Hardened Cement Paste Research Articles

Setting time acceleration of cement concrete with addition of termite mound clay

As part of government programs, strategies are put in place to militate against cost of construction, geared towards encouraging reduction in the use of construction inputs such as cement, aggregates, steel, reinforcement steel, etc. The objective of this research is based on the use of wastes and natural local materials as substitutes to the conventional materials. It is observed that termite clay mound (TMC) can be used as an accelerator. The setting times decreased as the percentage addition of cement by TMC increased, this is because the dissolved TMC form a close network that acts as dense barrier for the needle (used to measure the setting time) to penetrate the paste and thus, indicate accelerated setting time. The setting of cement paste is honored to be caused by adding volume of hydration products and leads to a decrease in the distance between individual particles until plastic flow is confined by cohesive forces. The water affinity of TMC cement pastes increased as the dosage increased. With this behaviour, the termite mound clay can be classified as an accelerator and thus it can be used for cold weather concreting. Along with this, the strength of concrete by the addition of TMC is analysed.

Mechanism Analysis on Carbonation of Hardened Cement Paste with XCT

Mechanism Analysis on Carbonation of Hardened Cement Paste with XCT

Effect of Crushing Method on the Properties of Produced Recycled Concrete Aggregates

Construction and Demolition Waste (C&DW) is generated around the world and its quantity will increase in the future. Recycling has become the favored method of dealing with concrete waste but, to avoid its downcycling, it is important to develop a recycling process which is able to produce high-grade recycled concrete aggregates (RCA). To that end, studying the influence of the production process on the properties of RCA can prove to be a crucial step toward a more circular construction industry. In this study, the influence of the crushing method is investigated. Samples of five laboratory-made concretes have been crushed using the most common mechanical crushing methods (impact crusher and jaw crusher), and the particle size distribution, morphology, hardened cement paste content and water absorption of the produced RCA have been measured and analyzed. The findings indicate that the use of impact crushers results in the production of RCA possessing more spherical geometric characteristics, albeit with a broader particle size distribution and a relatively higher content of fine particles as compared to those obtained from jaw crushers. Additionally, it is observed that the employed crushing technique seemingly exerts no discernible impact on the hardened cement paste content and the water absorption in the context of the studied concretes.

Study of Microstructure, Crystallographic Phases and Setting Time Evolution over Time of Portland Cement, Coarse Silica Fume, and Limestone (PC-SF-LS) Ternary Portland Cements

The use of silica fume as a partial replacement for Ordinary Portland Cement provides a wide variety of benefits, such as reduced pressure on natural resources, reduced CO2 footprint, and improved mechanical and durability properties. The formation of more stable crystallographic phases in the hardened cement paste can promote resistance to concrete attacks. However, using coarse silica fume may result in lower expenses and shorter workdays. In this work, coarse silica fume was used as a partial replacement of cement, by weight, at 3%, 5%, and 7%, and it was used as limestone filler at different particle sizes. The size of coarse silica fume used was 238 μm. The microstructural, compositional analysis, and crystalline phase content of mixed cements at different ages were evaluated. The addition of coarse silica fume and limestone promoted pore refinement of the composites and increased the calcium and silica content. The filling effect of fine limestone and coarse silica fume particles, as well as the formation of CSH gel, was found to be the main reason for the densified microstructure. The contributions of combined coarse silica fume and limestone improve the stability of CSH gels and pozzolanic reaction.

The Effect of Natural and Synthesised Zeolites on Cement-Based Materials Hydration and Hardened State Properties.

