Strength Development Of Cement Paste Research Articles

Dispersion quality of aqueously dispersed MWCNT affected by step sonication process and its impact on mechanical strength of cement paste: A comparison between polycarboxylate based high range water reducers and air entraining agent

It has been widely reported that the application of high-level ultrasonic energy during dispersion process of carbon nanotube causes damage on the carbon nanotube and reduces its mechanical performance. However, if the amount of ultrasonic energy is insufficient, the quality of carbon nanotube dispersion becomes poorer so that the effect caused by carbon nanotube reinforcement is minimized. In this work, the step sonication process, which divides a prolonged sonication into a few separate cycles while maintaining the same level of ultrasonic energy, is applied since this process can improve dispersion of carbon nanotube without causing serious damage. Commercially available polycarboxylate ester, polycarboxylate, and air entraining agent were used as dispersing agents. The effect caused by step sonication and type of dispersing agent on dispersion quality of multi-walled carbon nanotube was evaluated, and their impacts on mechanical strength development of cement paste were analyzed. According to the experimental results, step sonication process improved the quality of multi-walled carbon nanotube dispersion when polycarboxylate ester and polycarboxylate were used. Step sonication process was found to be more effective for multi-walled carbon nanotube suspension with higher concentration. Among all dispersing agents, air entraining agent was found to be the best dispersing agent showing 2 times higher UV absorption peak than polycarboxylate and polycarboxylate ester, but the application of air entraining agent caused 40–65% reduction in compressive strength due to the air entrainment. It was also found that dispersion quality of multi-walled carbon nanotube affected flexural strength more significantly than compressive strength.

Influence of negative temperature hardening on hydration, pore structure evolution and strength development of cement paste

The pre-cured 24-h demolded cement pastes were directly cured at −5 °C and +20 °C for different ages to investigate the influence of negative temperature hardening on hydration, pore structure evolution, and strength development. Thermogravimetric (TG) results show that the inhibition effect of negative temperature hardening on the hydration degree and C–S–H gel content is different. As the hydration age increases to 120 days, the hydration degree and gel-space ratio of negative temperature hardening cement paste can reach the positive temperature hardening level of 120 days, while the C–S–H gel content is still lower than the positive temperature hardening level of 120 days. Mercury intrusion porosimetry (MIP) results indicated that negative temperature hardening increases all types of pore volumes, especially the pore volume of <10 nm. Similar to the hydration degree, by giving sufficient negative temperature hardening time (120 days), the pore characteristic parameters of negative temperature hardening cement pastes can approach the level of 120-day positive temperature hardening. Also, the fractal analysis shows that negative temperature hardening increases the proportion of large-diameter pores in the range of <100 nm, and decreases the proportion of large-diameter pores in the range of >3000 nm. In addition, the micro-mechanisms of limiting strength development are different at different negative temperature hardening stages. The early strength development is affected by the coupled effects of hydration and pore structure, while the later strength development is limited due to the lower C–S–H gel content.

Roles of diatomite in hydration, microstructure and strength development of cement paste

The production of cement has a significant environmental impact, and the use of mineral admixtures can mitigate this impact by reducing the clinker rate. Diatomite powder (DP) possesses unique properties, such as micronized and sub-micronized porous structures, and a large specific surface area, making it an ideal cement replacement. To gain a better understanding of the mechanisms underlying its performance as a cement replacement, the reference used was quartz powder (QP) in this study, and zeta potential tests were conducted to characterize the surface chemical properties of DP. Subsequently, how it affects the cement paste hydration, microstructure and strength development was investigated. Results revealed that DP, owing to its sub-micronized porous structure and chemical composition, exhibited a higher affinity for Ca2+ ions compared to QP, which significantly affects the formation of C-S-H. This heightened affinity facilitated the nucleation and growth of C-S-H, leading to a shortened hydration induction period. Furthermore, the incorporation of DP into cement paste resulted in a refinement of the pore structure. This enhancement is due to the relatively small particle size of DP, its chemical reactivity, and its surface affinity for uniform distribution of hydration product. Notably, the DP-hydration interface exhibited significantly higher strength compared to the interface between QP and hydration products. As a result of improving pore structure and strengthening the DP-hydrate interface, the cement paste blended with DP demonstrated compressive strength comparable to that of pure cement paste and considerably higher than the cement paste incorporating QP.

