Microstructure Of Cement-based Materials Research Articles

Effect of steam curing regime on the mechanical properties, drying shrinkage, and microstructure of mortar in high-altitude areas with low air pressure

Low air pressure in plateau regions impacts the microstructure and macroscopic properties of cement-based materials by altering water transport. However, the efficiency of steam curing, a frequently-used curing approach, under these conditions has been inadequately studied. This work compares the effects of steam curing on the mechanical properties, drying shrinkage, and microstructure of cement-based materials at low air pressure. Results show that low air pressure decreases the compressive strength and increases the drying shrinkage of steam-cured mortar due to reduced hydration and increased total pore volume. Optimally, a steam curing temperature of 40°C to 60°C minimizes thermal damage, and a pre-curing time of 3 hours yields the best mechanical properties and drying shrinkage. Additionally, a field case confirms the effectiveness of the proposed steam curing regime; the demolding strength of precast beams meets engineering standards, and drying shrinkage fluctuates significantly with daily temperature changes. Notably, a 25.30 % difference in 90-day drying shrinkage between steam-cured and non-steam-cured specimens underscores the necessity of steam curing in high-altitude areas. These findings are crucial for optimizing steam curing practices in low-pressure environments.

Microstructure evolution of cement-based materials under Cl−-SO42- coupling erosion in simulated ocean tidal area

To disclose the corrosion principle of cement-based materials used in ocean tidal surrounding, the effects of chloride ion (Cl−) and sulfate (SO42−) coupling erosion on the microstructure of cement-based materials were mainly investigated. In the presence of Cl− solution at a concentration of 0.6 mol/L, the phase composition, pore structure distribution, and microscopic morphology of the hardened slurry of composite cementitious material containing 30 % fly ash (FA), or 30 % ground granulated blast furnace slag (GGBS), or mixed with 15 % FA and 15 % GGBS with varying concentrations of SO42− were investigated, respectively. The impact of SO42− concentration and mineral admixtures on the chemical binding Cl− and microstructure was examined from the microscopic level. The results showed that adding GGBS or FA can resist Cl− and SO42− erosion; however, GGBS is more effective at chemically solidifying Cl− than that of FA. The erosion of Cl− and SO42− influence each other and contain each other. Furthermore, Friedel salt (FS) transforms into Ettringite (AFt) after the introduction of SO42−, which releases bonded Cl− into the pore solution thereby increasing free Cl− content. In higher concentration (0.8 mol/L) of Na2SO4 solutions, the cement-based material is subjected to the superimposed destruction of physical crystallization and chemical erosion of the salt solution.

Effects of reactive MgO on durability and microstructure of cement-based materials: Considering carbonation and pH value

Reactive MgO is a type of eco-friendly material with a lower carbon footprint, as it is produced by calcination of magnesite (MgCO3) at 750 ℃, much less than that (i.e. 1450℃) needed for calcination of (CaCO3) to produce clinker for Portland cement. Carbonation can enhance the mechanical properties of reactive MgO-based cementitious materials but will lower the pH value, leading to the neutralization of cement-based materials and further deterioration in rebar passivation. In this study, varying amounts of reactive MgO (0%, 10%, 20%, and 30%) were incorporated into mortars as cement replacements. By adjusting the duration of CO2 curing (i.e. carbonation curing) and standard curing, it was found that the curing regime involving 36 hours of carbonation curing plus 26.5 days of standard curing led to a higher pH and compressive strength of cement mortars. The pore size distribution was improved as the internal pores of the mortar were filled with carbonation products, which hindered the reaction between CO2 and alkaline hydration products inside specimens and was conducive to maintaining the alkalinity within the mortar. Therefore, the chloride ion penetration resistance and the mechanical properties of mortar were enhanced. SEM and EDS characterization results indicate that the excessive MgO did not fully participate in carbonation and continuously hydrated to generate Mg(OH)2. The expansion of Mg(OH)2 led to crack formation inside the mortar and reduced its strength, which accelerated CO2 diffusion and further reduced the pH value of reactive MgO-based cementitious materials.

