This article was submitted to Structural Materials, a section of the journal Frontiers in Materials
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Using waste molecular sieves (MS) instead of sand as water-absorbing fine aggregates in cement-based materials can effectively deal with factory adsorption waste and reduce sand consumption. In this article, the industrial waste molecular sieve is recycled and incorporated into cement-based materials. The effect of the molecular sieve as a hydration internal curing agent on the performance of cement-based materials is studied. A series of experiments are designed to find out the appropriate ratio and to evaluate and analyze the internal curing effect of waste molecular sieves. Compressive strength, flexural strength, and dry shrinkage properties of mortar with different dosages of the molecular sieve are tested. The water release behavior of the molecular sieve in mortar is comprehensively analyzed combined with the desorption test of the molecular sieve. Results show that the compressive and flexural strength increase by 5% and 10%, respectively, and the drying shrinkage decrease by 6% when 10% of sand is replaced by a molecular sieve under the same total water content. The hydration behavior of the sample is characterized by a microcosmic test of paste. Thermogravimetric analysis is used to calculate the content of corresponding hydration products and quantitatively describe the hydration degree of the internal curing paste mixed with MS. Results show that the content of hydration products is improved by the addition of the molecular sieve, which provides a theoretical basis for the enhancement of mortar to a certain extent.
Internal curing technology has been extensively and systematically studied due to the improvement on concrete properties. Internal curing can effectively reduce the reduction of internal humidity of cement-based materials, reduce dry shrinkage cracks, and prevent acid corrosion on concrete surfaces (
Commonly used internal curing materials include lightweight aggregate (PLA) (
At present, the common water-absorbing light aggregates are pumice (
A molecular sieve is a crystalline aluminosilicate composed of a silicon–oxygen tetrahedron or aluminum–oxygen tetrahedron connected by oxygen bridges to form a molecular-sized channel and cavity system. It is widely used in the adsorption and separation process of the petrochemical industry and the removal of SO2 and NOx from industrial flue gas due to its unique sieving molecules and catalytic functions (
The possible use of industrial adsorption waste as an internal curing agent for cement-based materials was explored in this article. In order to find a suitable way in mortar mixed with MS, mortar specimens with different dosages of wet MS were designed, and their compressive strength, flexural strength, and dry shrinkage properties were tested. To evaluate the curing efficiency of the MS for cement-based materials, the water release capacity of the MS under different relative humidity levels was tested by a desorption experiment. The hydration behavior of the sample was characterized by a microcosmic test of paste. Thermogravimetric analysis was used to calculate the content of corresponding hydration products and quantitatively describe the hydration degree of internal curing paste mixed MS. The purpose of this article is to incorporate industrial adsorption waste into concrete to achieve the purpose of recycling industrial waste. This study provides a theoretical basis for the application of waste in engineering and is of great significance in reducing environmental pollution.
The mineral composition and physical properties of Portland cement (Grade 42.5) used in this study are shown in
Chemical composition of cement.
Composition | SiO2 | Al2O3 | Fe2O3 | CaO | MgO | SO3 | Na2Oeq | f-CaO | C3S | C2S | C3A |
---|---|---|---|---|---|---|---|---|---|---|---|
Content (%) | 21.36 | 5.20 | 3.68 | 64.57 | 1.70 | 0.97 | 0.56 | 0.80 | 57.55 | 17.82 | 7.54 |
Physical properties of cement.
Fineness % | Specific surface area m2/kg | Density kg/m3 | Standard consistency % | Loss % |
---|---|---|---|---|
0.4 | 354 | 3120 | 24.6 | 1.76 |
XRD pattern of the molecular sieve.
Image of the molecular sieve.
The strength contrast experiment of mortar was designed to find out the proper usage of industrial adsorption waste. The mix proportion for the strength test is shown in
Mix ratio of mortar.
Group | C (g) | W (g) | WE (g) | S (g) | MS (g) |
---|---|---|---|---|---|
M0 | 450 | 225 | 0 | 1350 | 0 |
IB15 | 450 | 225 | 0 | 1148 | 45 |
IS15 | 450 | 225 | 30 | 1148 | 45 |
TIS15 | 450 | 195 | 30 | 1148 | 45 |
TES15 | 450 | 195 | 30 | 1350 | 45 |
TIS10 | 450 | 205 | 20 | 1215 | 30 |
TIS20 | 450 | 185 | 40 | 1018 | 60 |
a) WE is for the extra water stored in the MS; b) In the symbol of mortar, the total water content is the sum of the water content and the extra water content, and “T” means that the total water content is the same as M0. “I” and “E” represent internal and external mixing methods, respectively, “B” and “S” represent blank molecular sieve and saturated molecular sieve, respectively; c) The number represents the volume dosage, for example, “15” means the volume dosage is 15%.
