Different anomalies of two-stage water absorption in carbonated and non-carbonated cement-based materials
Zhidong Zhang, Ueli Angst
Abstract
It was generally recognized that water absorption into cementitious materials shows two stages, the early stage with fast water uptake and the late stage with the reduced water absorption rate. Two types of anomalous moisture transport can be classified based on the shape of the water absorption curve. To investigate the main causes of different types of anomalous moisture transport, this study carried out water and isopropanol (IPA) absorption measurements for specimens of non‑carbonated and carbonated cement-based materials made of two types of cements (CEM II B-LL and CEM II/B-M). The results showed that the microstructures of the non‑carbonated materials are sensitive to the environmental conditions, so the exposure surfaces were modified by carbonation or leaching of ions due to the variation of the external moisture, while the surface layer modification does not affect the late stage of water absorption. The numerical simulation results of moisture transport further confirm that the microstructural alteration could be one of the causes of the anomaly in the early stage of water absorption. For carbonated materials, the early stages of water and IPA absorption follow the traditional square root of time function and the intrinsic sorptivities of water and IPA in the early stages are very similar, implying that the microstructure of carbonated mortars is barely altered by water and IPA; therefore, water absorption curves of the early stage can be well predicted by the conventional moisture transport models. This study concluded that anomalous moisture transport observed in non‑carbonated cement-based materials without air entrainment may be caused by microstructural alteration, complex microstructure and the heterogeneous porosity distribution, while for carbonated materials, it is most likely only from the complex microstructure which leads to different moisture transport speeds in different sizes of pores.