Strength and Microstructural Evolution of Magnesium Phosphate Cement Mortar in Plateau Environment

Climatic conditions in plateau areas can enormously affect the properties and microstructure of cement-based materials. This research investigates the strength development and microstructural changes in magnesium phosphate cement (MPC) mortars in a plateau environment. Experiments were conducted in parallel in a plateau area (Lhasa) and a plain area (Chengdu) to evaluate the effects of the water-to-binder ratio (w/b = 0.12, 0.14 and 0.16) and sand-to-binder ratio (s/b = 0.5, 0.75 and 1) on the compressive and flexural strength of MPC mortars. At the same time, hydration products were characterized via XRD, TGA, and SEM/EDX micro-analyses, and the porosity of the materials was also analyzed via MIP. The results demonstrated that curing in a plateau environment resulted in a decrease in workability and yielded higher strength at an early age (before 1 day) but degraded the long-term (180-day) strength of MPC mortars when compared with curing in a plain environment, irrespective of w/b and s/b ratios. Unlike the plain group, the plateau group revealed the deterioration of microstructures over time, including the decrease in struvite content, the morphology change in struvite crystals, and the increase in porosity, which resulted in the degradation of mechanical properties between 1 and 180 days. The strength loss can be effectively alleviated at lower w/b and s/b ratios.

Influence of Polyaluminum Chloride Residue on the Strength and Microstructure of Cement-Based Materials

Influence of Polyaluminum Chloride Residue on the Strength and Microstructure of Cement-Based Materials

Design method of high transport performance biochar-cement based materials based on particle size distribution and permeation probability calculation model

Cement-based materials are the most widely used and largest quantity of engineering materials. Enhancing carbon fixation efficiency and reducing carbon emissions in cement-based materials is an active method of coping with climate change, which has important environmental and social benefits. The amount of carbon dioxide transported into the cement-based material represents the maximum carbon fixation capacity. Enhancing the carbon transport performance of cement-based materials is the material guarantee and basic premise for improving their carbon fixation levels. Considering the impact of particle connections on the maximum cluster, a permeation probability calculation model of porous media system was established, providing the changing rule of the permeation probability with the mathematical expectation of the Poisson distribution, which was verified to be reasonable through experimental studies. The experimental results showed that under the same cement-based conditions, the water transport properties of biochar cement-based materials increased as the mathematical expectation increased. The carbon dioxide transport properties were influenced by the surface area of the biochar particles and exhibited a trend of first decreasing and then increasing with an increase in the mathematical expectation. When the water-cement ratio decreased, the differences in the microstructure of cement-based materials with different biochar particle size distributions increased, the change rate of permeation probability therefore was more sensitive, resulting that the differential effects of different particle size distributions on the transport properties were amplified, and the influence of the particle size distribution of biochar on the transport properties of cement-based materials was more significant.

Effects of MEA Type and Curing Temperature on the Autogenous Deformation, Mechanical Properties, and Microstructure of Cement-Based Materials.

MgO expansive agent (MEA) has the potential to meet the shrinkage compensation demands for concrete in different types of structures due to its designable reactivity and expansion properties. This study investigated the impact of three types of MEAs with different reactivities as well as curing temperature on the autogenous deformation, mechanical properties, and the microstructure of cement-based materials. The results showed that MEA type R exhibits a faster and larger hydration degree and expansion in cement mortars than MEA type M or type S in early ages under 20 °C, while when the curing temperature increases to 40 °C and 60 °C, MEA type M and type S present with significant accelerations in the hydration degree, leading to accelerated expansion rates and significantly increased expansion values compared to MEA type R. Under 40 °C, 5% MEA type M and type S present with 2.2 times and 1.1 times higher expansion in mortars than 5% MEA type R, respectively, and 8% MEA type M and type S present with 7.1 times and 5.6 times higher expansion in mortars than 8% MEA type R, respectively. Under 60 °C, 5% MEA type M and type S present 4.0 times and 3.1 times higher expansion in mortars than 5% MEA type R, respectively, and 8% MEA type M and type S present 7.0 times and 6.6 times higher expansion in mortars than 8% MEA type R, respectively. However, the increase in porosity, especially for large pores with pore size greater than 50 nm as well as the microcracks induced by the 8% dosage of MEA type M, type S, and high curing temperature of 60 °C, result in a decrease in strength of about 30% for the cement mortars. The results indicate that MEA type R is more suitable for shrinkage compensation of cement-based materials with lower temperatures, while MEA type M and type S are more suitable for shrinkage compensation of cement-based materials with higher temperatures. Under high-temperature and low-constraint conditions, the dosage of MEA needs to be strictly controlled to prevent negative effects on the microstructure and strength of cement-based materials.

