Effects of calcium silicate hydrates/polycarboxylate on the rheological behaviour and hydration performance of cement paste

The tensile strength of cement paste is one of the most important mechanical properties that influence shrinkage cracks in cementitious materials. Cement pastes that exhibit low tensile strength tend to exhibit greater shrinkage crack potential and reduced durability. Increasing the tensile strength in cement paste can minimize the shrinkage cracking potential. It is believed that the strength and cohesion of cement paste are controlled by the formation of calcium silicate hydrate (C-S-H) gel. To enhance macroscopic mechanical properties (tensile strength), it is necessary to understand the structure and behavior of C-S-H gel at the atomic level. Previously, molecular statics was used to determine minimal potential energy and the mechanical properties of crystalline C-S-H structures. From this study, a plausible atomic structure of C-S-H gel is proposed. This research effort builds on the aforementioned work by using molecular dynamics to derive tensile and compressive strengths of C-S-H structures from uniaxial stress–strain data. The results from the molecular dynamics simulations showed that the maximum strengths (i.e., compressive and tensile) for the proposed C-S-H structures are three orders of magnitude higher than the strength at the macrolevel. However, the tensile strength of the proposed C-S-H gel is 23% of the compressive strength. This research also concludes that electrostatic forces and bond forces in the silicate chains are the main contributors to cement strength at the atomic level and that breakage in silicate chains leads to low tensile strength in C-S-H gel.

Formation of synthetic C-S-H in the presence of triethanolamine and/or polycarboxylate polymers

This study investigates the hydrothermal synthesis of calcium silicate hydrate (C-S-H) seeds using nano-size glass powder (NGP) derived from siliceous raw materials and CaO as the calcareous precursor. The influence of varying NGP concentrations on the particle size, microstructure, and phase composition of the C-S-H seeds was systematically analyzed. The synthesized C-S-H seeds were subsequently employed as additives to evaluate their effects on the early hydration process and compressive strength development of cement paste. The results indicate that increasing the NGP concentration from 0.2 mol/L to 0.8 mol/L leads to an exponential increase in the median particle size of the C-S-H seeds from 147 nm to 1328 nm and a corresponding morphological transformation from flocculent to granular structures. Additionally, the purity of the C-S-H sample decreased from 74.8% to 54.5% as the NGP concentration increased. The addition of C-S-H seeds can increase the 12h compressive strength of cement pastes by 15.5%–53.5%. This improvement is due to the accelerated early hydration kinetics induced by the C-S-H seeds, which act as nucleation sites, thereby increasing the nucleation and growth rates by 31.4% and 53.2%, respectively.

In order to improve the early age strength of ordinary Portland cement-based materials, many early strength agents were applied in different conditions. Different from previous research, the nano calcium silicate hydrate (C-S-H) particles used in this study were synthesized through the chemical reaction of CaO, SiO2, and H2O under 120 °C using the hydrothermal method, and the prepared nano C-S-H particles were highly crystalized. The influences of different amounts of nano C-S-H particles (0%, 0.5%, 1%, 2% and 3% by weight of cement) on the setting time, compressive strength, and hydration heat of cement paste were studied. The hydration products and microstructure of the cement paste with different additions of nano C-S-H particles were investigated through thermogravimetry-differential thermal analysis (TG-DTA), X-ray powder diffraction (XRD), and scanning electron microscope (SEM) tests. The results show that the nano C-S-H particles could be used as an early strength agent, and the early strength of cement paste can be increased by up to 43% through accelerating the hydration of tricalcium silicate (C3S). However, the addition of more than 2% nano C-S-H particles was unfavorable to the later strength development due to more space being left during the initial accelerated hydration process. It is suggested that the suitable content of the nano C-S-H particles is 0.5%−1% by weight of cement.

