Chemical activation of hybrid binders based on siliceous fly ash and Portland cement

The hydration of Portland cement (PC) blended with a high amount of a siliceous fly ash (70% fly ash, 30% PC) has been examined. The fly ash contributes significantly to the long-term strength development, when compared to a reference sample with quartz powder. However the long setting time and the poor early strength prevent the use of such binders. Therefore the effect of different activators (sodium carbonate, potassium sodium silicate, potassium citrate and sodium oxalate) on the setting, the hydration kinetics and the strength development of the fly ash-PC blend has been investigated.The addition of the activators increases the pH and decreases thus the calcium concentrations in the pore solution, which leads to a faster reaction of alite and thus to early setting and increased early strength. On the long term, the high alkali concentrations lower the compressive strength and lead to a (partial) destabilization of ettringite.Sodium oxalate and potassium sodium silicate accelerate both the setting of the fly ash-PC blend and increase the early compressive strength. Furthermore, they show better compressive strengths at later ages compared to the other activators. Based on these findings, they can be considered as the most suitable accelerators among the investigated activators.     

Influence of limestone and anhydrite on the hydration of Portland cements

The addition of CaCO₃ and CaSO₄ to Portland cement clinker influences the hydration and the strength development. An increase of the CaSO₄ content accelerates alite reaction during the first days and results in the formation of more ettringite, thus in a higher early compressive strength. The late compressive strength is decreased in Portland cements containing higher quantities of CaSO₄. The reduced late compressive strength seems to be related to an increase of the S/Si and Ca/Si content in the C–S–H.The presence of calcite leads to the formation of hemicarbonate and monocarbonate thus indirectly to more ettringite. Only a relatively small quantity of calcite reacts to form monocarbonate or hemicarbonate in Portland cement. Although hemicarbonate is thermodynamically less stable than monocarbonate, hemicarbonate formation is kinetically favored. Monocarbonate is present only after 1 week and longer independent of the quantity of calcite available and the content of sulphate in the cement.     

Influence of curing temperatures on the hydration of calcium aluminate cement/Portland cement/calcium sulfate blends

In this paper, the effects of curing temperature on the hydration of calcium aluminate cement (CAC) dominated ternary binders (studied CAC: Portland cement: calcium sulfate mass ratio were 22.5: 51.7: 25.8) were estimated at 0, 10, 20 and 40 °C, respectively. Both α-hemihydrate and natural anhydrite were employed as the main source of sulfate. The impacts of temperature on the phase assemblages, morphology and pore structure of pastes hydrated up to 3 days were determined by using X-ray diffraction (XRD), backscattered electron imaging (BEI) and mercury intrusion porosimetry (MIP). Results reveal that the main hydration products are firmly related to calcium sulphoaluminate based phases. Increasing temperature would result in a faster conversion from ettringite to plate-like monosulfate for both calcium sulfate doped systems. When the temperature increases to 40 °C, an extraordinary formation of strätlingite (C₂ASH₈) and aluminium hydroxide is observed in anhydrite doped pastes. Additionally, increased temperature exerts different effects on the pore structure, i.e. the critical pore diameter shifts to finer one for pastes prepared with α-hemihydrate, but changes to coarser one for those made with anhydrite. From the mechanical point of view, increased temperature accelerates the 1-day strength development prominently, while exerts marginal influence on the development of 3-day strength.     

A study on hydration,compressive strength,and porosity of Portland-limestone cement mixes containing SCMs

In this study, the hydration of Portland-limestone cement (PLC) pastes and the relationship between compressive strength and porosity of PLC mortar samples containing various levels of supplementary cementitious materials were examined using XRD and MIP techniques. The results revealed that part of the limestone portion of Portland-limestone cements reacts with the alumina phases and produces carboaluminates, which increases compressive strength and reduces porosity. There is an optimum level of limestone corresponding to the available amount of alumina in the binder. Addition of slag or metakaolin provided more alumina, causing more limestone to participate in the hydration reaction and increasing the optimum level of limestone.     

