Steam-cured Mortar Research Articles

Mechanical properties, permeability and microstructure of steam-cured fly ash mortar mixed with phosphogypsum

Incorporating proper amount of fly ash (FA) contributes to improve the long-term performance of steam-cured mortar. However, with the introduction of stringent environmental policies, the reserves of high-quality FAs are decreasing and the costs are increasing. In this study, phosphogypsum is used to replace part of FA to produce high-quality prefabricated components, and mechanical properties, water permeability, microstructure and environmental benefits of samples are investigated. The research results show that replacing FA with proper amount of PG can greatly increase mechanical properties of steam-cured FA mortar and improve its water impermeability. When 10% of PG is applied to the steam-cured FA mortar, the 1-d and 28-d compressive strengths of the samples increase by 11% and 5.37%, respectively, relative to the control mortar. The results of microstructure characterization show that this is due to the generation of swelling products and the increase of the reaction degree of FA. When PG and steam curing are applied in combination, FA is severely corroded and a gel phase product is generated, which contributes to increase the strength of specimen. In addition, using appropriate amount of PG also brings obvious economic and environmental benefits for prefabricated components.

A study on mechanical properties and permeability of steam-cured mortar with iron-copper tailings

With the development of construction industrialization, the use of steam curing to produce precast concrete is widely used. In order to recycle long-term stacked tailings waste, iron-copper tailings (ICT) waste is comprehensively utilized in steam-cured mortar (SCM). The effect of ICT substitution level on the mechanics and durability of steam-cured mortars is investigated in this study. The research results show that the use of appropriate ICT instead of river sand in the production of mortar is beneficial to the enhancement of the mechanics and durability of the mortar. When the replacement level of ICT is 10%, the 1-d and 28-d compressive strength of SCM are increased by 1.09% and 7.27% respectively compared with plain mortar. SCM mixed with 10–20% ICT has lower permeability compared to plain samples, and further increasing the substitution level of ICT is not benefit the development of mortar durability. Since ICT has better conductivity compared to river sand, it makes the resistivity of SCM decrease with the increase of ICT content. Microstructural studies show that the addition of a small amount of ICT reduces the porosity of SCM and refines its pore structure. Meanwhile, the ICT and cement matrix exhibits a more dense ITZ than the river sand and cement matrix. Recycling of ICT also shows better economic and environmental benefits in SCM. This study provides a theoretical basis for recycling composite tailings waste in prefabricated components.

A preliminary study on the efficiency of the steel slag-based spraying carbonation layer in improving the durability of cement-based products

The idea of using a steel slag-based spraying carbonation layer to improve the durability of cement-based products was proposed. The carbonation layer was prepared by spraying the steel slag-based slurry on the surface of cement-based products and then curing them with high-concentration CO2. The results showed that the carbonation layer had excellent sulfate resistance; additionally, it effectively protected the standard-cured and steam-cured mortars from sulfate attack and reduced the chloride ions migration coefficient of both standard-cured and steam-cured mortars significantly by 57.1% and 53.8%, respectively. The protection of the carbonation layer on mortars was attributed to the better pore structure of the carbonation layer and the dense interfacial transition zone of the carbonation layer and mortar matrix. The steel slag-based spraying carbonation layer recycled the steel slag, solidified CO2, and improved the durability of cement-based products, which had great environmental and engineering significance.

Pore structure evolution and sulfate attack of high-volume slag blended mortars under standard curing and steam curing

Partial replacement of cement with high-volume slag reduces the amount of cement in concrete and thereby lessens the environmental pressure caused by cement production. This paper investigates the pore structure evolution and sulfate attack of high-volume slag blended mortars under standard curing and steam curing, discusses the effects of slag content and pore structure on sulfate attack, and evaluates the methods for determining the sulfate resistance performance. The pore structure results show that the hydration of cement and/or slag reduces the porosity and most probable pore diameter (MPPD) and refines the pore structure of mortars. Steam curing primarily affects the pore size distribution but not the porosity for the mortars within 3 d. Increasing slag replacement increases the porosity for standard-cured and steam-cured mortars at the age of 28 d; however, it significantly reduces the air voids that are detrimental to sulfate resistance. Sulfate attack results show that the combination of the flexural strength test with the sulfate-ion content can evaluate the sulfate resistance well. The sulfate resistance improves with increasing slag content, and 70% of slag improves the sulfate resistance significantly for both standard-cured and steam-cured mortars. In addition, the MPPD, gel pores, and mesopores do not seem to determine the sulfate resistance. Reducing the volume fractions of large capillary pores, especially air voids, can improve sulfate resistance.

