Cement Mortar Specimens Research Articles

Influence of water re-curing on microstructure of heat-damaged cement mortar characterized by low-field NMR and MIP

Water re-curing has been recognized as a potential method to recover the mechanical strength and durability of heat-damaged cement-based materials, which benefits from the improvement of its microstructure due to the rehydration and carbonation of dehydration products. In this contribution, the influence of water re-curing on the microstructure of heat-damaged cement mortar was evaluated by combining Mercury Intrusion Porosimetry (MIP) with low-field Nuclear Magnetic Resonance (LF-NMR). The heat-damaged cement mortar specimens were prepared through exposing to a series of ascending temperatures (105 °C, 200 °C, 400 °C, 600 °C, 800 °C). First, the influence of water re-curing on the porosity and pore size of the heat-damaged cement mortar was analysed with the results of MIP tests. Then, the microstructural evolution of the heat-damaged cement mortar was monitored by means of LF-NMR in the process of water re-curing. At last, a conversion of T2 spectra into pore size distribution (PSD) curves was performed. Causes for the difference in the PSD curves determined by LF-NMR and MIP were discussed. Results demonstrated that LF-NMR was a powerful tool to follow the microstructural evolution of heat-damaged mortar specimens during water re-curing. When combined with MIP tests, the LF-NMR technique can provide quantitative information with respect to the evolution of PSD in a nondestructive way. Capillary pores can be segmented into smaller pores due to the filling of products generated by the rehydration of dehydrated products. The microdamage was partially healed after 10.5 days of water re-curing. The original exposure temperature has a noteworthy impact on the recovery rate and level of the heat-damaged mortar during water re-curing.

Effects of strain rate, temperature and freeze–thaw cycle on the mechanical properties of cement mortar composite specimen

AbstractThe effects of the strain rate, water/cement ratio, temperature and number of freeze–thaw cycle on the ultimate compressive strength, peak strain and elastic modulus of cement mortar composite specimens were studied using uniaxial compression test. Test results show that: (a) The strength difference of cement mortar composite specimens is not significant when the axial loading direction is parallel to the interface of two components with different water/cement ratios under low strain rate loading. The strength of cement mortar composite specimens mainly depends on the one with lower strength, and the strength of cement mortar specimen with high strength is more sensitive to the strain rate than that of cement mortar specimen with low strength. (b) With the increase of temperature from 25°C (room temperature) to 500°C, the strength of the specimen with high compressive strength will increase first and then decrease. Nevertheless the strength of the specimen with low compressive strength will gradually decrease. The strength of cement mortar composite specimen will decrease with increased number of freeze–thaw cycle. (c) Under the same strain rate, after the same high temperature or the same number of freeze–thaw cycle, the peak strain of the composite specimens composed of two water/cement ratio components is much larger than that of the specimen composed of only one component; conversely, the elastic modulus shows an opposite trend.

The effect of specimen size on deterioration due to external sodium sulphate attack in full immersion studies

A rapid and reliable performance test will enable fast and suitable selection of binder for each service environment. In order to evaluate the possibility of using smaller specimens to accelerate the sulphate resistance tests, Portland cement mortar specimens of sizes 20 × 20 × 160 mm, 25 × 25 × 280 mm and 40 × 40 × 160 mm were continuously immersed in 50 g/l sodium sulphate solution and various physical and microstructural characteristics were evaluated at different ages. Additionally, the depth of deterioration and the alteration in binder chemistry was investigated using microCT scan and SEM-BSE along with EDX analysis. The XRD pattern confirms that expansion of specimen is related to the fraction in the cross section altered by the formation of ettringite and gypsum. The expansion and evolution of flexural strength showed a close correlation, notwithstanding a concurrence between the depth of deterioration and evolution of dynamic modulus. This paper discusses the relevance or possibility of using various test methods and their consistency in sulphate attack measurements. The investigation shows that smaller specimens can be considered as a means to accelerate the typical immersion test method for sulphate attack.