Effect of C-S-H-PCE and TEA on performances of lithium slag-cement binder

One of the essential factors which affects the performance of the concrete buildings under earthquake loads is the degree of brittleness of the concrete. It is possible to investigate the brittleness directly by dynamic experiments, whereas the static experiments which make it possible to get some numerical values. In this work, the second way is choosen and the brittleness of the concrete is expressed as the ratio between the compressive strength and the tension strength of the concrete (The brittleness index). The relationship between the brittleness, the grading of the aggregate mixture and the pore structure of the cement paste is investigated. The degree of sensitivity of the values determining the brittleness to the pore structure of the cement paste is also determined; then the factors determining the sensitivity correlation constants are investigated. The correlations between properties are strong because their sensitivities to the pore structure of the cement paste are close. All concrete properties are related to the inner structure of the cement paste because the properties of the concrete depend on the reactions of the hydration occurring in the cement paste. In the modern engineering, understanding the effect of the inner structure of the cement paste or how the materials of the concrete change the inner structure of the cement paste or the effects of the interface between the cement paste and the aggregates are important. From the past to the present, within the help of the developing technology, the inner structure of the cement paste is continuosly researched. The first researches of the inner structure of the cement paste are the crystallization theory of Le Chatelier and the Michealis' colladial theory (1). Powers and his friends proved that as a result of the cement hydration, the fibres like Labermant, form in the cement paste and these fibres are in colladial dimension and they have a great inner surface. Also, the structure of these fibres is gel. Besides this gel, in the cement paste, there are calcium hydroxide crystals, the secondary components, unhydrated cement particles and the pores. These pores are surrounded by the gel. Since the size of these pores is capillary, they are called as capillary pores (2). There are 4 main components of the cement; C2S, C3S, C3A and C4AF. After the hydration, these components turn into the hydrated components. The hydrated components are C-S-H (Calcium Silicate Hydrate) and CH (Calcium Silicate). C-S-H supply the binding property of the cement. The inner structure of the cement paste is porous. The pores form as a result of the hydration reactions because of the formed hydrated products. The capillary pores are formed after the hydration when used water evaporates. The capillary porosity depends on the water/cement ratio and the hydration degree. As the hydration degree increases, the more hydrated products are formed and they fill the pores so the numbers of the pores decrease. However, when the water/cement ratio increases, it is impossible that the hydrated products fill the pores because the amount of the water and the numbers of the pores increases, so the formed hydrated products are not enough to fill these pores. The gel pores are in the hydration products and their numbers and the total volume increase within the hydration process. The dimension of the gel pores is smaller than the dimension of the capillary pores. Besides these pores, there are air voids, the dimension of which is smaller. The air voids are formed because of the insufficient settlement of the concrete or some additives which drag air in the concrete. The interface between the cement paste and the aggregate is called as the weakest chain in the concrete. The inner structure of the interface between the cement paste and the aggregate differs from the inner structure of the cement paste. There are various models for the micro structure of the interface. According to Barnes;

The goal was to search for a replacement of CaCl2 which presents the most widely used accelerator for oil well cement used in cold and arctic environments and sometimes in deepwater drilling. For this purpose, novel calcium silicate hydrate (C-S-H) nanoparticles were synthesized and tested. The C-S-H was synthesized by the precipitation method in an aqueous solution of polycarboxylate (PCE) comb polymer which is widely used as concrete superplasticizer. The resulting C-S-H-PCE suspension was tested in the UCA instrument as seeding material to initiate the crystallization of cement and thus accelerate cement hydration as well as shorten the thickening time at low temperature. It was found that in PCE solution, C-S-H precipitates first as nano-sized droplets (Ø ~20 - 50 nm) exhibiting a PCE shell. Following a rare, non-classical nucleation mechanism, the globules convert slowly to nanofoils (HR TEM images: l ~ 50 nm, d ~ 5 nm) which present excellent seeding materials for the formation of C-S-H from the silicate phases C3S/C2S present in cement. Thickening time tests performed at + 4 °C in an atmospheric consistometer revealed stronger acceleration than from CaCl2 while very low slurry viscosity was maintained, as was evidenced from rheological measurements. Accelerated strength development was checked on UCA cured at + 4 °C and under pressure, especially the wait on cement time was significantly reduced. Furthermore, combinations of C-S-H-PCE and HEC as well as an ATBS-based sulfonated fluid loss polymer were tested. It was found that this C-S-H- based nanocomposite is fully compatible with these additives. The novel accelerator based on a C-S-H-PCE nanocomposite solves the problems generally associated with CaCl2, namely undesired viscosity increase, poor compatibility with other additives and corrosiveness against steel pipes and casing.

