Study On Sulfate Attack Silicate Cementitious System In Weak Alkaline

Cement concrete products had become an important material basis for the development of China's national economy,after decades of hard work in the construction industry,cement concrete products industry from the development to the eight categories of dozens of products.With the development of China's science and technology and the further expansion of market demand,cement concrete products are developing in the direction of large scale,ecological and decoration.As one of the important raw materials in the construction industry,cement was widely used in defense,transportation,water conservancy,industrial production and other fields and its outstanding position.In recent years,the failure of concrete structure due to sulfate erosion had attracted extensive attention in engineering circles.On the other hand,with the rapid development of social industrialization and the improvement of living environment,there were many industrial wastes that bring great pressure to the environment,such as silica fume,fly ash,coal gangue and so on.These wastes take up a lot of land resources,and harmful ions penetrate to cause environmental pollution.Therefore,the industrial waste as a mineral admixture,in the process of reducing cement clinker using also improve the performance of concrete resistance to sulfate attack,become a new type,the ecological road.At present,the research on the corrosion of concrete sulfate was concentrated in the strong alkaline?pH?12.6?environment,the destruction of concrete were ettringite and carburistite type.However,more and more modern concrete structures are located in weak alkali environments such as rivers,lakes,saline soils and oceans containing sulfate ions.There is little research on the dihydrate gypsum of sulfate attack concrete under this environment.C-S-H gel as the main component of the hydration product of portland cement plays a key role in the performance of cement paste and concrete.The sulfates can cause damage to C-S-H gel in the later stages of corroding cement material.In this paper,a variety of test methods were used to study the sulfate attack silicate cementing system in weak alkaline,it is divided into the following aspects:?1?The cement stone was soaked in sodium sulfate solution and the pH of solution was adjusted with nitric acid.The main corrosion product was gypsum,the contents of dihydrate gypsum which cement paste samples immersed in Na₂SO₄ solution were calculated by using K value in X-ray diffraction?XRD?,the contents of free and total sulfate ion in the samples were determined by spectropHotometry.?2?The C-S-H gel were soaked in magnesium sulfate solution,and the co-erosion mechanism of Mg²⁺and SO₄^2-to gel were studied.?3?The mechanism of the effect of silica fume as an admixture to improve the anti-sulfate erosion of concrete was studied.?4?Quality change and compressive strength of cement mortar which socked in sulfate solution in weak alkaline had been studied.According to experimental studies,it is concluded that:?1?The results showed that the contents of dihydrate gypsum in the samples increased significantly with the increase of Na₂SO₄ concentration in the attack solution,the contents of dihydrate gypsum that samples immersed in the 5%and 10%Na₂SO₄solution were 3.291%and 3.957%,respectively.It was found that there was obvious dihydrate gypsum in the sample which soaked in 10%Na₂SO₄ solution.It was found by spectropHotometry that the free sulfate ion content of the samples immersed in 5%and 10%Na₂SO₄ solution was 81,85 times of that of the sample immersed in water,respectively.?2?The C-S-H gel in 5%MgSO₄ solution was destroyed by the double erosion of Mg²⁺and SO₄^2-,resulting in the continuous decomposition of C-S-H.?3?Silica fume on the expansion of the samples had a certain improvement effect,the expansion rates of the samples which 5%silica fume cement paste and 10%silica fume cement paste were 36%and 32%of the sample which mixed with 0%silica fume,respectively.The extent of damage had also been improved with the addition of silica fume.?4?The secondary hydration and filling of silica fume will help to reduce the quality increase of mortar and reduce the strength loss.