The synthesis of zeolites from difficult-to-utilise waste materials facilitates the creation of more financially attractive and efficient synthetic zeolites. These can be incorporated into construction materials, resulting in a reduction in cement usage and the production of superior, clean, and sustainable construction materials. The potential to enhance the hydration rate of fresh cement paste by substituting up to 10% of the cement with two synthetic zeolites-one commercially produced and the other synthesised from waste and natural zeolite-was explored. Due to a higher Al/Na ratio, newly sintered waste-based zeolite possesses six times higher electrical conductivity compared to industrially produced 4A zeolite and more than 20 times higher electrical conductivity compared to natural zeolite. As the sequence of this fact, substituting up to 10% of the cement with AX zeolite cement paste accelerates the maximum heat release rate time and increases the total heat by 8.5% after 48 h of hydration. The structure, compressive strength, and water absorption of the hardened cement paste depends on the Al/Na ratio, pH, and electrical conductivity values of the zeolite used. The findings revealed that AX zeolite, due to presence of mineral gibbsite, which speeds up hydration products, such as CSH development, increases the compressive strength up to 28.6% after 28 days of curing and reduces the water absorption by up to 1.5%. Newly synthesised waste-based AX zeolite is cheap because its production is based on waste materials and is mostly promising due to superior properties of created construction materials compared to the other presented zeolites.

Method for Concrete Structure Analysis by Microscopy of Hardened Cement Paste and Crack Segmentation Using a Convolutional Neural Network

In recent years, the trend of applying intelligent technologies at all stages of construction has become increasingly popular. Particular attention is paid to computer vision methods for detecting various aspects in monitoring the structural state of materials, products and structures. This paper considers the solution of a scientific problem in the area of construction flaw detection using the computer vision method. The convolutional neural network (CNN) U-Net to segment violations of the microstructure of the hardened cement paste that occurred after the application of the load is shown. The developed algorithm makes it possible to segment cracks and calculate their areas, which is necessary for the subsequent evaluation of the state of concrete by a process engineer. The proposed intelligent models, which are based on the U-Net CNN, allow segmentation of areas containing a defect with an accuracy level required for the researcher of 60%. It has been established that model 1 is able to detect both significant damage and small cracks. At the same time, model 2 demonstrates slightly better indicators of segmentation quality. The relationship between the formulation, the proportion of defects in the form of cracks in the microstructure of hardened cement paste samples and their compressive strength has been established. The use of crack segmentation in the microstructure of a hardened cement paste using a convolutional neural network makes it possible to automate the process of crack detection and calculation of their proportion in the studied samples of cement composites and can be used to assess the state of concrete.

Dispersion of hydrophobic waste amorphous carbon powder in cementitious environment and its effect on the permeability of cement paste

A Proper dispersion of powder-based hydrophobic admixtures in cementitious composites is challenging due to the presence of water. Studies have shown that amorphous carbon powder (ACP), a hydrophobic by-product of refining waste materials of paraffin production can be used as a hydrophobic agent. However, mixing methods and dispersing agents for modifying cementitious materials with ACP can significantly impact their mechanical properties, workability, water-repellency (hence the durability), and dispersion quality of ACP. Lack of information about the extent of their impact on different aspects mentioned above can hinder the practical use of ACP as a hydrophobic agent. To fill this gap, the objective of this study is to comprehensively investigate factors in the dispersion of ACP in cementitious materials and the tradeoff between mechanical properties and water permeability of ACP-modified cementitious materials when more than one mixing method is adopted. The impacts of different dispersing agents and mixing methods, including ultrasonication, dry mixing, and after mixing, on dispersion quality of ACP in cementitious environment analogous with aqueous solutions and hardened cement paste were assessed. Polycarboxylate ether-based superplasticizer (PCE) proved to be the most effective dispersing agent. Compressive strength of ACP modified cement paste can be increased or decreased with or without the workability reduction effect of ACP along with PCE respectively. Based on volume fraction of ACP clusters in X-ray micro-CT 3D reconstruction of hardened cement paste, ultrasonication was identified as the most effective mixing method and dry mixing was the worst in terms of achieving a uniform distribution of ACP in cement paste. However, all mixing methods presented more than 85% volume fraction of uniformly dispersed ACP areas with no major difference in terms of hydrophobic modification capability. Modifying cement paste with a higher amounts of ACP (5 to 15% by weight of cement) substantially increased water contact angle (up to 171% for w/c ratio of 0.35) and decreased water absorption. The hydrophobic modification capability of ACP increases with a decrease in the quality of cement paste and is diminished by the presence of PCE. Consequently, without PCE, water absorption rate was reduced by up to 92% in similar ACP modified cement paste samples, which also led to a major reduction in compressive strength.

