Effect of glass powder on mechanical properties and durability of modified polystyrene particle concrete

In this paper, glass powder (GP) was used as a partial replacement of cement and the effects of different levels of GP replacement on the mechanical and durability properties of modified polystyrene particle concrete were investigated, and the mechanism of action was analysed using x-ray diffraction, scanning electron microscopy and CT techniques. The study results show that GP reduces the early strength of modified polystyrene particle concrete. When the substitution rate of GP is not more than 20%, it can improve the late compressive strength and splitting tensile strength of modified polystyrene granular concrete and enhance the chloride ion permeability and frost resistance. When the substitution rate of GP reaches 30% and above, it is unfavourable to the mechanical properties and durability of modified polystyrene granular concrete.20% is the best substitution rate of GP in modified polystyrene grainy concrete. The thermal conductivity of modified polystyrene particulate concrete tends to decrease and then increase with the increase of the GP substitution rate. The best thermal performance is achieved when the GP substitution rate is 10%. It can be attributed to the fact that the appropriate amount of GP can improve the internal microstructure of the modified polystyrene particle concrete and optimise the pore structure.


Introduction
Concrete has solid and durable properties and is the world's most widely used building material [1].At the same time, the high density and brittleness of concrete are also cited as being one of the most consumed materials in the construction industry [2].Adjusting the raw materials of concrete is a standard method to reduce the dead weight of concrete, especially the selection of aggregates plays a vital role in changing the importance of concrete.Lightweight aggregate concrete, prepared from light materials such as coarse and fine aggregates, is increasingly in demand in modern construction [3].Polystyrene particles comprise 98% air and 2% polystyrene, an ultralightweight closed-cell material with high porosity.Polystyrene granule concrete composed of cement, water, sand, stone and polystyrene granules can reduce the mining of natural stone, reduce the cost, and is an artificial lightweight aggregate concrete with high economic, social and environmental benefits [4].Polystyrene particle concrete is favoured due to its low density, thermal conductivity, good deformation, and thermal and acoustic insulation properties [5,6].It is widely used in composite panels, wall materials, load-bearing concrete blocks, pavement base materials and floating marine structures [7,8].
Mechanical properties and durability are the basic properties of concrete that are closely related to the safety and service life of the structure.Research on the mechanical properties and durability of polystyrene granular concrete to improve the safety and service life of the design is one of the essential technical means to promote polystyrene grainy concrete.Due to the poor mechanical properties of polystyrene particles, the mechanical properties of polystyrene particle concrete are seriously weakened, so there is a big difference between its particle concrete under freeze-thaw cycling was analysed from the physical phase, microscopic and microscopic levels using x-ray diffraction (XRD), scanning electron microscopy (SEM) and computed tomography (CT) techniques.

Materials
This test used silicate cement with strength grade P-O 42.5, which conforms to the Chinese standard GB 175-2007.The composition of the adhesive and GP are detailed in table 1.The density of cement and GP is about 3057 kg m −13 and 2495 kg m −13 , respectively, based on the standard test method for determining the cement density using small specific gravity bottles according to the Chinese traditional GB/T 208-2014.By laser particle size analyser, the average particle length of cement and GP were 20.5 μm and 13.8 μm, respectively.GP particles are smaller than cement particles, and replacing cement with GP can play a role in concrete production.GP particles meet the requirement of GP particle size less than 300 μm proposed in the literature [33].In addition, the gradation curves of cement and GP were determined, as shown in figure 1.The glue and GP particles were scanned using a scanning electron microscope with a magnification of 5000 times, and the images are detailed in figures 2 and 3.The GP particles are irregular in shape and angular, which is conducive to the strength of hardened concrete, but affects the workability of freshly mixed concrete.
River sand with a fineness modulus of 2.3 and a specific gravity of 2438 kg m −13 were selected as the fine aggregate.
In this study, the Italian EIA additives were used to modify the polystyrene particles with a particle size of 3 ∼ 5 mm and a density of 8.4 kg m −3 , and the treated ones are shown in figure 4.

