Influence of magnesium oxide expander on mechanical and shrinkage properties of UHPC incorporating recycled sand

doi:10.21203/rs.3.rs-4743276/v1

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Influence of magnesium oxide expander on mechanical and shrinkage properties of UHPC incorporating recycled sand

https://doi.org/10.21203/rs.3.rs-4743276/v1

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The incorporation of MgO expansive agents and internal curing mechanisms has individually exhibited positive effects on mitigating shrinkage in Ultra-high performance concrete (UHPC), yet studies on their synergistic performance remain scarce. This paper explores the utilization of recycled sand as a substitute for quartz sand in UHPC production, incorporating various reactive (Type R and Type S) MgO expansive agents at concentrations of 4%, 6%, and 8% (by mass) into the recycled sand UHPC. The primary objective is to investigate the impacts of these MgO agents on the fluidity, mechanical properties, shrinkage behavior, chloride ion permeability, and the macroscopic and microscopic characteristics of the recycled sand UHPC. Utilizing Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TG), and Mercury Intrusion Porosimetry (MIP), we analyze the phase transformations, microscopic morphology, and pore structures of the recycled sand UHPC with different MgO expansive agents. Furthermore, we delve into the underlying mechanisms governing the effects of these MgO agents on the performance of recycled sand UHPC. The results reveal that both Type R and Type S MgO expansive agents reduce the fluidity, compressive strength, and chloride ion permeability of recycled sand UHPC, while significantly mitigating autogenous and drying shrinkages. Notably, Type R MgO demonstrates superior performance, reducing autogenous shrinkage by 26.4% after 7 days and drying shrinkage by 76.9% after 150 days. Microstructural analyses indicate that the highly reactive Type R MgO hydrates to form a greater amount of Mg(OH)₂ crystals, which contributes to a decrease in porosity, particularly in pores smaller than 50nm, thereby enhancing the compactness of the UHPC matrix. Based on a comprehensive evaluation of workability, mechanical properties, and shrinkage behavior, the incorporation of Type R MgO expansive agent at an optimal dosage of 6% is recommended for recycled UHPC.

UHPC

Mechanical properties

Autogenous shrinkage

Pore structure

As China's engineering construction enters a phase of rapid development, unprecedentedly large-scale infrastructural projects featuring ultra-high, ultra-long, and ultra-large structures have emerged, posing new and heightened demands on the strength, fluidity, and durability of concrete. Ultra-high performance concrete (UHPC) emerges as a superior material, transcending both conventional and advanced concretes in its properties, renowned for its unsurpassed strength, resilience, and longevity. UHPC distinguishes itself from conventional concrete through its distinctive feature of a significantly elevated proportion of cementitious binders and an exceptionally low water-to-binder ratio. This unique formulation underscores UHPC's advanced material properties, which are attributed to the abundant presence of hydraulic cementitious materials and the meticulous control of the water content during mixing, resulting in a denser, more durable, and higher-strength microstructure. The quintessential composition of UHPC typically comprises Ordinary Portland Cement (OPC) serving as the primary hydraulic binder, silica fume (SF) functioning as a pozzolanic enhancer for densification and refinement of the microstructure, quartz sand (QS) constituting the fine aggregate fraction, high-efficiency superplasticizers (SP) or high-range water-reducing admixtures (HRWRA) enabling superior workability and rheological properties, and steel fibers incorporated as discontinuous reinforcements to confer unparalleled tensile resilience and crack mitigation capabilities. [1–5]. The definition of UHPC varies across researchers, with Chinese standards (GBT31387-2015) specifying a minimum compressive strength of over 120 MPa, while international standards generally require strengths exceeding 150 MPa [3].

Widespread adoption in bridge rehabilitation and reinforcement has been achieved by UHPC, owing to its exceptional mechanical properties, pronounced impermeability, and heightened durability under varying environmental conditions. [6–8], concrete structures, and other architectural elements [5, 9–11]. In repair projects, UHPC effectively reduces structural dimensions and self-weight [12–14], while in bridge engineering, it enhances compressive strength and mitigates environmental chemical exposure, thereby safeguarding steel reinforcement against corrosion [15–17].

