The main chemical components of Portland cement: calcium oxide CaO, silicon dioxide SiO2, iron oxide Fe2O3, aluminum oxide Al2O3.
The main minerals of Portland cement: tricalcium silicate (3CaO·SiO2, simplified C3S), dicalcium silicate (2CaO·SiO2, simplified C2S), tricalcium aluminate (3CaO·Al2O3, simplified C3A), tetracalcium calcium (4CaO · Al2O3 · Fe2O3, short form C4AF).
chemical reaction
Setting and hardening of cement:
(1) 3CaO·SiO2+H2O→CaO·SiO2·YH2O ( gel )+Ca(OH)2;
(2) 2CaO·SiO2+H2O→CaO·SiO2·YH2O ( gel )+Ca(OH)2;
(3) 3CaO·Al2O3+6H2O→3CaO·Al2O3·6H2O (calcium aluminate hydrate, unstable);
3CaO·Al2O3+3CaSO4·2 H2O+26H2O→3CaO·Al2O3·3CaSO4·32H2O ( ettringite , calcium aluminate trisulfide hydrate);
3CaO·Al2O3·3CaSO4·32H2O+2〔3CaO·Al2O3〕+4 H2O→3〔3CaO·Al2O3·CaSO4·12H2O] (monosulfur calcium aluminate hydrate);
(4) 4CaO·Al2O3·Fe2O3+7H2O→3CaO·Al2O3·6H2O+CaO·Fe2O3·H2O.
Cement quick-setting refers to an abnormal early curing or premature hardening of cement. The high temperature causes the crystal water in the gypsum to be dehydrated and become a slurry, thereby losing the ability to adjust the setting time. False setting is related to many factors. It is generally believed that it is mainly due to the high temperature in the mill during cement grinding, which causes the dihydrate gypsum to be dehydrated into hemihydrate gypsum. When the cement is mixed with water, hemihydrate gypsum quickly reacts with water to form dihydrate gypsum, forming a needle-like crystal network structure, which causes the slurry to solidify. In addition, for some cements with high alkali content, potassium sulfate and dihydrate gypsum produce potassium gypsum quickly, which can also cause false setting. False coagulation is different from rapid coagulation. The former emits very little heat, and after vigorous stirring, the slurry can recover its plasticity and reach normal coagulation, which has no adverse effect on the strength.
Account not verified