Mixture proportion
To investigate the effect of GP on the frost resistance of modified polystyrene particle concrete, four types of GP substitution rates of 0%, 10%, 20% and 30% were designed to replace cement.Table 2 shows the details of the mixing proportions by weight of each constituent of the polystyrene particle concrete.The modified polystyrene particle concrete is produced according to the process flow shown in figure 5.

Test methods
The modified polystyrene granular concrete for each of the four working conditions was fabricated into 11 groups of specimens, with three models in each group.There were two sizes of samples, in which the cubic illustrations with the size of 100 mm × 100 mm × 100 mm were used for the compressive strength test, split tensile strength test, thermal performance test, and freeze-thaw test.The other specimen size is a cylinder with a diameter of 100 mm and a thickness of 100 mm for the chloride ion permeability test.When the specimen reaches the maintenance age, the cylinder is taken out, and its upper and lower bottom surfaces are each cut 25 mm to make a cylinder with a diameter of 100 mm and a thickness of 50 mm for the chloride ion permeability test.X-ray and scanning electron microscope (SEM) were used to analyze the hydration products and the microstructure of the interfacial transition zone of the modified polystyrene granular concrete specimens containing GP.The detailed experimental procedures of XRD and SEM were based on the literature [34].A high-precision industrial spiral CT tester combined with image processing technology was used to qualitatively and quantitatively analyze the microstructure of modified polystyrene particle concrete specimens containing GP.

Compressive strength
The compressive strengths of the modified polystyrene particle concrete containing different levels of GP to replace cement at 7 and 28 days are shown in figure 6.It can be seen from figure 6 that the 28-day compressive strength of the modified polystyrene particle concrete containing GP is significantly higher than the 7-day compressive strength.The improvement effect of GP on the 7-day strength of the modified polystyrene particle concrete is not ideal.The compressive strength of the modified polystyrene particle concrete first increases and then decreases with the rise of the GP substitution rate.The compressive strength of modified polystyrene particle concrete with a 20% GP substitution rate at seven days and 28 days is the highest, 9.38% and 16.15% higher than that of the control group, respectively.The compressive strength of the modified polystyrene particle concrete is higher than that of the control group when the GP replacement rate is not more than 20%.However, when the GP replacement rate reaches 30%, its compressive strength is lower than that of the control group.The results show that the reasonable use of GP instead of cement can improve the later power of modified polystyrene particle concrete to a certain extent, and it is feasible to use GP as a cementing material to replace part of the cement.The experimental results agree with the conclusions of the literature [27].It is mainly due to the slow reaction rate of the volcanic ash in the glass powder, which replaces the cement and reduces the early age strength.As the curing time increases, the amorphous silica in the glass powder slowly dissolves in the alkaline environment.It reacts with Ca2+ to form low alkalinity C-S-H, improving compressive strength later [28].It can reduce the amount of glue and alleviate the severe threat to the ecological environment caused by the sharp increase in industrial glass waste.

Splitting tensile strength
It can be seen from figure 7 that the seven days splitting tensile strength of modified polystyrene particle concrete with GP substitution rate of 10%, 20% and 30% is 3.91%, 9.5% and 12.85% lower than that of the control group.It shows that GP can't improve the early splitting tensile strength of modified polystyrene particle concrete but has the effect of reducing it.The splitting tensile strength of the modified polystyrene particle concrete at 28 days first increases and then decreases with the increase of the GP substitution rate.The splitting tensile strength of modified polystyrene granular concrete with a GP substitution rate of 20% was the largest.When the GP substitution rate is 20%, the splitting tensile strength of the modified polystyrene particle concrete is only 9.78% higher than that of the control group.In comparison, when the GP substitution rate is 30%, its splitting tensile strength is 15.21% lower than that of the control group.The reason for this is that a moderate amount of GP improves the densification of the cement matrix microstructure due to the volcanic ash activity so that the interfacial bond between the aggregate and the cement paste matrix is improved, and the splitting tensile strength is improved to a certain extent [27].However, a large amount of GP drastically reduces the amount of cement, so the power of the cement matrix of polystyrene granular concrete is reduced, and the internal microstructure is loose, which is not conducive to the splitting tensile strength of polystyrene powdery concrete.
It can be seen that the improvement effect of GP on the splitting tensile strength of modified polystyrene granular concrete is less satisfactory.