Influence of different curing conditions on water absorption of cement-based materials under dry and wet cycles

As the main structural material of a subway, the precast concrete segment is produced under the steam-curing condition, and frequently experienced dry–wet cycles environment. In this study, steam-curing and dry–wet cycling methods were used to simulate the water absorption process of cement-based materials under special circumstances. The results showed that the water absorption in cement-based materials decreased with an increase in the number of dry–wet cycles. Based on a micro-analysis, the proportion of large capillary pores (dimension >0.1 μm) in the steam-cured mortar increased during the water absorption process with the increase in the number of dry–wet cycles compared with the standard cured mortar. Through thermodynamic and kinetics analysis, the mechanism and model were calculated to explain the component transformation mechanism of cement-based materials under dry and wet cycles.

Properties and characterization of high-performance steam-cured cement-based materials modified by phase change materials

Steam-cured concrete is widely used in precast element production due to its high early strength and efficiency, but its long-term performance is often poor. Integrating phase change materials (PCMs) into steam-cured samples is a potential solution. The influences of six PCMs on the fluidity, mechanical strength, toughness, water absorption, and pore structure of steam-cured mortars are investigated by a series of experiments. PCMs employed are paraffin (P-45 and P-58), fatty acids (lauric acid, LA and myristic acid, MA), and polyethylene glycol (PEG1500 and PEG4000). The results show that PEG powder can improve the fluidity of fresh mortar, while the fluidity reduces as the amount of LA or MA increases. The addition of appropriate PCMs slightly reduces the strength of steam-cured mortar but effectively improves the toughness. Incorporating proper PCMs significantly decreases the capillary water absorption of the steam-cured mortar due to the modification of the surface characteristics of pore walls by paraffin and fatty acids. Compared with specimens containing PEG and fatty acids in this study, mortar containing paraffin have higher strength, especially for the one-day early strength. Employing an appropriate amount (not more than 5%) of paraffin is an excellent way to develop high-performance steam-cured cement-based materials, which extend the service life and reduce an environmental load of steam-cured cement-based materials.

Experimental investigation on the influence of phase change materials on properties and pore structure of steam-cured mortar

Steam-cured concrete with high early strength has been widely applied in high-speed railway. However, heat damage caused by the steam curing process brings adverse effects on the pore structure and durability of steam-cured concrete. It has been found that phase change materials (PCMs) help improve the durability of cement-based materials. Therefore, the influences of two PCMs on the strength, water absorption, and microstructure of steam-cured mortar are discussed in this study. Two PCMs used include pure paraffin and a composite phase change material composed of porous diatomite and paraffin (D/P). The results show that the addition of appropriate PCMs does not significantly reduce the strength of steam-cured mortar. The addition of a small amount of PCMs shows little influence on the pore structure of steam-cured mortar but effective in improving the water-penetration resistance. This is partially attributed to the improvement of hydrophobic characteristics in the pores, resulting from the adhesion of hydrophobic PCMs on the pores during the steam curing. Compared with specimens containing pure paraffin, the pore structure of specimens containing D/P is refined, resulting in higher compressive strength. This may be due to the uniform distribution of small particles D/P in the specimen and the pozzolanic reaction of diatomite in D/P. The use of suitable phase change materials will be helpful to improve the durability of steam-cured concrete.

PHYSICAL PROPERTIES AND ENVIRONMENTAL PERFORMANCE OF STEAM-CURED CONCRETE CONTAINING FLUE-GAS DESULFURIZATION GYPSUM

ABSTRACT Flue-gas desulfurization (FGD) gypsum has occasionally been used as an additive to cement. Consequently, appropriate facilities are required to ensure the environmentally safe processing of FGD gypsum and the resulting cement material properties. Such facilities are yet to be developed because the amount of FGD gypsum used is still small when compared with the vast amounts of FGD gypsum generated. In this study, we analyze the effect of FGD gypsum addition on the physical properties, stabilization, and radon count of steam-cured mortar and compare its performance with air-cured mortar. Our results show that the steam-cured pozzolanic hydration products of ettringite and C-S-H promote the densification of the mortar structure, thereby resulting in nanopore size reduction and increased strength of FGD gypsum mortar subsequent to the steam-cured hydration process. Further, our environmental test results indicate that steam-cured pozzolanic materials composed of FGD gypsum are environmentally safer than air-cured cementitious materials.