Relatively few studies have been performed on drying shrinkage of mortars with a constant total water content after adding additional water. According to strength values of different proportions, it was found that the mixing ratio with a constant total water content after adding additional water was more suitable for MS. Thus, mortars with the same total water content (M0, TIS10, TIS15, and TIS20) were selected for the drying shrinkage test. Also, the effect of MS content on the drying shrinkage of mortar was investigated.
To explore the micro properties of cement-based materials mixed with MS, three groups of cement pastes with dimensions of 40 mm × 40 mm × 40 mm were designed. W/B required for cementitious materials in the ideal hydration state was 0.42 (
P0 was the control group without MS. The cement paste mixed with 15% volume content of water-saturated MS was named P15. Similar to the mortar, a group of pure paste TP15 with the same total water content and a molecular sieve volume content of 15% is added. The mixing ratio of cement paste is shown in
Mix ratio of cement.
Group | C (g) | W (g) | WE (g) | MS (g) |
---|---|---|---|---|
P0 | 603 | 241 | 0 | 0 |
P15 | 603 | 241 | 24 | 37 |
TP15 | 603 | 217 | 24 | 37 |
Specimens of mortar and paste were mixed using a mortar mixer and a paste mixer at room temperature, respectively. Raw MS was immersed in water for 1 h in advance. The mixing process of mortar is shown as follows to solve the early release of water in cement:
Curing efficiency is not only related to water absorption but also the desorption capacity at high RH of MS (
Relative humidity of different salt solutions.
Salt solution | K2SO4 (%) | KNO3 (%) | KCl (%) | KBr (%) | NaCl (%) | NaBr (%) |
---|---|---|---|---|---|---|
RH | 97 | 93 | 85 | 81 | 75 | 59 |
Mortar was cast into a mold with a size of 40 mm × 40 mm × 160 mm. A plastic film was applied to the mold immediately to prevent loss of water. Specimens were demolded after curing for 1 day in the mold with the plastic film and cured for 28 days at a temperature of 20°C with a relative humidity greater than 90%. Compressive and flexural strength of specimens were tested in accordance with the GB/T 17671-1999 (
The dry shrinkage value was tested by the contact method, which was based on JGJ/T70-2009 (
A plastic film was used to cover molds to prevent water loss of mixtures, and molds were removed from specimens after curing for 3 days to prevent the copper nails from moving in the mortar during demolding and measurement. Then, the specimens were transferred to a dry environment with a temperature of 20°C and relative humidity of less than 60%. The length of the specimen at the early stage of drying shrinkage was measured once a day and extended to 2 days in the late stage of dry shrinkage. Three test blocks were poured for each group of samples, and the dry shrinkage value was obtained from the average value of the three test blocks. The method to dry the shrinkage expansion rate is as follows:
in which
Influence of MS on the phase composition of the cement paste in the late stage of hydration was investigated by the X-ray diffraction analysis (XRD). After curing for 28 days, the specimens were crushed, and the sample block was taken out of the central part. To prevent the peak of hydration products from being covered by the peak of MS, the particle of MS in the sample block was removed. Then, the sample was ground into powder for XRD. All powder samples with a particle size of about 80 μm were soaked in anhydrous ethanol to stop the hydration reaction and dried in a vacuum drying oven at 60°C for 24 h before the experiment. Representing different hydration products in the four groups of samples, the peaks in the diffraction pattern were compared, and the hydration degree of samples could also be characterized.
Thermal gravity (TG) analysis is used to analyze the relative content of phases in a sample by the reduction in mass of samples at different temperatures (
The microstructure of cement paste can be observed by using a scanning electron microscope (SEM). Samples cured for 28 days were smashed into uniform fragments with a flat fracture surface and about 1 cm in size. Then, fragments were immersed in an anhydrous ethanol solution to prevent the surface from being hydrated or carbonized. It should be noted that the samples should be dried in a vacuum drying oven at 60°C for 24 h before the experiment begins. In addition, the surface of samples was coated by low vacuum sputter Au to prevent the accumulation of static electric fields. Considering the desorption behavior of MS in specimens, the microstructure of the interfacial transition zone (ITZ) between MS and paste was mainly studied.
The desorption curves of MS in different RH are shown in
Desorption curve with time.
The final residual water content is also one of the important properties of the internal curing agent. The remaining water after the analytical curve reaches equilibrium will be left inside the molecular sieve and cannot be released. Then, 27% of water was left in the MS after water releasing. In addition, compared with other absorption aggregates in
Results of compressive and flexural strength at different ages are presented in
Compressive strength of mortar at 1, 3, 7, and 28 days.
Flexural strength of mortar at 1, 3, 7, and 28 days.
