Delayed ettringite formation in heat cured cementitious systems

The objectives of this research were to study the influence of cement composition and curing history on the development of delayed ettringite formation (DEF) and to explore the mechanisms of DEF. Mortar bars and cubes were prepared using the cements made in the Civil Engineering materials laboratory of Purdue University by blending six different clinkers with Terra Alba gypsum. The optimum amount of SO3 for each clinker was determined by ASTM C 563. Cements were also made with the total amount of SO3 1 percent below and 1 percent above the optimum level. Ottawa sand was used as an aggregate to avoid the possibility of developing reaction between aggregate and alkalies (ASR). The mortar specimens were cured either continuously at the room temperature or were initially cured at high temperature of 55°C and 85°C respectively. After heat curing, the mortar specimens were stored in a fog room in enclosed containers with 100% relative humidity. The characteristics of mortar specimens, including expansion, ettringite content, weight change, dynamic modulus of elasticity, and microstructure, were systematically studied. Some of the techniques used were differential scanning calorimetry (DSC), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The results showed that deleterious expansion caused by DEF tends to take place in specimens made with cements with high contents of both C 3A and SO3. In addition to the contents of C3A and SO3, the molar ratio of SO3 to Al2O 3 is also important. Microstructural and microanalytical studies revealed that the significant expansion of the mortar bar specimens occurs when large amounts of sulfate and aluminate are released from C-S-H; it is the result of significant increase of ettringite content in the specimens, and is concurrent with the appearance of ettringite bands around aggregates and in the cement paste. Some components needed in the formation of the ettringite bands are provided by monosulfate or monosulfate-like materials in an area close to the forming bands in a relatively dense area or in a broader area in a relatively porous area. The results did not support the homogeneous paste expansion theory, they clearly showed the significant expansion is due to the formation of ettringite bands around aggregates and in the cement paste