Experimental Investigation into Lightweight High Strength Concrete with Shale and Clay Ceramsite for Offshore Structures

: To develop lightweight high-strength concrete (LWHSC) for offshore structures in a harsh seawater environment, LWHSC with shale and clay ceramsites was designed. LWHSC was experimentally investigated in terms of density, compressive strength, and durability in a coastal environment. Then, its feasibility for offshore structures was also assessed. The results show that the compressive strength and oven dry density of LWHSC appropriately improve with increases in cement content, while they are reduced by the replacement of shale ceramsite with clay ceramsite. The compressive strength of LWHSC also increases ﬁrst and then decreases with an increase in the pre-wetting of shale and clay ceramsites. Their optional pre-wetting time is about 0.5 h. LWHSC exhibits a higher brittleness compared with conventional concrete. LWHSC has increases in the resistances of freeze–thaw, carbonization, water penetration, and chloride penetration when the shale and clay ceramsite light aggregates decrease in the concrete. The LWHSC prepared in this paper is suitable for the harsh seawater environment of offshore oil platforms but is limited to the southern region where there is no requirement for the freeze–thaw resistance of concrete. The results of this study can provide some reference for the application of LWHSC in offshore structures and other similar aspects of engineering.


Introduction
Lightweight high-strength concrete (LWHSC) has been widely used for long-span bridges, high-rise buildings, and floating and offshore structures.LWHSC has economic benefits such as its light self-weight, high strength, and excellent possible durability and creep resistance [1,2].Generally, LWHSC is commonly recognized as having a strength greater than 40 MPa with a density of less than 1950 kg/m 3 and is sensitive to its material constituents and preparation [3].LWHSC mixed with fly ash and volcanic slag has an increase in its resistance to chloride diffusivity and shrinkage [4,5].LWHSC also has increases in strength and density when using nano-scale silica modifier [6,7], steel and polypropylene fiber [8][9][10], and graphene oxide [11].LWHSC is also affected by curing conditions and the vibration mixing process [12].The compressive strength of LWHSC can be predicted by using SPSS [13], an artificial neural network [14], and a support vector machine [15].LWHSC has a reduction in its strength and density due to the incorporation of lightweight aggregates involving pumice [16], expanded slate and polystyrene [17,18], perlite [19], polymer emulsion [20], cenosphere [21], and straw and oil palm [22,23].
When LWHSC was applied to offshore structures such as oil platforms, LWHSC served in a harsh seawater environment and should have had a strength greater than 50 MPa and a unit weight less than 1950 kg/m 3 [24].This required LWHSC to have high strength and light unit weight as well as additional good durability.Concrete durability in a seawater environment involves freeze-thaw resistance, water penetration, carbonization, and chloride penetration.Shale and clay ceramsite were incorporated into LWHSC as lightweight aggregates.Evangelista et al. [25] found LWHSC with a compressive strength of up to 64.3 MPa, a dry density of up to 1.77 kg/dm 3 , and an efficiency factor of up to 36.3 MPa•dm 3 /kg after using expanded shale in place of coarsely expanded clay aggregate.Compared with clay ceramsite, shale ceramsite increased the LWHSC strength [25,26].The shale ceramsite with a high pre-wetting degree increased the strength growth rate of concrete compared with the one without pre-wetting [27,28].Li Beixing et al. [29] systematically investigated the effects of low water absorption high-strength shale ceramic granules on the mechanical properties, shrinkage deformation, and durability of highstrength lightweight aggregate concrete (HSLC) after pre-wetting treatment and found that the pre-wetting treatment improved the anti-stratification segregation properties and pumpability of HSLC, which resulted in increases in the compressive strength of HSLC by 3.2-3.5MPa, the splitting tensile strength by 0.26-0.55,and the modulus of elasticity by 0.6-0.7 GPa.Shale ceramsite had no effect on the peak bond strength of LWHSC but reduced the tensile and compressive strengths of LWHSC [30,31].At present, the effects of shale and clay ceramsite on LWHSC strength and durability have not been reported.The feasibility of LWHSC with shale and clay ceramsite for offshore structures has also not been assessed.
In order to successfully formulate LWHSC with a dry apparent density not greater than 1950 kg/m 3 and a 28-day compressive strength greater than or equal to 50 Mpa, and to make LWHSC serve in a harsh seawater environment, LWHSC with shale and clay ceramics was designed and experimentally investigated with the dual-control indexes of lowering the dry apparent density and increasing the 28-day compressive strength to continuously reduce the mass ratio of shale ceramics and simultaneously increase the mass ratio of clay ceramics to obtain a high-strength lightweight concrete mix ratio.Furthermore, the effects of material constituents on durability in a coastal environment involving freezethaw resistance, carbonization, water penetration, and chloride penetration were also investigated.Then, its feasibility for offshore structures was assessed.The results of the study not only help enhance the understanding of the mechanical properties of LWHSC with shale ceramsite and clay ceramsite and their developmental patterns but also provide references for understanding, optimizing, and successfully developing LWHSC concrete with shale ceramsite and clay ceramsite for use in engineering applications such as in the harsh seawater environments of offshore oil platforms.

