Hardened Cement Paste Research Articles

Influence of CSH grown in situ on steel slag powder on the performance of fresh and hardened cement pastes

In this paper, for improving the enhancement efficiency of synthetic CSH to cement pastes, it was grown in situ on steel slag powder by a chemical co-precipitation reaction. Compared with common CSH, in situ-grown CSH had less negative influence on the workability of fresh cement pastes. More importantly, it also had a more prominent accelerating effect to cement hydration, which raised the hydration rate and increased the chemically bonded water content. Therefore, it promoted the strength development of cement pastes more efficiently and heightened the early and long-term strength of hardened cement pastes more distinctly. Besides, it refined the pores in the hardened cement pastes, decreasing the porosity more remarkably. Consequently, it can be believed that in situ-grown CSH has more outstanding seeding and filling effects due to its improved dispersibility, so as to enhance the performance of cement pastes more efficiently.

Real-time monitoring the electrical properties of pastes to map the hydration induced microstructure change in cement-based materials

The effect of the supplementary materials (SCMs) on the moisture content and ion diffusivity at different hydration time is important for the service life modelling of modern concrete. This study designed a simple but valid method to monitor the microstructure change in pastes during hydration. A procedure easy to implement was proposed to detect the water content in pastes. The electrical conductivity of pore solution was evaluated by the evaporable water content in pastes and chemical composition in the binders. Results show that the electrical properties of pastes (conductivity, formation factor and its growth rate) can effectively indicate the hydration reactivity of binder, pore connectivity and volume of pore solution in the hardened pastes. The effect of water-binder ratio and SCMs on the structure of pastes are effectively indexed by the formation factor which is the conductivity of pore solution divided by that of paste. The inflection point of average growth rate of formation factor is a good index for the final setting of pastes. The relation between volume of evaporable water and formation factor is well demonstrated by the extended percolation theory. The real-time monitored electrical conductivity and formation factor of pastes can be used to calculate the chloride migration coefficient in hardened cement pastes.

MODIFICATION OF CEMENT-CONCRETE MIXTURES WITH POLYMER ADDITIVES, STRUCTURED CARBON NANOTUBES

The use of complex modifiers for cement-concrete mixtures and concretes is becoming increasingly popular in modern materials science. The paper presents studies of the effect of a polymer additive structured with carbon nanomaterial on the physical and mechanical characteristics of cement-concrete mixtures. IR spectroscopy and thermogravimetry revealed that the use of carbon nanomaterial significantly changes the structure of cement-concrete mixtures. As a result of the fact that high-strength nanomaterial is the center of crystallization of cement stone formations, a denser reinforced microstructure is formed, which significantly increases the strength characteristics of cement-concrete mixtures. Inclusion in the composition of cement-concrete mixtures of polymer complex additives leads to higher and longer plasticization, which plays an important role in the production of monolithic products. It is established that in the presence of a complex modifier (polymer additive structured with carbon nanotubes) the crystal structure of calcium hydrosilicates is compacted, which causes high physical and mechanical characteristics of modified cement-concrete mixtures. It is experimentally shown that the additive acts as an accelerator of hardening and hardening of cement paste, as well as increases its strength characteristics. In general, for all cement-concrete mixtures in this study there is a water-reducing effect of the additive. Water consumption decreases by 5 wt. %, while the strength increases by 19%. Formulations of cement-concrete mixtures modified with polymer additives, structured carbon nanotubes, with high performance characteristics have been developed.

Synergic effect between two pozzolans: Clinoptilolite and silica gel by-product in a ternary blend of a Portland cement system

Two types of SCMs such as clinoptilolite and silicagel by-product was incorporated in the OPC system and acted as pozzolanic materials. The individual effect of silicagel by-product led to the significant reduction of compressive strength due to soluble barrier layers made of CaF2 formed on the particles of OPC. In a ternary blend of a Portland cement system the synergetic effect of clinoptilolite and silicagel by-product led to the similar strength properties of hardened cement paste. Clinoptilolite acted as sorbent which sorb acidic fluoride ion impurities of silicagel by-product. This paper demonstrates the potential application of silicagel by-product and clinoptilolite as SCMs in the production of eco-friendly blended cements.