Influence of Deviatoric Stress on the Deformation and Damage Evolution of Surrounding Rock under Unloading Conditions

The study of the deformation and damage evolution behaviour of surrounding rock under excavation unloading conditions is of vital importance for a deep understanding of the mechanism of roadway failure. In this study, unloading testing using a partially hollow thick-walled cylinder cement mortar specimen with dimensions of 280 mm (height) × 200 mm (outer diameter) × 60 mm (inner diameter) and a solid height of 60 mm at the bottom was performed to investigate the deformation response and damage failure evolution characteristics of the surrounding rock. The experimental results showed that the higher deviatoric stress level accelerated the damage development caused by the unloading effect and improved the expansion rate of the internal cracks, which led to a higher radial strain rate, total strain, and acoustic emission hits. When deviatoric stress increased to a relatively higher level, the radial strain rates were highly unstable, and the surrounding rock near or at the opening free surface was damaged locally and regionally. During the failure process of the specimen, the generation of the deformation and damage in the unloading stage was more alive (as indicated by the growth rate). Nevertheless, the main deformation and damage to the surrounding rock were generated and accumulated in the maintaining stage after unloading.

Biological Self-Healing of Cement Paste and Mortar by Non-Ureolytic Bacteria Encapsulated in Alginate Hydrogel Capsules.

Crack formation in concrete is one of the main reasons for concrete degradation. Calcium alginate capsules containing biological self-healing agents for cementitious materials were studied for the self-healing of cement paste and mortars through in vitro characterizations such as healing agent survivability and retention, material stability, and biomineralization, followed by in situ self-healing observation in pre-cracked cement paste and mortar specimens. Our results showed that bacterial spores fully survived the encapsulation process and would not leach out during cement mixing. Encapsulated bacteria precipitated CaCO3 when exposed to water, oxygen, and calcium under alkaline conditions by releasing CO32− ions into the cement environment. Capsule rupture is not required for the initiation of the healing process, but exposure to the right conditions are. After 56 days of wet–dry cycles, the capsules resulted in flexural strength regain as high as 39.6% for the cement mortar and 32.5% for the cement paste specimens. Full crack closure was observed at 28 days for cement mortars with the healing agents. The self-healing system acted as a biological CO32− pump that can keep the bio-agents retained, protected, and active for up to 56 days of wet-dry incubation. This promising self-healing strategy requires further research and optimization.

Effect of dispersibility of carbon nanotubes by silica fume on material properties of cement mortars: Hydration, pore structure, mechanical properties, self-desiccation, and autogenous shrinkage

In this study, the effect of dispersibility of carbon nanotubes (CNTs) by silica fume on the material properties of cement mortars were experimentally analyzed. Cement mortar specimens incorporating different proportions of silica fume and CNTs were fabricated, and tests were conducted to measure their hydration kinetics, pore structure, compressive strength, elastic modulus, internal relative humidity, and autogenous shrinkage. Additionally, scanning electron microscopy analysis was performed to observe the morphology of the CNTs in the cement matrix. The results revealed that the dispersibility of the CNTs was enhanced with the increasing replacement ratio of silica fume. Furthermore, well-dispersed CNTs resulted in a decrease in the self-desiccation by varying the pore structure and hydration kinetics, and an increase in the modulus of elasticity, thereby reducing the autogenous shrinkage of the cement mortar specimens.

Electromagnetic radiation detection from cubical mortar sample and its theoretical model

Electromagnetic radiation (EMR) signals have been detected from cubical cement mortar specimens during quasi-static compression and impact loading. In case of quasi-static compression EMR voltage vary from 45 to 13 mV while for impact height of 18 cm EMR voltage was found to vary between 434 and 228 mV. Nature of variation of EMR voltage and evaporable water content with increase in number of curing days is observed to be similar. Based on the EMR signals obtained during repetitive impacts mathematical model for EMR under impact loading is presented. Ions contained in the capillary and gel pores of the cement form dipoles and vibrations of dipoles under impact has been considered as the possible source of the observed emissions. EMR signals obtained using proposed model are in good agreement with the experimental ones in terms of nature as well as in amplitude and can be used for deformation monitoring.