A new enhanced carbonation curing method using monoethanolamine (MEA) solution: Their effects on hydration and microstructure of cement-based materials

CO2 curing can improve the properties of cement-based materials and absorb the CO2, which is an effective method for pre-cast concrete. However, the application of CO2 curing is limited for the on-site concrete because the CO2 is hard to diffuse into inner fresh concrete. A new enhanced carbonation curing method has been proposed by using monoethanolamine (MEA) solution in this study. In addition, the effect of new carbonation curing on the compressive strength, hydration and microstructure has been studied. The results showed that the MEA solution can delay the hydration of cement which affects the pore structure and reduce the compressive strength. The effect of CO2-rich MEA solution on the compressive strength of cement paste is quite different from that of MEA solution. The inhibition effect of CO2-rich MEA solution that absorbs carbon dioxide on the early hydration of cement is weak, and the promotion effect on the later hydration is more obvious. The CO2-rich MEA solution can chemically react with the calcium ions in the cement slurry pore solution to generate calcium carbonate, and the small amount of calcium carbonate generated can promote the later hydration of cement.

Effect of Glutinous Rice Flour on Mechanical Properties and Microstructure of Cement-based Materials

Effect of Glutinous Rice Flour on Mechanical Properties and Microstructure of Cement-based Materials

Effects of waste printed circuit board powder on strength, durability and microstructure of cement-based materials

A large number of waste printed circuit boards(WPCB) have caused extremely serious pollution problems to the environment. Two different particle sizes of waste printed circuit boards powders are incorporated into the cement-based material as an admixture to study its effect on the mechanical strength and durability of the cement-based material. The results show that WPCB powder can improve the compressive strength and flexural strength of cement-based materials. At the same time, WPCB powder can improve the freeze-thaw resistance and sulfate corrosion resistance of cement-based materials. The study also finds that when the mechanical strength and durability of the mortar are optimal, the content of fibrous WPCB powder is 1%, while the content of granular WPCB powder is 2.5%. The article also analyzes the improvement mechanism of WPCB powder on the mechanical properties and durability of cement mortar through porosity test, FTIR test and SEM test. The test results provide further theoretical research on the application of waste circuit boards in cement-based materials, and provide a new solution for reducing the pollution of WPCB to the environment.

Influence of low vacuum condition on mechanical performance and microstructure of hardened cement paste at early age

• The mechanical performance of early-age paste after low vacuum exposure is evaluated. • The influence of low vacuum on phase composition and pore structure is analyzed. • The evolution of the mechanical performance of paste is explained from the perspective of multi-scale pore structure. To explore the influence of low vacuum on the mechanical performance and microstructure of cement-based materials at early age, the mechanical properties, phase composition and multi-scale pore structure of 7d-aged hardened cement pastes exposed to low vacuum and normal air environment were studied, and the relation between mechanical performance and microstructure was analyzed. The results show that low vacuum can significantly retard the strength growth of paste at early age, and lead to the decline of strength with exposure time increasing. Moreover, the mechanical performance of cement pastes in low vacuum condition was generally weaker than that under normal air environment. Furthermore, the number of gel pores decreased, while the most probable aperture and total porosity increased after low vacuum exposure, and more micron-scale macropores and millimeter-scale defects appeared. The multi-scale variation of pore structure is the dominating reason for the delayed development and degradation of mechanical performance under low vacuum environment. In addition, the phase composition of cement pastes did not change during low vacuum exposure, and the strength of pastes was highly correlated with the total amount of some key hydration products (e.g. C-S-H gel, ettringite, calcite et al.).

