Study on Mix Proportion Design Based on Strength and Sulfate Resistance of 100% Recycled Aggregate Concrete

: This paper presents the mechanical properties of 100% recycled aggregate concrete (RAC), and the results and analysis of the dry–wet cycle accelerated sulfate attack test. The results show that recycled concrete aggregate (RCA) can replace the coarse and ﬁne aggregate. The recycled clay brick aggregate (RCBA) is not suitable for use as a coarse aggregate because the water absorption exceeds the standard. RCA replaces the coarse aggregate; and RCBA returns the ﬁne aggregate to prepare 100% recycled concrete aggregate (RAC). The water–cement ratio is the most signiﬁcant factor affecting the compressive strength of 100% RAC. The results of the mechanical properties analysis show that the compressive strength of RAC is less than that of NAC, and the difference in compressive strength between 100% RAC and NAC decreases with age. The splitting tensile strength of 100% RAC was slightly higher than that of NAC except for 7 d. The results of the dry–wet cycle accelerated sulfate attack test showed that the performance of 100% RAC was lower than that of NAC under the dry–wet process and sulfate attack coupling. Still, the loss rate was less than 5%, which met the standard resistance to the dry–wet cycle accelerating the sulfate attack.


Introduction
Concrete is the essential building material in civil engineering, and ordinary concrete generally contains 60-75% aggregate [1].The consumption of natural aggregate (NA) accounts for the vast majority of raw materials for concrete production.The coarse aggregate for concrete is pebble and gravel, and the fine aggregate is river sand and mountain sand.Aggregate sources mainly rely on mining, and excessive exploitation of non-renewable resources seriously endangers the ecological balance [2].Moreover, with the accelerated development of urban construction, a large amount of construction waste will be generated from abandoned buildings demolished every year.The traditional method to solve construction waste is direct landfill or open-air stacking, and the total stock in China is now more than 20 billion tons [3].The storage of excessive construction waste encroaches on the land, and pollutes soil and water [4].A method is urgently needed to solve the ecological damage caused by the shortage of natural aggregate resources and construction waste, with different waste materials that can be used in concrete, such as plastic, glass, and fibers [5,6].Recycled concrete (RAC), made of recycled aggregate (RCA) instead of natural aggregate, has been widely discussed by scholars in various countries [7,8].RCA's physical and mechanical properties differ from the realistic total [9].First, recycled aggregate covers many old mortars, so it has high water absorption, large porosity, high permeability, and low density.Under the same water-cement ratio, the slump of concrete prepared by RCA is less than that of ordinary concrete, and significantly decreases the functional performance.Secondly, the original natural aggregate in RCA is impacted during the crushing process to produce cracks, reducing the strength [10,11].The strength and durability of RCA concrete are lower than ordinary concrete [12,13].
In China's 1950s-1960s, buildings used about 20 billion m 3 of clay bricks in civil construction, which has now gradually reached its service limit.The classification of construction waste in China shows that 41% of the total construction waste is waste concrete, and about 40% is waste brick [14].It is inevitable and difficult to separate the waste clay brick mixed with the recycled concrete in construction demolition projects.Due to the uncertain source and complex composition of construction waste, the cost of classified recycling is high, and unified recycling is usually used for processing and reusing [15,16].Therefore, in recent years, many scholars have researched recycled clay brick aggregate (RCBA) [17].Clay brick's porosity and water absorption are higher than waste concrete's.Waste clay brick as coarse aggregate will lead to a substantial mechanical strength decrease [18,19].However, RCBA replaces natural sand as the fine aggregate, and the low crushing rate of clay brick leads to the formation of many clay differentials in the production process.Under the action of alkali excitation, the volcanic ash effect increases the compactness of the interfacial transition zone (ITZ), compensating for some strength loss [20].Compared with the research on waste concrete, the research on the recycling of waste clay bricks is not sufficient.
When the concrete structure exposes to the water system or soil containing a high concentration of sulfate ions, sulfate ions penetrate the material through the pores of the concrete, which seriously damages the internal structure of the concrete [21].Sulfate ions react with solid hydrates to produce ettringite, gypsum, and ettringite.The ettringite and gypsum are expansive, and the volume growth of solidified concrete creates stress.When the pressure exceeds the tensile strength of concrete, the concrete matrix cracks, and physical damage occurs.Crack penetration intensifies the sulfate ion intrusion.Chemical corrosion is more serious [22].Therefore, the sulfate corrosion of concrete is a double failure mechanism of chemical and physical damage-the process of corrosion reduces the strength and durability of concrete materials.The research on RAC resistance to sulfate erosion is very complex, with many influencing factors, and the research conclusion is still controversial, so it is a crucial durability index worthy of study for RAC.
In summary, existing studies show that the utilization rate of recycled aggregate is low, and separating recycled clay brick from recycled concrete is difficult.The research objective of this paper is to design 100% recycled aggregate concrete.The RCA replaces natural gravel as the coarse aggregate, and the RCBA replaces natural sand as the fine aggregate.The orthogonal test design RAC is based on long-term compressive strength.By measuring the loss of dynamic elastic modulus, mass loss, and compressive strength of 100% RAC before and after erosion, the mechanical and durability properties of 100% RAC can be evaluated.Finally, the influence mechanism of recycled aggregate on the mechanical properties and durability of fresh concrete is explained from the micro level by the micro test method.

