Deterioration mechanism of sulfoaluminate concrete at high temperature

In order to explore the deterioration law and mechanism of sulfoaluminate concrete under high temperature environment, this paper studied the deterioration law of sulfoaluminate concrete by observing the crack changes of sulfoaluminate concrete samples under different water/cement ratio, different ages and different temperatures after experiencing different high temperatures, and measuring the residual mass ratio, infrared temperature rise, ultrasonic pulse speed and compressive strength. The results show that the loss of strength and mass is the fastest when the temperature of sulfoaluminate concrete is 200 °C, and the loss of strength and mass becomes slower and slower with the increase in temperature. The main reasons for the deterioration of sulfoaluminate concrete are the destruction of microstructure by ettringite and Ca(OH)2 by dehydration at high temperature. The higher the initial strength of sulfoaluminate concrete, the greater the strength loss after high temperature, but the higher the residual strength.


Introduction
Sulfoaluminate cement is a cement independently researched and invented by China Academy of Building Materials Science.It has excellent properties such as anti-freezing, anti-seepage, early strength, high strength, corrosion resistance and low alkalinity [1] .In addition, sulfoaluminate cement consumes fewer resources re-sources and produces less CO2 than Portland cement, which is more environmentally friendly [2][3] .In recent years, sulfoaluminate cement has become a new target of research and development in cement field at home and abroad, and many scholars have carried out indepth research on it [4][5][6][7] .Most researchers have studied the early strength and anti-freezing and impermeability of sulfoaluminate cement, but few scholars have studied the high temperature resistance of sulfoaluminate cement, and the deterioration rule of sulfoaluminate concrete at high temperature has not been given.However, studies have proved that high temperature has great damage to concrete [8][9][10] .In order to explore the law of high temperature deterioration of sulfoaluminate concrete, this paper analyzes the influence of high temperature on sulfoaluminate concrete by studying the change law of residual mass ratio, infrared temperature rise, ultrasonic pulse speed and impressive strength of 1) Cement: quick-drying sulphoaluminate cement produced by Tangshan Polar Bear Building Materials Limited company, with a specific surface area of 350m² /kg or greater.
2) Fine aggregate: The fine aggregates used in the test is the standard sand produced by Xiamen Aisiou Standard Sand Limited company, which is tested in accordance with the "Standard for Testing Concrete (ISO 679:2009) ".
3) Coarse aggregate: limestone with particle size of 10 ~ 30mm.4) Water: ordinary tap water in the laboratory.5) Water reducing agent: polycarboxylic acid liquid water reducing agent.Formatting author names

Formatting author affiliations
Table 1 show the mix ratio of each material of sulfoaluminate concrete.The water/cement mass ratio is 0.30, 0.35 and 0.40, respectively.The concrete was poured into 100mm×100mm×100mm standard parts, and the test blocks were placed in curing boxes with humidity above 90% and temperature of 20 ±3℃ for 1 day, 3 days and 28 days, respectively, and then divided into 5 groups and placed in resistance furnace, and heated to 200℃, 400℃, 600℃ and 800℃ for 2h.Sulfoaluminate concrete samples were divided into 45 groups according to different water/cement ratios, curing time and heating temperature, which were marked as W/C-T-H.The residual mass ratio, infrared temperature rise rate, ultrasonic pulse speed and compressive strength of the specimens were measured in accordance with CECS02--2020 "Technical Regulations for testing concrete Strength with ultra-sonic rebound method" and "Standard for Testing Concrete" (ISO 679:2009).
Table 1.Mixture of concrete materials.

Cracking degree and bursting rate of specimens
According to the experiment, under the same temperature increase rate, crack change of 0.30 water/cement ratio part is the most obvious, while that of 0.40 water/cement ratio part is the least obvious.According to the results in Table 2, the lower the water/cement ratio of sulfoaluminate concrete is, the higher the heating temperature is, and the more likely it is to burst.According to the principle of steam pressure [11][12] , the higher the water/cement ratio, the higher the water content in concrete, and the more cracks and holes in concrete after solidification.Therefore, when heated at high temperature, because there are more holes, the internal water vapor is more likely to volatilize and not easily burst.When the water/cement ratio is low, the cracks and holes in the specimen will be fewer, and the concrete is relatively dense.However, when heated by high temperature, the internal water

Residual mass ratio
The change of residual mass ratio is shown in Figure 1.Because all specimens in group 0.35-28-800 burst, due to the high steam pressure inside the specimens, the residual mass ratio of specimens in this group was not obtained (the same below).It can be seen from Figure 1 that the larger the water/cement ratio of sulfoaluminate concrete, the shorter the age, the higher the heating temperature, and the smaller the residual mass ratio.It is also found that the mass loss of sulfoaluminate concrete occurs obviously when the heating temperature is 200℃ and 400℃ respectively.The main reason for the mass loss is that the hydration product of sulfoaluminate cement is a system dominated by ettringite.The dehydration rated of ettringite is the highest at 160℃, and the decomposition of ettringite is carried out gradually [13][14] .Other causes of mass loss are the breakdown of calcium hydroxide and the breakdown of cement components.Water-cement ratio

