Calculation and analysis of capillary tension and disjoining pressure in cementitious materials

Autogenous shrinkage in high-performance cement paste: An evaluation of basic mechanisms

The relationship between autogenous deformation and internal relative humidity (IRH) of high-strength concrete and high-strength expansive concrete were investigated. The experimental results indicate that, there exists a good linear relationship between autogenous shrinkage and IRH of high-strength concrete but a nonlinear relationship between autogenous deformation and IRH of high-strength expansive concrete with expansive agent. The new autogenous deformation curve can be obtained by transforming the autogenous deformation data of high-strength expansive concrete, and there exists linear relationship between the autogenous deformation and IRH. The concept of “critical internal relative humidity” was proposed, which is defined as the value of IRH when autogenous deformation is zero, to effectively reflect the autogenous deformation characteristic of expansive concrete.

Concrete is a brittle composite material that easily fractures under tension. Due to the fact that the early-age deformation of the concrete member is restrained by adjoining structures, cracking can occur throughout the concrete prior to application of any load. The cracks would provide preferential access for aggressive agents penetrating in the concrete and then cause corrosion of reinforcement and degradation of concrete. As a result, the service life of concrete structure would be decreased. There are many different types of early-age deformation of concrete, e.g. temperature induced strain, drying shrinkage and autogenous shrinkage. Among these types of early-age deformation, autogenous shrinkage is a consequence of the self-desiccation during the cement hydration process. For a long time autogenous shrinkage was considered negligible compared with drying shrinkage. In recent years, autogenous shrinkage has drawn more and more attention due to the increasing use of concretes with low water-binder ratios. Despite the fact that phenomenon of autogenous shrinkage has been recognized for several decades, the mechanism behind it is still not fully understood and no consensus has yet been reached. Three is a general agreement about the existence of a relationship between autogenous deformation and relative humidity change in the capillary pores of the hardening cement paste. Many simulation models were built based on this relationship to predict the development of autogenous shrinkage. The reliability of these predictions, however, is not always satisfactory. The discrepancy between the measured and calculated autogenous deformation becomes very pronounced at later ages. In those simulation models, cement paste was considered as an elastic material and only the elastic part of autogenous shrinkage was predicted. In fact, cement paste is not ideal elastic material. When a cement paste is subjected to a sustained load, it will deform elastically and continue to deform further with time, which process is known as creep. Creep plays an important role in autogenous shrinkage of hydrating cement paste. The ignorance of creep would lead to an underestimation of the autogenous shrinkage. The aim of this project is to study the autogenous shrinkage of Portland cement pastes and blended pastes with supplementary materials. The autogenous shrinkage is supposed to consist of two parts, elastic part and time-dependent part (creep), which are simulated separately. Based on the autogenous shrinkage of cement pastes, autogenous shrinkage of cement mortars and concretes were simulated by taking the restraining effect of rigid sand/aggregate particles into consideration.

This paper investigates the influence of temperature on autogenous deformation and early-age stress (EAS) evolution in ordinary Portland cement paste using a recently developed Mini Temperature Stress Testing Machine (Mini-TSTM) and Mini Autogenous Deformation Testing Machine (Mini-ADTM). In the Mini-TSTM/ ADTM, CEM I 42.5 N paste with a water-cement ratio of 0.30 was tested under a curing temperature of 10, 15, 20, 25, 30, and 40 °C. X-Ray diffraction (XRD) tests were conducted to measure the amount of ettringite and calcium hydroxide, which reveals the micro-scale mechanisms of autogenous expansion. The applicability of the Maturity Concept (MC) for the prediction of autogenous deformation and relaxation modulus under different temperatures was also examined by the experimental data and the viscoelastic model. This paper leads to the following findings: 1) The autogenous deformation of ordinary Portland cement paste is a four-stage process comprising the initial shrinkage, autogenous expansion, plateau, and autogenous shrinkage; 2) Higher temperature leads to higher early-age cracking (EAC) risk because it accelerates the transitions through the first three stages and causes the autogenous shrinkage stage to start earlier. Moreover, higher temperatures also result in increased rates of autogenous shrinkage and EAS in the autogenous shrinkage stage; 3) Autogenous expansion and plateau are attributed to the crystallization pressure induced by CH. Temperature-dependent CH formation rates determine the duration of the plateau stage; 4) Low-temperature curing can delay but not completely prevent the EAC induced by autogenous deformation; 5) The MC cannot predict the autogenous deformation at different temperatures but can be used to calculate the relaxation modulus, which in turn aids in EAS prediction based on autogenous deformation data.

Various strategies have been developed to mitigate the autogenous shrinkage of cement-based materials (CBMs), including internal curing and shrinkage compensation. However, the sole use of internal curing or shrinkage compensation is insufficient in many cases. In this study, magnesia-based expansive additives (MEAs) coupled with a superabsorbent polymer (SAP) were investigated as internal curing agents to mitigate the autogenous shrinkage of cement pastes. The internal relative humidity (RH), autogenous deformation and compressive strength of cement pastes containing two different types of MEAs and SAP were examined. The results showed that, with the incorporation of SAP, the RH in the cement paste was maintained at a higher level than that in cement pastes without SAP, and hence autogenous shrinkage was reduced. The combination of SAP and MEA provided very effective compensation for the autogenous shrinkage of cement paste. Moreover, the cement paste containing 8 wt% MEA and 0.2 wt% SAP produced a gentle expansion of 111 microstrain rather than shrinkage. This was attributed to the increased hydration degree of magnesia owing to the additional water supply provided by the SAP. This study provides a novel and efficient way to mitigate the autogenous shrinkage of CBMs.