Cement
As an excellent cementing material, cement plays a bonding role in concrete materials.The test adopted 42.5 silicate cement, produced by Shanshui Cement Co., Ltd.(Jinan, China).The test cement's physical properties were in accordance with SL237-1999 [23].The test results are shown in Table 1.The chemical composition of the adhesive is shown in Table 2, in which the main components of the silicate cement are CaO, SiO 2 , and Al 2 O 3 .The test results of the basic mechanical properties of the cement are shown in Table 3.The natural coarse aggregate selection gravel had a diameter of 10-20 mm.The fine aggregate selected was river sand, medium-rough, with a fineness modulus of 2.7. Figure 1a of the RCA originates from waste buildings demolished in Zibo, China.Figure 1b of the RCBA is derived from the wall of the demolition building in Zibo, China, and the original strength is unknown.The recycled aggregate was crushed using a two-stage crushing method: first, a frontal crusher is used on the whole piece of coarse waste, which is broken into large blocks and soaked for 60 min after drying to the saturated surface.Then, the hammer crusher is broken into the used aggregate, which can eliminate the influence of the high water absorption of the recycled aggregate on the water-cement ratio.

Others
The water-reducing agent is used to increase the functional performance of the concrete, which is produced by Shandong Longhuaxin Environmental Protection Technology Co., Ltd.(Zibo, China).The dosage is 0.5% of cementitious material.The mixed water is municipal tap water.

Optimum Mix Design
The orthogonal test selects the cementitious material content, water-cement ratio, and curing temperature as three factors, expressed as A, B, and C. Each element is divided into four levels, defined as 1, 2, 3, and 4. The cement dosage range is 15-30% of the aggregate mass; the degree of water-cement ratio is 0.35-0.5; the curing temperature is between 40 • C and 55 • C. The factor level is shown in Table 4.The orthogonal experimental design of RAC adopts three factors and four orthogonal practical tables.Table 5 shows the mixed proportion of the orthogonal tests.

Specimen Preparation
The coarse and fine aggregates were put into the mixer for dry mixing for 1 min, and then, the cement was added for continuous mixing.The appropriate amount of water mixed with the polycarboxylate water reducer was added into the dry mixing material for constant mixing for 2 min.After discharging into the mold, vibration compaction was performed.The mixture with the mold was maintained in a water bath for 24 h and put into saturated lime water at (20 ± 3) • C until the test age.
The compressive and splitting tensile strength were tested according to GB/T50081-2002 (Similar to EN12390-3:2019) [24].The sulfate resistance test is based on the GB/T50082-2009 (similar to ASTM C876-09) [25], and the specimen specifications are shown in Table 6.Apparent density refers to the ratio of the dry mass per unit volume to the drainage volume of material in the natural state; packing density refers to the mass per unit volume of granular materials in the packing form.The low density of the cement mortar layer on the recycled aggregate surface directly affects the performance of fresh mortar and fresh concrete.Therefore, the apparent density and bulk density of the recycled aggregate are significant indicators for judging the classification conditions of the recycled aggregate.The apparent aggregate density is determined according to GB/T14685-2011 (similar to BS EN 933-5-1998) [26].

Crush Index
The crushing index value reflects the ability of an aggregate to resist crushing.This aggregate parameter is closely related to the mechanical properties of the recycled concrete, and is an essential indicator of the recycled aggregate.The crushing index test was carried out according to GB/T14685-2011 (similar to BS EN 933-5-1998) [26].