Despite UHPC's remarkable material characteristics, its practical application still confronts numerous challenges deserving of exploration. Notably, UHPC's shrinkage deformation poses a significant threat to its volumetric stability [18–21]. Compared to ordinary concrete, UHPC's extremely low W/C ratio and high cementitious material content exacerbate self-shrinkage due to capillary tension in a semi-saturated state, generating substantial internal stress and deformation that may lead to cracking, thereby compromising structural safety and durability [21, 22].

Employing expansive agents to compensate for shrinkage during cement hydration represents one of the most direct and effective strategies to prevent UHPC from cracking [23]. Based on their active constituents, three primary types of expansive admixtures exist in concrete: calcium oxide-based, calcium sulfoaluminate-based, and magnesium oxide-based (MEA) [24, 25]. Distinguished from admixtures comprising calcium oxide and calcium sulfoaluminate, MEA displays advantageous traits, characterized by a reduced hydration water requirement, stable physicochemical attributes of its hydration products, and a customizable design for the expansion process. These attributes render MEA aptly suited for compensating shrinkage in concretes formulated with low water-to-binder ratios. [26–28]. The reactivity of magnesium oxide has been found to be modifiable through adjustments to calcination conditions, namely temperature and duration, as discovered by Mo et al [29]. Upon calcination at elevated temperatures or for extended periods, magnesium oxide particles undergo enlargement with a reduction in crystalline defects, leading to diminished hydration reactivity and consequently, a decelerated expansion process. Huang et al. [30] evaluated the impact of varying MgO admixture concentrations, ranging from 0–8%, on the mechanical properties and volumetric alterations of concrete at high altitudes. Their analysis indicated that the volumetric expansion triggered by MgO hydration effectively mitigated concrete shrinkage, albeit with a marginal decrement in compressive strength. This expansion, however, resulted in the narrow of transverse crack widths and a subsequent enhancement of pavement performance. Li et al. [31] conducted extensive research on the expansibility and crack resistance of MgO admixtures three varying degrees of reactivity (90s, 150s, 250s) within mass concrete, revealing a strong correlation between the expansion behavior of these admixtures and their respective reactivity values. Their findings emphasized that MgO admixtures with heightened reactivity are more fitting for concrete undergoing gradual temperature elevation.

Internal curing, which involves using pre-wetted lightweight aggregates or materials capable of releasing water from their interiors within the cementitious mixture, also contributes to mitigating autogenous shrinkage [32–35]. This approach aims to alleviate self-desiccation, a decrease in concrete's internal relative humidity due to progressive cement hydration, considered the primary driver of autogenous shrinkage and maintain hydration [36, 37]. Self-desiccation is a pronounced issue in High-Performance Concrete (HPC) with low W/C ratios [38, 39]. Moreover, maintaining hydration is crucial not only in HPC but also in concretes with high W/C ratios or difficult exterior curing conditions. Internal curing can counteract poor cement hydration caused by limited water supply during exterior curing [40–42]. Recycled sand (RS), characterized by high water absorption and porosity due to its internal or surface content of aged cement mortar [43–45], emerges as a promising internal curing material.

It is obvious that a notable lack of research has been observed concerning the combined effects of internal curing mechanisms and MgO expansive agents on the mitigation of shrinkage within UHPC. To address this knowledge gap, the current research endeavor adopts an innovative approach by substituting quartz sand with recycled sand in the formulation of UHPC. This substitution is accompanied by the strategic incorporation of MEA of varying reactivity (Types R and S) at precise mass fractions of 4%, 6%, and 8%. A rigorous and comprehensive examination is then undertaken to quantify the effects of these MEA on various macro-scale properties of the recycled sand UHPC, including its flowability, mechanical strength, shrinkage characteristics, and resistance to chloride ion penetration. The objective of this investigation is to present valuable insights pertaining to the optimization of performance in UHPC that incorporates RS and MEA. In this study, thermogravimetric analysis (TG) has been adopted to systematically examine the phase evolution of UHPC matrix. Additionally, an automated mercury intrusion porosimeter (MIP) has been utilized to meticulously characterize its pore structure, while scanning electron microscopy (SEM) has been implemented to provide a detailed insight into the microscopic morphology of UHPC matrix. These methods offer a window into the intricate pore structure, phase evolution, and micro-topography of the UHPC, enabling a detailed examination of the mechanisms by which MEA influence the shrinkage behavior of the recycled sand UHPC. Ultimately, this study seeks to unravel the intricate interplay between the reactive MEA, RS, and the UHPC matrix, contributing to the development of sustainable and high-performance construction materials with enhanced durability and reduced environmental impact.