Introduction: This study focuses on autogenous shrinkage in cement pastes and presents a novel calculation method considering variations in internal relative humidity (IRH). IRH significantly influences autogenous shrinkage, and its evolution is modeled based on decline curves. The proposed method accurately evaluates autogenous shrinkage and aligns well with experimental data. Additionally, we calculate capillary depression and meniscus radius using the Laplace–Kelvin equation. Methods: To address early autogenous shrinkage in construction materials, we developed our calculation method, emphasizing IRH variation. We analyzed decline curves to model IRH and validated our model using literature-based experimental data. Results: Our validated model for predicting IRH and autogenous shrinkage in Portland cement pastes, based on cement paste hydration degree, water-to-cement ratio (w/c), and the critical degree of hydration (αcr), closely aligns with experimental data and existing models.

Recent experiments and models for the spreading of liquids laden with nanoparticles have demonstrated particle layering at the three-phase contact line; this is associated with the structural component of the disjoining pressure. Effects driven by structural disjoining pressures occur on scales longer than the diameter of a particle, below which other disjoining pressure components such as van der Waals and electrostatic forces are dominant. Motivated by these experimental observations, we investigate the dynamic spreading of a droplet laden with nanoparticles in the presence of structural disjoining pressure effects. We use lubrication theory to derive evolution equations for the interfacial location and the concentration of particles. These equations account for the presence of the structural component of the disjoining pressure for film thicknesses exceeding the diameter of a nanoparticle; below such thicknesses, van der Waals forces are assumed to be operative. The resulting evolution equations, for the particle motion and free surface position, are solved allowing for the viscosity to vary as a function of nanoparticle concentration. The results of our numerical simulations demonstrate qualitative agreement with experimental observations of a "step" emerging from the contact line. The results are also relevant to a wide range of other phenomena involving layering, or terraced spreading of nanodroplets, or stepwise thinning of micellar thin films.

Shrinkage-reducing admixtures and early-age desiccation in cement pastes and mortars

Does enhanced hydration have impact on autogenous deformation of internally cued mortar?

Autogenous deformations of cement pastes: Part II. W/C effects, micro–macro correlations, and threshold values

Characterisation of inter-particle forces within agglomerated metallurgical powders

van der Waals forces, electrostatic interactions, and capillary forces are the dominant force interactions at the micro- and nanoscale. This complex ensemble of surface forces is oftentimes summarized as adhesion and is important for various applications and research fields. So far, numerous measurement techniques have evolved in this field. However, there is still a lack of experimental insight into the complex interplay of van der Waals, electrostatic, and capillary forces for small force ranges below 10 nN, as this is the order of magnitude of the latter, which can shadow other interactions in ambient and even inert gas environments. To exclude capillary forces and thus to turn the van der Waals and electrostatic forces into the most significant interactions, we develop an interferometric force spectroscopy setup based on a scanning probe technique, featuring a sub-nanonewton resolution, and integrate it into the vacuum chamber of a scanning electron microscope. In this work, we describe the setup integration, show the long-term drift behavior and resolution capabilities, and conduct first measurements of adhesion energies between a silica colloidal probe and a silicon substrate. The presented setup shows its capability to reliably measure adhesive interactions in vacuum and an ambient environment with a sub-nanonewton resolution proving its potential to allow for the investigation of the separate contribution of capillary, van der Waals, and electrostatic interactions to adhesion and for a systematic experimental validation of the established adhesion theories and approximations on the micro- and nanoscale.

Temperature Stress Testing Machine (TSTM) is a universal testing tool for many properties relevant to early-age cracking of cementitious materials. However, the complexity of TSTMs require heavy lab work and thus hinders a more thorough parametric study on a range of cementitious materials. This study presents the development and validation of a Mini-TSTM for efficiently testing the autogenous deformation (AD), viscoelastic properties, and their combined results, the early-age stress (EAS). The setup was validated through systematic tests of EAS, AD, elastic modulus, and creep. Besides, the heating/cooling capability of the setup was examined by tests of coefficient of thermal expansion by temperature cycles. The results of EAS correspond well to that of AD, which qualitatively validates the developed setup. To quantitatively validate the setup, a classical viscoelastic model was built, based on the scenario of a 1-D uniaxial restraint test, to predict the EAS results with the tested AD, elastic modulus, and creep of the same cementitious material as the input. The predicted EAS matched the testing results of Mini-TSTM with good accuracy in 6 different cases. The viscoelastic model also provided quantitative explanations for why variations in early AD do not influence the EAS results. The testing and modelling results together validate the developed Mini-TSTM setup as an efficient tool for studying early-age cracking of cementitious materials. At the end, the potential limitations of the Mini-TSTM are discussed and its applicability for concrete with aggregate size up to 22 mm is demonstrated.

Interpreting micromechanics of fluid-shale interactions with geochemical modelling and disjoining pressure: Implications for calcite-rich and quartz-rich shales

High-performance cementitious materials are sensitive to early age cracking, mainly due to the large magnitude of autogenous shrinkage, which is closely related to the internal relative humidity (RH) decrease and capillary pressure induced by self-desiccation in the cement matrix. However, there is debate about the determination of time-zero, the time at which autogenous shrinkage begins to develop, which causes great difficulty in comparing the results provided in the exiting researches. This study presents an accurate determination of time-zero based on the relationship between the internal RH and autogenous shrinkage of cementitious materials. According to the time-zero, the effect of replacements of cement by supplementary cementitious materials on the autogenous shrinkage was investigated for the early age cement pastes with low water/binder ratio. The autogenous shrinkage was conducted according to the standard method ASTM C1698. Internal RH was performed on the sealed cement pastes at very early age by conventional method of hygrometer. Setting time was determined by the Vicat needle apparatus according to the standard method ASTM C191. The results could potentially explain the mechanism of autogenous shrinkage at early age in mixtures with supplementary cementitious materials.