Cement
In this investigation, the cement used was camel P.O.42.5 Ordinary Portland Cement (OPC), which was supplied from the Tianjin cement factory.According to the manufacturer, the cement included 53.23% C3S, 21.96% C2S, 6.94% C3A, and 11.36% C4 AF, which was verified by Chinese standard GB 175-2007/XG3-2018 [32].Tables 1 and 2 tabulate the information about the cement's chemical and physical arrangement.

Supplementary Cementitious Materials
In pursuit of diminishing cement content within the LWHSC, a strategic integration of fly ash was effectuated as a judicious partial surrogate for ordinary Portland cement (OPC).Derived from the Tianjin Junliangcheng Power Plant situated in Tianjin, China, the fly ash in question manifests a comprehensive chemical composition, meticulously delineated in Table 1.Possessing a specific gravity of 2.00 and a unit weight of 0.625 g/cm 3 , the residual quantum of fly ash retained by the 0.045 mm sieve constitutes a modest 8%.Classified within the category of class F II fly ash, it conspicuously exhibits a noteworthy SiO 2 content of 50.17% and a SO 3 content of 1.0%.Importantly, these metrics impeccably align with the stipulations of the ASTM C-618 [33] standard, mandating a SiO 2 content surpassing 40% and a SiO 3 content subduing 3%, thereby definitively affirming the adherence of this fly ash variant to standardized benchmarks.
The granulated blast furnace slag powder was procured from Anhui Zhujiaqiao Cement Co., Ltd. in Wuhu, Anhui Province, China.Meticulously elucidated within Table 1, the chemical oxide constitution thereof delineates its distinctive attributes.Evidencing a specific surface area measuring 460 m 2 /kg, coupled with an ascertained unit weight of 2.91 g/cm 3 , it stands as a prototypical exemplar of the class S95 slag powder designation.

Aggregates
The fine aggregate employed in this study was sourced from natural river sand originating in Suizhong, Liaoning Province, China.The majority of the particles in this river sand were found to successfully pass through a 4.75 mm sieve.This particular river sand holds a classification as a class middle sand falling within zoneII, adhering to the guidelines stipulated by the Chinese standard GB/T 14684-2022 [34].For the coarse lightweight aggregates, a meticulous selection led to the incorporation of both clay ceramsite (Figure 1a) and shale ceramsite (Figure 1b).The clay ceramsite and shale ceramsite were produced in Guangxi province, China, and Yichang, Hubei province, China, respectively.In accordance with China's prevailing national standard "Lightweight Aggregate and its Test Methods Part 1: Lightweight Aggregate" (GB/T 17431.1-2010),both the clay ceramsite and shale ceramsite underwent meticulous screening processes, culminating in well-defined particle gradations delineated in Tables 3 and 4, respectively.The quantifiable physical attributes of these lightweight aggregates are comprehensively outlined in Table 5, and meticulously evaluated according to the parameters stipulated by Chinese Standard GBT 17431.2-2010[35].Refined from shale as raw material through high temperature and roasting.

Water Reducer
The high-performance water-reducer employed in this investigation pertained to the PCA (I) type polycarboxylate.This product was produced by Jiangsu Subote New Materials Co., Ltd.Nanjing, China.The manufacturer provides the physical and chemical properties of this product, as tabulated in Table 6.Refined from shale as raw material through high temperature and roasting.

