Reference Cement Paste Research Articles

Thermally induced cracking patterns of the MWCNTs modified cement paste

This paper investigates the influence of the multi-wall carbon nanotubes (MWCNTs) on the structure of cracking patterns (CPs), which is the main novelty, of cement pastes subjected to thermal loading up to 600 °C. The compressive and tensile strength, ultrasonic pulse velocity and weight loss during the thermal loading were investigated. The CPs structure was analyzed using a proprietary digital image segmentation procedure with the use of machine learning algorithms. The total crack area, crack density, fractal dimension and lacunarity of the CPs were determined. In addition, cross sections of cement paste samples were analyzed to determine the average crack depth. The results indicated that the most beneficial MWCNTs content is 0.05 % by weight of cement, for which the increase in the compressive strength was equal 7.8 %. In terms of CPs development, the total crack area and crack density decreased by as much as 41 % at temperatures above 500 °C. At temperatures below 400 °C, MWCNTs increased the crack depth by up to 47.2 %, while at higher temperatures the crack depth decreased to a difference of 1.5 %, compared to the reference cement paste.

3D printing concrete containing thermal responsive gelatin: Towards cold environment applications

Construction with cementitious materials via 3D printing requires a highly thixotropic behavior, which is a challenge for construction industry. In this work a facile and low-cost gelatin-modified thermal responsive smart cement paste formulation was developed for extrusion-based 3D printing in cold environments The paste's temperature-dependent rheological properties were characterised, and the underlying mechanism was explored. At room temperature, the modified paste presents similar rheological properties as the reference cement paste. After resting for 10 min at 5 °C, the modified cement paste has a yield stress of 1900 Pa which is 10 times that of the reference paste. Moreover, the mechanical properties of the modified paste are comparable to or higher than that of the reference paste at later age. Our approach may inspire a facile manipulation of paste rheology through temperature, which facilitates smartly controlled 3D concrete printings.

Sustainable development of basalt fiber-reinforced high-strength eco-friendly concrete with a modified composite binder

The aim of the paper is to development of basalt fiber-reinforced high-strength eco-friendly concrete with modified composite binder for sustainable construction. Cement composites based on a modified polymineral binder with the use of enriched aluminosilicates obtained from coal ash have been developed. A technology has been developed for extracting aluminosilicates from coal ash, which includes five stages. The choice of grinding technology of modified composite binder in a vario-planetary mill up to 550 m2/kg has been made, where the combined action of high impact energy, strong friction and centrifugal forces achieves the maximum mechanical activation of the binder. The developed fresh mixes are characterized by good flowability (slump 20 cm and slump flow 47–49 cm). The resulting composites are characterized by high 28-day compressive and flexural strengths of 59.1 and 13.3 MPa, respectively, which is 44% and 66% higher than that of the control sample. At the same time, the high ratio of 28-day flexural and compressive strengths, reaching 0.31 (for the control sample 0.19), proves the high potential of this material to work under shock and dynamic complex loading. These trends are also preserved for the ages of 1 and 7 days, which allows us to speak about the high early strength of the materials. Microstructural analysis using scanning electron microscopy, X-ray diffraction and energy dispersive spectroscopy showed that the modified cement matrix has a denser microstructure with a large amount of low-basic calcium silicate hydrates (C-S-H), while the reference cement paste contains more high-basic C-S-H and hexagonal portlandite plates. The formed materials are able to withstand up to 15 thermal cycles at a temperature of 700 °С (4 times higher than that of the control sample), 13 thermal cycles at a temperature of 900 °С (4.3 times higher), 8 thermal cycles at a temperature of 1100 °С (exceeding eight times the characteristics of the unmodified sample, which is destroyed without enduring a single thermal change).

