Hydration of Cement | Hydration of C3S | Hydration of C2S | Hydration of C3A by Dr K Mohan скачать в хорошем качестве

Hydration of Cement | Hydration of C3S | Hydration of C2S | Hydration of C3A by Dr K Mohan

Lecture-13 Hydration of Cement by Dr K Mohan Dr K Mohan is an internationally renowned and an acclaimed cement scientist. He is a former Director General of National Council for Cement and Building Materials (NCCBM). He holds a Ph.D. degree in Cement Chemistry from University of Aberdeen, Scotland, U.K. • The chemical reactions between cement and water is known as hydration of cement. • First, I would discuss hydration of all the four phases separately and later on I would discuss the hydration of cement.  Hydration of C3S • The following chemical reaction takes place when C3S reacts with water 2(3CaO.SiO2) + 6H2O  3CaO.2SiO2.3H2O + 3Ca(OH)2 Tricalcium Silicate Water Calcium Silicate Hydrate Calcium Hydroxide Or 2C3S + 6H  C3S2H3 + 3CH • The corresponding mass involved are: 100 + 24  75 + 49 • Calcium Silicate Hydrate is the principal hydration product. It is rather customary to refer to Calcium Silicate Hydrate simply as C-S-H. • The formula C3S2H3 for C-S-H is only approximate because the composition of C-S-H is actually variable over quite a wide range. • C-S-H has strength giving properties, whereas Ca(OH)2 has no contribution towards strength. • The composition of C-S-H is variable with different C/S and H/S ratios. • Hyphen has been put because no particular composition is implied. • More of C-S-H means more of strength. • C-S-H constitutes 50~60% of the volume of solids in a completely hydrated Cement paste. • Calcium Hydroxide makes up about 20~25% of the volume of solids in a completely hydrated Cement paste. • The only advantage of Calcium Hydroxide is its being alkaline in nature and maintaining a pH value of around 13 in the concrete which resists the corrosion of reinforcements.  Hydration of C2S • C2S hydrates in a similar manner as that of C3S. The same hydration products are formed. • The following chemical reaction takes place when C2S reacts with water 2(2CaO.SiO2) + 4H2O  3CaO.2SiO2.3H2O + Ca(OH)2 Dicalcium Silicate Water Calcium Silicate Hydrate Calcium Hydroxide Or 2C2S + 4H  C3S2H3 + CH The corresponding mass involved are: 100 + 21  99 + 22 • However, the hydration of C2S is much slower than C3S. It is less reactive compound than C3S. • C2S produces a comparatively higher quantity of C-S-H and lower quantity of Ca(OH)2 than that formed in the hydration of C3S. • In general, the quality and density of C-S-H produced due to hydration of C3S is slightly inferior to that formed by hydrationof C2S. The hydration product of C2S is rather dense.  Hydration of C3A • Without gypsum, C3A reacts very rapidly with water. C3A + 6 H2O (H)  C3AH6 • The reaction is so fast that it results in flash set. To prevent the flash set gypsum is added at the time of grinding the cement clinker. • With gypsum, the primary initial reaction of C3A with water is: C3A + 3 (CaSO4.2H20) + 26 H20  C3A.3CaSO4.32H2O Tricalcium Aluminate Gypsum Water Calcium Aluminate Trisulphate Hydrate Or C3A + 3 (CSH2) + 26 H  C6AS3H32 • The product calcium aluminate trisulphate hydrate is known as Ettringite. • Ettringite is a stable hydration product only while there is an ample supply of sulphate available. • If all the sulphate is consumed before the C3A has completely hydrated then ettringite becomes unstable and transforms to calcium aluminate monosulphate hydrate through the following reaction. C3A + CaSO4.2H20 + 16 H20  C3A.CaSO4.18H2O Tricalcium Aluminate Gypsum Water Calcium Aluminate Monosulphate Hydrate Or C3A + CSH2 + 16 H  C4ASH18 • The product calcium aluminate monosulphate hydrate is known as monosulphate, which is the final product of hydration of Portland cement.  Hydration of C4AF • On hydration, C4AF is believed to form hydrated calcium ferrite of the form C3FH6 without gypsum. • With gypsum, same sequence of hydration products as does C3A is formed. C4AF + 3 CSH2 + 21 H  C6(A,F)S3H32 + (A,F)H3 C4AF + 3 CSH2 + 19 H  C6(A,F)S3H18 + (A,F)H3 • The rates of hydration of phases are shown in the graph. C3A and C4AF phases have the highest rates of reaction, followed by C3S and C2S is the slowest to react. #Hydrationofcement #cementchemistry #Cementplant #Cementpdm #Clinkerchemistry