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

Författare

Mahsa Saeidpour

Summary, in English

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

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