Water Reducer
The high-performance water-reducer employed in this investigation pertained to the PCA (I) type polycarboxylate.This product was produced by Jiangsu Subote New Materials Co., Ltd.Nanjing, China.The manufacturer provides the physical and chemical properties of this product, as tabulated in Table 6.

Mix Proportions
Aiming to evaluate the factors contributing to enhanced strength, reduced weight, and heightened durability, adherence to Chinese Standard JGJ/T 12-2019 [36] dictated the mixture proportions of the LWHSC, as tabulated in Table 7.The water binder ratio (w/b) was fixed at 0.28 and the volume rate of Sand (βs) ranged from 0.35 to 0.4.To prepare lightweight and high-strength pumping concrete with the targeted slump of 100 ± 20 mm, the requisite water reducer volume was determined corresponding to a slump of 200 ± 20 mm, as stipulated by Chinese Standard GB/T50080-2002 [37].Based on the designated slump, all developed lightweight high-strength concrete formulations demonstrated commendable compatibility (i.e., fluidity, cohesion, and water retention).The reference mixture, designated as BM, featured an exclusion of clay ceramsite.Mixture SY1 introduced a combination of shale and clay ceramsite, while mixtures SY2, SY3, and SY4 exhibited variations in ceramsite content alongside a diminished water binder ratio in comparison to SY1 and SY2.

Specimen Preparation
In the fabrication of LWHSC, the initial step involved washing the river sand and resoaking the shale and clay ceramsite light aggregates.Subsequently, the cleaned river sand, pre-wetted shale, and clay ceramsite light aggregates were mixed using a mixer for approximately 2 min.After that, cement and supplementary cementitious materials (fly ash and slag powder) were introduced into the mixture and mixed for an additional 3-minute duration.Water, along with the water reducer, was then added into the mixture and blended for an extra 3 minutes, culminating in the formation of freshly prepared LWHSC.The water reducer dosage was determined based on achieving a slump of 100 ± 20 mm, as prescribed by the slump test outlined in Chinese Standard GB/T50080-2002 [37].Consequently, the freshly prepared concrete was cast in 100 mm cubical molds.After a curing period of 24 hours, the specimens were de-molded and subjected to continuous water curing (20 ± 2 • C) until the point of assessment of specimen strength, resistance to freeze-thaw cycles, and durability.

Experimental Methodology and Process
The strength, oven dry density, and durability of LWHSC constitute crucial parameters that determine its suitability as an offshore structural material.These factors were taken into account in this study.Utilizing a universal press, compressive strength tests were conducted on 100 × 100 × 100 mm LWHSC cubes at 3, 7, and 28 days on the basis of Chinese Standard GB/T 50081-2019 [38].The concrete's oven dry density was conducted according to Chinese Standard JGJ/T 12-2019 [36].Furthermore, the evaluation of LWHSC's resistance to freeze/thaw cycles, carbonization, water penetration, and chloride penetration was conducted in accordance with the methodologies prescribed by Chinese Standard GB/T 50082-2009 [39].