Thermal performance
Thermal conductivity is one of the leading indices of the thermal performance of concrete.It can be seen from figure 8 that as the GP substitution rate increases, the thermal conductivity of the modified polystyrene particle concrete shows a tendency to decrease and then increase.The modified polystyrene particle concrete with a 10% GP substitution rate has the lowest thermal conductivity.When the GP substitution rate is 10% and 20%, the thermal conductivity of the modified polystyrene particle concrete in the dry state is 13.67% and 4.54% lower than that of the control group.In comparison, the thermal conductivity of the concrete with a GP substitution rate of 30% is 7.36% higher than that of the control group.The thermal conductivity of modified polystyrene particle concrete with a GP substitution rate of 20% and 30% in the saturated state is 10.61% and 29.68% higher than that of the control group.The thermal conductivity of modified polystyrene particle concrete with a GP substitution rate of 10% is slightly lower than that of the control group.It is attributed to the fact that when the substitution rate of GP is low, it mainly acts as a micro-aggregate, increasing the number of tiny pores and the interfacial area, thus reducing the thermal conductivity of the concrete.As the substitution rate of GP increases, part of GP and calcium hydroxide will undergo a secondary hydration reaction after cement hydration to produce hydrated calcium silicate to improve the compactness of concrete.At the same time, the rest of GP is mainly used as a micro-aggregate.In addition, GP's thermal conductivity is higher than cement's.When the substitution rate of GP reaches 30%, the thermal conductivity of its modified polystyrene particle concrete is slightly higher than that of the control group.

Chloride ion permeability
Figure 9 shows that the chloride ion diffusion coefficient of modified polystyrene particle concrete shows a trend of first decreasing and then increasing with the increase of the amount of GP replacement cement.The chloride ion diffusion coefficient of the concrete was the lowest when the GP substitution rate was 20%.The chloride ion diffusion coefficient ratios of the modified polystyrene granular concrete with a GP substitution rate of 20% were lower than those of the control group by 11.23% and 14.57% at 28 days and 56 days, respectively.The chloride diffusion coefficients of modified polystyrene concrete with 30% GP substitution were 8.81% and 5.50% higher than those of the control group at 28 and 56 days, respectively.It suggests that a moderate amount of GP refines and fills the pores to some extent due to the hydration products of the volcanic ash reaction [35], which inhibits the migration of chloride ions and reduces the diffusion coefficient of chloride ions in modified polystyrene granular concrete [31,32].When GP replacement cement is around 20%, the best performance of improving the resistance to chloride ion penetration of modified polystyrene particle concrete is achieved.If the replacement rate of GP is 30% and above, it is unfavourable for the chloride ion permeation resistance of modified polystyrene granular concrete.

Frost resistance
The slow-freezing method uses the mass loss rate and strength loss rate as the concrete frost resistance evaluation index.As shown in figure 10, the strength and mass loss rates of modified polystyrene granular concrete showed a trend of increasing and then decreasing with the increase of the GP substitution rate.When the substitution rate of GP was 20%, the strength loss rate and mass loss rate of its modified polystyrene particle concrete were the most minor, 23.76% and 11.43% lower than that of the control group after 150 freeze-thaw cycles, respectively.When the replacement rate of GP was 30%, its modified polystyrene particle concrete had the most significant strength loss rate and mass loss rate, which were 9.39% and 20.36% higher than the control group after 150 freeze-thaw cycles, respectively.Therefore, increasing the amount of GP has a positive effect on the improvement of frost resistance of modified polystyrene granular concrete when the substitution rate of GP is not more than 20%.At the same time, it is unfavourable to the frost resistance of modified polystyrene granular concrete when the substitution rate of GP is 30%.The evaluation index of frost resistance of modified polystyrene granule concrete is more favourable when the substitution rate of GP is 20%.It is mainly because, on the one hand, GP fills pores in polystyrene granular concrete, effectively preventing the transport and flow of water in the concrete [28].On the other hand, the secondary hydration reaction of GP further improves the densification of the concrete microstructure and the strength of the cement matrix, which can better resist the action of freezing and expansion forces.