Investigation on Roles of Packing Density and Water Film Thickness in Synergistic Effects of Slag and Silica Fume.

The ternary blended cement with finer slag and silica fume (SF) could improve the packing density (PD) through the filling effect. The excess water (water more than needed for filling into voids between the cement particles) can be released to improve the fresh properties and densify the microstructure which is beneficial for improving the hardened properties. To verify the hypothesis and reveal how and why (cement + slag + SF) the ternary blends could bring such advantages, the binder pastes incorporating slag and SF with various water-to-binder ratios were produced to determine the PD experimentally. To evaluate the optimum water demand (OWD) for maximum wet density, the influence of the dispersion state of the binder on PD was investigated using the wet packing density approach. The effect of PD of various binary and ternary binder systems on water film thickness (WFT), fluidity, setting time, and compressive strength development of cement paste was also investigated. The results show that the ternary blends could improve the PD and decrease the water film thickness (WFT). The enhanced PD and altered WFT are able to increase fluidity and compressive strength. The ternary blends could improve the compressive strengths by increasing PD and exerting nucleation and pozzolanic effects.

Effect of Basalt Powder on Hydration, Rheology, and Strength Development of Cement Paste

Basalt materials (e.g., basalt powder, aggregate, and fiber) are commonly used in cement-based materials. To understand the mechanism of the influence of basalt on the properties of cement-based materials (i.e., fluidity, hydration, and strength), zeta potential tests with different Ca2+ concentrations were carried out using basalt powder (BP). It is found that BP has a weaker absorption for Ca2+ compared to cement and quartz particles, which is directly related to its surface chemical properties. This weak absorption has a significant influence on the rheology and early-age hydration of cement paste. Moreover, the morphology of hydrate on the surface of the material observed by scanning electron microscope (SEM) also shows that the growth of CSH on the surface of BP particles is smaller than that of cement particles, indicating that BP delays the formation of CSH. Rheological tests showed that the reduction of BP's adsorption of calcium ions weakened the electrostatic repulsion between particles, which led to the reduction of rheological properties. The influence of BP on the strength of cement paste was studied through crack characterization and fracture observation. The results show that the interfacial strength between BP and hydration products is very weak and does not increase with the hydration process, and the chemical reaction of BP is not obvious. In addition, the substitution of BP for cement leads to a dilution effect. These factors cause the strength of cement paste to decrease.

The Influence of Alkali-Free Shotcrete Accelerators on Early Age Hydration and Property Development within Cement Systems.

Fluoride-containing alkali-free setting accelerators are a common type of admixture used in tunnel shotcrete but few studies in the literature focus on the effect of their fluoride compounds on the setting and hardening properties of accelerated cement paste under low environment temperatures. Tunnel shotcrete in cold regions or winter construction periods would be obviously influenced by low environment temperatures, especially for its fast setting and quick support applications. The objective of this work is to evaluate the early age hydration behavior of different accelerated cement pastes under 20 °C and 5 °C environment temperatures. In this study, setting time measurement, early age strength development, hydration ion leaching concentration, isothermal calorimetry, X-ray diffraction, and ESEM were performed on cement systems prepared with a non-fluoride alkali-free accelerator (aluminum sulfate solution with over 60% solid content) and a designed fluoride-containing alkali-free setting accelerator (aluminum sulfate and fluoride compound). The results showed that the fluorides obtained in alkali-free accelerators promote C3S dissolution and massive ettringite needles together with monosulfoaluminate (AFm) hydrate formation, thus leading to a quicker setting effect and low sensitivity to low environment temperatures than in non- fluoride groups. However, the rate of mechanical strength development of cement pastes hydrated within 24 h was decreased obviously when fluorine-containing alkali-free accelerator was used. This phenomenon is mainly related to the crystallization of thin-plate shape calcium fluoride (CaF2) formations and promoted conversion of ettringite to monosulfoaluminate hydrate in the accelerating period, thus weakening the denseness of C-S-H gel and inhibiting alite further hydration.