Notably, the 28-day strength of TIS15 was the highest. In all mix ratios, there are two groups of mix ratios that need to be analyzed separately. Among M0, IB15, IS15, and TIS15 mortars, TIS15 has the highest strength. It is speculated that there are three factors causing this difference; they are the following: 1) The strength of mortar was affected by the water–cement ratio. The water absorption of the empty MS and the water release of the saturated MS led to the difference in the initial actual water–cement ratio in IB15 and IS15 mortars. It can be seen from the table of mix ratios that the initial water–cement ratio of TIS15 is the lowest, resulting in the higher strength of TIS15. 2) The lower strength of MS itself has a negative impact on the strength of the mortar. Many studies have shown that the higher porosity leads to a reduction in the strength of the water-absorbing lightweight aggregate itself, which negatively affects the later strength of concrete ( 3) The strength of the paste was strengthened by internal curing water. With the decrease of the relative humidity in mortar, the water absorbed in MS would be released, replenishing the pore water. Compared with external curing, this internal curing water could improve the hydration degree of the cement paste in the central part of the specimens (
The initial water–cement ratio of IS15 was relatively high, while the internal curing effect of blank MS in IB15 was too small. The higher strength of TIS15 was the result of a combination of these three factors. The actual water–cement ratio was lower, leading to the higher strength, and the internal curing water in MS enhanced the hydration. The negative effects of MS were offset by the aforementioned two reasons in the TIS15 mixture.
Under the same total water content, TIS15 also has the highest strength in M0, TIS10, TIS15, and TIS20. The reasons are as follows: 1) It can be seen from the table of mix ratios that the order of the initial water–cement ratio is TIS20 < TIS15 < TIS10 < M0. 2) The curing efficiency increases with the increase of MS dosage. The migration distance of internal curing water is limited, which is about 2–4 mm according to reports ( 3) The negative effect of MS on strength increases with the increase of dosage.
Finally, under the influence of the three factors, when the dosage was 15%, the strength was the highest.
It could be seen from the results that the mortar with the same total water content as M0 was stronger than those with additional water. Then, the strengths of four groups of mortars with the same total water content (TES15, TIS15, TIS10, and TIS20) were compared. Also, it could be seen that the internal mixing method may be more suitable for MS from the strength results of TIS15 and TES15. It was speculated that the appropriate incorporation method of molecular sieves in the mortar was as follows:
The mortars with the same total water content and different MS replacement rates were tested. The curves of the dry shrinkage value with age are shown in
Drying shrinkage value curve with age.
The drying shrinkage curve shows that the higher the dosage of MS was, the higher the growth rate was within the initial 2 days. At this time, the order of dry shrinkage values was as follows: TIS20 > TIS15 > TIS10 > M0. This may be related to the initial actual water–cement ratio in the mortar. The total water content of the four groups was the same, while the extra water carried by the MS was different. Thus, the order of the initial actual water–cement ratio was as follows: M0> TIS10 > TIS15 > TIS20. Drying shrinkage was attributed to the water loss from large pores of concrete in dry environments, which was one of the primary reasons for the generation of cracks in concrete (
In the next 8 days, compared with the control group, the drying shrinkage of TIS10, TIS15, and TIS20 was significantly alleviated, especially TIS20. This was closely related to the behavior of water migration in pre-absorbent MS. The internal curing water in MS supplemented pore water and reduced the liquid surface tension, which alleviated drying shrinkage. In addition, supplemented water could promote hydration reaction, and hydration products accumulated in paste pores, changing the pore structure in the paste. Also, it is well known that the pore structure strongly affects the mechanical properties, shrinkage behavior, and durability of concrete (
It is worth noting that the larger the dosage, the more obvious is the alleviating effect. The curing effect of the internal curing agent was improved with the decrease in the distribution distance (
It could be seen that the growth rate of the dry shrinkage value increased with the increase in the MS content after 40 days. This phenomenon was also mentioned in reference, and it may be related to the shrinkage of aggregates (
After 60 days of curing, the dry shrinkage value of TIS10 was the lowest and that of TIS15 and TIS20 were higher than M0 except for TIS10. There were four reasons for this result. On the one hand, according to the desorption curve, nearly 20% of absorbed water was still left in MS after releasing water. The higher the content of MS, the more water was left in MS. Although the total water content of the four groups was the same, the actual total available water for hydration and water migration was lower than M0, leading to greater shrinkage. On the other hand, the lower initial water–cement ratio intensified the early drying shrinkage of mortar, leading to a higher early shrinkage value, especially TIS20, which even increased by more than 60% compared with M0. Also, because the concrete at an early age may have more connected pores leading to the loss of free water, most of the dry shrinkage of concrete occurs in the early dry shrinkage stage (
The results show that the drying shrinkage value of TIS10 finally decreased by 6% and even decreased by 17.5% at the age of 30 days, while the compressive strength and flexural strength increased by 5% and 10.5%, respectively. Although the strength of TIS15 was higher than that of TIS10, the dry shrinkage value was decreased relative to M0.