Effects of calcium nitrate and triisopropanolamine on the setting and strength evolution of Portland cement pastes

The purpose of this work is to study the effect of using some admixtures such as calcium nitrate and triisopropanolamine on the setting and hardening process of cement pastes at 20°C temperature. Tests were performed on specimens cast from various mixtures prepared with two types of cements. The results indicate that calcium nitrate alone acts as a setting accelerator, but has relatively little beneficial effect on the long term period development of mechanical resistances. Regardless of the cement type used, triisopropanolamine used alone performed well as a hardening accelerator at all ages. The combined addition of calcium nitrate and triisopropanolamine produced at very early age significant and promising results with respect to both setting and hardening acceleration. Continuous compressive strength increase was observed with time.     

Mitigating the effects of system resolution on computer simulation of Portland cement hydration

CEMHYD3D is an advanced, three-dimensional computer model for simulating the hydration processes of cement, in which the microstructure of the hydrating cement paste is represented by digitized particles in a cubic domain. However, the system resolution (which is determined by the voxel size) has a prominent influence on the simulation results and, thus, is difficult to choose a priori. In this paper, it is shown that the effects of system resolution on the simulation results are mainly due to the lack of considerations of the diffusion-controlled reactions in the model. A new concept “hydration layer” is proposed for mitigating the effects of system resolution on the model predictions. By performing simulations with different system resolutions, the robustness of the improved model is demonstrated. Comparisons of model predictions with experimental measurements further demonstrate that the use of hydration layer can successfully mitigate the bias brought by the system resolution.     

Influence of phosphorus from phosphogypsum on the initial hydration of Portland cement in the presence of superplasticizers

Phosphogypsum is widely used for the total or partial substitution of natural gypsum in the production of Portland cement. However, contaminants from the phosphogypsum, such as chemicals that contain phosphorus, may affect the performance of the binder, especially when it is applied to concrete that uses chemical admixture. The goal of this study is to evaluate the impact of successive increments of Na₂HPO₄ on the hydration of cements that are produced with low or high phosphorus concentrations in the presence of ether–polycarboxylate–based (PCE) and sulfonated–naphthalene–formaldehyde–based (SNF) superplasticizers. Two binders with the same clinker size were produced in the laboratory with natural (phosphorus–free) gypsum and phosphogypsum (contaminated). The isothermal calorimetry and thermogravimetry (TG/DTG) techniques were used to evaluate the heat flow behavior and the formation of portlandite at the end of the induction period (Pi), at the maximum heat flow (Pmax), and at the final 72-h stage (P72). The results indicate that the greatest impact on hydration occurs at phosphorus concentrations between 0.83% and 1.64% in the form of P₂O₅ in the phosphogypsum and especially at a concentration of 1.13%. Nonetheless, in all cases, the formation of portlandite after 72 h is very similar.     

Effects of different nanomaterials on hardening and performance of ultra-high strength concrete (UHSC)

Nanomaterials have attracted much interest in cement-based materials during the past decade. In this study, the effects of different nano-CaCO₃ and nano-SiO₂ contents on flowability, heat of hydration, mechanical properties, phase change, and pore structure of ultra-high strength concrete (UHSC) were investigated. The dosages of nano-CaCO₃ were 0, 1.6%, 3.2%, 4.8%, and 6.4%, by the mass of cementitious materials, while the dosages of nano-SiO₂ were 0, 0.5%, 1.0%, 1.5%, and 2%. The results indicated that both nano-CaCO₃ and nano-SiO₂ decreased the flowability and increased the heat of hydration with the increase of their contents. The optimal dosages to enhance compressive and flexural strengths were 1.6%–4.8% for the nano-CaCO₃ and 0.5%–1.5% for the nano-SiO₂. Although compressive and flexural strengths were comparable for the two nanomaterials after 28 d, their strength development tendencies with age were different. UHSC mixtures with nano-SiO₂ showed continuous and sharp increase in strength with age up to 7 d, while those with nano-CaCO₃ showed almost constant strength between 3 and 7 d, but sharp increase thereafter. Thermal gravimetry (TG) analysis demonstrated that the calcium hydroxide (CH) content in UHSC samples decreased significantly with the increase of nano-SiO₂ content, but remained almost constant for those with nano-CaCO₃. Mercury intrusion porosimetry (MIP) results showed that both porosity and critical pore size decreased with the increase of hydration time as well as the increase of nanoparticles content to an optimal threshold, beyond which porosity decreased. The difference between them was that nano-CaCO₃ mainly reacted with C₃A to form carboaluminates, while nano-SiO₂ reacted with Ca(OH)₂ to form CSH. Both nano-CaCO₃ and nano-SiO₂ demonstrated nucleation and filling effects and resulted in less porous and more homogeneous structure.     