Deterioration of performances and structures of cement pastes under the action of thermal cycling fatigue

Concrete structures in the atmosphere are continuously subjected to thermal cycle (TC) inducedby varying environmental temperature. However, the influencing mechanism of thermal cycling fatigue on the mechanical properties and microstructure of cement-based materials has not been thoroughly documented. To address this issue, the relationship between the macro-properties and microstructure of cement paste under TC was analyzed using micro-properties measurement and quantitative microcrack evaluation. In this work, ordinary Poland cement pastes with three different water-to-cement (w/c) ratios (0.4, 0.5, and 0.6) were exposed to TC in the range of 5–45 °C. The mechanical properties, pore structures, and micromorphology of cement pastes after different cycle times were measured. The result indicated that as the number of cycles increased, TC negatively affected the macro-mechanical characteristics, pore structure, and micromorphology of cement pastes. Cement pastes with greater w/c ratios were more susceptible to TC-induced deterioration. In addition, thermal cycling fatigue caused the propagation of microcracks including crack width, length, and density. Thermal cycling fatigue also coarsened the pore structure of cement pastes, especially the large pores (i.e., diameter > 100 nm). The underlying damage mechanism caused by TCis the coupled actions of fluid flow, thermal deformation, and thermal fatigue. Moreover, a negative linear relationship was established between microcrack density and Vickers hardness of cement pastes.

The Effect of Additive ZRJ-S1 on Mechanical Properties of Portland Cement

For mechanical performance perfection of cement-based material, toughening materials are often added to the cement system, such as solid particles, carbon-based material, or other inorganic/organic materials. Compared with other materials, carbon-based materials have better compatibility and are more likely to form a cross-linked network structure in the cement system, which can significantly improve the mechanical properties of cement stone. In this review, a new type carbon-based material ZRJ-S1 was used as a mechanical additive to reinforce oil well cement. In this experiment, ZRJ-S1 was explored with microlevel scale and showed the thin-layer sheet morphology. With static mechanical-strength test, ZRJ-S1 reinforced cement-based material was found with excellent mechanical improvement when 0.05 wt% ZRJ-S1 additive was used, that is, compressive strength improved to 150.9% of that of control sample, splitting tensile strength improved to 134%, and flexural strength improved to 129.2%. Furthermore, the weight percentage superiority of ZRJ-S1 was researched by dynamic stress-train mechanical test and found the mechanical elasticity improvement in which Poisson ratio improved by 74% and formed elasticity modulus reduced to 65.4% when resisting to mechanical damage. The effect of ZRJ-S1 on the microstructure of cement-based materials was studied. As a result, ZRJ-S1 was found with the bridging effect for completion of cement-based material, crack deflection effect for controlling microcrack propagation, and pulling-out effect for preventing immediate fracture of cement-based material itself. Then, using about 0.05% of ZRJ-S1 achieved the accelerating effect on hydration, the speed of the cement-based material, in which more C-S-H gel was formed. Moreover, adding of microscale ZRJ-S1 showed denser spatial microstructure and excellent control ability to pore structure of cement-based material. Micropore percentage of more than 200 nm size, which was with serious damage to mechanical performance, was decreased from 17.40 to 11.89%, and porosity of cement-based material itself decreased to 16.20%. As a result, ZRJ-S1 additive showed excellent reinforcement effect on Portland cement.

