Surface Tension Of Pore Solution Research Articles

Effects of different activators on autogenous shrinkage of alkali-activated slag cement

Despite the exceptional early strength of sodium silicate or sodium hydroxide-activated slags, much emphasis has always been drawn to the volume instability brought on by their autogenous shrinkage. In this study, a ternary composite activation system was developed by introducing sodium sulfate (Na2SO4), and the effect of Na2SO4-Na2O·1.5SiO2-NaOH ternary composite activators system on the autogenous shrinkage, setting time, and mechanical properties of alkali-activated slag (AAS) was investigated. The effect mechanism of different activator compositions on the autogenous shrinkage of AAS was discussed through the internal relative humidity (IRH), surface tension of pore solution, and microstructure analysis. According to experimental results, the surface tension of pore solution can be reduced and IRH increased by increasing Na2SO4 content in AAS. Additionally, the formation of crystals (anhydrite, ettringite, and thenardite) with specific expansion properties can effectively mitigate autogenous shrinkage in AAS with Na2SO4. The proportions of Na2O·1.5SiO2 and NaOH determine the extent to which Na2SO4 impact the autogenous shrinkage of AAS. An optimal composite activator composition for AAS was determined using a model analysis: When the content of Na2O·1.5SiO2 ranged from 5 % to 17 %, Na2SO4 content ranged from 40 % to 55 %, and NaOH content ranged from 33 % to 56 %, the AAS mortar exhibited a compressive strength exceeding 42.5 MPa at 28 days. Furthermore, the autogenous shrinkage strain of AAS paste remained below 1500 microstrain within 7 days, with an initial setting time surpassing 45 min and a final setting time under 390 min.

Synthesis, performance and mechanism of shrinkage-reducing agents with water-reducing function for cement-based materials

Three shrinkage-reducing agents (SRAs) possessing a water-reducing capacity with different structures were synthesized by aqueous free radical copolymerization. Their structures were characterized by Gel Permeation Chromatography (GPC), specific anion charge density, and Fourier Transform Infrared Spectrometer (FTIR) measurements. Their abilities to reduce surface tension and their adsorption and dispersing effectiveness in cement paste as well as their impacts on cement hydration and compressive strength, autogenous shrinkage, drying shrinkage, and pore structure of cement mortar were investigated. Results clearly showed that the three synthesized SRAs with a water-reducing function reduced the surface tension of pore solution, with the water-reducing SRA with butyl exerting the most significant effect due to its lower molecular mass. They enhanced the flowability of cement paste because of their adsorption on cement grains. The water-reducing SRA with dimethyl amino exhibited higher adsorption amounts on cement grains due to higher charge density, resulting in higher initial flowability of cement paste and more significant hydration retardation. Their presence decreased the autogenous shrinkage of cement mortar by reducing the surface tension of pore solution, delaying the cement hydration, and decreasing the capillary pore volume. Their addition also mitigated the drying shrinkage due to the synergistic effect of surface tension reduction, pore structure coarsening, and porosity (2.5–50 nm) reduction. Among the three, the water-reducing SRA with butyl demonstrated a higher capability of reducing the drying shrinkage and had a favorable effect on the compressive strength of mortar at later ages.

Synthesis, performance and mechanism of novel polymer-type shrinkage reducing agents for cement-based materials

Simplifying the synthesis process and preparing a new type of shrinkage reducing agent (SRA) with a low dosage and high efficiency are beneficial for promoting the development of SRA and its application in cement-based materials. This study aims to synthesize three novel polymer-type SRAs with diverse structures through free radical copolymerization and to investigate their performance and working mechanisms in cement-based materials. Results clearly showed that the three synthesized SRAs reduced the surface tension of pore solution, and increased the flowability of cement paste. The SRAs-1 with butyl resulted in higher flowability of cement paste due to lower surface tension. Their addition at a low dosage of 0.5% by cement mass effectively mitigated the shrinkage of cement mortar. Among the three, SRAs-1 with butyl exhibited a higher capability of reducing drying shrinkage due to lower surface tension, lower porosity in the range of 2.5–50 nm, and lower total porosity.