The effect of pore microstructure on strength and chloride ingress in blended cement based on low kaolin clay

The objective of this study is to analyse the mechanical and durability properties of a high-performance mortars based on low grade kaolin clay and the effect of electrical conductivity and pore microstructure on the chloride penetration resistance. A total of six mixes were prepared with binary and ternary binders in which a high volume of cement was replaced by low-grade calcined kaolin clay, fly ash and limestone powder. The percentage of kaolinite in the clay used from the Southeast European region was less than 20%. The variations in compressive strength and chloride transport coefficient were analysed experimentally and related to the pore structure and electrical conductivity. Compressive strength, chloride migration and diffusion coefficients, and bulk conductivity were determined using mortar samples, while the pore size distribution was determined by mercury intrusion porosimetry (MIP) on hardened cement paste after 7,28, and 90 days of curing. To understand the capillary absorption of these mixtures, sorptivity indices were measured after 28 and 90 days. The experimental characteristics of the pore size distribution, such as mean pore entry radius (r0.5), permeable porosity (P), capillary porosity (ɸ), and critical pore diameter (rc), were calculated using the cumulative data of the intruded volume as a function of pressure obtained by MIP. In addition, the extrusion-intrusion curve was used to calculate the pore entrapment fraction (α) and the degree of inter-connectivity of the pore structure. The evolution of pore structure parameters was monitored for up to 90 days and their effects on strength and chloride penetration were studied in detail. The results showed that all mixes (even those with low kaolinite content) can be used in high performance cement systems and the pore structure has a limited effect on chloride penetration.

Performance of cement-based materials containing calcined coal gangue with different calcination regimes

The storage of coal gangue (CG) takes up a lot of lands and pollutes the environment. Calcined CG has the potential to be used as supplementary cementitious material. In this paper, the CG from Yibin, Sichuan province of China were treated with different calcined temperature, holding times and cooling modes, and the microtopography, phase composition and pozzolanic reactivity of calcined CG were studied. Meanwhile, the fluidity, mechanical properties, hydration products, and pore structure of cement paste and mortar with different calcined CG were analyzed. The apparent color of coal gangue under different treatment methods has also been studied to promote its better application for a wider range in cement-based materials. Results showed that there exists an optimal calcination condition of calcined CG at 800 °C and holding time of 2 h. Rapid cooling and slow cooling reduced the activity of calcined CG. Calcined CG powder would reduce the fluidity of cement mortar. However, proper calcination temperature and sufficient holding time could eliminate it to some extent. It was also found that calcination temperature has a greater impact on the early age reactivity of CG when compared to different temperature treated CG powder, C8T2 sample has the best early activity. Compared to rapid cooling, slow cooling and water-cooling mode, room temperature cooling would result in higher reactive CG. CG powder after proper thermal treatment could refine the pore structure of the hardened cement paste in the later age (28 days), reduce the harmful pores content (>100 nm), and increase harmless pores content (<100 nm) in hardened cement paste. It was found that the CG powder of calcined at 800 °C for 2 h, cooling in water and room temperature might be more suitable as the supplementary cementing material combined the apparent color, pozzolanic reactivity, and preparation process.

Use of Natural Zeolite and Glass Powder Mixture as Partial Replacement of Portland Cement: The Effect on Hydration, Properties and Porosity.