Semicircular bending fracture test to evaluate fracture properties and ductility of cement mortar reinforced by scrap tire recycled steel fiber

The existing test methods using single-edge notched-beam (SEN(B)) specimens to characterize the fracture properties of cementitious materials have some drawbacks. A novel approach using a single fracture test to assessing multiple cementitious materials properties with semi-circular bending (SCB) specimens is presented in this study. In comparison with the conventionally used SEN(B) specimens, the fabrication of the SCB geometry is less material- and labor- intensive. The geometry is also more suitable for field applications due to the ease of extracting cylindrical specimens. In this study, the utility of the SCB fracture method has been demonstrated by testing cement mortars reinforced by recycled steel fiber (RSF) from scrap tire. Based on the SCB fracture test, adding 2% RSF into the cement mortar increased the critical stress intensity factor (KIcs) and the critical crack tip opening displacement (CTODc) by up to 45% and 90%, respectively, and the fracture energy parameters (Gf and GF) by up to 128% and 475%. The ductility indexes were defined based on stress intensity factors rather than tensile strengths, and the results reveal the extremely high ductility of the RSF reinforced cement mortar. It is also found that early age cement mortar specimens are less brittle than mature specimens. Additionally, the theoretical tensile strength was determined. By comparing the theoretical tensile strength with the splitting tensile strength, the level of size effect was evaluated, which again verified that the cement mortar reinforced by 2% RSF is more ductile compared to the plain cement mortar.

A modified model for estimating peak shear displacement of artificial joints

The peak shear displacement (PSD) of artificial joints was studied by the shear test of cement mortar duplicate joints under constant normal load. First, the splitting rock joints were produced by the Brazilian splitting method, and the high-precision point cloud of joint surface morphology was acquired by three-dimensional (3D) scanning method. The polylactic acid (PLA) base mold was produced by 3D printing technology. The duplicate joints with the same joint surface as the splitting rock joints were obtained by pouring cement mortar on the PLA base mold. Then, shear tests of 20 cement mortar specimens with 5 groups of joint surface morphologies under 4 different normal loads were carried out, and the joint shear stress-displacement curves were obtained. The influencing factors of the PSD were studied. The results indicate that the PSD is inversely related to the joint roughness and positively related to the normal stress (NS). The limitation of Barton’s empirical formula is that it cannot predict (fit) the PSD of the joints with JRC = 0 and reflect the effect of NS on the PSD. To quantitatively study the PSD of joints, an empirical formula that can comprehensively consider the influence of surface roughness and NS is introduced based on the test results. The modified model overcomes the limitation of Barton’s empirical formula which ignored the NS and can well reflect the sensitivity of joint roughness coefficient (JRC) to PSD. The acquisition of PSD can provide a basis for the numerical simulation of rock engineering and provide quantitative research methods for the analysis of stick-slip events.

Effect of 3D Printed Spatial Reinforcement on Flexural Characteristics of Conventional Mortar.

In their fourth decade of development, additive manufacturing technologies are slowly entering research programs dedicated to building materials. While the majority of research effort is focused on using 3D printing of concrete, the authors propose using the technology for creation of spatial plastic reinforcement. Obviously, the strength properties of a 3D printed polymer are much lower than those of steel. Nevertheless, the unconventional spatial shape of a 3D printed reinforcement can substitute for much of the lower mechanical performance of polymer. Flexural characteristics of a cement mortar prism specimen reinforced by hexagon spatial elements were tested and analyzed in this paper. The hexagonal geometric shape was chosen due to its high rigidness. It was proven that it is possible to efficiently reinforce concrete beams by spatial 3D printed polymer elements. Directions of needed research were pointed and discussed.