Microstructure analysis
4.1.XRD analysis X-ray diffraction test helps to determine the relative intensity of diffraction peaks in XRD patterns.X-ray diffractometer was used to analyze the composition of powdered specimens of modified polystyrene particle concrete to study the microstructure of the cement matrix.As can be seen from figure 11, the physical phases of  the XRD spectra of four groups of modified polystyrene granular concrete with different amounts of GP dosing were the same at the age of 28 days and after 150 freeze-thaw cycles.The XRD spectra before and after freezing and thawing were also roughly similar, with diffraction peaks mainly of six phases, SiO 2 , Ca(OH) 2 , Ettringite, C 3 S, C 2 S and CaCO 3 [34].The intensities of the primary phase peaks of SiO 2 and Ca(OH) 2 (abbreviated as CH) showed a tendency to decrease and then increase with the increase of GP doping.In contrast, the intensities of the primary phase peaks of the physical phases of AFt, C-S-H, CaCO 3 , and C 3 S (C 2 S) tended to increase and decrease with GP doping.The phase angles underwent a small change.When the substitution rate of GP is, 20% is the turning point of the trend of the diffraction peaks of each physical phase.It is mainly due to the volcanic ash reaction between the amorphous SiO 2 in GP and CH produced by the hydration reaction of cement to generate additional C-S-H and C-A-H gels.And the modified polystyrene particle concrete specimens with a GP substitution rate of 20% showed the optimum condition of volcanic ash reaction.However, when the amount of GP continues to increase, the amount of cement decreases, the amount of CH generated is less, and its crystals cannot grow large enough due to the limited space, thus resulting in a less optimal volcanic ash reaction.Therefore, from the XRD patterns, it can be qualitatively stated that the hydration products of the cement reacted with the SiO2 in the glass powder in a volcanic ash reaction, and 20% was determined to be the optimum replacement rate of GP.This conclusion is consistent with the conclusion reached in the micromechanical tests.