Since the introduction of cementitious materials over 200 years ago, its use has multiplied many fold to be currently at 3 billion tonnes per year, accounting for about 8% of worldwide carbon emission which is forecast to double within the next 30 years. Upon hydration of cementitious materials, the principle product formed is Calcium Silicate Hydrate (C-S-H) that binds all the aggregates together. Solutions to reduce this Carbon footprint depend on being able to improve the development of early age strength, which is controlled by the self-limiting growth of the C-S-H, particularly when using alternative supplementary cementitious materials (SCMs) to replace Portland cement. The greatest difficulty to overcome this limitation is the lack of knowledge on the growth and structure of C-S-H. It is poorly crystalline and cannot be characterised by classical crystal techniques. Moreover, in the Portland cement system, the effect of different elements on C-S-H growth is unclear. Furthermore, there is a lack of kinetic analysis on C-S-H growth.This thesis work presents a synthetic method to produce uniform C-S-H phases with Ca:Si ratios from 1 to 2, with a ratio above 1.6 being achievable for the first time in any given synthetic system. Higher ratios are of particular importance when compared to a real system, where it exists around 1.75 (Avg.). This was achieved with a novel rapid precipitation method by controlling mixing and reaction chemistry. These synthetic C-S-H precipitates are chemically uniform at the nanoscale and allowed us to determine a better defined atomistic structure with the use of dynamic nuclear polarisation (DNP) NMR techniques and atomistic simulations. The discovered structures reveal the inclusion of a calcium site in the interlayer that bridges chain terminating silicate species. This site is associated with an environment of strong hydrogen bonding that stabilises the structure and, consequently, promotes high Ca:Si ratios in C-S-H. The major result is thus a clear relation between the atomic level defect structure and the high Ca:Si ratio in C-S-H. Further, the C-S-H phases were analysed by Raman micro-spectrometry and Fourier Transform Infrared spectroscopy (FTIR) and revealed a proximity to a defective tobermorite structure and the probability of a single solid solution model for the Ca:Si range from 1.25 to 2. A morphological transformation point is also reported to be near to pH 11, where C-S-H solid phases clearly change from nanoglobules (spherical) to nanofoils (platelets).The developed understanding of the thermodynamic conditions and its control in the formation of desired precipitated synthetic C-S-H phases was applied to the formation of C-S-H in the Portland cement system formed upon the hydration of Alite (an impure Tri-Calcium silicate main phase in cementitious materials). We could predict the calorimetric behaviour of the system based on the knowledge of the formation mechanism of synthetic C-S-H. Also, it allowed us to forecast desirable conditions to get a particular phase or response of the hydrating system. Kinetic data were collected from the synthetic precipitation experiments by following the Ca consumption from solution. Then with help of thermodynamics speciation modelling and Population Balance Modelling (PBM) an empirical growth model for C-S-H was developed. Overall, all these results together have given a much clearer picture about C-S-H formation and of its atomistic structure.

Herein, the performance of graphene oxide (GO) in improving mechanical properties and subsequently reducing the permeability of cement composites used in concrete pavement, is studied. A polycarboxylate superplasticizer was used to improve the dispersion of GO flakes in the cement. The mechanical strength of graphene-cement nanocomposites containing 0.1–2 wt% GO and 0.5 wt% superplasticizer was measured and compared with that of cement prepared without GO. We found that the tensile strength of the cement mortar increased with GO content, reaching 1.5%, a 48% increase in tensile strength. Ultra high-resolution field emission scanning electron microscopy (FE-SEM) used to observe the fracture surface of samples containing 1.5 wt% GO indicated that the nano GO flakes were well dispersed in the matrix, and no aggregates were observed. FE-SEM observation also revealed good bonding between the GO surfaces and the surrounding cement matrix. In addition, XRD diffraction data showed growth of the calcium silicate hydrates (C-S-H) gels in GO cement mortar compared with the normal cement mortar. Growths of the calcium silicate hydrates (C-S-H) gels causes reduce in permeability and consequently improvement in durability of the cement composite.