Performance of fly ash-based geopolymer incorporated with high magnesium nickel slag: Effects of different cooling conditions

High magnesium nickel slag (HMNS), as industrial by-product generated from the pyrometallurgical extraction of nickel, has the great potential to be used as the silica-rich precursor for the production of geopolymers. This work investigated the compressive strength and autogenous shrinkage of fly ash-based geopolymers incorporating HMNS with different cooling conditions. The results showed that HMNS additions could enhance the compressive strengths, while the autogenous shrinkage rates were also increased. The addition of air cooling HMNS with finer particle size could compact the pore structure. Larger proportion of medium-capillary pores formed in the binders raised the capillary pore pressure. Compared with the air cooling HMNS, the water quenching HMNS exhibited relatively higher reactivity. The Ca2+ and Mg2+ ions released from HMNS increased the surface tension of pore solution, resulting in a larger pore pressure developed in the binder. Furthermore, Mg/Si and Ca/Si ratios of binder gel phase were enhanced simultaneously, suggesting that HMNS addition facilitated the development of magnesium/calcium modified sodium aluminosilicate hydrate (N–(M)–A–S–H and N–(C)–A–S–H) gel phases.

Research on behavior mechanism and performance of concrete shrinkage reducing agent

The research on the function and mechanism of shrinkage reducing agent shows that shrinkage reducing agents can greatly reduce the shrinkage rate of concrete without adversely affecting the mechanical properties of concrete. The mechanism of the shrinkage-reducing agent was tested and analyzed from the water evaporation rate of the solution and the surface tension of the solution. The results showed that the addition of the shrinkage reducing agent could significantly reduce the surface tension of the aqueous solution, and had little effect on the evaporation rate of the water on the surface of the aqueous solution; the surface tension test shows that the mechanism of shrinkage reducing agent reducing drying shrinkage is mainly to reduce the surface tension of pore solution; the analysis from the water evaporation rate test shows that shrinkage reducing agent has no significant effect on water evaporation inhibition, and inhibiting concrete moisture evaporation is not the most commonly used shrinkage reducing agent reduction mechanism.

Water retention mechanism of cellulose ethers in calcium sulfoaluminate cement-based materials

To explore the mechanism to water retention of cellulose ether (CE) in calcium sulfoaluminate (CSA) cement-based materials, three cellulose ethers (CEs) with similar viscosity but different substitution groups were selected in this paper, namely hydroxyethyl cellulose (HEC) and hydroxyethyl methyl cellulose (HEMC) with high and low degree of substitution (DS). Water retention of CE modified CSA cement mortar with different CE dosages was characterized by filter paper method. The water state and content in CE modified CSA cement paste were determined by low field nuclear magnetic resonance (1H NMR). The adsorption and dispersion characteristics of CE in CSA cement paste were analyzed by measuring the organic carbon content in the pore solution. Combined with the measurement of dynamic viscosity and surface tension of pore solution, the mechanism to water retention of CEs in CSA cement-based materials was comprehensively analyzed. The results show that the addition of CE can significantly improve the water retention of CSA cement mortar. At the same dosage, the water retention effect of HEMC is better than HEC, and DS has little effect on the water retention capacity of HEMC. The mechanism to water retention of HEC and HEMC added less than 0.1% dosage in system is governed by water sorption effect of CE. While more than 0.1% dosage, HEC and HEMC in CSA cement-based materials achieve water retention respectively through the adsorption of CE on cement and the formation of three-dimensional polymer network in pore solution.

Effect of Surface Tension, Foaming Stabilizer, and Graphene Oxide on the Properties of Foamed Paste.

