Experimental studies of sorption and transport of moisture in cement based materials with supplementary cementitious materials

Detta är en avhandling från Printed in Sweden by Media-Tryck, Lund University

Sammanfattning: Most deterioration processes in cement based materials are closely related to moisture sorption and moisture transport properties. Therefore, it is important to study these properties, both theoretically and practically. This work is an experimental investigation in this field. Nowadays, the cement industry produces cements with increasing amounts of supplementary cementitious materials (SCMs) to limit CO2 emissions from concrete production. Knowledge about the moisture properties of concrete made from these blended cements is limited. This project has therefore been an attempt to further develop our understanding of the moisture properties of cement based materials, such as sorption isotherms and sorption transport properties in the presence of SCMs. This has been done by studying sorption isotherms mainly using the sorption balance method, and moisture transport coefficients using both the cup method and a sorption dynamic method. The experimental investigations were made on three types of hydrated cement pastes and mortars (OPC, OPC + 70% slag and OPC + 10% silica fume) with three different w/b –ratios (w/b) for cement paste (0.6, 0.5, 0.4) and two different w/b for cement mortar (0.5, 0.4). Sorption isotherms were determined for cement pastes and mortars in both hygroscopic and the super-hygroscopic relative humidity ranges using the sorption balance method, and the pressure plate method. The conclusion from this part of the study was that the desorption isotherms at low RH (0-30%) for different binders and different w/b-ratios are similar. At higher RHs the samples with silica fume and slag have higher moisture content than OPC samples. This is explained by that they have a higher amount of gel pores and a lower amount of capillary pores than OPC samples. The sorption isotherm at high RHs is difficult to validate experimentally, due to the critical RH of pore solutions. Steady-state and transient measurements of transport coefficients were also made. The dynamic sorption method was used to evaluate the diffusivity in small paste samples. The results show that Fick's law cannot completely describe the transport process in such small samples and sorption behavior is therefore anomalous with two processes with different time scales. One of these is macro-diffusion into the sample, which takes place on a shorter timescale in the small samples used. The second process takes place on longer timescales and it is possibly related to the sorption in nanometer-structure of materials. To better understand the transport properties in sorption cycles, steady-state diffusion coefficients of mortar samples were measured with a newly developed cup method set-up. The measurements were done on both the absorption and desorption limbs of sorption isotherms. For OPC samples the results show a clear difference between the diffusion coefficients in absorption and desorption with vapor content as potential (Dv) and presented as a function of relative humidity (RH). The Dv in desorption is higher than absorption especially at high RHs. For samples with SCMs the dependence of Dv on RH is small. The Dv:s were also recalculated to diffusivity (Dc) using the sorption isotherms to study the effect of different potentials on the effect of hysteresis on transport properties. Key words: Cement, Concrete, Moisture transport, Hysteresis, Supplementary cementitious materials, Water vapor sorption, Sorption isotherms, Anomalous sorption