Effects of defects on the tensile strength of short-fibre composite materials

Heterogeneous materials tend to fail at the weakest cross-section, where the presence of microstructural heterogeneities or defects controls the tensile strength. Short-fibre composites are an example of heterogeneous materials, where unwanted fibre agglomerates are likely to initiate tensile failure. In this study, the dimensions and orientation of fibre agglomerates have been analysed from three-dimensional images obtained by X-ray microtomography. The geometry of the specific agglomerate responsible for failure initiation has been identified and correlated with the strength. At the plane of fracture, a defect in the form of a large fibre agglomerate was almost inevitably found. These new experimental findings highlight a problem of some existing strength criteria, which are principally based on a rule of mixture of the strengths of constituent phases, and not on the weakest link. Only a weak correlation was found between stress concentration induced by the critical agglomerate and the strength. A strong correlation was however found between the stress intensity and the strength, which underlines the importance of the size of largest defects in formulation of improved failure criteria for short-fibre composites. The increased use of three-dimensional imaging will facilitate the quantification of dimensions of the critical flaws.     

On the relation of setting and early-age strength development to porosity and hydration in cement-based materials

Previous research has demonstrated a linear relationship between compressive strength (mortar cubes and concrete cylinders) and cumulative heat release normalized per unit volume of (mixing) water for a wide variety of cement-based mixtures at ages of 1 d and beyond. This paper utilizes concurrent ultrasonic reflection and calorimetry measurements to further explore this relationship from the time of specimen casting to 3 d. The ultrasonic measurements permit a continuous evaluation of thickening, setting, and strength development during this time period for comparison with the ongoing chemical reactions, as characterized by isothermal calorimetry measurements. Initially, the ultrasonic strength-heat release relation depends strongly on water-to-cement ratio, as well as admixture additions, with no universal behavior. Still, each individual strength-heat release curve is consistent with a percolation-based view of the cement setting process. However, beyond about 8 h for the systems investigated in the present study, the various strength-heat release curves merge towards a single relationship that broadly characterizes the development of strength as a function of heat released (fractional space filled), demonstrating that mortar and/or concrete strength at early ages can be effectively monitored using either ultrasonic or calorimetry measurements on small paste or mortar specimens.     

Synthesis of nanoparticles from slag and their enhancement effect on hydration properties of CaO/CaSO4-activated slag binder

Developing a low-cost and eco-friendly alternative to cement is of great significance for reducing the CO₂ emissions. CaO/CaSO₄-activated slag binder may only be served as a promising cementitious material when the severe defect in the early strength is overcame. In this study, gel-like nanoparticles with an average size of ～ 328 nm were prepared from the slag through dissolution at room temperature and reprecipitation at 50 °C. Subsequently, synthetic nanoparticles (SNPs) were added as a supplementary additive to enhance the strength of CaO/CaSO₄-activated slag binder. The effects of SNPs on the strength development, hydration kinetics, hydration products, and microstructure of the slag binders were investigated. The results indicated that the addition of moderate SNPs shortened the duration of induction period and improved the reaction rate in the acceleration period of the slag binders. As a result, large amounts of calcium aluminosilicate hydrate (C-A-S-H) gel was generated at early hydration ages. Meanwhile, SNPs increased the polymerization degree of this gel through the nucleation effect. Gel products’ well-filled the pore spaces between slag particles and yielded a compact microstructure, consequently enhancing the binder strength. The sample with adding 1.5 wt% SNPs exhibited the optimum strengths of 7.78 and 39.86 MPa after 1 and 28 days.     