SEM analysis
In general, the microstructure of concrete significantly affects its mechanical properties [36].This experiment used a scanning electron microscope to obtain SEM images of GP-modified polystyrene particle concrete specimens magnified 5000 times to analyze their microstructure before and after freeze-thaw.
Figure 12 shows that the microstructure of each modified polystyrene granular concrete specimen mainly includes unhydrated cementitious material particles, hydration products, aggregates, voids, microcracks, etc [37].The incorporation of GP resulted in significant changes in the microstructure of the modified polystyrene granular concrete.As shown in figure 12(a), (a) large number display a large number of hydrated calcium silicate (C-S-H) gels, a small number of CH crystals and needle and rod AFt crystals, and some dispersed large capillary pores and pores were also observed.These hydration products are interconnected and covered to form a relatively dense internal microstructure, indicating that the cement hydration reaction is sufficient.However, abundant granular and thick C-S-H gels, AFt crystals, disjointed capillaries and fine voids were found in figure 12(b), while CH crystals were not observed.It may be due to the volcanic ash reaction of the amorphous reactive SiO2 in GP with CH to generate additional C-S-H gels.As can be seen in figure 12(c), the internal microstructure of the modified polystyrene granular concrete specimens with 20% GP substitution is very dense.The C-S-H gels overlap and interconnect, and the unhydrated particles are surrounded by the absence of CH crystals and large-size capillary pores.On the one hand, with the increase of GP content, the volcanic ash reaction of GP is intensified, which can consume more CH crystals and produce more C-S-H gels.On the other hand, the microfilming effect of unreacted GP particles fills and refines large capillaries and voids more efficiently [38].As a result, the modified polystyrene particle specimens with 20% GP substitution showed the highest strength, resistance to chloride ion penetration, and frost resistance regarding macroscopic properties.The specimen's interior is shown in figure 12(d) with a reticulated C-S-H gel, some large capillaries, pores and microcracks.These capillaries, pores and microcracks are interconnected to divide the internal microstructure into two parts.It is because GP replaces 30% of cement and reduces the cement content, and the large number of GP particles increases the distance between the cement particles.The bonding between GP and cement particles will weaken [39].The reduction of hydration products on the surface of GP particles makes contact between CH and GP particles less likely, which leads to a relative decrease in the amount of C-S-H generated.The dispersing effect of GP particles is greater than the filling effect.It makes the hydration products less accessible, resulting in a relative reduction in the amount of C-S-H generated and filling impacts the number of voids between the hydration products increases.Therefore, the microstructure of the modified polystyrene particle concrete specimens containing 30% GP is poor, and the results in terms of macro properties such as strength, resistance to chloride ion permeability and frost resistance are not satisfactory.
The modified polystyrene particle concrete sample is magnified 5000 times by scanning electron microscope to obtain the SEM image after 150 freeze-thaw cycles, as shown in figure 13(a), which indicates that there are a large number of C-S-H cementitious blocks in the control group of modified polystyrene particle concrete.Compared to figure 12(a), the internal voids of the concrete in figure 13(a) are increased, and the interior is loose and not dense enough.It can be seen from figures 12(b) and 13(b) that the C-S-H gel inside the modified polystyrene particle concrete with a GP substitution rate of 10% is mainly reticular and flocculent after 150 freeze-thaw cycles, with more AFt crystals, larger voids and more pores.As can be seen in figure 13(c), the internal microstructure of the modified polystyrene particle concrete specimen with 20% GP substitution is very dense.Still, the void ratio increases somewhat without freezing and thawing.It can be seen from figures 12(d) and 13(d) that the C-S-H gel structure of the modified polystyrene particle concrete with a 30% GP substitution rate is loose, the number of voids increases, the voids increase, and microcracks appear after 150 freeze-thaw cycles.It is attributed to the excessive substitution rate of GP, which significantly reduces the amount of cement and forms a loose cement matrix, which is unfavourable to the freeze-thaw resistance of the modified polystyrene particle concrete.During the freeze-thaw cycle, the frost heave stress is concentrated in the defect area, and the loose cement matrix is insufficient to inhibit microcracks formation and propagation.Compared with figures 13(a) and (b), the results show that a suitable amount of GP can improve the freeze-thaw resistance of the modified polystyrene particle concrete.4.3.CT Image analysis 4.3.1.Qualitative analysis of two-dimensional section layer CT images at 5 mm from the upper surface were selected for each specimen to analyze the effect of GP on modified polystyrene particle concrete during freeze-thaw cycles.The CT-scanned two-dimensional tomograms of modified polystyrene powdery concrete without freeze-thaw and after 150 times of freeze-thaw cycles with a GP substitution rate of 20% are shown in figures 14(a) and (d), respectively.From figures 15(a) and (d), the modified polystyrene granular concrete specimens, after 150 times of freezing and thawing, showed apparent damage at the edges and corners.The number of pores increased, and the pores enlarged.It can be shown that the freezing and expansion forces during the freezing and thawing process have a significant effect on the modified polystyrene granular concrete.
Figure 14 shows that the surface integrity of the modified polystyrene particle concrete sample containing GP is good after 150 freeze-thaw cycles.There were no exposures, cracks or holes in the aggregate, and the interface between the crushed stone and the cement matrix, and between the polystyrene particles and the cement matrix, was tightly bonded with clear boundaries.However, all have some minor unevenness or corner loss or wear.In figure 14(d), the edges are uneven, and mortar peeling and corner dropping are the most severe phenomena.It leads to the loss of quality of the modified polystyrene particle concrete during the freeze-thaw cycle.In addition, the number of pores in figure 14(d) is the largest, and the pore size is large.However, the number of pores in figures 14(b) and (c) is significantly less than that in figure 14(a), and the number of pores in figure 14(c) is the smallest.It is attributed to the effective filling of the pores in the modified polystyrene particle concrete with a suitable amount of GP, and the pozzolanic effect improves the strength of the cement paste, enhances the impact of frost expansion resistance, and reduces the damage caused during the freezing and thawing process.When the GP substitution rate reaches 30%, the cement consumption is significantly reduced, resulting in insufficient hydrated calcium silicate and hydroxide.In addition, GP cannot fully undergo the secondary hydration reaction, resulting in low strength and looseness of the cement matrix within the modified polystyrene particle concrete, which is prone to frost heave damage.