The study investigates effect of the additive consisting of natural zeolite (clinoptilolite) and soda lime glass powder on the hydration, mechanical properties and porosity of Portland cement concrete. The effect of mineral additive on the technological, physical-mechanical properties and porosity of the mortar was investigated by increasing the content of natural zeolite and glass powder added to the mortar up to 20% by weight of cement in increments of 5% and different particles size of natural zeolite. The mixes with the best technological and mechanical properties were identified and further studies were conducted by replacing 10% and 15% of cement with natural zeolite and soda lime glass with an average grain size of 59.3 μm, 29.0 μm or 3.6 μm of zeolite, and 29.6 μm of glass powder. The hydration process and microstructure of hardened cement paste modified with the aforementioned mineral additives was analysed by microcalorimetry, X-ray diffraction tests and thermogravimetric analysis. The optimal composition of cement paste and particle size distribution of natural zeolite were determined to achieve the higher flexural and compressive strength and lower open porosity. The mixture of mineral additives has the highest effect in terms of flexural and compressive strength and open porosity when added at the proportion 75:15:10 (cement:natural zeolite:soda lime glass) and when zeolite with an average particle size of about 3.6 μm is used

Reusing recycled powder as eco-friendly binder for sustainable GGBS-based geopolymer considering the effects of recycled powder type and replacement rate

Utilizing recycled powder (RP) as binder for geopolymer reduces the construction waste and contributes to the development of sustainable geopolymer materials, but the effects of RP type and replacing rate on the characterization of sustainable geopolymer with RP received less consideration in previous investigations. Therefore, this investigation showed the upcycling of various RPs as an eco-friendly binder for sustainable GGBS-based geopolymer, and the micro-characteristics, mechanical strength and transport performance of geopolymer containing RP up to 100% were measured. The results indicated that RP from various construction waste mainly consisted of hydrated products, un-hydrated cement particle, quartz and calcite. Incorporating high-volume RP enlarged the pore structure of GGBS-based geopolymer, and substituting RP for 100% GGBS resulted in relatively loose micro-structure of sustainable geopolymer. Incorporating RP decreased the autogenous shrinkage of GGBS-based geopolymer, but the mix of high-volume RP increased the drying shrinkage of GGBS-based geopolymer. The mechanical strength and resistance to water ingress of GGBS-based geopolymer decreased as the RP replacing rate was increased, and substituting RP for 100% GGBS led to a significant reduction of mechanical strength and water absorption resistance. However, the mechanical strength of geopolymer with 100% RP from hardened cement paste waste was much better relative to geopolymer with 100% RP from mortar waste or concrete waste, because hydrated cement particle in RP has relatively high polymerization activity. Optimizing RP type and replacing rate can prepare sustainable geopolymer with good performance, and the upcycling of 100% RP for fully recycled geopolymer is feasible and sustainable.

Degradation of concrete in marine environment under coupled chloride and sulfate attack: A numerical and experimental study

The corrosion of offshore reinforced concrete structures under combined chloride and sulfate ions attack is a complex corrosion phenomenon. The purpose of this study is to develop numerical models in conjunction with experimental studies for long-term durability assessment of offshore concrete structures. A numerical model was developed to describe the simultaneously transport of chloride and sulfate ions considering the competitive binding by hardened cement pastes effect. The model was validated by a series of experimental studies. The validated model was implemented to predict the service life of the offshore RC structures under various chloride and sulfate solutions. The results show that chloride ions induced corrosion of steel bars in offshore RC structures is highly influenced by the concentration of sulfate ions. The sulfate ions induced concrete expansion and cracking from ettringite formation could potentially accelerate chloride ions induced corrosion of steel bars in concrete, ultimately the premature failure of the offshore RC structures.

External Sulfate Attack on Cementitious Binders: Limitations and Effects of Sample Geometry on the Quantification of Expansion Stress.

The hollow cylinder method was used to estimate the expansion stress that can occur in concrete due to the crystallisation pressure caused by the formation of ettringite and/or gypsum during external sulphate attack. Hardened cement paste hollow cylinders prepared with Portland cement were mounted in stress cells and exposed to sodium sulphate solutions with two different concentrations (3.0 g L SO42− and 30.0 g L SO42−). Microstructural analysis and finite element modelling was used to evaluate the experimental observations. The expansion stress calculation was verified for a range of diameter/length ratios (0.43–0.60). Thermodynamically predicted maximum expansion stresses are larger than expansion stresses observed in experiments because the latter are affected by the sample geometry, degree of restraint, pore size distribution and relaxation processes. The results indicate that differences in self-constraint at the concave inner and convex outer surfaces of the hollow cylinder lead to an asymmetric expansion stress when ettringite is formed. This leads to macroscopic longitudinal cracks and ultimately failure. Heavy structural components made of concrete are likely to support larger maximum expansion stresses than observed by the hollow cylinder method due to their self-constraint.