Recycling of Flash-Calcined Dredged Sediment for Concrete 3D Printing

Due to the large volumes of sediments dredged each year and their classification as waste materials, proper management is needed to efficiently dispose of or recycle them. This study aimed to recycle flash-calcined dredged sediment in the development of an eco-friendly 3D-printable mortar. Mortars with 0, 5, 10, 15, 20, and 30% of flash-calcined sediment were studied. Two tests were carried out to determine the printability of the mixtures. First, a manual gun device was used to examine the extrudability, then a modified minislump test was conducted to assess the buildability and shape-retention ability of the mixtures. Furthermore, the flow table test and the fall cone test were used to evaluate the workability and structural buildup, respectively. The compressive strength was also evaluated at 1, 7, and 28 days for printed and nonprinted mortar specimens. In addition, isothermal calorimetry measurements were conducted on corresponding cement pastes. The results showed that it was possible to print mortars with up to 10% of flash-calcined sediment with the preservation of extrudability and buildability. The results showed that flash-calcined sediment shortened the setting time, decreased the flowability, and enhanced the shape-retention ability. Nonprinted samples with 5% and 10% of flash-calcined sediment showed a similar to higher compressive strength compared to that of the reference mortar. However, printed samples recorded an equal to lower compressive strength than that of nonprinted samples. In addition, no significant change in the hydration process was detected for blended cement pastes compared to the reference cement paste.

Effect of High Calcium Fly Ash, Ladle Furnace Slag, and Limestone Filler on Packing Density, Consistency, and Strength of Cement Pastes.

Environmental considerations and technical benefits have directed research towards reducing cement clinker content in concrete, and one of the best ways to do this is to replace cement with supplementary cementitious materials. High calcium fly ash, ladle furnace slag, and limestone filler were investigated as supplementary cementitious materials in cement pastes, and binary mixtures were produced at 10%, 20%, and 30% cement replacement rates for each material. The water requirement for maximum packing and for normal consistency were obtained for each paste, and strength development was determined at 3, 7, 28, and 90 days for the 20% replacement rate. Furthermore, two ternary mixtures at 30% cement replacement were also prepared for maximum packing density and tested for compressive strength development. The results showed that high calcium fly ash decreased cement paste packing and increased water demand but contributed to strength development through reactivity. Ladle furnace slag and limestone filler, on the other hand, were less reactive and seemed to contribute to strength development through the filler effect. The ternary paste with 70% cement, 20% high calcium fly ash, and 10% limestone filler showed equivalent strength development to that of the reference cement paste.

Preparation and Performance of a Fluorine-Free and Alkali-Free Liquid Accelerator for Shotcrete

Based on aluminum sulfate, a fluorine-free and alkali-free liquid accelerator (FF-AF-A) was prepared in this study. The setting time and compressive strength of three cement types with different FF-AF-A dosages were fully investigated. The compatibility of the FF-AF-A with the superplasticizers were also investigated, and the early hydration behavior and morphology of the hydration products of reference cement paste with the FF-AF-A were explored by hydration heat, X-ray diffractometry (XRD), and scanning electron microscopy (SEM). Test results indicated that adding the FF-AF-A at 8 wt% of the cement weight resulted in 2 min 35 s initial setting time and 6 min 30 s final setting time. The 1-day compressive strength of the cement mortar with 8 wt% of FF-AF-A reached 13.5 MPa, which represents an increase of 35% as compared to the strength of cement mortar without the FF-AF-A, and the 28-day compressive strength ratio was 119%. In addition, the FF-AF-A also showed good compatibility with different superplasticizer dosages. The results show that, when the FF-AF-A was added to the cement paste, it promoted the formation of ettringite crystals due to the aluminum ions (Al3+) and sulfate ions (SO4 2-) reacted with gypsum in the cement, as well as promoted the hydration of tricalcium aluminate (C3A) and tricalcium silicate (C3S) leading to the overall structure becomes more compact. As a consequence, the hydration heat rate of the cement sharply increased, the cement paste setting time is shortened, and the compressive strength of cement mortar is improved.