Research on anisotropic permeability and porosity of columnar jointed rock masses during cyclic loading and unloading based on physical model experiments

In this paper, the permeability and porosity of self-prepared artificial columnar jointed rock masses (CJRM) with different columnar dip angles under different pore pressures during cyclic loading-unloading of confining pressure were measured. The experimental results indicate that the gas permeability of the artificial CJRM gradually decreases with the rise of pore pressure due to the existence of Klinkenberg effect, and Klinkenberg effect gradually decreases with the rise of columnar dip angle. The existence of it improves significantly the intrinsic permeability. Besides, the intrinsic permeability and porosity of artificial CJRM are more sensitive to the first loading than that of the intact cement mortar specimen. The permanent deformation of pores in the specimens mainly appears in the first cycle, and the subsequent cycles have smaller effects on the intrinsic permeability and porosity. The permanent deformation results in the intrinsic permeability, and porosity in the loading process is always higher than those in the unloading process. The intrinsic permeability gradually increases with the growth of columnar dip angle, exhibiting obvious permeability anisotropy. Under high confining pressure, the permeability anisotropy of artificial CJRM is less than that under low confining pressure. Meanwhile, under the same confining pressure, during the repeated loading-unloading cycles, the permeability anisotropy decreases gradually. The existence of a representative element volume (REV) for CJRM is verified; thus, the equivalent continuum media model can be utilized to analyse the seepage characteristics of CJRM.

Effect of particle characteristics stress on the mechanical properties of cement mortar with coral sand

The coral sand is widely used as a construction material in the world. The mechanical behavior of cement mortar made by coral sand would be quite different from that made by the Leighton Buzzard sand (LBS) due to the inherent properties of coral sand, such as large void ratio, irregular shape and particle breakage. The effect of particle size on the particle crushing stress and influence on the mechanical behavior of cement mortar are examined in this study. A series of single particle compression tests of coral sand particles were carried out. The results revealed that the characteristic stress of single particle for both coral sand and LBS decreased with the increasing of particle sizes. The exponent relationship between the average crushing characteristic stress and particle size was proposed. The crushing stresses of single particles were in a wide range which varied from 0.99 MPa to 38.25 MPa and 10.98 MPa to 89.41 MPa for coral sand and LBS, respectively. The uncertainly of the crushing stress for single particles was analyzed and considered as Gaussian distribution. A probability density function of characteristic crushing strengths for both coral sand and LBS particles were derived and further verified with experimental results. Finally, the cement mortar specimens with five different types of particle size distributions of coral sands were prepared and tested to investigate the effects of the particle characteristic stress on the compressive strength of the cement specimen. An exponent relationship between the compressive strength and the particle crushing strength was derived and proposed which could helpful for the concrete mix proportion design with coral sands.

Mechanical Properties of Cement Mortar after Dry–Wet Cycles and High Temperature

The dry–wet cycle and high temperature exposure are important factors affecting the normal use and durability of concrete structures. The objective of this work is to investigate the mechanical properties of cement mortar specimens after combinations of dry–wet cycles and high temperature exposures, uniaxial compressive tests on cement mortar specimens were carried out under the following two sets of conditions: (1) high temperature treatment followed by a dry–wet cycle and (2) a dry–wet cycle followed by high temperature treatment. The results show that the compressive strength of specimens increases with the number of dry–wet cycles. After a dry–wet cycle and then a high temperature treatment procedure, the compressive strength of a specimen will first decrease and then increase with the number of dry–wet cycles. The strain at the peak stress of cement mortar decreases as the number of dry–wet cycles increases. At present, there are few research results about the mechanical properties of concrete first after combinations of dry–wet cycles and high temperature exposures. The work in this paper can enrich the results in this area.