Surface properties of ceramic waste powder and its effect on the rheology, hydration and strength development of cement paste

To reduce carbon emissions and environmental impacts, ceramic waste powder (CWP) can be used to replace part of the cement in concrete. To understand the mechanisms of the influence of CWP on the properties (i.e. rheology, hydration and strength) of cement-based materials, a zeta potential test with different Ca2+ concentrations was conducted. It was found that CWP has a similar absorption for Ca2+ as cement and quartz particles, which is directly related to its surface chemical properties. The adsorption of Ca2+ on the particle surface affects the formation of C–S–H and the rheology of the cement paste. Based on the similarity in surface chemistry, it is elucidated that the effect of CWP on the rheology and early-age hydration of cement paste is attributed to its size or shape, not the surface chemical properties. In addition to strength measurement, the effect of CWP on the strength of cement paste was investigated by crack characterization and fracture surface observation. The results demonstrated that as hydration proceeded the interface strength between CWP and hydrates increased, and the chemical reaction of CWP became apparent. It also showed that CWP is an inert filler similar to quartz particles at early age. Nevertheless, at late age the pozzolanic reaction becomes obvious and improves the strength development of the cement paste.

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.

Strength Development of Cement Paste Using Nitrite Below Freezing Point

Strength Development of Cement Paste Using Nitrite Below Freezing Point

Early strength development and hydration of cement pastes at different temperatures or with superplasticiser characterised by electrical resistivity

The effects of the curing temperature and superplasticiser on the hydration rate and early strength development of cement pastes were investigated using their electrical resistivities. The cement pastes were prepared either at various temperatures or with different dosages of the superplasticiser. A general equation for electrical resistivity development was employed to monitor the paste rate of densification during the hardening period. The cement pastes obtained at a high temperature or optimal dosage of the superplasticiser had a faster densification than those of the pastes obtained at a low temperature or excessive dosage of the superplasticiser because of their higher rates of hydration. The electrical resistivity can be used to evaluate the factors that influence the hydration rate, microstructure, and early physical properties of the cement paste as well as for a better utilisation of admixtures and determination of the optimal dosage for hydration and paste structure formation.

Interaction of Basalt with Calcium Ion and Their Role in Hydration, Rheology and Strength Development of Cement Paste

Interaction of Basalt with Calcium Ion and Their Role in Hydration, Rheology and Strength Development of Cement Paste

A further understanding on the strength development of cement pastes in the presence of triisopropanolamine used in CRTS III slab track

Accelerators are commonly used in the self-compacting concrete of CRTS III slab track to advance its strength development in cold areas. Triisopropanolamine (TIPA) is a known accelerator, and its influences on the strength development of hardened cement pastes (HCPs) were discussed based on a further understanding of cement hydration and microstructure. The results show that the addition of TIPA visibly heightens hydration degree at each age because it triggers the reaction of C4AF and gently promotes C3S hydration. At low dosages, more hydrates deposited in capillary pores facilitate a refined pore structure and therefore, significantly increase the early and late strengths. On the contrary, a high dosage of TIPA results in a loose and porous structure at an early age owing to the emergence of numerous AFm platelets and the reduction of Ca(OH)2 crystals, so that the heightened hydration degree only slightly increases the early compressive and flexural strengths. At a later age, high dosages of TIPA cause an increase in the volume of harmful pores (greater than 50 nm), which may result from the appearance of some pores full of fractional AFm and the changed morphology of Ca(OH)2. Therefore, the late compressive strength of HCPs remains almost unchanged under the positive effect of the heightened hydration degree coupled with the negative effect of the increased volume of the harmful pores. In contrast, the late flexural strength sharply decreases, which is ascribed to both of the increased volume of the pores larger than 50 nm and the morphological changes of hydration products.