The Effect of Baghouse Dust from Secondary Aluminum Processing Waste on Cement Hydration and Mechanical Properties

This work studies the mechanical and chemical effects of utilizing baghouse dust (BHD) from secondary aluminum processing waste as a cement additive for potential use in concrete materials. The baghouse dust was added to cement pastes at replacements of 4 % and 8 % by cement mass. In addition, a combination of BHD and silica fume at different blended ratios were added to the cement mixture as a combined additive. Some reference proportions were also prepared for a comprehensive comparison. Unconfined compressive strength and the chemical composition of the extracted pore solution of the abovementioned hardened cement pastes were investigated. In addition, the effect of BHD on the hydration characteristics of the cement paste was also observed through isothermal calorimetry. It was found that an 8 % substitution of cement by BHD resulted in increased compressive strength after 1 day and 3 days of curing and a reduction of less than 2 % at 7 days of curing. Comparatively, samples with 8 % silica fume resulted in a strength increase of 17 %. As such, it was concluded that BHD addition of up to 8 % was not detrimental to concrete strength but did not improve performance either. This finding was supported by isothermal calorimetry data, which showed that the addition of BHD and the addition of silica fume both increased the initial peak of hydration and accelerated the hydration process but did not significantly impact the total energy of hydration over a period of 7 days. Finally, the high chloride content in BHD may promote corrosion in steel bars and increase concrete scaling potential.

Elastic constants of nano-scale hydrated cement paste composites using reactive molecular dynamics simulations to homogenization of hardened cement paste mechanical properties

Calcium-silicate-hydrates, portlandite and ettringite are the main hydrated cement paste phases. The study of these phases might lead to a better understanding of the mechanical properties of hardened cement paste. In this work, molecular dynamics simulations were used to simulate different composites of these main phases. The objective were to obtain their elastic constants and to use them as a reference to calculate the homogenization of simplified hardened cement paste using Mori-Tanaka scheme. The results of calcium-silicate-hydrate/calcium-silicate-hydrate, calcium-silicate-hydrate/portlandite and calcium-silicate-hydrate/ettringite composites with three different orientations and configurations were obtained via molecular dynamics simulations using a software called LAMMPS. The homogenization of simplified hardened Portland cement paste gave the value of Young’s modulus in the range from 10.1 GPa to 15.1 GPa. This method might be able to determine Young’s modulus of normal hardened cement paste by the calculations of elastic constants of different composites with other hydrated and unhydrated phases.

Hydration of Hardened Cement Paste Incorporates Nano-Palm Oil Fuel Ash at Later Age: The Microstructure Studies

Hydration of Hardened Cement Paste Incorporates Nano-Palm Oil Fuel Ash at Later Age: The Microstructure Studies

Effect of ISA and chloride on the uptake of niobium (V) by hardened cement paste and C-S-H phases: Quantitative description and mechanistic understanding

The uptake of niobium by hardened cement paste (HCP) and calcium silicate hydrate (C-S-H) phases was investigated with 93Nb and 95Nb (t1/2 = 35.0 days). Structural materials used in nuclear reactors as well as cements contain the naturally occurring isotope 93Nb, while radioactive 94Nb with t1/2 = 2 × 104 years is relevant in the context of nuclear waste disposal. Strong uptake of Nb was observed for both materials, confirming that C-S-H is the main sink of Nb in cement. Isotopic exchange with 93Nb in cement can play a role in the uptake of 94Nb under repository conditions. The formation of complexes with isosaccharinic acid (ISA) decreases the Nb uptake, although sorption remains strong up to [ISA]tot = 0.1 M. Chloride has a negligible effect on the uptake of Nb up to [NaCl] = 2 M. This work provides a sound basis for the quantitative description and mechanistic understanding of 94Nb retention in L/ILW repositories.