Water Absorption
The water absorption rate of the recycled aggregate reaches 80% in 5 min and 85% in 30 min.The water absorption rate of the recycled aggregate is tested in 60 min according to GB/T14685-2011 (similar to BS EN 933-5-1998) [26].

Mechanical Properties Test of Concrete Compression Test
The compressive strength is made of a WAW-600C microcomputer-controlled electrohydraulic servo universal testing machine (Jinan, China), operated following the relevant provisions of GB/T50081-2002 (similar to EN12390-3: 2019) [24].Five test blocks were taken per age, and the average was calculated.

Tensile Strength Test
The splitting tensile strength was operated according to the relevant provisions of GB/T50081-2002 (similar to EN12390-3: 2019) [24].The specimens were cured under standard conditions for 28 days, and the test equipment was similar to the compressive strength.

Resistance to Sulfate Attack Test
Referring to GB/T50082-2009 (similar to ASTM C876-09) [25], the specimen was taken out after 28 days of maintenance, put into the surface water, and dried at (80 ± 5) • C for 48 days.The specimen was taken out and placed in an indoor, cool, ventilating place, and naturally cooled to room temperature, with the weighing mass recorded as M 0 .The dry-wet cycle accelerated sulfate erosion test was started.The specimen was completely immersed in configure 5% sodium sulfate solution for 48 h, removed, and dried at (80 ± 5) • C for 24 d, every three days to complete a cycle, for a total of 100 times: the whole test process was 300 d.

Relative Dynamic Elastic Modulus
The ultrasonic wave propagates inside the solid medium-the propagation changes when it encounters substantial defects (such as holes and cracks).Concrete corrosion is usually characterized by changes in the spatial distribution of internal crystals, which can be sensitive to ultrasonic propagation.In this paper, the MC-6321 non-metallic ultrasonic detector (Nanjing China) produced by Nanjing Mingchuan Measurement and Control Technology Co., Ltd., was used to measure the sound time, T, of the concrete under the set cycle.According to Formula (1), the relative dynamic elastic modulus ratio can be replaced by the percentage of a good time: E rN is the relative dynamic elastic modulus; V 0 is the ultrasonic velocity before concrete corrosion, m/s; V N is the ultrasonic velocity of concrete after breakdown, m/s; T 0 is the sound before concrete deterioration, us; T N is sound after N corrosion, us.

Mass Loss
The weight of the concrete may change under sulfate ion corrosion.In this paper, the weight change of the concrete at different corrosion times was measured by an electronic balance of 0.1 g, according to the GB/T50082-2009 (similar to ASTM C876-09) [25].For frost resistance, the critical value of damage failure was 5% [27].The weight loss of the concrete specimens was calculated by Formula (2): where ∆M is the concrete specimen mass loss, %; M N is the quality of concrete specimens corroded by sulfate after N dry and wet cycles, g; M 0 is the quality of concrete illustration before corrosion, g.

Corrosion Resistance Coefficient of Compressive Strength
The compressive strength of concrete after a sulfate attack changes with corrosion time, which is calculated by the corrosion resistance coefficient of compressive force using Formula (3): where C N is the pressure and corrosion resistance coefficient after N dry-wet cycles; f cN is the compressive strength after N dry-wet cycles corroded by sulfate, MPa; f cD is compared with the exact age of the sulfate corrosion specimen compressive strength, MPa.

Micro-Analysis
Under the multi-factor coupling effect, the morphology and structural characteristics of the matrix hydration products and corrosion products before and after corrosion were observed by SEM.The specimens were dried in an oven at 80 • C for 18 h and taken out of a 1 cm 3 sample for metallographic analysis.A Quanta250 scanning electron microscope produced by FEI Company in Oregon, Hillsboro, United States, equipped with EDAX spectrum, can be used for the elemental analysis of the product.