Enhancement of triisopropanolamine on the compressive strength development of cement paste incorporated with high content of wasted clay brick powder and its working mechanism

Using ground clay brick powder (CBP) replacing cement to prepare blended binding system is an effective solution to reduce the accumulation of wasted clay bricks. However, the increased incorporation ratio of CBP causes a drawback of decreased mechanical property. The present research is aiming at using triisopropanolamine (TIPA) to alleviate the excessive loss of compressive strength of cement paste incorporated with high-content of CBP. Furthermore, the hydration behavior of cement-CBP-TIPA system is investigated in detail. The study indicated that the addition of TIPA improved cement hydration and pozzolanic reaction of CBP, thereby the compressive strength was improved at different ages. More specifically, the presence of TIPA accelerated the conversion of AFt to AFm as well as the dissolution of aluminum and iron ions from CBP, thus more calcium aluminosilicate hydrate was formed. In addition, the economic and environmental effect was also assessed. It was proved that 47% of energy consumption and 47% of carbon footprint can be saved by developing the cement-CBP-TIPA binding system.

Surface properties of clay brick powder and its influence on hydration and strength development of cement paste

Replacing part of cement with recycled clay brick powder (CBP) can ease the environmental impact caused by construction waste disposal and cement production. This research studied the surface properties of CBP and its effect on the hydration and strength development of cement paste at the micro and sub-micro scales. Zeta potential measurement was applied to study the surface chemical properties of CBP. Based on it, the interactions between the main ions in the cement paste solution and the surface of CBP were discussed. Then the effect of CBP on the nucleation and growth of hydrates, and CBP/hydrates interfacial strength was investigated. The results show that CBP has similar surface properties as quartz powder (QP), even though the chemical compositions of them are different. Silicate is the main phase at the surface of CBP. The interactions of CBP’s surface with Ca2+ are due to an electrostatic force, which has an important effect on the nucleation and growth of hydrates (mainly C-S-H) on the surface of CBP, and the adhesion between CBP and the hydrates. The morphology of hydrates on the CBP particles is similar to that on the QP particles. It suggests that the effect of CBP particles on the early hydration of cement is mainly due to its physical properties, rather than the chemical activities. The interface between CBP and hydrates is very weak, and the chemical reaction of CBP in the cement paste is rarely presented. The main role of clay brick used in this study playing in cement paste is a filler, similar to quartz particles.

Effect of pozzolanic micro and nanoparticles as secondary fillers in carbon nanotubes/cement composites

Incorporation of micro and nanoparticles as secondary fillers in cement based matrices improves the efficiency of self-sensing carbon nanotubes (CNT)/cement composites through the excluded volume effect. Pozzolanic particles can be seen as interesting secondary fillers for this purpose since they would increase the compacity of the granular matrix, increase the nanotube effective volume during the first hours of hydration, and densify the matrix through the growth of additional hydration products at latter ages. In this work, pozzolanic micro and nano particles (nanosilica, microsilica, metakaolin and nanoclay) were used as secondary fillers to investigate how their combined effect with CNT dispersion containing surfactant as dispersing agents affects the hydration kinetics, setting time and early strength development of Portland cement pastes. For this purpose, isothermal calorimetry and ultrasonic pulse velocity tests were performed. The change in solid fraction caused by the pozzolanic particles also was analyzed to understand the excluded volume effect in the matrix. It was found that the use of surfactants as dispersing agent for CNT has a retardation effect on the hydration and strength development of cement paste and this retardation is minimized by the presence of pozzolanic secondary fillers. It also was found that microparticles are much more effective than nanoparticles in increasing the compacity and effective nanotube volume in the matrix.

Surface characterization of carbonated recycled concrete fines and its effect on the rheology, hydration and strength development of cement paste

Carbonation treatment can effectively improve the performance of recycled concrete aggregate and fines due to the reactions of CO2 with CH and C–S–H gel of cement paste. To better understand the mechanisms involved in the performance improvement, the surface properties of carbonated recycled cement paste powder (CRP) and its effect on the rheology, hydration and strength development of cement paste was studied. The results showed that during the carbonation, the surface of CRP was covered by a layer of amorphous silica gel. The generated CaCO3 was wrapt by the silica gel and seldom exposed. The silica layer led to the poor flowability of CRP-cement paste due to that the silica gel on the surface of CRP has a strong affinity for H2O. During the very early hydration, the silica gel dissolved and then CaCO3 was exposed. CaCO3 is capable of chemically absorbing Ca2+, which facilitated the nucleation of C–S–H nuclei and stabilized the C–S–H phase. As a result, the C–S–H grew densely, uniformly and perpendicularly on the surface of CRP. In addition, the chemically absorbing Ca2+ enabled the chemical bond to be formed between CaCO3 and C–S–H. Due to increased C–S–H resulted from reactions of silica gel with CH at the interface and the stronger bond formed between CaCO3 and C–S–H, the interface between CRP and hydration products was much stronger than that between recycled cement paste powder (RP) and hydration products.

