Analysis of the role of gypsum in silicate cement

What exactly is cement?
the cement we usually see is also known as silicate cement. Chemically speaking, its main components are tricalculum silicate, calcium silicate, tricalcalculum aluminate and calcium ferroalate, the former two mainly.
cement clyceration composition
cement production to add gypsum
seal production process
cement production process can be summarized as 'two grinding one burn

. Cement is prepared by limestone, clay and iron ore powder for raw materials such as grinding, calcination, cooling and other processes.
carbonate (CaCO3), the main component of limestone, and, like other mineral stones, contains many impurities. The main component of clay is silicon dioxide (SiO2), iron ore powder is generally hetero-iron ore, the main component is iron trioxide Fe 2O3. At a high temperature of 1450 degrees C, limestone decomposes into calcium oxide and carbon dioxide, of which carbon dioxide is discharged, calcium oxide continues to react with clay minerals at high temperatures, fusion, forming a gel-based material, which is "cooked" by high temperature materials are visually referred to as "cedes".
these clypes also need to be mixed and ground with gypsum or slag, so why do you need to add gypsum to cement clyce?
what is gypsum:
gypsum is a mono-slope crystalline mineral, is the main chemical composition of calcium sulfate (CaSO4) hydrate.The slow coagulation of gypsum:
After water encounter, various minerals in cement clycerine, such as tricalculum silicate, calcium silicate, tricalcalculphate aluminumate and tylucal acid, began to react with water to produce hydrated silicate Calcium gels, calcium hydroxide crystals, aluminized aluminum acid tricalcalculum crystals
the hydration reaction rate and build strength of each mineral vary, which can also explain why there are different strengths and condensation time of cement, the faster the hydration reaction then the shorter the gel time.
of theoretical and academic studies show that the hydration of tricalculum aluminate is the fastest among the four minerals of cement clycerine. In order to ensure that cement has good construction performance (suitable condensation time), it is necessary to reduce and inhibit the disadvantages of the rapid hydration of tricalculumate aluminum acid by adding a recoagulant. Gypsum is a commonly used anticoagulant in cement.
tricalculum silicate: cement clycerine minerals, tricalculum silicate content is the highest. When tricalcalculum silicate and water act, the reaction is fast, the hydration heat is large, and the production of calcium hydrate and calcium hydroxide. Calcium hydrated silicate is almost insoluble in water and is immediately dissected from collomic particles and gradually condensed into a gel.
hydroxide is a six-party crystal, soluble in water.
: when calcium silicate and water act, the reaction is slower, the hydration heat is small, the production of calcium hydrate and calcium hydroxide.
. Tricalculum Aluminate: When acting with water, the reaction is extremely fast, hydration heating is the largest, the production of aluminized aluminum acid tricalcalculum, aluminized aluminum acid tricalcification is cubic crystal, soluble in water.
. Iron aluminum acid four calcium: iron aluminum acid four calcium and water action, the reaction is also faster, hydration heat medium, the production of aluminized aluminum acid tricalculum and calcium hydrated ironate, calcium hydrated ironate for the gel.
Among the four cement clycears, tricalcalculminate aluminumate is the most special, mainly reflected in two aspects: 1. the reaction speed with water is extremely fast, hydration heat release is the largest, that is, condensation time is short, 2. the resulting product is soluble in water, this characteristic causes tricalcalcate aluminum acid will not be like other clycerine shell and water production insoluble reaction speed, but will continue to speed up, so the reaction will start in a very short period of time. And the heat of this reaction will increase the temperature of the reaction system, and under higher temperature conditions, the rate of condensation reaction will be further accelerated and the condensation time will be shortened.
Cement clycerry in the case of not adding gypsum mixed with water will immediately produce condensation, in order to adjust the condensation time, the amount of gypsum added is too little, slow coagulation effect is not obvious, add too much, will cause poor cement stability, suitable addition of 3%-5% of cement quality. The added gypsum reacts with tricalcalculum aluminized aluminum acid to produce high-sulfur calcium sulfate, which is a needle-like crystal and insoluble in water.
gypsum is the use of its main components calcium sulfate and tricalculum aluminate reaction to produce calcium hydrated sulfur aluminumate (also known as calcite). Calcium is encased in the surface of tricalcalculum aluminate and inhibits its further hydration, thus prolonging the hydration process and ensuring that the cement has a suitable condensation time.
Under the conditions of gypsum and Ca (OH)2, the hydration process of C3A is divided into four stages: stage
: the dissolution of tricalculum acrylic C3A aluminum acid, the formation of calcite
(Aft), the rate of hydration slows down;stage
3: the cladding rupture, the hydration reaction accelerates again, the formation of calcite and make the rupture closed
;
the amount of gypsum
gypsum is one of the necessary groups of cement production, in cement mainly play a role in regulating the condensation time, that is, to do the use of anticoagulants. Article 7.1 of the technical requirements of GB175 General Silicate Cement clearly requires: "Common silicate cement, composite silicate cement, coal ash silicate cement sulfur trioxide (mass fraction) ≤3.5%." too much or too little gypsum can lead to abnormal condensation. In general, the gypsum is not enough to cause fast coagulation, but when the amount is increased to a certain extent, the effect on the condensation time becomes very small. When the gypsum doping (in SO3) is less than about 1.3%, the gypsum doping is too small and the cement produces fast coagulation. When the SO3 content is further increased, the gypsum appears to have a significant slow coagulation effect, but after the gypsum doping (in SO3 terms) exceeds 2.5%, the condensation time increase is very small. Many researchers have also pointed out that the appropriate amount of gypsum should be added water after about 24h can be exhausted.
When the gypsum is doped too much, after the cement hardens, the residual gypsum and calcium aluminate continue to react to produce calcite, which increases in volume by about 1.5 times and also causes the cement stone to crack.
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