Study Of The Basic Mechanisms Of Sulfate Attack On Cementitious Materials

Sulfate attack on concrete is a major issue within concrete durability. This detrimental process has been reported to be a cause of damage to concrete structures for over a century. However, due to the discrepancy between laboratory tests and field cases, sulfate attack on concrete is still a confused world.In order to avoid the above discrepancy, first, according the extensive and in-depth literature review on the analysis of field cases and the relationship between concrete structures and their environment, two types of sulfate attack on concrete were defined as follows:sulfate attack on fully buried concrete and sulfate attack on partially exposed concrete. Then, a series of tests based on the appearance of damaged field cases are performed. At last, the basic mechanisms of sulfate attack on these two types of concrete are proposed.Concerning the partially exposed concrete (a part of concrete in contact with sulfate environment and another part exposed to relative dry air), the part of concrete exposed to air is always severely damaged in a manner similar to that of freezing-and-thawing scaling, however the part of concrete in contact with sulfate environment is still sound. This appearance is normally taken for granted to be caused by physical sulfate attack on or salt crystallization in concrete. However, the field cases, the experimental results of long term field experiences, and the indoor laboratory studies do not show strong evidence to prove this view.In this thesis, pore solution expression disclosed that an aquiferous zone containing a large amount of high SO42- concentration (almost saturated) and high pH value (around 13.0)pore solution was generated in the upper parts of cement paste in contact with air due to wick action. On the other hand, results obtained by means of micro-analysis did not show any evidence to prove that salt crystallization can occur in the hydrated cement phase in spite of cement paste or concrete and in spite of constant exposure condition or sharply fluctuated exposure condition. However, the chemical reaction products, ettringite, gypsum and brucite, were the main causes that induce the failure of cementitious materials. Salt crystallization can occur in the calcite crystals, the carbonated products of hydrated cement phases. The mechanical performances of partially exposed concrete cylinders showed that mineral additions, concrete carbonation and high relative humidity exposure condition could accelerate the concrete damage by sulfate attack, and the surface coating could effectively improve the sulfate resisting performance of concrete.First, the reasons why salt crystallization distress cannot result in concrete damage are as follows:(1) Supersaturation is the key factor to generate salt crystallization, however the chemical reactions between sulfates and cement hydrated phases would consume a lot of sulfate ions and the higher concentration and the faster reaction rate. So, it is difficult to reach supersaturation in the concrete.(2) The stress existing in a single pore cannot cause failure, for fracture to occur the crystals must propagate through a region of the network with a lot of pores with different sizes and shapes. However, during the growth of crystals, the chemical adsorption between sulfates and hydrated cement phases makes the pore wall attractive to the salt, rather than repelling. This results in the situation that the thin liquid film between crystals and pore walls cannot be formed. Crystals will grow into contact with the pore walls, and will stop growing, so that the crystallization stress disappears.(3) During the process of wick action, it is difficult to form the dry-wet interface in the bulk of concrete to form sub-efflorescence due to the rich pore solution in the concrete.Secondly, based on the above results, the real damage mechanism of sulfate attack was proposed that the severely chemical attack occurring in the aquiferous zone containing almost saturated sulfate and high pH value pore solution caused the concrete damage. The following appearances can be logically explained by the above mechanism:(1) The blend concretes are always deteriorated more severely than the normal concretes when they are partially exposed to sulfate envrionment. There are two factors responsible for this appearance, due to wick action a more extensive aquiferous zone can be formed in the blend materials. On the other hand, reactive aluminum present in blended cementitious materials is activated by high concentration sodium sulfate solution with high pH value, as generated in the aquiferous zone. This leads to more ettringite formation, and a more severe deterioration than in case of ordinary cementitious materials.(2) When the partially exposed concrete is stored in a higher relative humidity environment, a more extensive aquiferous zone can be generated than in a lower relative humidity environment, this results in more severe damage.(3) The carbonation of concrete plays a negative role in the sulfate resisting performance of concrete. It enables salt crystallization in concrete. Furthermore, carbonation can improve the capillary suction of concrete and promote the formation of aquiferous zone in the upper part of concrete in contact with air.(4) The surface coating is the most effective way to improve the sulfate resisting performance of concrete because it can prevent the formation of aquiferous zone due to wick action in the concrete.At last, a two-stage model involving the formation of aquiferous zone and the progress of sulfate attack was proposed. First, the aquiferious zone can reach equilibrium in a relative short period. Then, during the process of sulfate attack, gypsum would accumulate in the interface zone and ettringite would be formed in the binding medium. Small piece of concrete spalled along the interface zone due to the accumulation gypsum is the main reason why the partially exposed concrete was damage in a manner similar to that of freezing-and-thawing scaling.Concerning sulfate attack on fully buried concrete (concrete fully burid under the earth surface or deeply buried in the soil), according to the survey of field cases, the thaumasite sulfate attack (TSA) was attributed to the failure of concrete. In this thesis, the effects of mixed sulfates solution and high concentration magnesium sulfate solution on the thaumasite formation were studied. Combining previous research and field experiences, it can be concluded that thaumasite sulfate attack took place in the perched sulfates solution with high pH value (>12.5) and high concentrations sulfate solution (higher than the monitored value of local groundwater).In summary, one major conclusion can be drawn. In the field case, if there is concrete damage by sulfate attack, whether it is in fully buried concrete or in partially exposed concrete, the main cause is the chemical sulfate attack occurred in a high concentration sulfate solution formed by a long time accumulation.