Infrared temperature rise
After high temperature, there are more pores and cracks on the surface of sulfoaluminate concrete, and when irradiated by infrared heat source, there is more heat accumulation on the surface, so the infrared thermal image temperature value of concrete after high temperature is larger [15] .It can be seen from Figure 2 that the infrared temperature rises of the surface of concrete specimens increases faster in the temperature range of 25~600℃, because the surface cracks of specimens grow the fastest at this time.When the heating temperature range is 600~800℃, the surface cracks basically do not change.And under the condition of constant heating temperature, the smaller the water/cement ratio, the longer the age, the higher the infrared heating rate.The analysis is because the lower the wa-ter-cement ratio, the longer the age, the denser the structure, and the water vapor is not easy to discharge when heated.Under the same heating condition, the internal accumulated pressure is larger, and the internal and surface are more likely to produce cracks, and the temperature rises faster under the irradiation of the red lamp.

Ultrasonic pulse velocity
If there are holes, honeycomb and other defects or cracks in the concrete, when the ultrasonic wave propagates through the cracks, the measured result will be lower than the sound velocity without cracks [16] .The ultrasonic pulse velocity of sulfoaluminate concrete samples measured after heating is shown in Figure 3.The change ruled of ultrasonic pulse velocity measured after high temperature is similar to the change rule of residual mass ratio.The ultrasonic pulse test results of concrete samples will decrease with the increase in temperature.Under the same heating condition, the lower the water/cement ratio, the longer the age, the higher the ultrasonic pulse rate.As can be seen from the table, the ultrasonic pulse speed of concrete samples decreases rapidly at 400℃ and 600℃, and obvious cracks appear on the surface and inside at this time.The more cracks, the lower the pulse speed.When the temperature reaches 800℃, the structure of the specimen is basically completely destroyed and the cracks no longer increase, so the ultrasonic pulse speed basically does not change.
The surface cracks of some specimens are shown in Figure 5.
Water-cement ratio

Compressive strength
As can be seen from the data in Figure 4, the lower the water/cement ratio at room temperature and the longer the age, the higher the compressive strength of sulfoaluminate concrete specimen.The compressive strength of sulfoaluminate concrete decreases with the increase of heating temperature.The higher the initial strength of sulfoaluminate concrete, the greater the strength loss after high temperature, but the better the high temperature resistance.When the heating temperature is 200℃, the strength of sulfoaluminate concrete decreases the fastest, and the strength decreases directly to 40 ~ 56% of the normal temperature.This is because the hydration product of sulfoaluminate cement is a system dominated by ettringite.Under the action of high temperature, ettringite dehydrates and transforms into the single sulfoaluminate calcium hydrate (AFr0) [17][18] , which damages the internal structure of concrete, which is the main reason for the strength loss of sulfoaluminate concrete.When the temperature rises to 600℃, the strength becomes less than 20% of the original.This is because the dehydration of Ca(OH)2 at this time makes the concrete structure further damaged and the concrete specimen basically loses its strength.When the temperature rises to 800℃, the strength loss is no longer obvious, indicating that the internal structure has completely been destroyed.

Conclusion
This study mainly draws the following conclusions: (1) When the temperature of sulphoaluminate concrete specimen is above 200℃, obvious cracks will occur and there is a probability of bursting.The higher the temperature, the longer the age and the smaller the water/cement ratio, the greater the bursting rate.The mass loss of sulfoaluminate concrete is the fastest at 200℃, and the mass loss slows down gradually with the increase in temperature.And the higher the water/cement ratio, the shorter the age, the more obvious the mass loss after high temperature.
(2) The higher the heating temperature of sulfoaluminate concrete, the smaller the water/cement ratio, the longer the age, the higher the infrared heating rate and the higher the ultrasonic pulse rate.
(3) The compressive strength of sulphoaluminate concrete decreases with the increase of heating temperature.At 200℃, the strength declines the fastest, while at 600℃, the strength declines more and more slowly.The dehydration of ettringite and Ca(OH)2 at high temperature is the main reason for the strength loss of sulfoaluminate concrete.The higher the initial strength of sulfoaluminate concrete, the greater the strength loss and residual strength after high temperature.

FIG 1 .
FIG 1. Variation of residual mass ratio of concrete sample after heating.

FIG 2 .
FIG 2. Infrared temperature rise of concrete sample after heating.

Table 2 .
Burst ratio of concrete specimens under different water/cement ratio and heating temperature.
Natural Science Foundation of Guangdong Province of China (Project no.2022A1515010404), University Teachers' Characteristic Innovation Research Project from Guangdong University Research Fundings Commercialization Centre (Project no.2022XCL10) and Laboratory Openning and Innovation Fund from Foshan University (Project no.KFCX2023-A5).