Compressive Strength and Densities
Table 8 tabulates the oven dry densities and compressive strengths of LWHSC affected by variations in cement content, shale, and clay ceramiste.As can be seen from the results in Table 8, the BM specimen exhibited superior 7-day and 28-day compressive strengths and densities compared to the SY1 specimen.Conversely, SY4 demonstrated reduced compressive strength and oven dry density in contrast to SY3 within the LWHSC.Notably, as the proportion of clay ceramsite within the lightweight aggregates increased, a consistent decline in both the compressive strength and oven dry density of LWHSC was observed.This observation can be attributed to the cylinder compressive strength of shale ceramsite as it was observed to surpass that of clay ceramsite (Table 2).Furthermore, particle size discrepancies contribute to the phenomenon.Notably, the particle size range of clay ceramsite (5 to 25 mm) exceeds that of shale ceramsite (5 to 20 mm).Thus, the shale ceramiste has a larger specific surface area and an enhanced cementitious bonding capacity.This distinctive attribute underpins the heightened compressive strength associated with shale-ceramsite-containing LWHSC.Consequently, the substitution of shale ceramsite with clay ceramsite engenders a reduction in compressive strength within the LWHSC matrix.
In comparison with SY1 and SY2, in Table 8, it can be seen that the 28-day compressive strength of LWHSC, along with its oven dry density, exhibits a notable enhancement with escalating cement content.The heightened amount of cementitious material yields increased heat of hydration, subsequently elevating the internal temperature of the concrete.This thermal effect fosters augmented cement hydration, culminating in the generation of more alkaline hydration products.These products, in turn, promote the pozzolanic activity of active admixtures and facilitate a secondary hydration reaction, thereby contributing to the improved compressive strength of LWHSC.
Comparing SY2 with SY3, in Table 6, it can be seen that a judicious augmentation in the absolute volume rate of sand effectively augments the compressive strength of LWHSC, albeit concurrently raising its oven dry density.
Figure 2 plots the influence of the varying pre-wetting durations on the compressive strength of LWHSC in the SY5 specimen.The dry ceramic particles underwent diverse pre-wetting treatments, encompassing no pre-wetting, as well as 0.5-h, 1-h, and 24-h water absorption intervals.These intervals corresponded to moisture content levels of 0%, 3.5%, 4.0%, and 4.0%, respectively.The pre-wetting process involved direct immersion, followed by draining excess water from the surface of the ceramic grains upon reaching the designated pre-wetting duration.This facilitated subsequent concrete mixing.During the mixing design process, to prevent potential test errors resulting from excessive water content, the pre-wetted ceramic concrete was blended using the dry ceramic ratio.The total water quantity for mixing was based on the net water consumption, accounting for the incremental water absorption of ceramic particles within the first hour (4% of dry ceramic particles).Observing Figure 2, it becomes evident that the 7-day and 28-day compressive strengths of LWHSC exhibit an initial increase followed by a decline as the pre-wetting duration extends.This trend stems from the pre-wetting of clay and shale ceramsite mitigating the adverse influence of dry lightweight aggregates on the water/binder ratio, consequently bolstering the compressive strength.However, once the pre-wetting period surpasses an optimal threshold, excessive water absorption by the lightweight aggregates results in surface softening.This, in turn, weakens the interface bond between cementitious materials and lightweight aggregates, leading to a subsequent deterioration in compressive strength.
Sustainability 2023, 15, x FOR PEER REVIEW 7 of 14 mixing design process, to prevent potential test errors resulting from excessive water con tent, the pre-wetted ceramic concrete was blended using the dry ceramic ratio.The tota water quantity for mixing was based on the net water consumption, accounting for the incremental water absorption of ceramic particles within the first hour (4% of dry ceramic particles).Observing Figure 2, it becomes evident that the 7-day and 28-day compressive strengths of LWHSC exhibit an initial increase followed by a decline as the pre-wetting duration extends.This trend stems from the pre-wetting of clay and shale ceramsite miti gating the adverse influence of dry lightweight aggregates on the water/binder ratio, con sequently bolstering the compressive strength.However, once the pre-wetting period sur passes an optimal threshold, excessive water absorption by the lightweight aggregates results in surface softening.This, in turn, weakens the interface bond between cementi tious materials and lightweight aggregates, leading to a subsequent deterioration in com pressive strength.In contrast to the NC detailed in Table 6, SY1 exhibits a lower unit weight while main taining a similar level of compressive strength to the normal-weight concrete.The 28 d oven dry density of LWHSC stands at approximately 83% of the corresponding value fo ordinary concrete.Figure 3 delineates the trajectories of the compressive strength devel opment for normal-weight concrete and LWHSC (SY1) against curing time.Figure 3 re veals that the LWHSC has a compressive strength of 57.5% at 3 d, 67.7% at 7 d, and 94.2% at 14d, in comparison to the strength at 28 d.In contrast, the normal-weight concrete ex hibits 49.4% at 3 d, 62.5% at 7 d, and 86.6% at 14 d in comparison with its strength at 28 d Consequently, LWHSC demonstrates a more rapid initial increase in compressive strength relative to normal-weight concrete.In contrast to the NC detailed in Table 6, SY1 exhibits a lower unit weight while maintaining a similar level of compressive strength to the normal-weight concrete.The 28 d oven dry density of LWHSC stands at approximately 83% of the corresponding value for ordinary concrete.Figure 3 delineates the trajectories of the compressive strength development for normal-weight concrete and LWHSC (SY1) against curing time.Figure 3 reveals that the LWHSC has a compressive strength of 57.5% at 3 d, 67.7% at 7 d, and 94.2% at 14 d, in comparison to the strength at 28 d.In contrast, the normal-weight concrete exhibits 49.4% at 3 d, 62.5% at 7 d, and 86.6% at 14 d in comparison with its strength at 28 d.Consequently, LWHSC demonstrates a more rapid initial increase in compressive strength relative to normal-weight concrete.
Figure 4 illustrates the compressive failure morphology of both normal-weight concrete and LWHSC(SY1).As the load on LWHSC increases, audible cracking emerges, accompanied by the initiation of vertical cracks near the mid-section of the non-compressed surface.These cracks then progressively widen until, eventually, specimen failure occurs, adopting a bipyramidal mode with opposing vertices, as depicted in Figure 4a.This specific pattern is absent during the loading sequence of normal-weight concrete.In the case of normal-weight concrete, failure transpires through the development of a crack on the compression surface under axial compression.Hence, it can be inferred that LWHSC exhibits significantly heightened brittleness in comparison to normal-weight concrete.Figure 4 illustrates the compressive failure morphology of both normal-weight concrete and LWHSC(SY1).As the load on LWHSC increases, audible cracking emerges, accompanied by the initiation of vertical cracks near the mid-section of the non-compressed surface.These cracks then progressively widen until, eventually, specimen failure occurs, adopting a bipyramidal mode with opposing vertices, as depicted in Figure 4a.This specific pattern is absent during the loading sequence of normal-weight concrete.In the case of normal-weight concrete, failure transpires through the development of a crack on the compression surface under axial compression.Hence, it can be inferred that LWHSC exhibits significantly heightened brittleness in comparison to normal-weight concrete.