Effects of the addition of nanosilica on the rheology,hydration and development of the compressive strength of cement mortars

This paper examines the rheology, hydration kinetics and development of the compressive strength of cement mortars including nanosilica and fly ash. The contents of these materials and the superplasticizer dosage are related to different rheological and strength parameters. Effects on rheology were analysed through yield stress and viscosity. Calorimetry tests were carried out to assess the variations in cement hydration kinetics, and the maximum and minimum heat release rates were analysed. Compressive strength was evaluated at different ages up to 56 days. The equations presented in this paper make it possible to optimize mortar proportionings that fulfil required performance levels in both fresh and hardened states.     

Effects of light crude oil contamination on the physical and mechanical properties of geopolymer cement mortar

Fly ash and oil contaminated sand are considered as the two waste materials that may affect environment. This paper investigated the suitability of producing geopolymer cement mortar using oil contaminated sand. A comparison between physical and mechanical properties of mortar produced using geopolymer and Ordinary Portland Cement (OPC), in terms of porosity, hydration and compressive strength, was conducted. The results showed that heat curing can increase the compressive strength of geopolymer mortar up to 54% compared to ambient curing situation. The geopolymer mortar with 1% of light crude oil contamination yielded a 20% higher compressive strength than OPC mortar containing sand with a saturated surface dry condition. Furthermore, the formation of efflorescence decreased as the level of oil contamination decreased. Moreover, the heat curing method increased the kinetic energy and degree of reaction for geopolymer cement mortar, which cause an increment of the density of the pore system and improving the mechanical properties of the resulting composites. From the results of this study, it was demonstrated that geopolymer mortar has the potential of utilizing oil contaminated sand, and reducing its environmental impacts.     

Ply angle effect on fiber composite wrapped reinforced concrete beam–column connections under combined axial and cyclic loads

This paper presents a numerical analysis to investigate the effect of ply angle on the improvement of shear capacity and ductility of beam–column connections strengthened with carbon fiber-reinforced polymer (CFRP) wraps under combined axial and cyclic loads. Three-dimensional nonlinear finite element models for the beam–column connections were developed and simulated with the Marc.Mentat™ 2001 finite element analysis (FEA) software. An experimental study on an FRP-wrapped beam–column connection, which was previously reported in the literature, was utilized to validate the accuracy of the proposed finite element models. The FEA study entailed profiling the behavior of three beam–column connections that were strengthened through the CFRP wrapping with various ply angle configurations. Analysis results indicated that four layers of wrapping placed successively at ±45° ply angles with respect to the horizontal axis is the most suitable upgrade scheme for improving shear capacity and ductility of beam–column connections under combined axial and cyclic loads.     

Effect of cement substitution by limestone on the hydration and microstructural development of ultra-high performance concrete (UHPC)

The effect of limestone on the hydration and microstructural development of ultra high performance concrete (UHPC) with different levels of replacement (34%, 54% and 74% by volume) was investigated. Up to 54% replacement of cement by limestone the mixes showed better workability and higher compressive strength (170 MPa at 56 days for 54% addition) compared to a classical mix (155 MPa) with no limestone replacement. The kinetics of hydration were compared for different replacement levels using isothermal calorimetry. The phase development was quantified by X-ray diffraction with Rietveld method combined with thermal gravimetric analysis. The pore structure was examined by mercury intrusion porosimetry. The composition of hydration products was determined by scanning electron microscopy with energy dispersive X-ray analysis. The results showed that the hydration degree of the cement is increased from 39% for classical UHPC to 66% for the UHPC with 54% of limestone.     

The influence of mechanical activation by vibro-milling on the early-age hydration and strength development of cement

This paper presents results from an experimental investigation that evaluated the mechanical activation of portland cement using vibro-milling. In this investigation, the duration of the vibro-milling was systematically varied and its influence was evaluated using mortar samples. In addition, the amount of activated cement used in the mortar samples was varied and evaluated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to evaluate differences in hydration products and the structure of activated cement and mortars. The activated cements were tested to determine the influence of activation on the rate of hydration and compressive strength development. The test results suggested that the use of mechanical activation can improve early-age structure formations and compressive strength. A 32% and 25% increase in 1-day strength were observed for the systems with Type I and Class H cements, respectively. This increase in 28-day strength was 16% and 58% for Type I and Class H cement, respectively. It was observed that longer milling times did not necessarily improve performance, and 15 min appeared to be sufficient vibro-milling time to provide valuable benefits.     