Test Results of Recycled Aggregate Performance
Table 7 shows the aggregate test results.The apparent density, bulk density, crushing index, and water absorption of the natural aggregate have the same trend.The natural aggregate has the maximum density, the minimum crushing index, and the minimum water absorption, which show good physical properties.This conclusion is consistent with the studies of Silva [28].The recycled clay brick has the most negligible density, extensive crushing index, considerable water absorption, and worst physical properties.It is worth noting that the water absorption rate of RCA increases with the decrease of particle size, which is nearly identical to the findings of Katz and Martín-Morales [29,30].According to GB/T25177-2010 [31]: the recycled concrete coarse aggregate (RCCA) apparent density of >2350 kg/m 3 belongs to the II-type aggregate; the recycled clay brick coarse aggregate (RCBCA) apparent density of >2250 kg/m 3 belongs to the IIItype aggregate.The crushing index of RCCA is between 12% and 20%, belonging to the class II aggregate; the RCBCA crushing index between 20-30% belongs to the III aggregate; the RCCA water absorption is between 8%, and should not be used as recycled coarse aggregate.
GB/T25176-2010 [32]: The apparent density of the recycled concrete fine aggregate (RCFA) is more significant than 2450 kg/m 3 , which belongs to the class I aggregate, and the apparent viscosity of the recycled clay brick fine aggregate (RCBFA) is more excellent than 2250 kg/m 3 , which belongs to the class III aggregate.The bulk density of the recycled concrete and recycled clay brick fine aggregate is more significant than 1200 kg/m 3 , belonging to the class III aggregate.The RCFA crushing index is less than 20% and belongs to the class I aggregate; the RCBFA crushing index is less than 30% and belongs to the III aggregate.
The indexes of the recycled concrete aggregate meet the requirements of the class III recycled aggregate.The water absorption rate of the recycled clay brick aggregate exceeds the standard, which is unsuitable for recycled coarse aggregate use.Therefore, the recycled concrete aggregate is selected as the coarse aggregate in the test, and the gradation range is 0.95-2 cm.The recycled clay brick aggregate is used as the fine aggregate, and the gradation range is less than 0.95.The grading curve is shown in Figure 2.

Results of Orthogonal Test
According to the orthogonal test design of 16 groups of test combinations, the concrete compressive strength results at 3 d, 7 d, 28 d, and 60 d are shown in Table 8.

Range Analysis of Test Results
Range analysis is an intuitive orthogonal test method, and the range reflects the size of the influence of various factors in the test.In the range analysis table, K j (j = 1, 2, 3, 4) represents the sum of the test indexes corresponding to a single factor at the level of j, and each point in Figure 3 is the average K jm of K j .K jm can be used to judge the changing trend between different levels of a single factor.Table 9 is the range analysis table of compressive strength.R j is the range of the elements in column j; that is, the difference between the maximum and minimum of the average index value at each level of column j; R j is used to judge the importance of the factors.The greater the R j value is, the more outstanding the contribution of the factors to the test [33].
Figure 3a: with the increase of A, cementitious material, the 3-d compressive strength increases gradually.With the rise of B, water-cement ratio, the 3-d compressive strength decreases slowly.With the increase in curing temperature, the 3-d compressive strength steadily increases.Figure 3b: with the rise of A, cementitious material, the 7-d compressive strength increases gradually.With the increase of B, water-cement ratio, the 7-d compressive strength decreases slowly.With the height of the curing temperature, the 7-d compressive strength rises gradually.Figure 3c: with the proliferation of A, cementitious material, the 28-d compressive strength increases slowly.With the increase of B, watercement ratio, the 28-d compressive strength decreases gradually.With the rise in initial curing temperature, the 28-d compressive strength decreases slowly.Figure 3d: with the increase of the amount of A, cementitious material, the 60-d compressive strength increases gradually.With the rise of B, water-cement ratio, the 60-d compressive strength decreases slowly.With the addition of the initial curing temperature, the 60-d compressive strength decreases gradually.The range analysis results in Table 9 show that the water-cement ratio, B, has the most significant impact on the 3-d compressive strength, followed by the initial curing temperature of C, and the amount of cementitious material, A, has the slightest effect.The most influential index of the 7-d compressive strength is B, water-cement ratio; followed by A, cementitious material dosage; and C, initial curing temperature, has the most negligible influence.The most influential index of 28-d compressive strength is B, water-cement ratio; followed by C, initial curing temperature; and A, cementitious material dosage, has the most negligible influence.The 60-d compressive strength of the most extensive index is B, water-cement ratio; followed by the initial curing temperature of C; and the effect of A, cementitious material dosage, is minimal.Comprehensive 3-60-d compressive strength range analysis results: B, water-cement ratio, tremendously influences each age.The more significant the water-cement proportion, the smaller the compressive strength.The amount of cementitious material has little effect on the 7-d compressive strength, and little on other ages.The more cementitious materials used, the higher the compressive strength of the concrete.C's initial curing temperature positively affects 3-d and 7-d compressive strength.However, the compressive strength after 28 d decreases with the increase of the initial curing temperature.
Range analysis is intuitive, and the calculation process is simple.However, there are significant errors in the study of test data, especially not being able to accurately identify the fluctuation of test data caused by objective reasons such as test conditions and operation errors.The optimal mix design of materials cannot accurately describe the errors caused by the horizontal change or the difference in the experimental operation when determining the optimal level of factors.Range analysis results are directly used as test results, with low reliability.Because of the lack of a statistical basis, range analysis can only be used to determine the size of the single factor affecting each level, which cannot determine the significance of this effect on the test results.Therefore, this paper continues to use variance analysis to assist in processing compressive strength data.