Freeze-Thaw Resistance
The freeze-thaw cycles of LWHSC were subjected to optical observation.After 25 freeze-thawing cycles, the SY1 and SY4 specimens exhibited intactness without any visible epidermal detachment.After 75 cycles, observable epidermis shedding and the initiation of hemp surface changes were noted.Upon reaching 125 cycles, a substantial area of epidermis shedding and pronounced hemp surface alterations led to the exposure of coarse aggregates.After 150 cycles, the SY1 specimen was broken and the test was finished, while the SY4 specimen only manifested as aggregate shedding and the surface was marked by multiple "pits".Figure 4 illustrates the compressive failure morphology of both normal-weight concrete and LWHSC(SY1).As the load on LWHSC increases, audible cracking emerges, accompanied by the initiation of vertical cracks near the mid-section of the non-compressed surface.These cracks then progressively widen until, eventually, specimen failure occurs, adopting a bipyramidal mode with opposing vertices, as depicted in Figure 4a.This specific pattern is absent during the loading sequence of normal-weight concrete.In the case of normal-weight concrete, failure transpires through the development of a crack on the compression surface under axial compression.Hence, it can be inferred that LWHSC exhibits significantly heightened brittleness in comparison to normal-weight concrete.

Freeze-Thaw Resistance
The freeze-thaw cycles of LWHSC were subjected to optical observation.After 25 freeze-thawing cycles, the SY1 and SY4 specimens exhibited intactness without any visible epidermal detachment.After 75 cycles, observable epidermis shedding and the initiation of hemp surface changes were noted.Upon reaching 125 cycles, a substantial area of epidermis shedding and pronounced hemp surface alterations led to the exposure of coarse aggregates.After 150 cycles, the SY1 specimen was broken and the test was finished, while the SY4 specimen only manifested as aggregate shedding and the surface was marked by multiple "pits".