Identification of viscoelastic C-S-H behavior in mature cement paste by FFT-based homogenization method

This paper presents the effect of nanosilica (NS) on compressive strength and microstructure of cement paste containing high volume slag and high volume slag-fly ash blend as partial replacement of ordinary Portland cement (OPC). Results show that high volume slag (HVS) cement paste containing 60% slag exhibited about 4% higher compressive strength than control cement paste, while the HVS cement paste containing 70% slag maintained the similar compressive strength to control cement paste. However, about 9% and 37% reduction in compressive strength in HVS cement pastes is observed due to use of 80% and 90% slag, respectively. The high volume slag-fly ash (HVSFA) cement pastes containing total slag and fly ash content of 60% exhibited about 5%-16% higher compressive strength than control cement paste. However, significant reduction in compressive strength is observed in higher slag-fly ash blends with increasing in fly ash contents. Results also show that the addition of 1-4% NS improves the compressive strength of HVS cement paste containing 70% slag by about 9-24%. However, at higher slag contents of 80% and 90% this improvement is even higher e.g. 11-29% and 17-41%, respectively. The NS addition also improves the compressive strength by about 1-59% and 5-21% in high volume slag-fly ash cement pastes containing 21% fly ash+49%slag and 24% fly ash+56%slag, respectively. The thermogravimetric analysis (TGA) results confirm the reduction of calcium hydroxide (CH) in HVS/HVSFA pastes containing NS indicating the formation of additional calcium silicate hydrate (CSH) gels in the system. By combining slag, fly ash and NS in high volumes e.g. 70-80%, the carbon footprint of cement paste is reduced by 66-76% while maintains the similar compressive strength of control cement paste. Keywords: high volume slag, nanosilica, compressive strength, TGA, high volume slag-fly ash blend, CO2 emission.

Characteristics of CSH under carbonation and its effects on the hydration and microstructure of cement paste

Dynamic mechanical thermoanalysis (DMTA) was conducted on compacted specimens of calcium silicate hydrates (C-S-H), 1.4 nm tobermorite, jennite, and compacted hydrated Portland cement paste powders, as well as hardened cement paste. The synthetic silicates are key elements for compositional models of the hydrated calcium silicates present in cement paste. The study focuses on the nanostructural effects due to the removal of water from the 11 % RH condition. The DMTA results (E′ and tan∂ versus temperature curves) in the 25–110 °C range mimicked those of DMA (E′ and tan∂ versus mass loss curves) conducted at room temperature for C-S-H and cement paste. In addition, the DMTA curves for 1.4 nm tobermorite and jennite in the temperature range 110–300 °C were sensitive to phase changes including the transition of 1.4 nm tobermorite to 1.1 nm tobermorite and other forms, as well as the transition of jennite to metajennite. The DMTA curves of a 50/50 mixture of 1.4 nm tobermorite and jennite exhibit similarities and differences to that of hydrated cement paste that are influenced by porosity and the amorphous nature of C-S-H in the cement paste. The study provides useful data for evaluating Taylor’s concept of a possible tobermorite-jennite model for the C-S-H present in hydrated cement paste.

Influences of rehydration conditions on the mechanical and atomic structural recovery characteristics of Portland cement paste exposed to elevated temperatures

Cesium-137 is a common radioactive byproduct found in nuclear spent fuel. Given its 30 year half life, its interactions with potential storage materials---such as cement paste---is of crucial importance. In this paper, simulations are used to establish the interaction of calcium silicate hydrates (C-S-H)---the main binding phase of cement paste---with Cs at the nano- and mesoscale. Different C-S-H compositions are explored, including a range of Ca/Si ratios from 1.0 to 2.0. These calculations are based on a set of 150 atomistic models, which qualitatively and quantitatively reproduce a number of experimentally measured features of C-S-H---within limits intrinsic to the approximations imposed by classical molecular dynamics and the steps followed when building the models. A procedure where hydrated ${\mathrm{Ca}}^{2+}$ ions are swapped for ${\mathrm{Cs}}^{1+}$ ions shows that Cs adsorption in the C-S-H interlayer is preferred to Cs adsorption at the nanopore surface when Cs concentrations are lower than 0.19 Mol/kg. Interlayer sorption decreases as the Ca/Si ratio increases. The activation relaxation technique nouveau is used to access timescales out of the reach of traditional molecular dynamics (MD). It indicates that characteristic diffusion time for ${\mathrm{Cs}}^{1+}$ in the C-S-H interlayer is on the order of a few hours. Cs uptake in the interlayer has little impact on the elastic response of C-S-H. It leads to swelling of the C-S-H grains, but mesoscale calculations that access length scales out of the range of MD indicate that this leads to practically negligible expansive pressures for Cs concentrations relevant to nuclear waste repositories.