2.1 Materials and mix proportion

The cement utilized was of the P.Ⅱ 52.5 grade, originating from Huashan Cement Plant situated in Huangshi, Hubei province. It possessed a density measuring 2080 kg/m³ and exhibited a specific surface area of 385 m²/kg. The SF obtained from Chengdu Dongxing was classified as Eken 95 level, exhibiting an apparent density of 2250 kg/m³ and a substantial surface area of 17,900 m²/kg. The commercially sourced Fly Ash (FA) possessed a density of 2280 kg/m³ and a surface area measuring 457 m²/kg. Wuhan Sanyuan Special New Materials Co., Ltd. provided two magnesium oxide (MgO) expansive agents, R and S types, with reaction times of 72 s and 210 s, respectively. The chemical details of the binders and additives can be found in Table 1. UHPC formulations were created using RS and QS as primary aggregates. The RS, derived from the rigorous process of crushing and sieving waste concrete, is characterized by a particle size range spanning from 1.18 to 2.36 millimeters, boasting an apparent density of 2277 kilograms per cubic meter, alongside a notable water absorption rate of 11.3 percent. In contrast, the QS, which falls within the dimensional range of 0.45 to 0.90 millimeters, exhibits an apparent density of 2650 kilograms per cubic meter and comprises a silica content exceeding 95 percent. As for the admixture, it is furnished by Jiangsu Subot Co., Ltd., and is distinguished by a solid content of 33 percent and a remarkable water-reduction efficiency of 35 percent. Furthermore, the incorporation of cold-drawn steel fibers (CSF), featuring a diameter of 0.3 millimeters, a length of 15 millimeters, and a formidable tensile strength of 3500 MPa. To prepare UHPC, RS was replaced at a 20% rate for QS. Test groups, such as R4 (4% R-type MgO) and S4 (4% S-type MgO), were formed by substituting equal-weight portions of the expansive agents for the cement, corresponding to 4%, 6%, and 8% of the total binder weight. The standard composition comprised cement: SF: QS at 6.5:2.5:1. The water-to-binder ratio remained constant at 0.18. Detailed mix ratios can be found in Table 2, with a reference group (Ref) without expansive agents, followed by additional test series following this structure.

Table 1

Chemical composition of binder materials and MgO expansive agent (%).
NO.	SiO₂	CaO	Al₂O₃	Fe₂O₃	MgO	SO₃	L.O.I
Cement	21.48	62.88	5.22	3.08	1.63	2.31	1.89
SF	95.08	0.21	0.35	0.23	0.51	0.58	2.29
FA	42.35	6.82	28.15	6.91	1.42	1.51	0.81
R-MgO	2.31	0.75	0.72	91.65	0.30	3.04
S-MgO	1.25	0.10	0.54	90.88	0.23	2.58

Table 2

Mix proportion of UHPC mortar.
NO.	Cement( kg·m^− 3)	MgO( kg·m^− 3)	SF( kg·m^− 3)	FA( kg·m^− 3)	QS( kg·m^− 3)	RS( kg·m^− 3)	Water( kg·m^− 3)	CSF( kg·m^− 3)	SP( kg·m^− 3)
Ref	650	0	250	100	810	190	180	156	4.5
R4	610	40	250	100	810	190	180	156	4.5
R6	590	60	250	100	810	190	180	156	4.5
R8	570	80	250	100	810	190	180	156	4.5
S4	610	40	250	100	810	190	180	156	4.5
S6	590	60	250	100	810	190	180	156	4.5
S8	570	80	250	100	810	190	180	156	4.5