The XRD patterns of pastes are shown in
XRD patterns of pastes at 28 days.
Hydration products’ composition of paste was not altered after mixing MS. However, there existed an influence of MS on the intensity of hydration peaks, which was related to the degree of hydration in the paste. Also, the quantitative analysis of the hydration degree was mainly concentrated in TG analysis.
To compare the composition content of the four groups of cement paste, the derivative curve corresponding to thermogravimetric loss at the age of 28 days is shown in
Results of TGA at 28 days: dehydration of C-S-H (40–200°C), dehydroxylation of CH (370–500°C), and decarbonization of calcium carbonate (500–700°C).
The first peak in the derivative curve appeared at about 80°C. Weightlessness occurs before 200°C due to the evaporation of physically bound water from ettringite (Aft) and calcium silicate hydrate (C-S-H) in samples (
The second peak in the derivative curve appeared at about 430°C. Limited by the differences in instruments, conditions, and the specimen preparation process in the experiment, the decomposition of CH occurred at a different temperature range in studies by many scholars. But it was certain that the mass loss generally occurred at about 450°C (
The content of hydration products can reflect the degree of hydration (
Relative content of CH in different samples.
The CH contents of P15 and TP15 increased by 29% and 28%, respectively. The content of hydration products could represent the hydration degree of paste to a certain extent (
It is worth noting that the total water consumption in TP15 is the same as P0 and lower than P15. The desorption test of MS showed that 28% of water absorbed by saturated MS could not be released due to the small pores. Thus, the actual total water consumption of TP15 was even lower than P0. However, the hydration degree of TP15 exceeded P0. It was speculated that the MS reduced the initial water–cement ratio, slowed down the initial water volatilization in the paste, and reduced the water loss.
The analysis of mass loss demonstrated that the incorporation of MS into cement-based materials promoted the hydration of cement-based materials. Also, it verified the speculations and conclusions of the early strength of mortar.
The microscopic morphology of different samples is shown in
Morphology of different groups at 28 days:
In this study, the effect of waste MS on mechanical properties, dry shrinkage properties, and micro properties of internally cured cement-based materials was systematically studied. The main conclusions were summarized as follows: (1) The water release rate of MS increased with the decrease in relative humidity. MS had a higher water absorption capacity than the other internal curing agents. However, the residual water content of MS was also higher than that of the other internal curing agents due to the nanoscale pore size. According to the results of the drying shrinkage experiment and analysis of degree of hydration, the water desorption performance of MS can meet the requirements as an internal curing agent. (2) According to the results of the strength test, the internal mixing method was more suitable as the mixing method of MS. The low strength of waste MS itself would adversely affect the strength of mortar. However, when the total water content in the mix ratio remains unchanged after adding saturated MS, mortar strength would exceed that of the blank control mortar, and the adverse effects of MS can be offset by internal curing. (3) A suitable mixture ratio of mortar containing factory adsorption waste MS was found. When the total water content was unchanged and the content of saturated MS was 10%, the compressive and flexural strength of mortar increased by 5% and 10%, respectively, while the drying shrinkage value finally decreased by 6% and even decreased by 17.5% at the age of 30 days. Although compressive and flexural strength were the highest when the dosage was 15%, the dry shrinkage value increased compared with the blank control group. There was no necessary relationship between dry shrinkage and strength. (4) Water-saturated waste MS had no effect on the physical composition of cement-based materials. According to the results of the TG test, hydration product content was increased even though the total water content was unchanged after the incorporation of saturated MS. It means that the hydration degree of cement-based materials had been strengthened under the influence of saturated MS. The increase of hydration products may be the mechanism of mortar strength enhancement. In addition, SEM images show that the hydration products are enriched in the transition zone near MS, which was consistent with the results of the TG test.
The original contributions presented in the study are included in the article/
PS: investigation, visualization, and writing—original draft. ZL: conceptualization, methodology, resources, and funding acquisition. XC: writing—review and editing, data curation, and validation. LZ: supervision and project administration. RH: resources.
This work was financially supported by the Department of Science and Technology of Hubei Province (grant number: 2021BCA130), the Hubei Key Laboratory of Water System Science for Sponge City Construction (Wuhan University) (grant numbers: 2020-04), and the Hubei Provincial Department of Education (grant number: Q20201304).
Author XC is employed by the China Western Construction Academy of Building Materials Co., LTD, and author RH is employed by Hubei Chufeng Jianke Group Jingzhou Kaiyuan New Materials Limited by Share LTD.
The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
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