USES OF DRY VINASSE AS A FRIENDLY ENVIRONMENTAL MATERIAL TO IMPROVE PROPERTIES OF CEMENT MORTAR AND CONCRETE

This study aims at using the sugar by-product (dry vinasse), as one of the concrete production requirements, instead of getting rid of them in vain. Dry vinasse was obtained by drying liquid vinasse at 250 ° C. In this study, dry vinasse is used as a friendly environmental material to improve properties of cement mortar and concrete, through studying its effect on setting time and compressive strength of cement mortar and concrete. Effect of dry vinasse on compressive strength of cement mortar and concrete by using different doses of dry vinasse 0 %, 0.2 %, 0.3 %, 0.4 %, and 0.5 % by weight content of cement is studied. Compressive strength was determined by standard specimens (40x40x160mm) for cement mortar and standard cubes specimens (150X150X150mm) for concrete. By cylinder specimens (150X300mm), the splitting strength was determined and the flexural strength was determined by beam specimens (100X100X500mm). All the specimens were cured in water and tested for 7 and 28days for cement mortar and concrete. The splitting strength and flexural strength were done for 28 days. It is found that, dry vinasse addition causes increase in setting times. With comparing the results of compressive strength of cement mortar and concrete with and without dry vinasse, it is showed that, there are noticeable increases in the results of compressive strength, splitting strength and flexural strength. Keywords: Dry Vinasse, Compressive Strength, Splitting strength, flexural strength, cement mortar, Concrete

Effects of wetting and drying on alkalinity and strength of fly ash/slag-activated materials

The alkalinity of alkali-activated materials, so-called geopolymer (GP), is attributed to the alkali activator remained in the pores. For the application of GP in reinforced concrete as binder, it is necessary to investigate the alkalinity change of GP concrete in various environments. In this study, after GP concrete (GPC) and GP mortar (GPM) specimens, which used active fillers of fly ash (FA) and ground granulated blast furnace slag (GGBS), were stored in water, wet-dry repeating, accelerated carbonation (AC), and AC-dry repeating environments for a certain period, we observed the coloring reaction to the spraying phenophthalein indicator, measured pH values in different depths and the compressive strengths, and did micro-structure analysis by SEM. A comparison was also performed between GPM and Portland cement (PC) mortar specimens. It was found that, the alkalinity of GP mortar is much easier to decrease no matter in any of above environments, compared to PC mortar. Heat-cured GP has a higher alkali leaching resistance in the wet-dry repeating environment than ambient air-cured GP. Carbonation only decreases the alkalinity in the outer zone, but drying and wetting redistributes the alkalis and promotes the movement of internal alkalis to the outer zone even to the surface of GP, resulting in an alkalinity drop in both the outer zone and the inner zone. The AC-dry repeating causes a greater alkalinity drop than the wet-dry repeating. The AC results in a faster and greater alkalinity drop in the outer zone, compared to the water immersion. Carbonation and water immersion can raise GP’s strength, but intense drying causes cracks to harm the strength of GP.

A potential damage mechanism of rubberized cement under freeze-thaw cycle

Abstract The recycling of scrap tires has attracted attention under the increasingly severe environmental pressures. Rubberized cement, a hardened cement paste with part of the aggregate replaced by rubber particles, shows non-brittle damage differing from the normal concrete. Therefore, there have been mounting interests in the durability of rubberized cement, especially the property against the freeze-thaw cycles. The changed behavior of rubberized cement may be attributed to the effect of rubber particles packing of cement mortar. However, very little progress has been made in exploring the failure mechanisms due to the complex interactions among the material components. The objective of this study is to investigate the damage mechanism of treated cement mortar with rubber particles in terms of freeze-thaw procedure. The cement mortar specimens were divided into two main components: cement paste and rubber particle, which are represented by two parameters: Relative Skeleton Strength (RSS) & Compressive Ratio (CR) respectively. The analysis is based on the Canonical correlation method and Spearman correlation method. The statistical analysis reveals that CR has a high correlation with the residual strength of the specimens after the freeze-thaw cycle.

