Influence Of Anions On Hydration And Properties Of Ordinary Portland Cement

Limestone powder has been widely used in cement production and concrete preparation.Moreover,nitrate,nitrite and sulfate are also common admixtures widely used in concrete,which can introduce CO₃^2-,NO₃^-,NO₂^-and SO₄^2-into cement-based materials including concrete.Furthermore,Cl^-can probably be introduced to concrete due to the use of water or sand.These anions can affect cement hydration,especially the formation of AFm phase and ettringite,so their influence on the properties of cement-based materials can not be ignored.It is of great practical significance to study the effect of the anions on the performance of cement-based materials.In this thesis,calcium carbonate,calcium nitrate,sodium nitrite,sodium chloride and sodium sulfate are used as the introduction sources of the above anions,and the chemical substances were added into the ordinary Portland cement（OPC）respectively.The influence of the anions on the hydration of cement was studied by XRD and thermal analysis.Additionally,their influence on the macro properties of cement was comprehensively and systematically studied by testing compressive strength,flexural strength of cement mortars and linear expansion rate of the cement pastes.The specific conclusions are given as follows:1)CO₃^2-,NO₃^-,NO₂^-and Cl^-can form their corresponding anionic AFm phases during cement hydration.The stability of these AFm phases is higher than that of SO₄-AFm,so these anions will react with C₃A prior to ettringite,thus inhibiting the transformation of ettringite to SO₄-AFm and increasing ettringite content.SO₄^2-can also increase ettringite content,but the mechanism is different.SO₄^2-reacts with Ca（OH）₂to form finer secondary gypsum,which accelerates the transformation of C₃A to ettringite and enhances the alkalinity of the system.As a result,ettringite is formed rapidly and massively.Although SO₄^2-also participates in the formation of SO₄-AFm before 28 days,most of the SO₄-AFm is transformed into ettringite at 56 days.2)CO₃^2-,NO₃^-,NO₂^-,Cl^-and SO₄^2-can affect the strength of cement mortar.An appropriate amount of anions is beneficial to enhance the mortar strength except NO₂^-.The results show that 3%dosage of calcium carbonate is advantageous for mortar strength improvement,while 6%dosage of calcium carbonate can only improve the mortar strength at 56 days.1.5%dosage of calcium nitrate can also enhance the mortar strength beyond 3 days.NO₂^-can reduce the mortar strength.Cl^-can enhance the mortar strength at3 and 7 days,but it is harmful to the development of later strength.The appropriate dosage of sodium chloride for mortar strength improvement is 1.5%.SO₄^2-also has activation effect for the early strength,but only at 3 days,and its effect is not as good as Cl^-.The excessive expansion caused by SO₄^2-will cause strength retraction.3)NO₃^-,NO₂^-,Cl^-and SO₄^2-can increase the linear expansion rate of cement pastes within the range of 9%salt dosage.However,calcium carbonate can only increase the linear expansion rate at 3%dosage,and the linear expansion rate will decrease when the dosage exceeds 6%.CO₃^2-,NO₃^-,NO₂^-,Cl^-and SO₄^2-can participate in cement hydration,and inhibit the transformation of ettringite to SO₄-AFm by forming more stable AFm phase,so as to increase ettringite content and improve cement performance.However,NO₂^-is harmful to mortar strength.Cl^-and SO₄^2-can improve the mortar strength only at early ages,but it is harmful to the later strength development.