Mechanical properties and water absorption of steam-cured mortar containing phase change composites

Steam curing is a common method used in the production of precast concrete components. However, the formation of microcracks and large pores during steam curing process are very detrimental to the long-term performance of cement-based materials. The use of phase change materials may be a potential method for improving disadvantages caused by steam curing. Phase change composites are prepared by implanting paraffin into diatomite, expanded graphite, and mixture of diatomite and expanded graphite. They are represented by D/P, EG/P and D/EG/P in turn. The thermal properties and static water contact angle of phase change composite (CPCM) are tested. The mechanical and water absorption properties of steam-cured mortars with CPCM are investigated. Results indicate that the latent heat values of EG/P, D/P and D/EG/P are 80%, 30% and 36% of the values for pure paraffin, respectively. The CPCMs exhibit hydrophobic properties due to the roughness of solid surface. The presence of CPCM greatly improve the water resistance of steam-cured mortar without sacrificing mechanical properties, and steam-cured mortar with 1 vol% D/P has the best performance. The possible reason is that D/P reduces the porosity and improves porous microstructure of steam-cured mortar. The cracks and holes in the mortar matrix can be covered and filled with paraffin from D/P by SEM analysis. There is no interfacial gap between D/P and the mortar matrix through SEM analysis, which promotes the formation of a dense and strong mortar matrix. Therefore, the addition of an appropriate CPCM is helpful to improve the durability of the steam-cured cementitious materials.

Sulfate resistance and hydration products of steam cured steel slag blended cement mortar under dry–wet cycle

In this investigation, the dry–wet-cycled sulfate resistance and hydration products of steam-cured and standard-cured steel-slag mortars were investigated. Strength loss, porosity change, and microstructure of mortars prepared by replacing 20% of cement with steel slag (SS) were analyzed and compared with pure cement (PC) mortars. The results revealed that SS-blended cement mortar shows worse sulfate resistance than PC mortar regardless of the initial curing condition. Steam-cured PC mortar presents inferior sulfate resistance to standard-cured PC mortar, on the contrast, steam curing improves the sulfate resistance of SS mortar, when compared with the standard-cured mortars. Besides the large amount of C-S-H gels, the formation of hemicarbonate, C-A-S-H and relative lower CH content under steam curing can account for the improved sulfate resistance of initially steam-cured SS mortars, although the high porosity caused by steam curing brings negative impact to the sulfate resistance.

Acoustic emission analysis of characteristics of healing products in steam-cured cementitious materials with mineral additives

The objective of this work is to investigate the self-healing performances of steam-cured mortars with mineral additives (e.g., calcium-based materials, chemical expansive agents and crystalline components). The self-healing capabilities of crack was evaluated by transport tests and acoustic emission (AE) analysis for cracked mortars. The two representative AE parameters, i.e., amplitude and energy, were used to reveal the characteristics of healing products. The results showed that the improvement of transport properties of cracked steam-cured mortars were more effective compared to standard-cured mortars. The healing efficiency can be further improved by utilizing mineral additives. The differences of damage stage between healing products and host matrix can be distinguished by AE signals. Upon reloading of steam-cured mortar with mineral materials, the number of amplitude were much larger and the cumulative energy was much higher compared to other mortar. It indicated that the steam-cured mortar with mineral materials possessed larger amount and much higher energy healing products in the crack to some extent, which were well consistent with that of transport properties and microscopic observation.

Effect of Expansive Additive and Shrinkage Reducing Agent on Plastic Shrinkage of Steam-Cured Mortar

This research is part of the mortar’s plastic shrinkage study. Contents in this article is related to how the Expansive Additive (EX), Shrinkage Reducing Agent (SRA), and Fly Ash (FA) help to reduce and control the shrinkage and to compare the effectiveness of these substances used in the normal curing, i.e., at 30 °C and in the steam curing process by using the factorial design with 3 factors and to be divided into 2 levels. Factors to be studied are the amount of EX, SRA and FA replacement. The test of plastic shrinkage was conducted in accordance with the ASTM C1579-06 standard, placing the strain gauge 0.5 centimeters beneath the surface in the middle of the mold, recording the shrinkage rate starting from the initial setting time for 24 hours. The results showed that, in normal curing, the EX influences the expansion while, in steam curing, the EX and SRA significantly influences the expansion. To add the FA in high volume along with the EX significantly effects the expansion for both the normal and steam curing. Furthermore, the study model and equation for plastic shrinkage of mortar are presented in the form of factor proportion to be considered from the factorial design study basis.

