SCMs and their impact on Portland clinker hydration. Insights derived from fundamental work using synthetic SCMs, dissolution experiments and model systems are discussed as well as recent work studying the impact of common SCMs on hydration and microstructure of blended cements. Particular attention is paid to recent work on calcined clays, which are currently receiving substantial interest.
A series of β-cyclodextrin (β-CD) modified polycarboxylate (PCE) was prepared. β-CD was first grafted onto polyethylene oxide chain to produce β-CD-HPEG, and β-CD-HPEG was then adopted to replace ...part of HPEG to fabricate β-CD modified PCE. The adsorption-dispersing performance of PCEs and cement hydration properties were investigated. Results showed that the insertion of β-CD into the side chain led to impaired adsorption behavior of PCE polymer onto cement particle, but performed higher steric hindrance effect. The dispersing behavior of β-CD modified PCE varied with the insertion ratio of β-CD modified HPEG. β-CD modified PCE exhibited enhanced retardation effect on cement hydration, resulting in prolonged induction period, decreased hydrates at early age. But, due to the enhanced dispersing performance of β-CD modified PCE, the hydration degree was promoted at 28d, resulting in more hydrates formed in the matrix and denser microstructure.
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•Insertion of β-CD into the side chain led to decreased adsorption.•β-CD modified PCE exhibited enhanced retardation effect on cement hydration.•β-CD modified PCE led to enhanced hydration degree and denser microstructure.
Progress in understanding hydration mechanisms of alite and Portland cement is reviewed. Up to the end of the induction period, dissolution rates determined by the undersaturation of the solution ...dominate the reaction, but, better understanding is needed about the alite solution interface. The main heat evolution peak hydration is dominated by the growth of outer C-S-H with a spiky or “needle” like morphology. Growth is rapid over several hours (acceleration period) and then slows (deceleration period). At later ages the consumption of water and lack of water filled pores above about 10 nm, along with the consumption of anhydrous material are major factors leading to the continual reduction in the rate of reaction. There is no evidence that diffusion becomes the rate controlling mechanism even at this stage. The microstructure of cement differs significantly from that of alite, largely due to the influence of alumina on C-S-H growth and distribution.
A promising approach to accelerate cement hydration known as “seeding technology” has been discovered using nano‐particles to provide additional nucleation sites for growing of C‐S‐H. Two different ...types of polymer, polycarboxylate (PCE) and polysulfonate (PSE) were used as stabilizer to synthesize nano‐C‐S‐H via co‐precipitation process. The obtained C‐S‐H‐polymer composites were characterized by means of XRD, FTIR, thermogravimetric analysis (TGA), TEM, dynamic laser scattering (DLS), and BET. DLS measurement shows that the particle size of the obtained C‐S‐H‐polymer suspension ranges from 82.6 to 589.9 nm. The results of DLS and BET show that the particle size of the C‐S‐H particles synthesized using PCE polymer as stabilizer is smaller than those synthesized with PSE polymer, and hence the specific surface area is much higher. FTIR and TGA results confirm the presence of the polymers in the obtained C‐S‐H composites particles. XRD results indicate that the presence of the polymers reduces the crystallinity of C‐S‐H due to the absence of the d002 peak at 2θ of 7°. The calorimetry results show that the main hydration peak of cement is dramatically increased by the addition of the C‐S‐H‐polymer composites. It is interestingly found that the acceleration effect of the C‐S‐H‐polymer composites is linearly proportional to the total surface area of the nanoparticles introduced into the cement pastes. At the same time, it is found that the secondary hydration peak, usually known as the sulfate‐depletion peak, is greatly advanced by addition of the C‐S‐H nano‐particles in comparison with the blank cement paste. The acceleration effect of the nano‐C‐S‐H is further verified in a pure C3S system.
Municipal solid waste incineration fly ash (IFA) is theoretically more suitable for forming a composite cementitious system with cement and calcined clay compared to inert limestone due to its ...pozzolanic activity. In addition, IFA contains high levels of alkaline calcium compounds that have the potential for carbonation. The study examined the improvement of CaCO3 content within IFA facilitated by CO2-active (CIFA), replacing limestone in limestone calcined clay cement (LC3), and creating sustainable cementitious materials. The hydration behavior, heavy metal morphologies and mechanical properties of (C)FC3 were studied. The research indicates that introducing IFA adversely affects the initial hydration process of the system, but this inhibition is alleviated by using CIFA. This is facilitated by the reduction in sulfate dissolution rate attributed to the formation of carbonate shells. The incorporation of IFA into the system leads to the effective utilization of Ca(OH)2, thereby enhancing the pozzolanic reaction of the calcined clay. Consequently, (C)FC3 show higher compressive strength than LC3 at 56 d, with L15, F15 and CF15 achieving 29.9 MPa, 39.6 MPa and 46.8 MPa, respectively. While there is an increase in the exchangeable and acid-soluble states in CFC3, the elevation of oxidizable and residual states contributes to a reduced potential for total leaching in harsh environments. The excellent mechanical properties and environmental stability confirm the feasibility of using (C)IFA for LC3.
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•It is feasible to use IFA to replace limestone in LC3.•CO2-active treatment can promote the activity and utilization rate of IFA.•(C)IFA promotes the pozzolanic reactions of CC and strength of blends.•(C)FC3 is environmentally acceptable.
Digital fabrication with concrete is a field of rapidly developing building processes – including layered extrusion - where specific yield stress evolution over time is needed, owing to concrete ...loading during fabrication. By combining physico-chemical principles of concrete yield stress evolution with strength requirements imposed by the building process an avenue is devised for an effective development of these processes. Most notably, strength should grow linearly with time to ensure strength-based self-support, while it should growth with its third power to avoid self-weight induced buckling. It is argued that the needed evolution can be achieved by adequately controlling cement hydration during fabrication. Specifically, we outline the Set on Demand approach, relying on a combination of admixtures adequate for the process under study. This strategy is implemented with newly developed methods capable of measuring concrete yield stress evolution at rest, thus providing the link from chemistry to processing and mechanical stability.
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