Multifunctional performance of core–shell rGO@Fe3O4 on the mechanical, electrical/thermal, EMI, and microstructure properties of cement-based composites

This research study focuses on the development of a multifunctional cement nanocomposite using reduced graphene oxide (rGO) nanosheets. To address the challenges of rGO dispersion, a core–shell structure of rGO@Fe3O4 (rGO@F) was investigated in cement paste. The rGO@F nanohybrid was thoroughly characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). The main objectives of this study were to improve the mechanical properties, electrical conductivity, thermal conductivity, and chemical properties of the cement composite through the incorporation of rGO@F. The performance of the cement paste containing rGO0.1@Fe3O4 (0.1 wt% rGO and 1 wt% Fe3O4) was evaluated, and it was found that its toughness and compressive strength were significantly enhanced compared to the reference sample. Furthermore, the addition of rGO@F led to a remarkable increase in the electrical and thermal conductivity of the modified cement composite, attributed to the formation of conductive pathways facilitated by the rGO nanosheets. Comprehensive analyses including XRD, TGA, differential scanning calorimetry (DSC), thermally assisted microscopy (TAM), FTIR, and X-ray photoelectron spectroscopy (XPS) confirmed that the rGO@F nanohybrid had a significant impact on the chemical properties and hydration process of the hardened cement paste. SEM images revealed a more compact microstructure of the cement paste in the presence of rGO@F, demonstrating its role in crack inhibition. Additionally, an increase in the formation of improved hydration crystals such as tobermorite and jennite was observed. The core–shell structure of rGO@F exhibited exceptional electromagnetic wave absorption properties due to its layered configuration, effectively trapping the electromagnetic waves. Overall, this research study highlights the successful fabrication of a multifunctional cement nanocomposite through the incorporation of rGO@F. The findings demonstrate significant improvements in mechanical strength, electrical conductivity, thermal conductivity, chemical properties, crack inhibition, and electromagnetic wave absorption. This research contributes to the advancement of cement-based materials with enhanced performance and opens up avenues for various applications in fields such as construction, infrastructure, and telecommunications.

Resource recycling and carbon sequestration by carbonizing concrete washing wastewater of ready-mix concrete plant

Recycling the concrete washing wastewater (CWW) of ready-mix concrete plants is significantly important for saving hundreds of millions of tons of water and avoiding water and soil pollution. However, to directly recycle CWW to mix fresh concrete is particularly difficult due to its high alkalinity and high solid content with unstable compositions. To respond these challenges, a feasible approach involving carbonation treatment of CWW and subsequent recycling of the carbonized CWW (CCWW) for concrete mixing is proposed in this paper. To this end, the CWW carbonation test was firstly carried out and the compositional analysis showed that the CCWW became a stable suspension with uniform distribution of fine CaCO3 particles, stable ions contents and neural pH value. The effects of CWW, CCWW and the tap water on the fresh cement pastes hydration and the mechanical and deformation properties of hardened cement pastes were then comparatively analyzed. The results supported that CCWW contributed to the improvement of workability of fresh paste, the increase of strength, the decrease of dry shrinkage and the pore refinement of harden paste. Approximate 16.6 million tons of CO2 per year was estimated to be captured by this approach wemix concrete plants. Benefiting from the improved mechanical properties and extraordinary CO2 sequestration capability, recycling CCWW for concrete mixing resulted in nearly half reduction in CO2 emission.

Diffusion potentials in saturated hardened cement paste upon chloride exposure

The diffusion potentials can cause significant errors in corrosion-related investigations of reinforced concrete structures (half-cell potential mapping, potentiometric sensors). Therefore, an improved understanding of the diffusion potentials in cement-based materials is needed. This study investigates the permselective behavior and its implication for the arising diffusion potentials. A diffusion cell is used to study the diffusion potentials in hardened cement pastes with imposed NaCl gradients. The cement pastes consist of ordinary Portland cement (OPC) and blast furnace cement (BFC) with water-cement ratios of 0.30–0.70. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is used to determine the concentration profiles of Cl, Na, K and Ca in the cement pastes with a high spatial resolution (100 µm). For the BFC pastes, considerable differences in the Cl− and Na+ mobilities are found, indicating their permselective behavior. Despite the permselective behavior, the measured diffusion potentials are small (− 6 to + 3 mV) for all investigated cement pastes due to the high pH levels (13–14) in the pore solutions. However, when using the diffusion cell, the pH differences interfere with the measured diffusion potentials. The interfering pH differences need to be considered for an accurate measurement of the diffusion potentials in cement pastes.