Foamed paste has attracted much attention because of its excellent thermal insulation performance and diverse applications in infrastructure projects. However, there are still some shortcomings hindering the further application of foamed paste, such as the low mechanical strength and the lack of effective methods to evaluate the properties of foaming bubbles. In this study, surface tension was used as the key parameter to characterize the properties of bubbles. A novel nanomaterial, graphene oxide was employed to enhance the mechanical strength of foamed paste, which was also effective in decreasing the surface tension of aqueous solution. A central composite design scheme was employed to evaluate the influence of three selected factors, surface tension, Sodium Phosphate/foaming reagents mass ratio, and graphene oxide/binder mass ratio, on the engineering properties of foamed paste. Additionally, mercury intrusion porosimetry and scanning electron microscope were employed to elucidate the structure of pores, X-ray diffraction and thermogravimetric analysis were employed to further analyze the hydration products at the microscopic scale. This study reveals that surface tension holds great potential in predicting the engineering properties or performances of foamed paste, and a new mechanism may be developed for explaining the influence of graphene oxide on the pore structure of cementitious materials by evaluating the surface tension of pore solution.

Effect of hydrophobicity on autogenous shrinkage and carbonation of alkali activated slag

The aim of this study is to investigate the feasibility of applying super-hydrophobic slag (s-slag) to reduce the autogenous shrinkage and enhance resistance to carbonation of alkali-activated slag (AAS). The s-slag is produced by milling slag with stearic acid. The influence of the s-slag on the wetting behaviour is characterized by the water contact angle. It was found that the addition of s-slag has a significant influence on the hygroscopic and water absorption properties of the AAS. The hydration kinetics, reaction products, internal humidity, surface tension of pore solution and carbonation depth are investigated in order to explain the mitigation mechanism of the autogenous shrinkage and carbonation test. After applying 20% s-slag, the autogenous shrinkage and carbonation depth of AAS can be reduced by 68% and 70%, respectively compared to the reference. The surface coating by stearic acid reduces the contact between the slag and the activated solution, which decreases the heat release of AAS. The hydrophobic modification alters the wetting property of the AAS which is helpful to decrease the capillary pressure and obstruct contact with CO2. As a result, the autogenous shrinkage and carbonation depth are dramatically decreased. In addition, the introduction of the s-slag increases the compressive strength and flexural strength of AAS.

Shrinkage-reducing measures and mechanisms analysis for alkali-activated coal gangue-slag mortar at room temperature

By using slag as a calcium source, high-strength alkali-activated coal gangue-slag (AACGS) cementitious material can be prepared. Given that higher drying shrinkage at room temperature is detrimental to the expanded engineering application of the material, this paper studies several methods for reducing drying shrinkage such as shrinkage-reducing admixture (SRA), polypropylene fiber (PPF), sulfate-enriched materials (gypsum, high-performance concrete expansive admixture (HCSA), U-type expansive admixture (UEA)) and water retaining admixture (WRA). Meanwhile, the mechanisms for drying shrinkage compensation were deeply analyzed by mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Experimental results show that the addition of admixtures to reduce the drying shrinkage of AACGS mortar is related to the change of pore size of 2–5 nm, and reduces the volume fraction of the minimum aperture. At room temperature, reducing the surface tension of pore solution, generating expansion products, increasing compressive strength and improving pore structure distribution by adding admixtures are important measures to reduce drying shrinkage. SRA exhibits the strongest capacity to compensate for drying shrinkage, followed by UEA, WRA, HCSA, PPF and gypsum. Despite better compensation for drying shrinkage with the increase in SRA dosage, the reduction of strength value becomes more drastic. The findings of this study provide substantial experimental foundations and in-depth theoretical analyses for the drying shrinkage compensation of AACGS binary cementitious materials.