Investigation of Low-Calcium Circulating Fluidized Bed Fly Ash on the Mechanical Strength and Microstructure of Cement-Based Material

This present study mainly focuses on the influence of low-calcium circulating fluidized bed fly ash (LCFA) on the mechanical property and microstructure of cement-based materials under different curing conditions. The mechanical properties test was conducted by changing variable parameters, such as LCFA content, the internal mixing ratio of LCFA and fly ash (FA), and dry and water curing conditions. Further, the hydration products and pore structure were analyzed using XRD, FT-IR, TG-DTG, NI, SEM, and BET micro-testing technology. The strength development law of LCFA on cement-based materials is discussed. The research results show that LCFA has a certain degree of self-hardening and can be used as a cementitious material in cement-based materials. Still, the loose and porous microstructure of LCFA leads to higher water requirements, which reduces the fluidity of cement-based pastes. Water curing is favorable for promoting the development of LCFA on the long-term compressive strength of cement-based materials. When the LCFA was added to the cement, the optimal substitution ratio was 20%, and the compressive strength at 91 days reached 101 MPa. In the case of compounding LCFA and FA, when the internal mixing ratio of LCFA/FA was 3 and the total content was 20%, the mechanical properties were the highest, and the compressive strength at 91 days was 92 MPa. The microscopic analysis result shows that the cumulative hydration heat of the samples decreased significantly with the increase of dosage of LCFA. The main hydration products of cement-based materials mixed with LCFA were AFt, C-S-H gel, and Ca(OH)2. AFt and C-S-H gels are critical to the strength development of OPC-LCFA samples. The active Al2O3 and active SiO2 in LCFA were involved in hydration reactions to promote the formation of C-A-H and C-S-H gel and effectively promote the development of the mechanical properties. Overdosages of LCFA would reduce the ettringite formation rate. FA is not conducive to AFt formation in the hydration process of OPC-FA samples.

The role of temperature in thaumasite formation under the immersion of magnesium sulfate solution

To clarify the role of temperature in thaumasite formation of cement mortar under magnesium sulfate solution at two different temperatures, the corrosion products and microstructure of cement-based materials with different amounts and particle sizes of limestone powder were quantitatively analysed by Fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray diffraction, scanning electron microscopy and energy-dispersive spectroscopy. At 5°C, the main corrosion products of cement mortar were gypsum and thaumasite. At 20°C, the main corrosion products of cement mortar were gypsum and ettringite. When the temperature increased from 5°C to 20°C, the contents of ettringite, thaumasite and gypsum changed from 0.3, 12.3 and 64.6% to 4.6, 0 and 57.0%, respectively. The formation of thaumasite was the combination of direct reaction with ettringite transformation. The incorporation of limestone powder accelerated the corrosion of mortars, and the change coefficient of compressive strength of mortars decreased from 100% to 47.3% when its content increased from 0% to 30%. Low temperature and incorporation of finer limestone powder enhanced the corrosion of magnesium sulfate solution.

Upcycling waste flavedo into a bio-admixture of set retarder and compressive strength enhancer for cement-based materials

In this study, we propose a new and clean route to upcycle waste flavedo into a novel bio-admixture for cement-based materials. The effects of waste flavedo on the hydration properities, compressive strength, and microstructure of cement-based materials were investigated by setting time, heat flow calorimetry, X-ray diffractometer, thermogravimetric-differential thermal analysis (TGA-DTG) and scanning electron microscopy (SEM). The experimental results confirmed that the flavedo powder simply prepared by drying and grinding procedures acted as both a set retarder and a compressive strength enhancer for cement-based materials. The addition of 0.5‰ flavedo powder improved the compressive strength of the mortars by 9.91%, 10.5%, and 5.91% at 3d, 7d and 28d, respectively. Both hydration impeding and accelerating effects were observed when flavedo powder was added as a bio-admixture. In particular, the addition of flavedo powder impeded the formation of ettringite; however, it accelerated the generation of Ca(OH)2. Moreover, the flavedo powder helped to refine the sizes of the cement hydration products and facilitated the formation of finer particles. In summary, this study provides a new insight into the clean reuse flavedo powder into a local available and low-cost bio-admixture, and would be benefit for relieving the needs on fossil fuel industry of chemical admixtures.