The effect of orientation on the shock response of a carbon fibre–epoxy composite

The effect of fibre orientation on the shock response of a two-dimensional carbon fibre–epoxy composite has been studied using the technique of plate impact. In the through-thickness orientation, it appears that the material behaves as though it is a simple polymer. When one of the fibre directions is orientated parallel to the loading axis, very different behaviour is observed. The stress pulse has a pronounced ramp, before at sufficiently high stresses, a much faster rising shock occurs above it. Examination of the wave velocities suggests that the start of the ramp travels at a near constant velocity of ca. 7.0 mm μs⁻¹, whilst the shock velocity in this orientation converges with that of the shock velocity of the through-thickness orientation. Therefore, we believe that the stress pulse is separated into a fast component that travels down the fibres, with the rest travelling at the shock velocity in the matrix between the 0° fibres (epoxy plus fibres normal to the loading axis). Finally, from the Hugoniot, we observed that at low shock intensities, the 0° orientation was significantly stiffer than the through-thickness orientation. As the severity of the shock increased, the Hugoniots of the two orientations converged. Therefore, it would appear that orientation only effects the shock equation of state at lower shock stresses.     

Comments on “Is the cause of size effect on structural strength fractal or energetic-statistical?” by Ba?ant & Yavari [Engng Fract Mech 2005;72:1-31]

Aim of this letter to the Editor is at replying to the criticisms raised by Ba?ant and Yavari [Ba?ant ZP, Yavari A. Is the cause of size effect on structural strength fractal or energetic - statistical? Engng Fract Mech 2005;72:1-31] against the fractal approach to the size-scale effects on the mechanical properties of materials and the concept of the Multi-Fractal Scaling Law presented by Carpinteri [Carpinteri A. Scaling laws and renormalization groups for strength and toughness of disordered materials. Int J Solids Struct 1994;31:291-302]. These criticisms will be analysed thoroughly, showing how they also contain some mistakes and misunderstandings. The presented elucidations should redirect the discussion to a more correct scientific debate.     

Geopolymers based on a coarse low-purity kaolin mineral: Mechanical strength as a function of the chemical composition and temperature

A coarse mineral with 70% kaolinite and 30% quartz was calcined and chemically activated by alkaline solutions of Na₂SiO₃ and NaOH. The compressive strength evolution was investigated as a function of the curing temperature at 20 and 80 °C, and the molar ratios of SiO₂/Al₂O₃ (2.64-4.04) and Na₂O/Al₂O₃ (0.62-1.54). For curing at 20 °C, the best composition was SiO₂/Al₂O₃ = 2.96 and NaO/Al₂O₃ = 0.62, reaching 85 MPa at 28 days. Curing at 80 °C had a positive effect on the strength development only in the first 3 days. X-ray diffraction of the geopolymeric formulations showed the formation of amorphous silicoaluminates of similar nature. The microstructure consisted of unreacted quartz and metakaolinite particles in a matrix of silicoaluminate polymer and condensed silica gel from the unreacted sodium silicate.     

Study of the effect of in-plane and interlaminar stresses on damping in fluid-filled laminated composite cylinders

The modal strain energy method is used in conjunction with a three-dimensional finite element analysis in the characterization of the effects of in-plane and interlaminar stresses in fluid-filled composite laminate cylindrical shells. A semi-analytical, 8-noded isoparametric finite element, which includes both the symmetric and antisymmetric modes in the circumferential direction, is used in the analysis. The effects of fiber angle, contained fluid height, size parameter of the shell, and stacking sequence on the contribution of in-plane and interlaminar stresses to the overall system damping in fluid-filled, composite laminate cylindrical shells are studied.