Qualitative analysis of three-dimensional model degradation laws
The three-dimensional images of the CT scan reconstructed pores of modified polystyrene granular concrete with a GP substitution rate of 20% when unfrozen and thawed and after 150 freeze-thaw cycles are shown in figures 15(a) and (d), respectively.As can be seen from the two figures, the modified polystyrene granular concrete, without freezing and thawing, has fewer pores inside, and the pores are relatively sparse.The density of pores in figure 15(d) after 150 cycles of freezing and thawing of the modified polystyrene granular concrete with a GP substitution rate of 20% is much larger than in figure 15(a) when the concrete was not frozen and thawed.
Figure 15 shows that the pores are spherical or elliptical, and the pores are distributed more in the centre of the specimen, while those at the edges or corners are fewer, and the pore sizes are smaller.It is mainly because the cement mortar overflowed to fill the voids at the edges and corners of the specimen during the vibration process of the modified polystyrene granular concrete.However, the freezing and thawing damage is a process from the outside to the inside, and the specimen edges, corners and surfaces have the most significant damage under the action of the freezing and expansion force.Their cement matrix is prone to loosening, abrasion, or even falling off the specimen's corners, edges, and surfaces.The pores in figure 15(e) are very dense, the number of pores is large, and the diameter is also larger.The pores in figure 15(d) are sparsely distributed, and the number of pores is less than that in figure 15(b).It indicates that the incorporation of GP has changed the internal structure of the modified polystyrene particle concrete.A reasonable amount of GP can optimise the interior design of modified polystyrene particle concrete, reduce the porosity, improve the compaction and, to some extent, improve the mechanical properties and durability.An excessive amount of GP will deteriorate the frost resistance of modified polystyrene concrete.The pore volume, surface area, porosity and pore diameter of GP-modified polystyrene granular concrete before and after 150 freeze-thaw cycles are shown in table 3.As shown in table 3, the total pore volume and the average pore diameter of the modified polystyrene granular concrete after 150 freeze-thaw cycles show a trend of increasing and then decreasing with the increase of the GP substitution rate.The average pore diameter, porosity and pore surface area of modified polystyrene granular concrete with a 20% GP substitution rate after 150 freeze-thaw cycles increased by 10%, 19.84% and 21.72%, respectively, compared with that of unfrozen-thawed concrete.The average pore size of modified polystyrene concrete with a 20% GP substitution rate was 9.09% and 12.28% lower than that of modified polystyrene concrete with a 10% GP substitution rate.The control group, respectively, and the average pore size of modified polystyrene concrete with a 30% GP substitution rate was the same as that of the control group.It indicates that the GP admixture fills part of the pores, reduces the pore size and changes the large pores into tiny pores, which is consistent with the conclusion of the macroscopic mechanical properties.The volumetric porosity and pore surface area of the modified polystyrene granular concrete after 150 freeze-thaw cycles showed a trend of decreasing and then increasing with the increase of the GP substitution rate.When the GP substitution rate was 20%, its concrete's porosity and pore surface area was the smallest, which were 22.22% and 27.03% lower than those of the control group, respectively.And the porosity and pore surface area of the modified polystyrene particle concrete with a GP substitution rate of 30% was the largest, 11.11% and 11.48% higher than those of the control group, respectively.It indicates that the pore size and pore number of modified polystyrene particle concrete are greatly improved under the action of freezing and expansion force during the freeze-thaw cycle, and the GP substitution rate of 20% has a significant effect on enhancing the frost resistance of modified polystyrene particles.