Freeze-Thaw Resistance
The freeze-thaw cycles of LWHSC were subjected to optical observation.After 25 freeze-thawing cycles, the SY1 and SY4 specimens exhibited intactness without any visible epidermal detachment.After 75 cycles, observable epidermis shedding and the initiation of hemp surface changes were noted.Upon reaching 125 cycles, a substantial area of epidermis shedding and pronounced hemp surface alterations led to the exposure of coarse aggregates.After 150 cycles, the SY1 specimen was broken and the test was finished, while the SY4 specimen only manifested as aggregate shedding and the surface was marked by multiple "pits".
Figure 5 represents the weight loss rate and relative dynamic modulus of the elasticity of LWHSC after rapid freeze-thaw cycles.It can be seen from Figure 4 that, after 125 freezethaw cycles, the SY1 and SY4 specimens have a weight loss of 4.8% and 3.6%, and relative dynamic modulus of elasticity reduction of 41.1% and 15.6%, respectively.These findings suggest superior freeze-thaw resistance in the SY4 specimen relative to SY1.Table 7 shows that the SY4 specimen boasted elevated cementitious materials (cement, fly ash, and slag powder) while sharing the same aggregate composition (sand, clay, and shale ceramsite with SY1.From this, it can be deduced that an increase in cementitious material potentially enhances the freeze-thaw resistance of LWHSC to some extent.This augmentation can be rationalized through the lens of concrete freeze-thaw failure mechanisms, wherein microcracks form on LWHSC pore walls under the combined pressure of ice expansion and unfrozen water osmotic pressure.The elevated cementitious content effectively counters this expansion and osmotic pressure, thereby amplifying freeze-thaw resistance.This implies that the freeze-thaw resistance of LWHSC diminishes with escalating proportions of shale and clay ceramsite lightweight aggregates.
relative dynamic modulus of elasticity reduction of 41.1% and 15.6%, respectively.These findings suggest superior freeze-thaw resistance in the SY4 specimen relative to SY1.Table 7 shows that the SY4 specimen boasted elevated cementitious materials (cement, fly ash, and slag powder) while sharing the same aggregate composition (sand, clay, and shale ceramsite with SY1.From this, it can be deduced that an increase in cementitious material potentially enhances the freeze-thaw resistance of LWHSC to some extent.This augmentation can be rationalized through the lens of concrete freeze-thaw failure mechanisms, wherein microcracks form on LWHSC pore walls under the combined pressure of ice expansion and unfrozen water osmotic pressure.The elevated cementitious content effectively counters this expansion and osmotic pressure, thereby amplifying freeze-thaw resistance.This implies that the freeze-thaw resistance of LWHSC diminishes with escalating proportions of shale and clay ceramsite lightweight aggregates.

Water Penetration Resistance
Figure 6 plots the water penetration depth of LWHSC.As is evident from Figure 6, the SY1 specimen exhibits deficient water penetration performance, manifesting a penetration depth of 35 mm.In contrast, the SY4 specimen displays a relatively limited water penetration depth.Water penetration is primarily governed by diffusion and intricately tied to concrete porosity.Diminished porosity results in a reduced water diffusion coefficient.Notably, an increased cementitious content translates to decreased porosity.Therefore, the increase in the cementitious material of LWHSC engenders an enhanced water penetration resistance.As a result, this effect leads to a reduction in water penetration depth.
Figure 6 plots the water penetration depth of LWHSC.As is evident from Figure 6, the SY1 specimen exhibits deficient water penetration performance, manifesting a penetration depth of 35 mm.In contrast, the SY4 specimen displays a relatively limited water penetration depth.Water penetration is primarily governed by diffusion and intricately tied to concrete porosity.Diminished porosity results in a reduced water diffusion coefficient.Notably, an increased cementitious content translates to decreased porosity.Therefore, the increase in the cementitious material of LWHSC engenders an enhanced water penetration resistance.As a result, this effect leads to a reduction in water penetration depth.