Variance Analysis of Test Results
The variance analysis method can make up for the deficiency of range analysis.Its basic idea is to divide the total fluctuation of data into two parts.One part is the fluctuation caused by the change of factor level, and the other part reflects the oscillation caused by test errors.The F-test is conducted on whether each factor significantly influences the test index through the sum of the squares of average deviation.Variance analysis was performed using the univariate and multivariate analysis method of IBM SPSS Statistics R26.0.0.0.The variance analysis results in are shown in Table 10.The A, cementitious material dosage, 3-d, 7-d, 28-d, and 60-d sig.values are more significant than 0.05, indicating that the impact of the A index on compressive strength is not apparent.Combined with the range analysis in Figure 2a, the increasing trend of level 4 is more evident than in level 3. Therefore, with the cement dosage of the fourth level, the quality of the recycled aggregate is 30% of the best performance.The hydration of cementitious materials forms cohesive substances.The cohesiveness is stronger when the cementitious materials are hydrated better and entirely.However, increasing the number of cementitious materials contributes little to the strength after the cementitious materials are sufficient to cover all the aggregates.
The influence of B, water-cement ratio, on the compressive strength of 3 d, 7 d, and 28 d was less than 0.05, indicating that the F-test was significant at a = 0.05 level, and the influence of the water-cement ratio on the final compressive strength was substantial.The sig. value of 60 d is more effective than 0.05, indicating that the water-cement balance has no significant effect on the long-term compressive strength.Combined with the range analysis results, a 0.35 water-cement ratio is the most suitable for RAC, and the influence is apparent.Some free water may be left for the pre-water absorption saturation of the recycled aggregate, so the water-cement ratio is one level, 0.35.Before the final strength, the greater the water-cement percentage, the smaller the power of the concrete.These observations follow the findings of Thomas and Gesoglu [34,35].Hydrating active substances, granular substance disintegration, and the regeneration of cementitious substances increase concrete strength.The bonding force of cementitious materials combines the aggregates to produce stability.However, excessive water is unable to be thoroughly connected with cementitious materials, forming free water in the concrete, and the evaporation of water during curing and drying forms pores, resulting in a reduced compactness of the concrete.After 28 d, the hydration ended, and the water-cement ratio at this time was not prominent.
The sig. value of C, curing temperature, on the compressive strength of each age is more significant than 0.05, indicating that this index is not a prominent factor affecting the compressive strength of concrete.However, appropriate early heating and curing are beneficial to improve the early strength of RAC, combined with range analysis.Especially for 3-7 d, the compressive strength increased significantly, but later, there was a power reduction effect.Early heating and curing were suitable for the concrete with high early strength requirements.High-temperature curing causes a large amount of ettringite in the early stage, which benefits the early strength.However, excessive consumption of cementitious ions leads to slow strength growth and poor strength compensation in the later stage, which quickly leads to insufficient final power.High early strength is conducive to mold turnover and accelerated construction.Therefore, considering the economic benefits and strength requirements, the third level is selected at 50 • C. The high-temperature curing time was reduced to 6 h.In summary, the material consumption per cubic meter of 100% RAC is shown in Table 11.