Hygric deformation of hardened cement paste with limestone-rich cements under cyclic ad- and desorption

Substitution of Portland cement clinker by limestone powder can significantly reduce the environmental impact of cement and concrete. At the same time, shrinkage will decrease. Within this study, the shrinkage behavior was analyzed on cement pastes made with limestone-rich cements during cyclic ad- and desorption. With an increasing limestone volume, a higher ratio of irreversible shrinkage was observed. A high influence of the clay content of the limestone on the ad- and desorption behavior of limestone-rich cements was revealed, notwithstanding of the minor influence on the pore structure. Water vapor sorption isotherm measurements showed, that the pores coarsened by first drying and cannot be refilled totally later at 95 %RH. Mercury intrusion porosimetry and helium pycnometry indicated a higher proportion of meso- and macropores by increasing limestone volume at constant w/c-ratio. Finally, an analytical prediction model is presented to predict the reversible shrinkage strain of cement pastes made with limestone-rich cements.

Loading-rate-dependent effects of colloidal nanosilica on the mechanical properties of cement composites

Colloidal nanosilica (CNS) has shown great potential in the fabrication of high-performance cement composites by improving their quasi-static mechanical properties. However, the effects of CNS under higher strain rates (i.e. dynamic loads) have been seldom explored. In this study, we thoroughly investigated the loading-rate-dependent effects of CNS on the mechanical performance of Portland cement paste. A Split Hopkinson Pressure Bar (SHPB) equipped with a high-speed camera was utilised to conduct dynamic testing under two different strain rates, namely ≈40 (1/s) and ≈120 (1/s), to analyse dynamic compressive strength, modulus of elasticity, failure strain, energy absorption, and fragment size distribution of the composites. Although CNS significantly improved the quasi-static mechanical properties, it had detrimental effects on the dynamic strength, stiffness, energy absorption, and fragmentation behaviour of the cement composites. These negative effects were attributed to the rapid flocculation of CNS upon mixing with the alkaline environment of cement, which led to the formation of a weak and porous hydration phase in the hardened cement paste, as corroborated by microscopy analysis. These findings demonstrate the importance of dynamic characterisations to fully understand the mechanical effects of nanomaterials because quasi-static investigations might not reveal their potential side effects.

Quantification of the Hardened Cement Paste Content in Fine Recycled Concrete Aggregates by Means of Salicylic Acid Dissolution.

Adherent hardened cement paste attached to recycled concrete aggregates (RCA) generally presents a higher porosity than natural aggregates, which induces a lower porosity in the properties of RCA. The characterization of the adherent hardened cement paste content (HCPC) in the fine RCA would promote better applications of RCA in concrete, but the determination of HCPC in fine RCA is not well established. A simple method based on salicylic acid dissolution was specifically developed to quantify the HCPC in RCA, especially for RCA containing limestone aggregates. The results demonstrated that the soluble fraction in salicylic acid (SFSA) was equal to the HCPC for white cement and slightly lower for grey Portland cement, which was also confirmed by a theoretical approach using modelling the hydration of cement paste with the chemical equations and the stoichiometric ratios. The physical and mechanical properties of RCA (e.g., water absorption) were strongly correlated to the SFSA. For industrial RCA, SFSA did not give the exact value of HCPC, but it was sufficient to correlate HCPC with the other properties of RCA. The water absorption could be estimated with good accuracy for very fine RCA (laboratory-manufactured RCA or industrial RCA) by extrapolating the relationship between water absorption and HCPC, which is very important for concrete formulation.