Carbonization Resistance
Figure 7 indicates the carbonization depth of LWHSC.As depicted, the carbonization depth of the SY1 and SY4 specimens are 2.4 mm and 3.6 mm at 7d and 11.9 mm and 11.2 mm at 28d, respectively.The 28-day carbonization depth of SY1 is greater than that of SY4.SY1 has a lower cementitious material than SY4, resulting in a higher proportion of aggregates.These findings indicate that the increase in cementitious material of LWHSC improves its carbonization resistance.This enhancement stems from the concrete carbonization mechanism, wherein a reduction in alkalinity occurs due to the reaction of gaseous CO2 with Ca(OH)2, forming the neutral product CaCO3.One of the main carbonization factors is the diffusion of CO2, which is determined by the concrete porosity.The low porosity results in a diminished diffusion coefficient of CO2.The increase in cementitious material in LWHSC augments its carbonization resistance, ultimately resulting in a reduced carbonization depth.

Carbonization Resistance
Figure 7 indicates the carbonization depth of LWHSC.As depicted, the carbonization depth of the SY1 and SY4 specimens are 2.4 mm and 3.6 mm at 7 d and 11.9 mm and 11.2 mm at 28 d, respectively.The 28-day carbonization depth of SY1 is greater than that of SY4.SY1 has a lower cementitious material than SY4, resulting in a higher proportion of aggregates.These findings indicate that the increase in cementitious material of LWHSC improves its carbonization resistance.This enhancement stems from the concrete carbonization mechanism, wherein a reduction in alkalinity occurs due to the reaction of gaseous CO 2 with Ca(OH) 2 , forming the neutral product CaCO 3 .One of the main carbonization factors is the diffusion of CO 2 , which is determined by the concrete porosity.The low porosity results in a diminished diffusion coefficient of CO 2 .The increase in cementitious material in LWHSC augments its carbonization resistance, ultimately resulting in a reduced carbonization depth.

Chloride Penetration Resistance
Figure 8 portrays the electric flux of LWHSC.A higher electric flux corresponds to diminished chloride penetration resistance.As illustrated in Figure 8, the electric flux values for the SY1 and SY4 specimens are 670C and 540C, respectively.It is inferred that heightened cementitious material within LWHSC could potentially enhance its resistance to chloride penetration.This behavior is rooted in the diffusion of chloride within concrete.An elevated cementitious material content yields reduced porosity, resulting in limited chloride diffusion, as evidenced by the lower electric flux values.

Chloride Penetration Resistance
Figure 8 portrays the electric flux of LWHSC.A higher electric flux corresponds to diminished chloride penetration resistance.As illustrated in Figure 8, the electric flux values for the SY1 and SY4 specimens are 670C and 540C, respectively.It is inferred that heightened cementitious material within LWHSC could potentially enhance its resistance to chloride penetration.This behavior is rooted in the diffusion of chloride within concrete.An elevated cementitious material content yields reduced porosity, resulting in limited chloride diffusion, as evidenced by the lower electric flux values.

Discussions
From the data presented in Figures 5-8, it can be seen that, compared with the SY4 specimen, SY1 showcases less resistance against freeze-thaw, carbonization, water penetration, and chloride penetration.These findings may be due to the greater amounts of

Discussions
From the data presented in Figures 5-8, it can be seen that, compared with the SY4 specimen, SY1 showcases less resistance against freeze-thaw, carbonization, water penetration, and chloride penetration.These findings may be due to the greater amounts of cement and active supplementary cementitious materials in LWHSC, and the corresponding reduction in shale and clay lightweight coarse aggregates.The inclusion of fly ash and slag powder prompts a secondary hydration reaction with cement hydration products, yielding calcium silicate gel that enhances concrete compactness.This augmented compactness acts as a barrier against the ingress of moisture, CO 2 , and chloride ions, consequently augmenting LWHSC's durability.
The LWHSC formulated herein exhibits commendable freeze-thaw resistance, excellent carbon resistance, water penetration resistance, and chloride penetration resistance.Therefore, it is well-suited for demanding seawater environments, such as offshore structures like oil platforms.However, the attained freeze-thaw resistances for the LWHSC graded as F100 and F125, respectively, fall short of the F350 grade, making it suitable mainly for regions without stringent freeze-thaw resistance requirements.
In fact, this paper diverges from other scholars' studies in the following three innovations: i.
Attempting to address the current domestic and international applications of lightweight aggregate concrete, where breakthroughs in strength grade have been elusive, with the highest strength grade remaining LC60.Achieving lightweight aggregate concrete with a dry apparent density below 1950 kg/m 3 at 28 days and a standardized strength exceeding 50 MPa remains an arduous endeavor.ii.Employing a novel approach-combining clay and shale ceramic particles as coarse aggregate-unprecedented in the literature (Table 7).This innovation not only reduces concrete's self-weight but also optimizes aggregate gradation and diminishes voids, ensuring concrete strength.iii.Distinguishing itself by systematically investigating the durability aspects of lightweight aggregate concrete in marine environments, encompassing freeze-thaw resistance, water infiltration resistance, carbonation, and chloride ion infiltration resistance, which have received limited attention in existing research.These comprehensive durability tests are meticulously conducted in Sections 5.2-5.5.