Influence of water-to-binder ratio on the optimum percentage of nano-TiO2 addition in terms of compressive strength of mortars: A laboratory and virtual experimental study based on ANN model

This research aims to study the effect of water-to-binder ratio (w/b) as a potential key factor that changes the optimum percentage of nano-TiO2 addition in terms of compressive strength of mortars. Data from previous literature were collected and analyzed. An artificial neuron network (ANN) model was trained and validated to predict the compressive strength of mortars as a function of the level of nano-TiO2, w/b, cement content, aggregate-to-binder ratio, and age. Then, a virtual experimental campaign evaluating 45,015 case scenarios, and using the ANN model, was performed to study the effect of the w/b on the optimum percentage of nano-TiO2. Results suggest that the higher the w/b and the lower the age, the higher the optimum percentage of nano-TiO2. Laboratory experimental compressive strength results are in agreement with these trends. Thermogravimetric analysis and 3D X-Ray scans show that the use of nano-TiO2 presents a double positive effect on compressive strength, reducing the porosity and increasing the hydration product content when the initial porosity of the reference cement paste is high (due to high w/b). However, when the porosity of reference samples is very low (due to low w/b), the use of nano-TiO2 may lead to a reduction of hydration products content due to the lack of space for the CH to grow. Since the initial porosity was already very low, the porosity reduction due to the use of nano-TiO2 would not be enough to overcome the negative effect on hydration. Therefore, it may lead to a decrease in the compressive strength of mortars. These results explain the dependence of the optimum percentage of nano-TiO2 on w/b and age.

Chemical properties of pore solution of hardened cement pastes with mineral additions

This paper presents the influence of mineral additions in blended cement, or ‘green’ concrete, on the divalent ions of the cement paste pore solution. Because of the lack of data concerning the direct comparison between the pore solution of cement and mineral additions-based material, six types of hardened cement pastes were tested. For this purpose, Portland cement (CEM I or CEM V) and mineral additions (limestone filler, fly ash, blast-furnace slag and silica fume) were used. Results from chemical analyses by ionic chromatography show significant concentrations of divalent ions, such as sulfates and calcium, in the pore solution. In fact, the reference cement paste based on CEM I contains about 1.8 mmol/l of calcium and 1.6 mmol/l of sulfates. However, the use of mineral addition significantly modifies the pore solution. In particular, a substitution of cement by 10% of silica fume increases the calcium and sulfates concentrations to 12.2 mmol/l and 6.7 mmol/l, respectively. This change in the chemical composition, the pH and the ionic strength should be taken into account in multispecies transfer modelling and the thermodynamic equilibrium. The time evolution of the chemical composition, pH and ionic strength of the pore solutions was also investigated.

Effect of potassium humate as humic substances from river sediments on the rheology, the hydration and the strength development of a cement paste

Dredged river sediments may contain various types of organic matter that can affect the properties of a cement matrix, with the most representative part of such organic matter consisting of humic substances (HS). This work seeks to investigate the effects of humic substances on the rheology, the hydration and the strength development of a cement paste with different curing times, ranging from 1 to 90 days. Results show that adding organic matter to the cement paste offers greater workability by decreasing the yield stress. This addition also serves to retard the hydration of cement particles. Setting times increase to beyond one week when raising the amount of humic substances in the mixture, while the released heat ultimately reaches values comparable to those of the reference cement paste. Therefore, the addition of organic matter negatively affects strength development of the paste, especially at an early age; no resistance at 1-day has been noticed for HS content greater than 1%. Nevertheless, there was no significant reduction in the compressive strength up to 20% for HS content less than 1% at 90-day. In parallel, a study on pastes containing calcium lignosulfonate (water reducer) is carried out and the same effects are observed.