Mechanical Properties Evaluation of 100% RAC
Figure 4 shows the strength comparison between RAC and the reference concrete.
The compressive strength of 100% RAC is lower than that of ordinary concrete.The 3-d difference is 9.37%, the 7-d difference is 35.51%, the 28-d difference is 21.96%, and the 60-d difference is 14.49%.This conclusion is the same as [36,37].Compressive strength depends on the aggregate strength and interface strength.The recycled aggregate has large initial damage, a large crushing index, and less strength than the natural aggregate.The damage in the secondary crushing process in the recycled aggregate is also an important reason for the decrease in strength.With the extension of curing age, the decreasing trend of the compressive strength of 100% RAC slowed down.The results were similar to the research findings of [38,39], and reached the same conclusion.Long-age curing is beneficial to improve the compressive strength of 100% RAC.The difference in the compressive strength between RAC and NAC gradually decreases, which may be due to a large amount of water in the pre-saturation of the clay brick.With the release of more water during curing, the compressive strength of the concrete is compensated.The literature [17] also shows that RCA concrete's strength develops faster than ordinary aggregate concrete's after 28 days.The splitting tensile strength of the total recycled aggregate is slightly larger than that of ordinary concrete, except for seven days, which is a disadvantage.The difference between 3 days is 59.64%, the difference between 7 days is 9.54%, the difference between 28 days is 26.62%, and the difference between 60 days is 16.06%.References [11,40] also reached the same conclusion.The splitting tensile strength mainly depends on the bonding effect between the various parts of the concrete.The crushing index of the clay brick in the total recycled concrete is extensive.In the crushing process, some nano-scale differential clay bricks will be generated, increasing the ITZ's compactness between the aggregate and the matrix.The density of the ITZ is the reason for the high splitting tensile strength of 7 d.The overall RAC splitting tensile strength analysis is still less than ordinary concrete.Still, the downward trend with the increase of curing age weakened, and the strength difference can be compensated by long-term maintenance.This conclusion is the same as in the reference [22].

Results of Sulfate Resistance Test
The relative dynamic elastic modulus of concrete after a dry-wet cycle accelerated sulfate attack is shown in Figure 5a.The relative dynamic elastic modulus of 100% RAC is divided into three stages: descending stage, ascending stage, and secondary descending stage.In the first stage, ordinary concrete has 0-20 cycles between dry and wet, and RAC has 0-10 cycles between dry and wet.Compared with standard concrete, the initial dynamic elastic modulus of total RAC decreases more rapidly.At the early stage of corrosion, the corrosion effect of a dry-wet cycle on concrete is greater than that of sulfate ions.The matrix of concrete is loose, and the relative dynamic elastic modulus decreases under the action of a dry-wet cycle.The initial density of RAC is less than ordinary concrete, so the relative dynamic elastic modulus decreases faster than regular concrete.In the second stage, standard concrete has 20-30 wet and dry cycles, and RAC has 10-40 wet and dry cycles.The relative dynamic elastic modulus of 100% RAC is greater than that of ordinary concrete, indicating that the density of fully RAC increased significantly during this period, which is more than that of regular concrete.The third stage is the second descent of the dynamic elastic modulus.The dynamic elastic modulus of the whole three steps of RAC is greater than that of ordinary concrete, indicating that the density of the recycled aggregate is always greater than standard concrete.The downward trend between the two is the same.The conclusion is consistent with the study of Jaturapitakkul [41].However, Zega [42] conducted a ten-year survey of concrete corrosion under a sulfate environment, and proved that the relative dynamic elastic modulus of the 100% recycled aggregate and natural aggregate concrete was not significantly different.The difference from the conclusion of this paper was that the concrete was wholly immersed in a single corrosion under natural sulfate corrosion in the literature.This paper used a high-sulfate solution and a dry-wet cycle coupling test.The two mechanisms were different, and the damage principle of concrete was different.Figure 5b: the mass damage of ordinary concrete and 100% RAC after sulfate corrosion, accelerated by dry-wet cycles, showed a similar trend with the development of dry-wet processes, which was divided into two stages.In the first stage, the mass loss of ordinary concrete in 40 dry-wet cycles was positive.The total recycled aggregate in 30 cycles was positive, indicating that hydration and sulfate ion erosion led to increased concrete quality.In the second stage, the mass loss of ordinary concrete after 40 dry-wet cycles with an accelerated sulfate attack was hostile, indicating that the mass decreased.For the total RAC, this process appeared after 30 cycles.The mass loss rate of the total RAC is greater than that of ordinary concrete, indicating that the internal damage of the full RAC after a sulfate attack is more significant than that of standard concrete.The result is nearly identical to the findings of [43].After 100 dry-wet cycles with an accelerated sulfate attack, the mass loss of 100% RAC was less than 5%, which met corrosion resistance requirements.
Figure 5c: the compressive strength of RAC after the sulfate attack accelerated by dry-wet cycles was lower than that of ordinary concrete.These observations follow the findings of Hwang [44].After the dry-wet cycle accelerates sulfate corrosion, the corrosion resistance coefficient of concrete compressive strength changes continuously, which can be divided into three stages: stable decline, rapid decline, and stable secondary decline.For the ordinary definite corrosion resistance coefficient, 0-30 is the first stage, 30-50 is the second stage, and 50-100 is the third stage.For the RAC compressive strength corrosion resistance coefficient, 0-40 dry-wet cycles is the first stage, 40-60 cycles is the second stage, and 60-100 dry-wet cycles is the third stage.According to the test data, the three-stage model can be used to describe the corrosion resistance coefficient of the compressive strength of ordinary concrete.The 100% RAC combines the first three stages into a stable decline section, and the second stage is a rapid decline section.The two-stage model can be used to describe the corrosion resistance coefficient of the compressive strength of concrete.The concrete corrosion resistance coefficient model is shown in Table 12, where N is the number of dry-wet cycles.

