A Quantitative Thermogravimetric Study on the Nonevaporable Water in Mature Silica Fume Concrete: Influence of Carbonation and Moisture Conditions
The primary goal of the project presented here was to study and quantify the influence of materials parameters (cement type, silica fume content, w/c ratio, type and dose of superplasticizers) and environmental parameters (carbonation and moisture history) on the nonevaporable water in hydrated cement composites. In order to achieve this, a thermogravimetric method which separates different types of nonevaporable water (bound in different hydrates and hydroxide) and carbonation products was developed. This method was adapted to a thermogravimetric analyzer, which could be programmed to give the weight loss in five separate temperature steps up to 1000 °C. In each temperature step, the temperature remained constant until weight equilibrium was achieved. The analysis gave reproducible results that could be used for quantification.
The experiments were mainly carried out on well cured mortar specimens, but some pastes and concretes were also included. The w/c ratio varied between 0.30 and 0.60 and the silica fume addition from 0 to 25% (CSF/C by weight). The cement type and the dose and type of superplasticizer were also varied.
The experimental results show that when silica fume is incorporated into a cement mix, the major part of it reacts pozzolanically with the Ca(OH)2 released during the cement hydration, combining with the existing CSH phase. In this polymerization process, all the water from the reacted Ca(OH)2 is converted into evaporable water. The traditional relation between nonevaporable water content and cement hydration (Wn=0.25.alpha.C) is therefore not relevant for silica fume mixes; it has to be modified with a factor depending on the silica fume addition. It was also shown that, if sufficient water is supplied to a system with moderate silica fume contents, the degree of cement hydration of a standard Portland cement will not be significantly influenced.
Silica fume does not prevent carbonation as such. However, it may influence the amount of evaporable water, which in turn affects the ingress of carbon dioxide in gas form into the pores. The proportions between different types of carbonates will change, due to a changed ratio between CSH and Ca(OH)2 in the gel phase.
Carbonation of the CSH also involves a polymerization phase which may cause dehydration. The degree of dehydration caused by carbonation of the CSH reflects the ratio between H2O and CaO in the CSH gel. In an already highly polymerized CSF blend, further polymerization becomes difficult and carbonation takes place without dehydration. A certain amount of the polymerization/dehydration that took place can not be attributed to carbonation. It was probably induced by the strains caused by the increased surface tension of the pore water at intermediate relative humidities.
The developed thermogravimetric method has been a useful tool to clarify the nature and quantify the chemical influence of silica fume and carbonation. It could be used to study and quantify the influence of other reactive additives and other degradation mechanisms as well as any additive influencing the cement hydration process.