2.2 Test methods

The workability of UHPC was assessed in accordance with the "Method for Measuring Flow of Cement Mortar" (GB/T 2419 − 2005); its compressive strength was determined using a YAW-300 fully automatic pressure testing machine on 100 mm × 100 mm × 100 mm mortar samples. Early self-shrinkage was measured using an SBT-AS200 self-shrinkage tester (wound tube method) based on "Methods for Testing the Net-Curing and Shrinkage of Concrete Paste and Mortar" (ASTM C189-09-2014). Long-term drying shrinkage was evaluated using a vertical concrete shrinkometer with 40 mm × 40 mm × 160 mm specimens. The performance of UHPC without steel fibers was tested according to "Basic Properties and Test Methods for Ultra-High Performance Concrete" (T/CBMF 37-2018), employing 100 mm × 50 mm cylindrical samples. After 28 days of curing, the samples were cut into approximately 5 mm × 5 mm × 5 mm cubes, with some ground and analyzed through simultaneous thermal analysis (PerkinElmer STA 6000) for thermogravimetric-differential thermogravimetry (TG-DTG), while others underwent pore structure analysis with a fully automatic mercury intrusion porosimeter (Micromeritics AutoPore V 9620). Surface morphology was observed under scanning electron microscopy (Zeiss Sigma 300) after grinding and polishing.

3.1 Macroscopic Performance

3.1.1 Workability

The effect of magnesium oxide (MgO) expanders on the workability of UHPC incorporating recycled sand is presented in Fig. 1. It can be seen that with the increase of MgO dosage, the flowability of UHPC decreases. Compared to the control group, the use of R-type MgO expanders led to a decline of 9.21–14.89% in flowability for recycled sand UHPC, whereas S-type MgO expanders resulted in a reduction of 4.34–11.92%. Higher activity R-type MgO exhibited a more pronounced detrimental effect on workability at equivalent dosages. This is attributed to MgO replacing cement by mass, with a higher surface area that absorbs free water. Consequently, fewer saturated hydration films are formed due to reduced water availability, leading to decreased flowability. Since R-type MgO has a larger surface area compared to S-type, more water is required for hydration during mixing, which exacerbates the loss of workability.

3.1.2 Compressive Strength

The effect of MEA on the compressive strength of recycled sand UHPC is depicted in Fig. 2. As evident from the figure, the incorporation of various reactive MEA consistently reduced the compressive strength of recycled sand UHPC at different ages. The intensification of the compressive strength reduction was noted to be correlated with an escalation in the dosage of MEA. Specifically, compared to the Ref group, the 28-day compressive strength of recycled sand UHPC incorporating R-type MEA declined by 2.31–7.89%, while that with S-type MEA decreased by 1.57–2.39%. The addition of MEA is observed to have a detrimental effect on the compressive strength of recycled sand UHPC, with the extent of this detrimental impact becoming increasingly pronounced at higher dosages, a finding that is in accordance with the previous research conducted by Li. [46]. The inclusion of MEA primarily results in a reduction of the relative cement content, thereby initiating a competition for water resources with cement, which in turn hinders the hydration reaction. This hindered hydration reaction subsequently leads to a diminished production of cement hydration products, with the end result being a compromise in the mechanical properties of the material. Additionally, the finer crystals formed as hydration products by MEA contribute less to mechanical performance than cement, further diminishing the compressive strength of recycled sand UHPC. Notably, at equal dosages, the highly reactive R-type MEA more readily competes for water with cement particles, exerting a more significant adverse impact on cement hydration. Conversely, the less reactive S-type MEA exhibits a weaker competition for water and thus imposes lesser detrimental effects on cement hydration. Consequently, the influence of highly reactive R-type MEA on the compressive strength of recycled sand UHPC is more pronounced.