A method to determine water absorption of recycled fine aggregate in paste for design and quality control of fresh mortar

The high absorption of recycled fine aggregate (RFA) directly affects the effective W/C ((W/C)eff) of paste in cement-based materials, giving poor fresh state consistency and affecting the mechanical behavior in the hardened state. A novel method was proposed to determine the water absorption of RFA in fresh paste (WAp) at plastic stage in this paper, which was calculated by the difference of total water content of paste between the mixture incorporating the RFA and the reference cement paste. Three series of mortars were prepared respectively for investigating the influence factors on the WAp, considering the absorption capacity of RFA in water at 24 h (WA24h), W/C (0.35, 0.45, 0.55) of fresh paste and A/C (1, 2, 3) of the proportions. The results show that the water absorption of RFA in paste and its evolution is lower than that in water, being up to 44.38%–80.18% of WA24h at 1 h. In the first 5 min, the WAp reaches quickly to the 36.34%–57.61% of WA24h. And it decreases when using RFA with smaller WA24h, and in the mixture proportions with a lower W/C and higher A/C. Based on the relationship between the WAp and these factors, the (W/C)eff of mortar with dry RFA can be estimated in different mixtures. Conversely, the additional water for a better workability can be also calculated with the proposed W/C, which has been verified by the testing results for total water content of paste in various mortars. The corresponding reduction of water absorption in paste should be taken into account during the mixture design with RFA.

Characterization of carbonation-cured cement paste using X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) was used to examine the microstructure of cement pastes following early-age CO2 curing. The samples were hydrated for 18 h, and, then, were exposed to high-purity gaseous CO2 at a pressure of 0.15 MPa for 6 h. They were subsequently hydrated until the age of 28 d. The samples were tested right after the carbonation process at the age of 1 d, as well as after the subsequent hydration. The results were compared with those obtained for the reference cement pastes which were not exposed to the carbonation curing. It was found that the calcium-silicate-hydrate (C-S-H) produced by the early-age carbonation had a more disordered and highly polymerized structure compared to that in the conventionally hydrated cement paste. The subsequent hydration significantly changed this structure, and resulted in the incorporation of additional calcium ions. The improved compressive strengths at both test ages were attributed to the higher polymerization of silicates in the cement pastes due to the early-age carbonation curing. Based on the quantitative X-ray diffraction results, more than 40% of C2S phase of cement was reacted in early carbonation at the age of 1 d. This accelerated reaction appeared to be a primary mechanism of the higher compressive strength at both early and late ages.

Influence of carbon nanofiber clustering in cement pastes exposed to sulfate attack

The effects of CNFs and CNF clustering on the microstructural evolution and mechanical response of cement pastes subjected to sulfate attack were studied. Portland cement pastes with and without 0.5% CNFs were exposed to sodium sulfate for a total duration of 550 days. The presence of CNFs and CNF clusters led to pore refinement, limited sulfate-induced microstructural changes, and provided a reinforcing network that delayed the onset of cracking and spalling and increased the cement paste expansion capacity. At all stages of exposure, the cement paste with CNFs experienced less sulfate-induced changes in flexural strength than the reference cement paste.

Effects of nano-SiO2 on the permeability-related properties of cement-based composites with different water/cement ratios

To find the suitable conditions under which nano-SiO2 can exhibit a significant impermeability enhancement effect and the mechanism underlying this effect, comparisons between the permeability-related properties of a nano-SiO2-filled cement paste and those of a reference cement paste composed of different water/cement (W/C) ratios were carried out in this research. Permeability-related properties of cement paste, such as the chloride-ion penetration coefficient (D nssm), water permeability coefficient (K p), and initial water sorptivity coefficient (S i), were tested. Furthermore, Power’s model, mercury intrusion porosimetry data, and the general effective media theory were also applied to analyse the evolution mechanism. The results indicate that the effect of nano-SiO2 on the enhancement of the impermeability becomes more remarkable at a lower W/C ratio. The decreasing rates of D nssm, K p, and S i increase as the W/C ratio decreases. Furthermore, it can be concluded that the effects of nano-SiO2 on promoting the hydration, refining the pore structure, narrowing the width of microcrack and thus enhancing the impermeability of cement paste become much clearer as the W/C ratio decreases.