Effect of graphene oxide on rheology, hydration and strength development of cement paste

The aim of this paper is to investigate the effect of a well-dispersed graphene oxide (GO) on workability, rheological properties, hydration kinetics, density of hydration products, and compressive strength of cement paste. Paste mixtures were prepared with 0.42 water-to-cement ratio, and a well-dispersed GO was incorporated at contents of 0.04%, 0.15%, and 0.29%, by mass of cement. When coupled with sonication time of 48 to 56 min, the optimum active superplasticizer (SP) dosage was found to be four times that of the GO. Compared to paste made with 0.16% SP and no GO, the addition of 0.04% GO increased yield stress and viscosity coefficient values of paste, total heat of hydration at 70 h, density of hydration products at 28 d, and compressive strength at 1, 3, and 7 d. The increase in rheological properties can be caused by the reduction in free water due to water absorption by the GO, while the increase in total heat and compressive strength can be associated with the nucleation effect of the GO. However, increasing the GO content from 0 to 0.15% with the SP dosage fixed at a higher dosage of 0.6% showed no effect on hydration kinetics but reduced yield stress, viscosity coefficient, and 1-d compressive strength. This behavior was attributed to the high dosage of the SP needed for dispersing the high content of GO. The beneficial effect of using 0.15% GO was evident from the increase in compressive strength at 3 and 7 d. Overall, the addition of well-dispersed GO improved the rheological properties, hydration kinetics, density of hydrates, and compressive strength of cement paste. However, the improvement achieved with GO at early age (1 d) was hindered by the retarding effect of SP used to maintain proper dispersion.

Strength development of cement pastes with alkali-activated dehydrated sewage sludge

The application of the highly dehydrated sewage sludge (HDSS) with high organic content was discussed through various alkali-activated cement binders. The fresh properties and the strength developments of the cement pastes with 5% HDSS by volume and a liquid-solid ratio of 0.6 mL/g were investigated. Results indicated that the 28-day compressive strength was recovered by the geopolymerization with economic advantages. The costs of such treatment are lower than the landfill by 3.6% or incineration by 30%.

Impact of sodium gluconate on white cement-slag systems with Na2SO4

Sodium gluconate has been shown to compensate the impact of alkalis on the long term strength development of cement pastes. This work studies the impact of sodium gluconate combined with Na2SO4 on the microstructural development of white cement-slag systems compared to the addition of only Na2SO4 to better understand its effect on strength development. At long ages the addition of sodium gluconate in systems with Na2SO4 increases the strength compared to the systems with only Na2SO4. Differences in strength cannot be explained by changes in the degree of hydration of cement, degree of reaction of slag, phase assemblage or C-S-H morphology and chemical composition, since these parameters are not affected by sodium gluconate. 1H NMR shows that sodium gluconate increases the water amount in C-S-H and image analysis confirms the formation of a less dense C-S-H, which can better fill the space. Consequently, the porosity decreases and strength development increases.

Effect of nano-silica particles on the hydration, the rheology and the strength development of a blended cement paste

The aim of the present work is to evaluate the effect of nano-silica (NS) on the hydration, the rheology and the strength development of cement pastes. The advance of chemical reactions is monitored by mean of isothermal calorimetry and thermogravimetric analysis: adding nano-silica particles speeds up the hydration of the cement paste but alter its workability. Indeed, the effect of the nano-silica particles on the hydration kinetics can be modelled by accounting for its high specific surface and a flocculation model based on the DLVO theory is proposed so as to investigate the stability of nano-silica suspensions in the fresh cement paste. As a consequence, the dosage of nano-silica can be optimized to promote the early age strength. Lastly, a ternary blend incorporating fly ash can be designed so as to provide an early age strength similar to that of the cement while lowering the induced CO2 emissions.