3.1.3 Autogenous shrinkage

Figure 3 shows the influence of MEA on early autogenous shrinkage of recycled sand UHPC. It can be seen that different active MEA can effectively mitigate for the early autogenous shrinkage of recycled sand UHPC, and the autogenous shrinkage of recycled sand UHPC decreases gradually with the increase of MEA content. Under the same dosage, high-activity MEA has a more obvious compensation effect on the 7-day autogenous shrinkage of recycled sand UHPC compared to low-activity MEA, and the compensation effect becomes more significant with the increase of dosage. Compared with the Ref group, the autogenous shrinkage of R4, R6, and R8 groups decreased by 20.68%, 23.97%, and 26.49% respectively at 7 days; the autogenous shrinkage of S4, S6, and S8 groups decreased by 11.76%, 15.65%, and 20.71% respectively at 7 days. The primary reason for this phenomenon lies in the scarcity of available water within the low water-binder ratio recycled sand UHPC system, which constrains the hydration process of both cement and MEA. Specifically, the hydration degree of the low-activity S-type MEA is inherently inferior to that of the high-activity R-type MEA, thereby eliciting a slower expansion rate of MgO. [46, 47]. Therefore, within the context of the low water-binder ratio recycled sand UHPC system, the presence of high-activity R-type MEA exerts a pronounced mitigating effect on the early autogenous shrinkage exhibited by UHPC.

3.1.4 Drying shrinkage

The influence of MgO expansive agent on the 150-day drying shrinkage of recycled sand UHPC is shown in Fig. 4. As shown in Fig. 4, the utilization of diverse active MEA has been found to effectively mitigate the drying shrinkage exhibited by recycled sand UHPC. Under the same dosage, high-activity MEA has a more obvious compensation effect on the drying shrinkage of recycled sand UHPC compared to low-activity MEA, and the trend of reducing the drying shrinkage of recycled sand UHPC becomes more significant with the increase of dosage. Compared with Ref group, the drying shrinkage of R4, R6, and R8 groups decreased by 47.82%, 55.73%, and 76.91% respectively at 150 days; the drying shrinkage of S4, S6, and S8 groups increased by 10.01%, 23.92%, and 36.58% respectively at 150 days. The addition of MEA can reduce the drying shrinkage of recycled sand UHPC. The primary rationale for this phenomenon lies in the intricate "water competition effect" that arises between the MEA and cement in the recycled sand UHPC matrix. This competition leads to a reduction in the available water-to-binder ratio, which in turn, diminishes the extent of cement hydration. Consequently, there is a decreased consumption of internal water, resulting in a mitigated capillary tension. These cumulative effects contribute significantly to the observed reduction in drying shrinkage within the system. In addition, the volume of Mg(OH)₂ crystals generated by the hydration reaction of MEA is larger than that of cement hydration products, which to some extent increases the volume of the matrix [48]. The high-activity R-type MEA has higher activity and shorter reaction time, resulting in more expansion crystals of Mg(OH)₂, which has a better inhibitory effect on the drying shrinkage of UHPC matrix.

3.1.5 Chloride ion permeability

The influence of MEA on the chloride ion diffusion coefficient of 28-day recycled sand UHPC is shown in Fig. 5. It can be seen that the inclusion of MEA diminishes the resistance to chloride ion penetration in recycled sand UHPC, manifested by a progressive decline in the anti-chloride ion permeability as the dosage of MEA increases. Under the same dosage, high-activity MEA has a more obvious trend of reducing the diffusion coefficient at 28 days. Compared with the control group, the chloride ion diffusion coefficients of R4 and R6 groups increased by 65.19% and 53.88% respectively, while the chloride ion diffusion coefficient of R8 group decreased by 14.84%; the chloride ion diffusion coefficients of S4, S6, and S8 groups increased by 83.57%, 69.98%, and 56.07% respectively. The primary reason for this observation stems from the inferior chloride ion permeation resistance exhibited by Mg(OH)₂, which is formed through the hydration of MEA, in comparison to the hydration products of cement. Nonetheless, as the quantity of MEA increases, the expansion crystals of Mg(OH)₂ become prevalent, effectively filling the pores within the recycled sand UHPC. This process leads to a reduction in porosity and diminishes the availability of chloride ion transport channels, ultimately enhancing the resistance to chloride ion permeability within the recycled sand UHPC [49]. Among the various types of MEA, the R-type variant exhibits the utmost reactivity, yielding an abundance of Mg(OH)₂ crystals. These crystals exhibit a pronounced filling effect within the pores of the recycled sand UHPC matrix, thereby significantly enhancing its resistance to chloride ion penetration. Consequently, the utilization of R-type MEA results in a more impermeable and durable UHPC material.