Modeling heat and moisture transport in steam-cured mortar: Application to AASHTO Type VI beams

During steam curing of concrete, temperature and moisture gradients are developed, which are difficult to measure experimentally and can adversely affect the durability of concrete. In this research, a model of cement hydration coupled to moisture and heat transport was used to simulate the process of steam curing of mortars with water-to-cement (w/c) ratios by mass of 0.30 and 0.45, considering natural convection boundary conditions in mortar and concrete specimens of AASHTO Type VI beams. The primary variables of the model were moisture content, temperature, and degree of hydration. Moisture content profiles of mortar specimens (40mm in diameter and 50mm in height) were measured by magnetic resonance imaging. The degree of hydration was obtained by mass-based measurements of loss on ignition to 1000°C. The results indicate that the model correctly simulates the moisture distribution and degree of hydration in mortar specimens. Application of the model to the steam curing of an AASHTO Type VI beam indicates temperature differences (between the surface and the center) higher than 20°C during the cooling stage, and internal temperatures higher than 70°C that may compromise the durability of the concrete.

Influence of alkali activators on the early hydration of cement-based binders under steam curing condition

The effects of water glass, NaOH, Na2SO4, and Na2CO3 on the early hydration of plain cement and two composite binders incorporating 40 mass% ground granulated blast furnace slag (GGBS) and 40 mass% fly ash under the steam curing temperature of 60 °C were investigated. Meanwhile, a 20 °C curing condition was set as a reference. The results showed that water glass, NaOH, and Na2CO3 can improve the exothermic rates of the binders at the acceleration period and promote the reaction of GGBS and fly ash. However, the alkali activators inhibited the hydration of Portland cement at the deceleration period, which was more significant at 60 °C. Thus, the alkali activators cannot efficiently increase the cumulative hydration heat within 16 h under steam curing or even appear to decrease. Na2SO4 shows a better performance than the other three alkalis in the early hydration of the composite binders due to its promotion on the formation of AFt, but it cannot efficiently increase the cumulative hydration heat too. The results also showed that all of the alkali activators significantly decreased the form-removal strength of cement–GGBS composite binder. For cement–fly ash composite binder, the form-removal strengths of the samples activated by a low concentration of water glass and high concentration of Na2SO4 were close to that reacted with water, while the form-removal strengths of the samples activated by other alkaline solutions were lower. Overall, the four alkali activators cannot improve the form-removal strength of steam-cured mortar containing a large volume of mineral admixtures.

Effects of Long-Term Dry Curing on Strength Development of Initially Combined Wet/Dry-Cured and Steam-Cured Hardener-Free Epoxy-Modified Mortars

It is clarified by the previous studies that even without any hardener epoxy resin can harden in the presence of hydroxide ions from calcium hydroxide in the cement mortars and concretes at ambient temperature. The strength development through the long-term curing of hardener-free epoxy-modified mortars and concretes has hardly been examined till now. Hardener-free epoxy-modified mortars using an epoxy resin (diglycidyl ether of bisphenol A) without any hardener are prepared with various polymer-cement ratios, subjected to (1) initial combined wet/dry curing plus long-term dry curing and (2) initial steam curing plus long-term dry curing, and tested for hardening degree of epoxy resin and strength development. As a result, the application of the initial steam curing rather than the initial combined wet/dry curing causes a marked strength development with increasing polymer-cement ratio and additional dry curing period. The flexural and compressive strengths of the initially steam-cured plus 336-d dry-cured hardener-free epoxy-modified mortars with polymer-cement ratios of 10% or more are about 1.5 to 2.0 times higher than those of the initially steam-cured unmodified mortar. This is proved by a marked increase in the degree of hardening of the hardener-free epoxy resin in the epoxy-modified mortars with additional dry curing period.

STUDIES ON THE ACTION OF SULPHATES ON PORTLAND CEMENT: III. THE EFFECT OF THE ADDITION OF SILICA GEL TO PORTLAND CEMENT MORTARS ON THEIR RESISTANCE TO SULPHATE ACTION

The effect of substituting silica gel for a portion of the Portland cement in standard and graded-sand mortars on the expansion and loss in strength of the mortars in sulphate solutions was determined. Portland cement silica gel sand mortars were cured in steam at 100 °C. and the effect on their sulphate resistance measured; the behavior of lime-silica gel sand mortar in solutions of sodium and magnesium sulphate was also studied. It was found that the addition of silica gel to the mortar, very effective in preventing expansion and maintaining the tensile strength of the mortar in solutions of sodium, and calcium sulphate, was not so effective in solutions of magnesium sulphate. Steam-cured mortars containing silica gel to the extent of 20% of the cement present showed a slightly greater resistance to the action of solutions of sodium and calcium sulphates, but less resistance in solutions of magnesium sulphate, than similar steam-cured mortars containing no silica gel. Lime-silica gel sand mortars behaved very similarly in sulphate solutions as Portland cement mortars containing silica gel. The possible causes of the effects produced by the addition of silica gel to Portland cement mortars are considered, and several explanations discussed.