Comparative study on the early properties of cement modified with different ionic polyacrylamides

Polyacrylamide (PAM) has been widely applied as a viscosity modifier to optimize the workability of cement-based materials, but PAMs with different ionic types may show various effects, which is often ignored in practical engineering. In this paper, the early properties including the rheology, flowability, and setting of cement pastes modified by three ionic PAMs (anionic APAM, cationic CPAM, and nonionic NPAM) were contrastively studied. The absorption and hydration of cement pastes with different ionic PAMs were further investigated. Moreover, the contributions of physical flocculation and chemical hydration on the structural build-up of cement pastes were quantified by combining the static yield stress and heat release at the same time scale. The results indicated that three ionic PAMs showed different effects on the structural build-up, fluidity and setting of fresh cement pastes, which were dominated by the different physical flocculation and hydration retardation effects of three ionic PAMs on the cement pastes. The order of flocculation and hydration retardation effects of these three ionic PAMs was APAM ≫ NPAM > CPAM, which was concentration-dependence and evolved over times. The reason is mainly due to the distinct molecular structures and physicochemical properties of three ionic PAMs and their different adsorption behaviors with cements particles.

Effect of electroless plating time and temperature on the formation and antibacterial ability of Cu-plated cement-based material

Microbial induced corrosion (MIC) is an increasing challenge in concrete wastewater conveyance systems. Preventing MIC usually requires a corrosion-resistant chemical/antibacterial coating on cement-based materials. This study investigated the effects of electroless plating time and temperature on the preparation of the Cu-plated hardened cement paste (HCP) and successfully applied electroless copper plating to modify the HCP surface for antibacterial properties. The results indicated that the main components of the coating were metallic copper nanoparticles and phosphate sediment. The HCP plated for 4 h resulted in a relatively higher mass gain (0.76%) and Vickers hardness (163.6 HV). 65 °C was more favorable for reaction kinetics, displaying better mass gain and Vickers hardness. Besides, the antibacterial assays and accelerated bio-corrosion experiments indicated that the Cu-plated HCP had a significant improvement in the antibacterial performance and MIC resistance. Therefore, the proposed electroless plating process is a viable method for cement-based material to meet the MIC challenge.

Surface area development of Portland cement paste during hydration: Direct comparison with 1H NMR relaxometry and water vapor/nitrogen sorption

The specific surface areas of sealed-cured hardened cement pastes (HCP) were evaluated during the hydration process using three types of Portland cement and with two different water-to-cement ratios. The BET surface area was measured by water vapor/nitrogen sorption and directly compared with that obtained by the fast exchange model measured by 1H NMR relaxometry. The results confirmed that the total surface area evaluated by 1H NMR was consistently 2–2.5 times larger than the water vapor BET surface area for different HCPs. This difference is attributed to the local stacking of C-S-H layers: the surface between adjacent stacked layers is not measurable by water vapor due to the nature of the pre-treatment, while 1H NMR is able to measure the entire interlayer surface. The nitrogen surface area is better able to include the majority of the HCP gel pore surface by 1H NMR for white and high-early-strength Portland cement.

Speeding Up Reactive Transport Simulations in Cement Systems by Surrogate Geochemical Modeling: Deep Neural Networks and k-Nearest Neighbors

We accelerate reactive transport (RT) simulation by replacing the geochemical solver in the RT code by a surrogate model or emulator, considering either a trained deep neural network (DNN) or a k-nearest neighbor (kNN) regressor. We focus on 2D leaching of hardened cement paste under diffusive or advective-dispersive transport conditions, a solid solution representation of the calcium silicate hydrates and either 4 or 7 chemical components, and use the HPx (coupled Hydrus-PHREEQC model) reactive transport code as baseline. We find that after training, both our DNN-based and kNN-based codes, \(\hbox {HPx}_{\rm{py}}\)-DNN and \(\hbox {HPx}_{\rm{py}}\)-kNN, can make satisfactorily to very accurate predictions while providing either a 3 to 9 speedup factor compared to HPx with parallelized geochemical calculations over 4 cores. Benchmarking against single-threaded HPx, these speedup factors become 8 to 33. Overall, \(\hbox {HPx}_{\rm{py}}\)-DNN and \(\hbox {HPx}_{\rm{py}}\)-kNN are found to achieve a close to optimal speedup when DNN regression and kNN search are performed on a GPU. Importantly, for the more complex 7-components cement system, no emulator that is globally accurate over the full space of possible geochemical conditions could be devised. Instead we therefore build “local” emulators that are only valid over a relevant fraction of the input parameter space. This space is identified by running a coarse and thus computationally cheap full RT simulation, and subsequently explored by kernel density sampling. Future work will focus on improving accuracy for this type of cement systems.

