Changes in the Microstructure of Cement Paste and Concrete due to Calcium Nitrate Addition

Calcium nitrate (CN) has been increasingly used as a chloride free set accelerator in later years. Mature cement pastes and concrete have been subjected to microstructure investigations in order to identify possible and long term changes when a high dosage of CN was added to the fresh mixes. The following changes ere found for 2 year old cement pastes (w/c = 0.50) based on ordinary Portland cement (OPC) and sulphate resistant Portland cement (SRPC) when 5.26% CN was added: (1) The degree of hydration was only marginally lower. (2) The amount of calcium hydroxide (CH) was significantly lowered (= 10%) in the case of OPC and unaltered for SRPC. (3) The amount of chemically bound water in both pastes was increased. (4) The average length of the polysilicate anions in the amorphous CSH-gel was prolonged (17% for OPC and 5% for SRPC). (5) The porosity of the OPC paste was increased (+7%) and inhomogeneously distributed, while it was decreased (-2%) for the SRPC paste. (6) The morphology of calcium hydroxide (CH) in the pastes was changed from being evenly distributed to be gathered in larger (=50um diameter) clusters. This phenomenon may be explained by restricted diffusion due to the high Ca2+ concentration supplied by CN. This latter effect was also observed for a plain concrete (w/c = 0.57) based on high strength Portland cement (HSPC) and 3.86% CN>