Shrinkage and strength development of UHSC incorporating a hybrid system of SAP and SRA

The individual use of super-absorbent polymer (SAP) or shrinkage reducing admixture (SRA) impairs some properties required for ultra-high strength concrete (UHSC). This study used SAP and SRA in hybrid systems in order to enhance their role on autogenous shrinkage, drying shrinkage and compressive strength of UHSC. Accordingly, the water absorption and the surface tension of pore solution containing the hybrid system of SAP and SRA were measured, and the effect on internal relative humidity (RH), porosity and hydration degree were examined. The results showed that the interaction of SAP and SRA reduced the water absorption capacity of SAP and limited the reducing effect of SRA on the surface tension. However, the hybrid system of SAP and SRA compensated the adverse effect of each on the properties of UHSC. It was also found that the surface tension of pore solution was more effective than the internal RH on the autogenous shrinkage of UHSC. Overall, the specimens with 0.3% SAP and 2% SRA had the minimum autogenous and drying shrinkage.

Mechanism Research of Drying Shrinkage of Cement Pastes Based on the Contact Angle

Capillary tention is one of the driving forces of shrinkage of cement-based materials, which is related to distribution of pore size, surface tension of pore water, contact angle of pore walls and so on. There are some appropriate test methods for pore diameter and surface tension of cement-based materials, but the contact angle of pore water is rarely related to. This paper showed a new method to test the contact angle of cement-based materials which is based on the principle of thin-layer wicking about surface physical chemistry. Combining Washburn equation x2=(Refft/2) , the contact angle of analog pore solution to cement paste was obtained. The influence of superplasticizers and mineral admixtures on the contact angle and drying shrinkage of cement pastes also were researched. The results showed that the larger contact angle leads to the smaller surface tension of pore solution, which also caused that the capillary wall wass infiltrated more difficultly. As a result, the pressure of pore and the drying shrinkage were both small, vice versa.

Effect of mineral admixtures and viscosity modifying admixtures on plastic shrinkage cracking of cementitious composites

The effects of mineral admixtures (MAs) and cellulose-based viscosity modifying admixtures (VMA) on bleeding and plastic shrinkage cracking of cementitious mortar exposed to a windy environment were examined for this article. Cementitious composites containing 25% of three types of MAs, i.e. fly ash and two finenesses of slag were incorporated with two molecular weights of VMA with various mixing ratios in the mortar specimens. Plastic shrinkage cracking tests were conducted on the restrained plate subject to a constant evaporation rate of 1000 g/(m2 h) using a speed adjustable fan in an environment chamber. Bleeding and total crack areas (TCA) and crack widths and crack width distribution were evaluated. In addition, the viscosity of cementitious composite and surface tension of pore solution were also investigated. Without VMA, with the addition of slag, viscosity increased and bleeding reduced, and thus TCA increased. The opposite was found when fly ash was added. With the addition of VMA, bleeding and surface tension reduced significantly. Cementitious mortars with specified VMA dosages appeared to be significantly effective in reducing TCAs, average crack width, and large cracks. The crack reductions are fairly well correlated with surface tension, VMA dosage, and polymer film formation.

Effect of viscosity modifying agent on plastic shrinkage cracking of cementitious composites

The effect of cellulose-based viscosity modifying agent (VMA) on plastic shrinkage cracking of cementitious composites exposed to windy environment was examined in this paper. Two molecular weights of VMA with various mixing ratios were employed in the mortar specimens. Plastic shrinkage cracking tests were conducted on a restrained plate subject to a constant evaporation rate of 1,000 g/(m2 h) using a speed adjustable fan in an environmental chamber. Crack pattern, crack width and total crack area were evaluated. In addition, the viscosity of cement paste, surface tension of pore solution, bleeding, and weight changes due to evaporation were also investigated. Addition of VMA affected both surface tension and bleeding, and thereby resulted in the change of evaporation rates and polymer depositions on the drying surface. The cementitious composites with specified VMA doses were found to be very effective in reducing both crack area and crack width and the crack reductions were fairly correlated with surface tension, bleeding and weight changes.