Evaluation on effect of carbonation on chloride binding capacity and stability of cement hydration products with multiple testing methods

Carbonation leads to decrease in chloride (Cl-) binding capacity of hardened cement paste, and subsequently endangers safety of reinforced concrete structures. During the research process, the results obtained by different standards and methods used to quantify the deterioration differ greatly and are unsuitable for comparison. To solve this issue, multiple standards, appointed with various conditioning regimes, for testing acid- and water-soluble Cl- contents in hardened cement paste were followed to evaluate binding capacity and stability of monosulfoaluminate (AFm) and tricalcium aluminate (C3AH6) on premixed Cl- exposed to external CO2. Results show that the methods used in three standards can completely dissolve the acid-soluble Cl-; the different conditioning temperatures lead to different water-soluble Cl- content measured, resulting in significance difference in the AFm chloride binding results, while the difference can be slight and negligible in the C3AH6. Cl- binding products of AFm was relatively stable within room temperature to 70 °C, and the stability decreased once higher than 70 °C; the products of C3AH6 showed relatively good stability between 20 and 100 °C. The Cl- binding capacity and stability of both AFm and C3AH6 were greatly reduced after a carbonation of 3 d. Detailed results for evaluating changes in the binding capacity of AFm before and after carbonation were related to the standards selected, while that for the C3AH6 was quite close.

Performance of Silica Fume on Preventing Strength Retrogression in Hardened Cement Paste and Mortar at Elevated Temperatures

This study examines the effect of silica fume (SF) as a partial replacement for cement to prevent strength retrogression in hardened cement paste (HCP) and mortar at elevated temperatures. An experimental procedure was conducted on 320 specimens, replacing 0, 10, 20, and 30% of the cement by weight with SF. The residual compressive strength of the specimens was evaluated at room temperature (25 °C) and at 100, 200, 300, and 400 °C for 7, 28, and 56 days. The results indicate that the addition of SF to the cement paste and mortar improves the compressive strength both at 25 °C and at temperatures up to 400 °C. That is attributed to the formation of C-S-H phases, such as tobermorite and xonotlite. Additionally, the optimal residual compressive strength was achieved by adding 30% of SF. Therefore, XRD, SEM, and EDS techniques were employed to evaluate the microstructure of HCP specimens with 30% of SF. The results show that adding SF leads to a denser microstructure and lower porosity, resulting in more durable cement paste and mortar at ambient and elevated temperatures. In conclusion, using SF as a partial replacement for cement can be an effective way of developing sustainable fire-resistant construction materials.

Experimental and Estimated Evaluation of Drying Shrinkage of Concrete Made with Fine Recycled Aggregates

Using fine recycled concrete aggregates (FRCA) in concrete manufacturing points towards achieving sustainability in recycled aggregate valorisation. The higher absorption and amount of hardened cement paste of FRCA may impair concrete performance. One of the most influenced properties is drying shrinkage; this is because of the extra cement paste content and higher porosity and deformability of FRCA when compared to natural sand. Thus, the influence of FRCA on shrinkage appears to depend on the quality of FRCA and how its absorption is considered during mix design. In this study, the influence of FRCA mineralogy and quality on drying shrinkage is evaluated, also considering the compensation of FRCA absorption rates. In addition, the feasibility of different models to predict the ultimate shrinkage is also analysed. The quality of FRCA and the compensation of water absorption cause different effects on concrete according to the property evaluated. The storage of water inside the FRCA particles causes no influence (or even a beneficial influence) on the shrinkage of concretes. Models used to estimate the drying shrinkage show they are still reliable with the use of FRCA.