3.2 Microstructure

3.2.1 TG-DTG

Figure 6 shows the DTG curve of 28d recycled sand UHPC with MEA. From Fig. 6(a), it can be observed that the DTG curve exhibits four absorption peaks in the temperature ranges of 60–200°C, 310–400°C, 400–500°C, and 600–800°C, which correspond to the decomposition of the hydration products calcium aluminate and hydrated calcium silicate gel, Mg(OH)₂, Ca(OH)₂, and CaCO₃, respectively. No exothermic peak corresponding to Mg(OH)₂ was detected in the Ref group. Based on the formula proposed by Mo et al. [50], the mass of generated MH (W_(MH)) and the hydration degree of MgO (H_(MgO)) can be calculated, as shown in Table 3.

Table 3

Hydration products contents of UHPC containing recycled sand with MgO expansion agents addition (%).
NO.	Bound water	Mg(OH)₂	Ca(OH)₂	W_(MH)	H_(MgO)
Ref	7.61	-	0.59	-	-
R6	7.56	0.99	0.61	2.89	45.28
R8	7.67	1.18	0.78	3.19	52.14
S6	6.81	0.92	0.61	2.90	28.35
S8	6.45	0.95	0.57	3.06	30.91

As evident from Table 3, the concentration of Mg(OH)₂ within the UHPC matrix undergoes a proportional increase with the escalation of MEA dosage. Additionally, under constant dosage conditions, an elevation in the reactivity of MgO corresponds to a heightened Mg(OH)₂ content within the UHPC matrix, indicative of improved MgO hydration. Notably, while high-reactivity MEAs exert a marginal influence on the water-binder ratio of the recycled sand UHPC matrix, they significantly contribute to the Mg(OH)₂ formation. In contrast to sample R8, sample S80 exhibits a notable decrease in Mg(OH)₂ content and MgO hydration degree by 19.49% and 40.71%, respectively. Furthermore, the analysis of Table 3 underscores the limited impact of MgO hydration degree on the recycled sand UHPC, with minimal variations observed in the quantity of hydration products generated. Consequently, it is inferred that the heightened reactivity of MEA directly correlates with an increased generation of Mg(OH)₂ within the UHPC matrix.

3.2.2 MIP

The variation in pore size distribution within the 28d recycled sand UHPC, as influenced by various levels of MEA activity, is depicted in Fig. 7. A notable observation is that the incorporation of 6% of high-reactivity MEA leads to a refinement of pore sizes below 100 nm within the recycled sand UHPC, consequently resulting in a reduction in overall porosity. However, the addition of low-reactivity MEA caused a slight shift of the median pore size towards larger values, indicating coarsening of the pore structure, but without increasing the porosity. Compared to the Ref group, both the 6% R-type MEA and the 6% S-type MEAt led to a reduction in the porosity of the recycled sand UHPC matrix, with a more significant decrease observed for the high-reactivity R-type MEA. This can be attributed to the larger surface area and higher reactivity of high-reactivity R-type MEA, allowing for earlier and greater hydration reaction with water, leading to more effective filling of pores with hydration product Mg(OH)₂, resulting in finer pore structure and lower porosity in the matrix [51] .