Dynamic compressive response of high-performance fiber-reinforced cement composites

Composite effect of hybrid fibers with different types containing steel fibers of two different length-diameter ratio, and polymer (PE) fiber was experimentally investigated for high-performance fiber-reinforced cement composites (HPFRCCs). Both of cylindrical cement paste and mortar specimens reinforced with 1.5% vol. hybrid fiber were carried out using a Φ75mm diameter split Hopkinson pressure bar ranged strain rates from 67 s−1 to 219 s−1. A dynamic relationship of compressive stress–strain were tested. Results shows that different fiber reinforcement and strain-rate have a great effect for dynamic behavior of HPFRCCs. The dynamic compressive strength and ultimate strain all generally increase with increasing strain rate. At the similar strain rates, HPFRCCs with larger amount of PE fiber had higher compressive strength. Addition of steel fiber has improved the deformation performance and energy absorption. HPFRCCs with a larger amount of long steel fibers have shown higher dynamic strength.

Vehicular soot for improvement of chemical stability of cement composites towards acid rain and sewage like atmospheres

This study explores the use of vehicular soot, an otherwise waste product, for protecting cement based composites against corrosion and degradation due to acid rain and sewage like atmospheres. Cement mortar cubes (volume 70.7 × 70.7 × 70.7 mm3) were coated with colloidal vehicular soot dispersed in ethanol medium using a simple drop casting technique. The coated cement mortar specimens were characterized using scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction. The protective role of soot coating against acid rain and sewage atmospheres were assessed by crack propagation and water absorption tests. Specimens, previously subjected to stress until minor cracks become visible, were employed to study crack-propagation as a function of time. The reasoning for the observed variation between the coated and the reference sample are discussed. The mechanism of enhanced acid resistance property of coated mortars is explained based on the hydrophobic and water absorptive behavior of soot. Present study presents a novel and economical technique for corrosion protection of cement based composites.

Assessment of molybdenum mine tailings as filler in cement mortar

The present study investigates the mechanical properties and durability of cement mortar incorporating molybdenum mine tailings as a filler material in an effort to report any suitable recycling potential. Molybdenum mine tailings are a type of mineral waste obtained from the mining industry. The proper disposal of mine tailings is an important challenge for sustainable mining practices. These fine-sized mine tailings (<300 μm) can be incorporated into building materials. Cement mortar specimens were fabricated with additions of 0% to 20% of molybdenum mine tailings. The mechanical behavior was observed by compressive strength, flexural strength, ultrasonic pulse velocity, dynamic modulus of elasticity, and bulk and apparent density tests. The durability behavior was tested by assessing the water absorption by immersion, total voids, water absorption by the capillary rise, and resistance to chemical (sulfate and acid) attacks. The addition of molybdenum mine tailings was found to be able to improve the performance of cement mortar according to all parameters examined here. The incorporation of 20% mine tailings resulted in 16% increase for 28 days mechanical performance while 10% mine tailings led to a 20% reduction in total voids in cement mortar. Furthermore, the mine tailings act as a buffer shield to resist aggressive chemical attacks. Based on the outcomes of the present study, the production of cement mortar containing molybdenum mine tailings is a viable solution for the recycling of these materials. Up to 10% molybdenum mine tailings can be used to produce masonry mortar with properties similar to that of conventional mortar.

Tests on the Mechanical Properties of Corroded Cement Mortar after High Temperature

Durability of cement mortar and concrete materials under sea water condition is always an important research topic. The objective of this work is to understand the mechanical properties of corroded cement mortar after high temperature, the cement mortar specimens after high temperature were placed in water and sodium sulfate solution, and then the uniaxial compression tests were carried out on the cement mortar specimens after corroded. Test results show that both the differences of compressive strength and strain at the peak stress after high temperature caused by high temperature, are relatively small when the specimens are eroded in water, and the differences are relatively high when the specimens are eroded in sodium sulfate solution. The compressive strength of the cement mortar specimens under normal temperature eroded in sodium sulfate solution is highest, and that eroded in water is lowest. The compressive strength of specimen after high temperature eroded in water is highest and that eroded in sodium sulfate solution is lowest. The strain at the peak stress of specimen, whether after high temperature or not, is highest when eroded in sodium sulfate solution, and that eroded in water is lowest. At present, there are few research results about the mechanical properties of concrete first after high temperature and then after sea water corrosion. The work in this paper can enrich the results in this area.