Pore diameter and quantity distribution characteristics
Figure 16 shows that most of the pores in the modified polystyrene particle concrete range from 0 to 3 mm.The number of pores in the GP-modified polystyrene particle concrete after 150 freeze-thaw cycles shows a trend of first increasing and then decreasing with the increase of pore size, and the number of pores in the range of 0.7 mm to 0.8 mm is the largest.With the GP substitution rate rise, the number of pores in the modified polystyrene particle concrete with a pore diameter range of 0 to 1 mm increases and then decreases with the height of the GP substitution rate.The number of pores with a pore diameter of 1 ∼ 3 mm first decreases and then increases with the increase of the GP substitution rate.When the GP substitution rate is 20%, the percentage of pores in the regions with pore diameters of 0 ∼ 0.5 mm, 0.6 ∼ 1 mm and 1 ∼ 3 mm is 8.69%, 53.89% and 37.42%, respectively, which are the maximum, maximum and minimum values of the corresponding pore diameters in the CT scan samples of GP modified polystyrene particle concrete.It is attributed to the fact that when the GP substitution rate is 20%, the role of GP as a micro-aggregate to fill the pores and the pozzolanic effect reduces the pore size to a certain extent, reducing the large pore size.On the other hand, when the GP substitution rate is 30%, the cement matrix inside the modified polystyrene particle concrete becomes loose, which promotes the conversion from small pore diameter to large pore diameter during the pore melting cycle.
Figure 17 shows that the most significant number of pores is in the range of 0.6 mm to 1 mm pore size, followed by the interval of 1.1 mm to 1.5 mm pore size; the smallest number of pores is in the gap of 0 mm to 0.5 mm.The trend of the percentage of the number of pores and the pore size of the modified polystyrene granular concrete specimens with a 20% replacement rate without freezing and thawing is similar to the trend after 150 times of freeze-thaw cycles.The GP replacement rate of 20% modified polystyrene granular concrete specimens after 150 freeze-thaw cycles, the number of pores with pore size in the interval of 0 ∼ 0.5 mm  decreased dramatically, which was 12.8% lower than that in the unfrozen-thawed state.And the number of pores in the gap of pore size 0.6 ∼ 3 mm increased, especially in the interval of 0.6 ∼ 1 mm, the number of its pores increased by 9.43%.It indicates that the modified polystyrene particle concrete not only produced micropores after 150 freeze-thaw cycles, but the pore size of the pores increased, especially the pore size of the pores in the interval of 0 ∼ 0.5 mm gradually increased to the pores with a pore size of 0.6 ∼ 1 mm under the action of freezing and expanding force.This conclusion is consistent with the macroscopic properties and the findings of SEM microanalysis.
As shown in figure 17, the most significant number of pores is in the range of 0.6 mm to 1 mm pore size, followed by the interval of 1.1 mm to 1.5 mm pore size; the smallest number of pores is in the gap of 0 mm to 0.5 mm.The trend of the percentage of the number of pores and the pore size of the unfrozen and thawed modified polystyrene granular concrete specimens with a replacement rate of 20% was similar to that of the frozen and thawed samples after 150 times freeze-thawing cycles.The trend of the number of pores and the pore size of the GP replacement rate of 20% modified polystyrene granular concrete specimens after 150 cycles of freeze-thawing, the number of pores with pore size in the interval of 0 ∼ 0.5 mm decreased dramatically, which was 12.8% lower than that in the unfrozen-thawed condition.And the number of pores in the gap of pore size 0.6 ∼ 3 mm increased, especially in the interval of 0.6 ∼ 1 mm, the number of its pores increased by 9.43%.It indicates that modified polystyrene particle concrete not only produced micropores after 150 freeze-thaw cycles, but the pore size of the pores increased, especially the pore size of the pores in the interval of 0 ∼ 0.5 mm gradually increased to the pores with a pore size of 0.6 ∼ 1 mm under the action of freezing and expansion force.This conclusion is consistent with the macroscopic properties and SEM microanalysis conclusions.