Conclusions
This paper introduces an innovative approach involving a composite mixture of clay ceramsites and shale ceramsites as coarse aggregates in the production of LWHSC.The effects of key factors (cement content, shale ceramsites, and clay ceramsites ratios) on various LWHSC parameters, encompassing compressive strength, density, freeze-thaw resistance, carbonation resistance, water permeation resistance, and chloride permeation resistance, were investigated.The findings contribute to a comprehensive understanding of the mechanical properties and developmental trends of LWHSC, incorporating shale and clay ceramsites and offering insights for engineering applications of concrete utilizing shale and clay vitrified particles.The main conclusions are as follows: (1) Substituting clay ceramsite for shale ceramsite leads to a decrease in compressive strength and oven dry density by 2.3-14.2Mpa and 50-110 kg/m 3 , respectively.Conversely, an increase in cement content results in an enhancement of 3.2-12.7Mpa and 40 kg/m 3 .(2) The compressive strength of LWHSC increases first and then decreases with an increase in pre-wetting time.The optional pre-wetting time of approximately 0.5 hours is determined for shale ceramsite by clay ceramsite.(3) LWHSC exhibits a higher brittleness compared to conventional concrete.(4) Decreasing the proportions of shale and clay ceramsite light aggregates leads to an increased resistance against freeze-thaw, carbonation, water penetration, and chloride penetration.(5) The LWHSC prepared herein is suitable for the harsh seawater environment of offshore oil platforms, although its applicability is constrained to regions with minimal freezethaw resistance requirements.
The results of this paper suggest that LWHSC incorporating shale and clay ceramsites has great potential for application in construction projects in the harsh seawater environment of offshore oil platforms.However, before transitioning from laboratory-scale experimentation to field applications, rigorous controlled experiments on pivotal factors, encompassing water binder ratio, sand volume rate, cementitious material proportion, and shale and clay ceramsite ratios, among others, are indispensable.Such investigations will aid in identifying the dominant factors, facilitating the optimization and successful development of LWHSC employing shale and clay ceramsites for offshore oil platform environments.This research also provides valuable reference data for offshore structures and similar projects.

Figure 2 .
Figure 2. Relationship between pre-wetting time and compressive strength of SY5 specimen.

Figure 2 .
Figure 2. Relationship between pre-wetting time and compressive strength of SY5 specimen.

Figure 3 .
Figure 3.The growths of the compressive strength of normal-weight concrete and LWHSC(SY1).

Figure 3 .
Figure 3.The growths of the compressive strength of normal-weight concrete and LWHSC(SY1).

Figure 3 .
Figure 3.The growths of the compressive strength of normal-weight concrete and LWHSC(SY1).

Figure 5 .
Figure 5. Effects of freeze-thaw cycle on LWHSC.(a) Weight loss rate, (b) Relative dynamic modulus of elasticity.

Table 1 .
Chemical arrangements of cement, fly ash, and slag powder.

Table 2 .
Physical arrangement of cement.

Table 5 .
Physical and mineralogical characteristics of lightweight aggregate.

Table 6 .
Polycarboxylate properties by the manufacturer.

Table 5 .
Physical and mineralogical characteristics of lightweight aggregate.

Table 6 .
Polycarboxylate properties by the manufacturer.

Table 7 .
Mixture proportions of the LWHSC.
Notes: NC represents normal-weight concrete, * denotes that the shale ceramist is replaced by crushed stone in normal-weight concrete.

Table 8 .
Densities and compressive strengths of LWHSC.