Micromorphology Analysis
Figure 6 is the SEM contrast diagram of the reference concrete and the 100% RAC after curing for 28 d. Figure 6a shows the morphology of the natural aggregate and ITZ.The natural aggregate had a smooth surface with basically no attachment.The interfacial transition region is evident and dense, and the connection between the aggregate and matrix is strong.The results are similar to the research of [45,46].Figure 6b shows the recycled aggregate and its surrounding morphology.The surface of the recycled aggregate is rough, and a granular hydrated calcium silicate gel is attached to it.Many coral-like C-S-H gels are scattered around the entirety, and a small amount of ettringite is arranged alternately.There is no apparent interfacial transition region.The texture is sparser than ordinary concrete.The study of concrete ITZ proves that the compactness and strength of ITZ are the prominent factors affecting the strength of concrete [47].Many pore structures of RAC ITZ reduce the material's overall continuity, making it easy for the concrete to produce micro-cracks and form high-stress zones at the initial loading stage.The cracks expand rapidly, and the cracks run through the entire specimen.The concrete is damaged, which is consistent with the conclusion that the strength of RAC is lower than that of ordinary aggregate concrete.

SEM-EDS Analysis of Sulfate Corrosion Products
The recycled concrete specimens were immersed in a 5% Na 2 SO 4 solution.After drying and wetting cycles of 0 d, 90 d, and 300 d, the concrete microstructure evolved, as shown in Figure 7.  Figure 7a,b shows the microstructure of ordinary concrete and 100% RAC before erosion.Before concrete corrosion, there was little acicular ettringite and a short columnar gypsum.Additionally, there was a large number of hexagonal plate-like calcium hydroxides, as well as calcium silicate hydrate gel (C-S-H) and alumina hydrate gel (Al 2 O 3 •3H 2 O).The internal components of concrete are regular hydration products [48].
Figure 7c,d is the microstructure of ordinary concrete and recycled concrete after 30 cycles (90 d) of the dry-wet process accelerated the sulfate attack.The concrete immersed in SO 4 2− has needle-like ettringite (AFt) and columnar gypsum CaSO 4 •2H 2 O. Figure 7c: a large amount of gypsum and ettringite is interlaced in the corrosion products of ordinary concrete.Figure 7d: the RAC is mainly composed of ettringite, which explains why the standard concrete relative dynamic elastic modulus is less than the RAC during this period (Figure 5a; step II).CaSO 4 •2H 2 O and AFt are expansive substances, which will cause an increase in concrete quality and density, which is consistent with the conclusion of the sulfate resistance test that the relative mass loss is positive (Figure 5b Step II) and the corrosion resistance coefficient of compressive strength increases (Figure 5c).RAC has a small initial density and many pores, and the filling effect of ettringite and gypsum is more pronounced.Therefore, the relative mass loss and dynamic elastic modulus of RAC are higher than those of ordinary concrete under the sulfate attack accelerated by 30 dry-wet cycles.
Figure 7e,f shows the microstructure of ordinary concrete and recycled concrete after 100 cycles (300 d) of the sulfate attack accelerated by dry-wet cycles.The concrete is filled with pores and cracks after corrosion.Many fine filaments are scattered around the pores.It is not easy to find the existence of calcium hydroxide in some calcium carbonate.The dynamic elastic modulus and relative mass loss of concrete decreased rapidly during this period (Figure 5a,b; step III).EDS analysis showed that the filamentous material was ettringite (Figure 7g).The formation of ettringite is considered the leading cause of concrete degradation.The participation of gypsum in expansion is still controversial [49].The crystallization pressure generated by the growth of ettringite in concrete micro-holes acts on the pore wall of concrete.After exceeding the tensile strength of concrete, the matrix generates micro-cracks, which eventually lead to changes in the mechanical and durability of concrete [16].When calcium hydroxide reacts with sulfate to form gypsum, calcium hydroxide is dissolved simultaneously, and the volume of the two is similar, which may not expand.

