C4AF ettringite and calorific synergic effect contribution

The need for cements or other cementitious materials that afford high early age mechanical strength has led to the use of extremely reactive pozzolanic additions such as silica fume, nanosilica, metakaolin and similar. The inclusion of the right proportion of such pozzolanic additions stimulates portland cement hydration, i.e., directly, as they are initially moistened by the mixing water, non-directly when they act as “seed crystals”, and indirectly, because of the pozzolanic reaction between the addition particles and the portlandite forming from the portland cement components hydration; since this reaction is characterized by its intensity and speed, when its occurs it prevails over the other two. Indirect stimulation also causes the fraction of portland cement in the blend to release more heat of hydration than pure portland cement, and its does so on a scale consistent with the existence of a calorific synergic effect. Such greater heat is released in the early stages of hydration primarily by C3A and C3S that react with the mixing water to respectively generate ettringite and hydrated calcium silicates. When portland cements have a low to nil C3A content, less heat of hydration is released due to the absence of an AFt phase that could be transformed into AFm. However, when extremely active pozzolanic additions, such as silica fume, are used, ettringite forms from C4AF, further contributing to origin amounts of hydration heat released comparable to the above calorific synergic effect.