Figure 8 shows the comparative volumetric distribution of pores across various size ranges within the recycled sand UHPC, subject to varying degrees of MEA activity. When contrasted with the Ref group, the incorporation of MEA was found to elevate the abundance of pores within the 3–10 nm spectrum, concurrently diminishing those within the 10–50 nm range. Conversely, an augmentation in pore quantity was observed in the 50–100 nm interval, whereas a decrement was noted in the 100–5000 nm and beyond 5000 nm categories. Notably, pores smaller than 50 nm hold paramount significance in influencing the autogenous shrinkage behavior of UHPC. Specifically, samples treated with high-reactivity MEA exhibited a heightened proportion of pores within the 10–50 nm range, in comparison to their low-reactivity MEA, suggesting a more pronounced macroscale mitigation of shrinkage phenomena. Therefore, UHPC prepared using high-reactivity R-type MEA exhibited lower autogenous shrinkage and drying shrinkage. Additionally, the UHPC samples with high-reactivity R-type MEA showed fewer large pores (> 100 nm), and pores larger than 100 nm have a detrimental effect on the chloride ion permeability, indicating that UHPC prepared using high-reactivity R-type MEA has superior resistance to chloride ion penetration.

3.2.3 SEM

Figure 9 displays the SEM images of recycled sand UHPC with different levels of activity MgO expansion agents. From Fig. 9(a), it can be observed that the reference group produced a significant amount of hydration products, including hydrated calcium silicate gel, ettringite and Ca(OH)₂. In contrast, the samples with MEA exhibited small Mg(OH)₂ crystals, which effectively filled the pores inside the UHPC matrix, resulting in a denser matrix, consistent with the aforementioned pore structure results. As shown in Fig. 9(b), more compact Mg(OH)₂ crystals were observed in the R6 group, indicating a denser microstructure. The reduction in the quantity of small pore sizes due to the addition of high-reactivity MEA is beneficial for compensating the early shrinkage of UHPC. However, Fig. 9(c) showed a higher amount of plate-like Ca(OH)₂ crystals, which hinder the bonding between hydration products and result in a more dispersed spatial structure. This may be the reason why recycled sand UHPC with low-reactivity MEA exhibits higher resistance to permeability.

(1) The workability, mechanical properties, and resistance to chloride ion penetration of recycled sand UHPC decrease with the addition of MEA. However, at an appropriate dosage, the impact of high-reactivity MEA on workability, mechanical properties, and resistance to chloride ion penetration is relatively minor.

(2) The addition of MEA effectively reduces the shrinkage of recycled sand UHPC, with a more significant improvement observed at higher dosages. High-reactivity MEA can reduce the shrinkage of recycled sand UHPC by 26.49% and 76.91%, respectively.

(3) Microscopic analysis indicates that high-reactivity MEA enhance the hydration degree of MgO in the recycled sand UHPC matrix, improve the pore distribution, refine pore size, and generate fine Mg(OH)₂ crystals that effectively fill the pores inside the UHPC matrix, resulting in a denser matrix.

(4) Considering the workability, mechanical properties, resistance to chloride ion penetration, and shrinkage performance, it is recommended to use high-reactivity MEA with an optimal dosage of 6%.

CRediT authorship contribution statement

Shukai Cheng: Conceptualization, Methodology, Resources, Formal analysis, Writing-review & editing. Qianyong Liang: Data curation, Formal analysis, Investigation, Validation, Writing-original draft, Testing and testing assistance. Qiwen Liu: Conceptualization, Methodology, Formal analysis, Writing-review & editing. Kang Chen: Investigation, Testing and testing assistance, Writing-review & editing. Hongtao Wang: Conceptualization, Supervision, Writing-review & editing.

Declaration of Competing Interest

The article is original, has not been published previously and has been written by the stated authors who are all aware of its content and approve its submission. It is not under consideration for publication elsewhere, no conflict of interest exists, and if accepted, the article will not be published elsewhere in the same form, in any language, without the written consent of the publisher.

Acknowledgements

The authors gratefully acknowledge the financial supports from the National Science Foundation of China (No. 52208286), Innovation Group Project of Hubei Province (No. 2024AFA033), Natural Science Foundation of Hubei Province (No. 2022CFB672), Knowledge Innovation Program of Wuhan-Shuguang Project (No. 2022020801020362), Guizhou Provincial Science and Technology Projects (No. 2023 − 207).

Data Availability Statement:

The data presented in this study are available on request from the corresponding author.

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Influence of magnesium oxide expander on mechanical and shrinkage properties of UHPC incorporating recycled sand

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