1.
The recycled concrete coarse aggregate apparent density is >2350 kg/m 3 , the crushing index is between 12-20%, the water absorption is 2250 kg/m 3 , the crushing index is between 20-30%, and the water absorption is >8%.The water absorption of RCBA exceeds the standard and is unsuitable for recycled coarse aggregate use.

2.
The amount of cementitious material has no significant effect on the compressive strength of concrete.The greater the amount of cementitious material, the higher the strength of concrete, but the degree of strength increase decreases with the rise in cementitious material.The effect of the water-cement ratio on the 3-d-28-d compressive strength is significant, but the impact on long-term compressive strength is not essential.The water-cement balance is inversely proportional to compressive strength.Curing temperature has no significant effect on the compressive strength of each age.

3.
The compressive strength of 100% RAC is lower than that of ordinary concrete.The difference was 9.37% on 3 d, 35.51% on 7 d, 21.96% on 28 d, and 14.49% on 60 d.The splitting tensile strength of the fully recycled aggregate is slightly larger than that of ordinary concrete, except for 7 d; the difference between 3 d is 59.64%, the difference between 7 d is 9.54, the difference of 28 d is 26.62%, and the difference between 60 d is 16.06%.SEM morphology analysis results show that the interfacial transition zone of 100% RAC is sparser and more porous than NAC, which explains the low strength.4.
The density, mass, and strength degradation of 100% RAC after the dry-wet cycle accelerated the sulfate attack was more evident than that of NAC.The initial thickness of 100% RAC is small, and the filling effect of the product after corrosion is more pronounced, but the strength is not improved.Microscopic analysis shows that the expansion of the corrosion product, ettringite, is an important reason for the deterioration of the sulfate attack durability of recycled aggregate concrete.
In summary, separating waste clay bricks from waste concrete in building demolition projects is inevitable and challenging.Due to the uncertain sources and complex composition of construction waste, the cost of classification and recycling is high.Through systematic experimental and theoretical research, this paper realizes the waste-free utilization of construction waste.As has been shown in this work, RAC is highly complex due to differences in source and wood quality.In this study, we first established a systematic mix design theory.The whole-age strength index determines the mix proportion.This design idea has a broad application prospect.In the second part, based on the macroscopic mechanical properties test, the compressive strength of RAC at 28 d decreased by 21.96%, and the splitting tensile strength decreased by 26.62% compared with NAC with the same mix ratio.In the third part, a two-stage compressive strength loss model of 100% RAC was established by a sulfate resistance test.The results show that the sulfate resistance of 100% RAC is lower than that of NAC, but meets the specification requirements.Based on

Figure 1 .
Figure 1.Morphology of recycled aggregate: (a) waste concrete and broken coarse aggregate; (b) recycled clay bricks and broken fine aggregate.

Figure 2 .
Figure 2. The particle size distribution of aggregates; (a) the fine aggregate; (b) the coarse aggregate.

Figure 5 .
Figure 5.The change rule of sulfate resistance index with wetting and drying cycles: (a) relative dynamic elastic modulus; (b) quality loss; (c) corrosion resistance coefficient of compressive strength.

Figure 6 .
Figure 6.The SEM diagram of influence for reclaimed aggregate on concrete.(a) NAC, (b) RAC.

Table 1 .
Physical properties of cement.

Table 2 .
Chemical composition of cement.

Table 3 .
Technical properties of cement.

Table 7 .
Test results of aggregate.

Table 8 .
Results of compressive strength.

Table 9 .
Range analysis table of compressive strength.

Table 10 .
Range analysis of compressive strength.

Table 12 .
Corrosion resistance coefficient model of concrete compressive strength.