Mechanical, Physico-Chemical and Morphological Characterization of Energy Optimised Furnace (EOF) Steel Slag as Coarse Aggregate in Concrete.

This research tests energy optimised furnace (EOF) steel slag as substitution for natural coarse aggregate in concrete. Steel slag’s usefulness as a substitute for natural coarse aggregate in concrete is the primary goal of this research. According to IS:2386-1963, the characterization of EOF steel slag, as coarse, is done by examining the shape and size of a particle, mechanical properties, physical properties, soundness, and alkali-aggregate reactivity. Tests for detection of staining material in steel slag and hardness of inter-facial transition zone in hardened cement paste were also carried out. The chemical analysis of the steel slag reveals the stability of oxides present in the steel slag. Microstructural characterization by SEM (scanning electron microscope) analysis of steel slag aggregate was also employed to support the characterization and XRD analysis, and it was found that the EOF steel slag is crystalline. The digital image processing technique (DIP) is adopted to study the shape indices, circularity, sphericity, shape factor, and roundness of natural and EOF steel slag aggregate. According to the characterization and strength investigation, steel slag aggregate outperforms natural coarse aggregate.

Degradation Mechanism and Numerical Simulation of Pervious Concrete under Salt Freezing-Thawing Cycle.

In order to explore the occurrence area of pervious concrete freeze-thaw deterioration, the mass loss, strength deterioration, ultrasonic longitudinal wave velocity and dynamic elastic modulus attenuation of pervious concrete under freeze-thaw cycles were measured, and a prediction model of freeze-thaw damage was established. The mechanical properties of hardened cement pastes with the same W/C ratio under freeze-thaw cycles were also measured. Mercury intrusion porosimetry (MIP) was used to measure the pore structure characteristic parameters and pore size distribution changes of cement paste under freeze-thaw cycle, and the microstructure evolution of interfacial transition zone (ITZ) of paste and aggregate was observed by SEM scanning electron microscopy. Finally, a pervious concrete model was established by DEM to analyze the relationship between the number of freeze-thaw cycles and the mesoscopic parameters. The results indicated that the quality, strength and dynamic elastic modulus of pervious concrete deteriorate to different degrees under the conditions of water freezing and salt freezing. The damage sensitivity and strength loss of freeze-thaw damage is greater than the dynamic elastic modulus loss, which is greater than mass loss. In the pervious concrete paste which underwent 100 freeze-thaw cycles, the pore structure and macro strength had no obvious change, and hardened paste and the aggregate-interface-generated defects increased with the increase in freezing and thawing times, indicating that the deterioration of pervious concrete performance under freeze-thaw cycles was closely related to the deterioration of the interface strength of the aggregate and hardened paste. The pervious concrete model established by DEM can accurately simulate the change of the compressive modulus and the strength of pervious concrete during freeze-thaw cycles.

Laboratory Test and Geochemical Modeling of Cement Paste Degradation, in Contact with Ammonium Chloride Solution.

Concrete tanks, in coke wastewater treatment plants, are exposed to aggressive wastewater with high ammonium and chloride content, deteriorating the concrete binder. Due to this, toxic compounds may migrate to the environment. The results of the experimental work presented confirmed the changes in the phase, microstructure and concentration of chlorides caused by the penetration of NH4Cl into the hardened cement paste in dry conditions. Geochemical modeling of the interactions between the aggressive solution, the cement stone matrix and the pore water was performed in order to track the destruction process effects. The results are useful for condition assessment of the structures operating under occasional immersion.