# Deterioration Process of Cementitious Material Properties under Internal Sulphate Attack
## 1 Introduction  
In recent years, the structural durability of concrete has become a hot research topic in materials science. Its importance to the social and economic benefits of our national economy and to engineering safety has received increasing attention [1, 2]. Engineering safety accidents due to deterioration problems in concrete structures frequently occur [3, 4, 5, 6], and sulphate attack is one of the important causes of structural durability problems and shortened service life of concrete materials [7]. Sulphate attack is a relatively complex physicochemical reaction process, and there are many sources of sulphate ions. In general, ions enter the cementitious material from the external environment and react with substances in the cement, such as seawater, industrial wastewater, acid rain, and sulphate ions in the soil; however, nowadays, due to the special requirements of different projects for cementitious materials as well as raw materials taken nearby, often with some chemical admixtures and misuse of gypsum-containing aggregates, sulphate ions are already present in the matrix when the concrete is mixed [8]. The ions can react directly with the hydration products of cement and damage cementitious materials. Therefore, we can classify external sulphate attack (ESA) and ISA according to how sulphate ions are sourced [9]. [PERSON] and others have pointed out that among the deterioration processes to which concretestructures are susceptible, internal attack by sulphate (ISA) is prominent [10]. Currently, the hot direction of research on sulphate attacks has moved from external to internal [11]. ISA occurs due to the iron sulfide minerals in the aggregate, which react with oxygen, water and cement hydrates [12].  
Gypsum rocks are widely distributed in China [13], so this type of rock will often be misused in engineering construction, causing safety problems and economic losses [14; 15; 16]. This problem has occurred in dams and tunnels in Middle Eastern countries [17] and some European countries [18]. A similar problem occurred in the Cross Pass Tunnel in Hubei, China [19; 20], which started construction in October 2002 and was forced to close for remediation after an inspection found that the tunnel's concrete had swelling, cracking and damage such as softening after commissioning in July 2004 maintenance. [PERSON] [21] and others studied the concrete removed through an on-site collection of remediation and found that the concrete contained gypsum dolomite aggregate; it is this aggregate that caused internal sulphate attack [22]. A large number of calcium alumina crystals were found in the transition zone of the interface between the cement paste and the aggregate; and carbon sulfur silica calcium products were also found, making the paste soften and lose its cementitious properties without strength. sulphate attack with gypsum-containing aggregates also occurred in China's Chengkun Railway tunnel [23; 24], causing the train to stop running.  
At present, the research on sulphate erosion of cementitious materials is mostly on external sulphate erosion, while the research on internal sulphate erosion is not very comprehensive. However, many cases of sulphate erosion of gypsum-containing aggregates have occurred in actual projects, yet we are not very clear about its erosion mechanism. There are few methods to evaluate the index of such internal erosion, and only a few methods to evaluate external sulphate erosion are suitable for evaluating internal sulphate erosion. Therefore, this paper investigates the effects of gypsum admixture, gypsum particle size and maintenance temperature on the performance of cementitious materials. It also investigates the types of erosion products generated and the effects on microstructure and analyses the process and mechanism of material performance deterioration under internal sulphate erosion.