A Comparative Study on the Mechanical Properties and Microstructure of Cement-Based Materials by Direct Electric Curing and Steam Curing

Direct electric curing (EC) is a new green curing method for cement-based materials that improves the early mechanical properties via the uniform high temperature produced by Joule heating. To understand the effects of EC and steam curing (SC) on the mechanical properties and microstructure of cement-based materials, the mortar was cured at different temperature-controlled curing regimes (40 °C, 60 °C, and 80 °C). Meanwhile, the mechanical properties, hydrates and pore structures of the specimens were investigated. The energy consumption of the curing methods was compared. The results showed that the EC specimens had higher and more stable growth of mechanical strength. The hydration degree and products of EC samples were similar to that of SC samples. However, the pore structure of EC specimens was finer than that of SC specimens at different curing ages. Moreover, the energy consumption of EC was much lower than that of SC. This study provides an important technical support for the EC in the production of energy-saving and high early-strength concrete precast components.

Modelling of the fully coupled chemo-mechanical degradation for cement-based materials

The coupling of chemically induced degradation and mechanical damage is fundamental to investigate the durability of load-bearing cement-based materials subject to aggressive environments. This study is devoted to the incorporation of a reactive transport model and non-local damage model to integrate the complete process of chemical degradation and appropriate propagation of mechanical damage in the framework of coupled chemo-mechanical degradation. The chemical degradation of cement-based materials subject to leaching solution is principally concluded as the dissolution of portlandite and decalcification of C–S–H, and the reduction of Young's modulus is evaluated based on the microstructure of cement-based materials by the Mori–Tanaka scheme. The displacement-based non-local damage model is incorporated to capture the post-peak behaviour of the leached mortar beam in three-point bending test. The significance of coupling chemical and mechanical damage for the degraded beam is demonstrated by comparing the fully coupled damage with outcomes of the non-coupled case.

The Effect of Graphene Nanoplatelets on Chloride Binding and Chloride Migration in Cement Paste

Abstract This paper aims to study the influence of graphene nanoplatelets (GNPs) on chloride binding and chloride migration in cement paste. GNPs were dispersed in cement paste in the form of a GNP solution with the help of gum Arabic powder as a dispersant. The microstructure of cement-based materials was investigated via field-emission scanning electron microscopy (FE-SEM). FE-SEM analysis indicated that GNPs can improve the compactness of the cement paste through its filling effect, bridging, and network effects. Adding 0.10 % GNP content improved the chloride binding by 32 % and reduced chloride migration by 45 %. The addition of GNPs can not only improve chloride binding within cement paste, it can also impede the in-migration of chloride from the outside.

Recycling use of sulfate-rich sewage sludge ash (SR-SSA) in cement-based materials: Assessment on the basic properties, volume deformation and microstructure of SR-SSA blended cement pastes

Sewage sludge ash (SSA) is the waste obtained from the incineration of wastewater sludge. SSA could be recycled in cement-based materials, but normally SSA in China is sulfate rich, and how the sulfate-rich sewage sludge ash (SR-SSA) influence the properties and microstructure of cement-based materials is still unknown. This study aimed to investigate the possibility of recycling use SR-SSA in cement-based materials. The effect of SR-SSA on the basic properties, volume deformation and microstructure of the cement paste was experimentally studied. The findings of this study indicated that the addition of SR-SSA reduced the workability, compressive strength and flexural strength, retarded the setting times, reduced the autogenous shrinkage and caused greater long-term drying shrinkage and higher potential expansion of the cement paste. The formation of excessive ettringite and the coarse pore structure contributed to the properties reduction and volume instability of cement pastes with excessive SR-SSA. However, replacing 5% cement with SR-SSA didn’t induce much detrimental influence on the properties of cement paste. It is feasible to recycle SR-SSA in cement-based materials by replacing up to 5% cement. At the same time, replacing 5% cement with SR-SSA could result in 4.98% lower CO2 emission and 25.42% lower financial cost. In case of recycling all SR-SSA in China, it is possible to expect annual reduction in CO2 emission of 6.8 billion kg CO2-e and financial cost of 16.1 billion RMB yuan. This study could help to recycle SR-SSA and improve the sustainability of cement-based materials.