Carbonation and related behaviors of hardened cement pastes under different hydration degrees

This paper develops a kind of molded disc samples to investigate the carbonation and related behaviors of hardened cement pastes under different previous hydration degrees. Weight and length changes of cement pastes over time are monitored during a multistep process including carbonation, drying, rewetting, and redrying. The combination of X-ray diffraction (XRD) and thermogravimetric analysis (TGA) is used to identify and quantify the mineral compositions of carbonated cement pastes. An exponential function between CO2 uptake capacity and hydration time of cement pastes is established, which shows that the CO2 uptake capacity of cement pastes decreases dramatically at the very beginning days of hydration and then remaining relatively stable as hydration time is prolonged. Two reasons for this finding are revealed: i) the equilibrium between the carbonation and the post-carbonation reaction of carbonation product, i.e., silica-alumina gel; ii) refining of pore structures by hydration products which hinders carbonation. A clearer zonation of carbonation areas is proposed, and the spatial distribution equations of CO2 absorption are initially established. By monitoring carbonation and drying behavior of cement pastes with different hydration ages, it is revealed that carbonation reduces drying shrinkage of cement pastes especially for early-age samples, whereas drying increases carbonation shrinkage. By investigating the water changes during the multistep process, it is found that water is little released during the carbonation of C–S–H gels. New insight into mechanism of carbonation shrinkage is provided by a newly proposed model.

Modelling of carbonation of hardened cement pastes with premixed Cl− and resulting Cl− redistribution

Concrete manufactured with desalinated sea sand undergoes carbonation. Consequently, pH decreases and chloride ion (Cl−) is redistributed, interactively impacting the safety of embedded reinforcements. In this study, a physical model considering the processes of cement hydration, pore structure formation, substances transport, and chemical reactions is established. Six key indicators, including the characteristics of carbonation degrees and Cl− distribution, prove the model’s high accuracy. Subsequently, durability-related design suggestions, such as avoiding reinforcement corrosion under the combined impact of carbonation and premixed Cl−, are implemented. The allowable Cl− in desalinated sea sand must be not more than 1/3 of the present allowable values. To ensure a 50-year service life of a concrete with a 50-mm-thick reinforcement cover, the water-cement ratios under the influence of the coupling interaction and single carbonation must not exceed 0.28 and 0.36, respectively.

Corrosion mechanism of recycled mortar prepared from CO2-treated hardened cement paste powder

CO2-treated recycled powder improves the mechanical properties of concrete. However, its corrosion mechanisms are not thoroughly understood. This study focused on the carbonation-induced corrosion mechanism of recycled mortar prepared from CO2-treated hardened cement paste powder (CCPP). First, the corrosion behavior of the steel reinforcement in a CCPP mortar (CCPPM) was evaluated using electrochemical and corrosion mass loss tests. Furthermore, the carbonation resistance of the CCPPM was quantified by testing the carbonation coefficient, and a corrosion model was established to predict corrosion. Finally, the microproperties of CCPPM and CCPP were investigated to reveal the corrosion mechanism. The results show that the corrosion rate of the steel reinforcement in the CCPPM is controlled by the combined effect of the concentration of OH– ions around the steel reinforcement and mortar porosity; the former plays a more critical role than the latter. Moreover, the variation in the w/c ratio of the CCPP base material had a slight effect on the compressive strength and carbonation resistance of the CCPPM and the corrosion rate of the steel reinforcement. The low concentration of OH– ions in the CCPPM increases the corrosion current density of the steel reinforcement by 15.91%–26.79%, consistent with the 10.96%–18.67% higher corrosion mass loss of the steel reinforcement embedded in the CCPPM cured for 540 days relative to that of the hardened cement paste powder mortar. Although incorporating CCPP increases the steel corrosion risk, the corrosion model developed herein demonstrates that CCPP can be used to guarantee a service life of 50 years for mortars when its replacement ratio is less than 20%.