Use of a plant based polymeric material as a low cost chemical admixture in cement mortar and concrete preparations

Aqueous extract of okra called here as bio-admixture was characterized and tested for using as a sustainable chemical admixture in the production of cement mortar and concrete. Several fresh and hardened states properties of cement mortars and concrete with and without the presence of bio-admixture were evaluated. Interactions of bio-admixture with cement particles during hydrations were also investigated by using conductometric and FTIR spectroscopic methods. Moreover, an improved method was developed to determine water retention capacity of cement pastes. Results indicate that the investigated bio-admixture exhibits viscosity enhancing property. In comparison to the reference cement paste, bio-admixture containing cement pastes have shorter setting times due to increasing hydration rate of cement particles. Solution chemistry and FTIR spectroscopic investigation indicate that the addition of bio-admixture enhances cement hydration rate. Increase hydration rate can be related with the interactions of pectin type heteropolysaccharides present in bio-admixture with Ca2+ ions of cement paste. Compressive strengths of bio-admixture containing mortar and concrete are higher than that of the reference specimens at the investigated curing period. Deterioration of strength of bio-admixture based mortars cured in MgSO4-NaCl solution is distinctly less in comparison to reference, depicting higher durability under such conditions. Most of the properties of cement mortar and concrete are also dependent on the concentrations of bio-admixture in aqueous solution. Therefore, investigated bio-admixture can be considered as a low-cost, environmentally benign viscosity enhancing admixture for producing sustainable cement composites having improved mechanical and durability properties.

HYDRATION PROCESS AND MECHANICAL PROPERTIES OF CEMENT PASTE WITH RECYCLED CONCRETE POWDER AND SILICA SAND POWDER

Recycled concrete powder (RCP) mostly consisting of cement paste could be reused as partial cement replacement. The aim of this paper is to compare hydration and mechanical properties of RCP and two types of silica sand powder (SSP). Comparison of those materials combined with cement can highlight the binder properties of recycled concrete powder. Using of two types of SSP also show an influence of their fines on hydration process and mechanical properties. Particle size analysis and calorimetric measurement were carried out and mechanical properties such as bulk density, dynamic Young’s modulus and compression strength were examine. Calorimetric measurement proves the presence of exposed non-hydrated particles in RCP that can react again. However lower density of old cement paste in RCP overweight the mentioned potential of RCP and mechanical properties are decreasing compared with reference cement paste and cement paste SSP.

Influence of increasing amount of recycled concrete powder on mechanical properties of cement paste

This paper deals with using fine recycled concrete powder in cement composites as micro-filler and partial cement replacement. Binder properties of recycled concrete powder are given by exposed non-hydrated cement grains, which can hydrate again and in small amount replace cement or improve some mechanical properties. Concrete powder used in the experiments was obtained from old railway sleepers. Infrastructure offer more sources of old concrete and they can be recycled directly on building site and used again. Experimental part of this paper focuses on influence of increasing amount of concrete powder on mechanical properties of cement paste. Bulk density, shrinkage, dynamic Young’s modulus, compression and flexural strength are observed during research. This will help to determine limiting amount of concrete powder when decrease of mechanical properties outweighs the benefits of cement replacement. The shrinkage, dynamic Young’s modulus and flexural strength of samples with 20 to 30 wt. % of concrete powder are comparable with reference cement paste or even better. Negative effect of concrete powder mainly influenced the compression strength. Only a 10 % cement replacement reduced compression strength by about 25 % and further decrease was almost linear.