Damage analysis
The damage-sensitive zone is the most intensive damage area for pore development, crack sprouting and expansion within the modified polystyrene granular concrete specimen, which shows dark low-density bands in the process of CT images using digital image processing techniques.No damage zone with low-density bars or pore zones in clusters was found in the unfrozen and thawed specimens of modified polystyrene granular concrete with a GP substitution rate of 20%.The damage-sensitive zone of GP-modified polystyrene granular concrete at 150 freeze-thaw cycles is shown in figure 18.As shown in figure 18, the modified polystyrene granular concrete with 30% GP substitution rate and the control group had two low-density damage-sensitive zones that were almost continuous throughout the specimen.There is a low-density damage zone in the 3D images of the modified polystyrene granular concrete with 10% and 20% GP substitution rates, and the damage area is relatively small.

Conclusion
In this paper, the effect of GP on the mechanical properties, thermal properties, frost resistance and chloride ion impermeability of modified polystyrene granular concrete was investigated, and the internal structure was analysed at the physical, micro and acceptable levels using experimental results from x-ray diffraction, scanning electron microscope and CT techniques.The following conclusions have been drawn: • GP Partial replacement of cement decreases the early strength of modified polystyrene granular concrete and increases the late strength of modified polystyrene powdery concrete to a certain extent.• The compressive strength and splitting tensile strength of polystyrene granular concrete tended to increase and then decrease with the increase of GP substitution rate, in which the strength value of concrete with a 20% GP substitution rate was the highest.Its 28-day compressive strength and splitting tensile strength were increased by 16.15% and 9.78%, respectively.
• The thermal conductivity of the modified polystyrene granular concrete increases with the increase of the GP substitution rate, in which the thermal conductivity of the GP substitution rate 20% concrete in the dry state increases by 7.36%.In comparison, the thermal conductivity in the saturated state rises by 29.68%.
• when the GP substitution rate is not more than 20%, it can improve the anti-chlorine ion penetration property and the anti-freezing property of modified polystyrene granular concrete.20% is the optimum substitution rate of GP in modified polystyrene powdery concrete to replace cement.
• Results of pore analysis in XRD analysis, SEM image analysis and CT image when the GP substitution rate is not more than 20% can improve the intensity of the diffraction peak of the hydration product, improve the internal microstructure of modified polystyrene granular concrete, reduce the number of pores and optimise the pore structure.It is practicable to use GP as a cementitious material to replace not more than 20% of the cement in modified polystyrene granular concrete.

Figure 1 .
Figure 1.Gradation curve of cement and GP.

Figure 5 .
Figure 5. Preparation flow chart of GP modified polystyrene particle concrete.

Figure 6 .
Figure 6.Effect of GP on compressive strength of modified polystyrene particle concrete.

Figure 7 .
Figure 7. Effect of GP on splitting tensile strength of modified polystyrene particle concrete.

Figure 8 .
Figure 8.Effect of GP on thermal conductivity of modified polystyrene particle concrete.

Figure 9 .
Figure 9.Effect of GP on chloride diffusion coefficient of modified polystyrene particle concrete.

Figure 10 .
Figure 10.Relationship between freeze-thaw index and freeze-thaw cycle number of GP modified polystyrene particle concrete (a) Effect of GP on mass loss rate; (b) Effect of GP on strength loss rate.

Figure 17 .
Figure 17.Stacked histogram of pore diameter and pore quantity proportion of GP modified polystyrene particle concrete.

Table 2 .
Mix designs of the polystyrene particle concrete used in the experiments (in weight).

Table 3 .
Statistical table of pore structure data of modified polystyrene particle concrete.