Hydration Heat Evolution of the Cement Paste with Recycled Concrete: Influence of Grain Size Distribution of Recycled Concrete Powder

The paper deals with the influence of level of grinding of the recycled concrete. Level of grinding mostly influence the hydration heat evolution of cement paste. Measurement of hydration heat evolution is one of the ways to detect reactive properties of recycled concrete powder modified by high speed mill device. Basic characteristics that most influence the hydraulic properties of recycled concrete powder are grin size diagram and chemical composition of the original concrete. Non-hydrated inner part of cement grains could further react if it is exposed to water. In this experiment the recycled concrete from recycled railroad sleepers and structural layers of the D2 highway was used. Recycled concrete powder from highway was divided into 4 more variants which differ by grinding process during production. To detect difference in hydration process four mixtures of cement paste with different types of recycled concrete powder were made and further subjected to calorimetric measurement. The results were after compared with the hydration heat rate of reference cement paste. According to results the type of milling process had minor influence on hydraulic properties of recycled concrete powder compared with influence of origin of recycled concrete.

Influence of Finely Ground Recycled Concrete on Microstructure of Cement-Based Composite Material

This article deals with using recycled concrete in cement-based composite materials. The recent studies were mostly focused on utilization of recycled concrete in the form of an aggregate filler. In this study we are investigating the possibility of using finely ground recycled concrete as microfiller or partial substitution for binder. In particular, we focus on changes in microstructure of the cement paste which differed in amount of finely ground recycled concrete (FGRC). We used four mixtures of cement pastes containing 0, 33, 50 and 67 wt. % of FGRC, respectively. For the examination of their microstructure, phase distinction and determination of FGRC influence, the images obtained using optical and electron microscope were used. The first results indicate that cement paste with 33 wt. % of FGRC has similar mechanical properties as reference cement paste. Partial replacement of cement by finely ground recycled concrete leads to a cost reduction of cement-based composites and also can reduce environmental impacts of construction waste disposal and cement production.

Replacement of Cement with Finely Ground Recycled Concrete: Influence on Mechanical Properties

The waste production from construction sites become very serious problem. Recycling is the best option for disposal of such waste, and the proper sorting and knowledge of the recycled concrete history allows its further use in the construction. The current studies are mostly focused on utilization of recycled concrete in the form of aggregate. The presented work is focused on the utilization of Finely Ground Recycled Concrete (FGRC) used as a filler and partial substitution for binder. Recycled concrete was ground from concrete railway sleepers in the Lavaris Company (Czech Republic). Through the testing of mechanical properties, we demonstrate the influence of FGRC’s amount in cement paste on mechanical properties of the composite. To clearly show the relationship between the amount of FGRC and the composite properties, samples with 33, 67 and 100 wt. % of cement replaced by FGRC were tested. The composite with 33 wt. % of FGRC attained the compressive and flexural strength comparable with reference cement paste without any FGRC additions. The results indicate that the partial substitution of cement by FGRC could lead to a cost reduction of cement composites with minimal impact on their mechanical properties.

Use of wood waste (Aleppo pine) as a superplasticiser in self-compacting mortars

This study investigated the effect of waste of Aleppo pine wood on the rheological and mechanical properties of self-compacting concrete. Aleppo pine wood is a compound of cellulose, hemicelluloses and more than 25% lignin. The latter belongs to the admixture family used as plasticiser in concrete. An experimental study was conducted to evaluate the rheological and mechanical properties of mortar containing the coupons of Aleppo pine wood (in liquid form) at different dosages. According to the results, the main rheological parameters of cementitious pastes (shear stress and plastic viscosity) decreased with increasing dosage of waste Aleppo pine. The cementitious pastes containing 5% waste, became more fluid, compared to the reference cement paste (containing 1·0% superplasticiser). Also, compared to the reference mortar prepared with superplasticiser, a decrease in compressive strength of mortars with increasing dosage of Aleppo pine wood was found. However, the resistance values obtained by the mortars (5% of Aleppo pine) were very close and gave a mortar with 30 MPa strength, which is an acceptable value for construction.