The effect of blast furnace slag chemistry on carbonation characteristics of cement-slag systems

Abstract

In order to compensate the limited availability of raw material resources and meet the growing demand for decreasing CO2 emissions during cement and concrete productions, a practical method is to decrease the clinker content in cement. This strategy mainly consists of substituting a part of the clinker with supplementary cementitious materials (SCMs) at the cement and concrete production levels. As a mature addition in cement industry, blast furnace slag is a high-performance alternative that has been widely used in Europe and North America, as a SCM. Cement blended with slag is known to exhibit a high resistance to many chemical deteriorations such as alkali silica reaction, sulfate attack, and chloride ingress. An exception is carbonation, which renders a poor microstructure at the skin area of slag-rich concrete. The main aim of the thesis was to investigate the connection among slag chemistry, reactivity, and carbonation resistance of slag-rich cement paste. For this reason, the variation of slag composition was firstly identified through (1) literature review (Chapter 2) and (2) examining unhydrated slag grains existing in old slag concrete structures with different service life. Therefore, Chapter 3 studied the feasibility of using EDS microanalysis as a tool for quantitative measurement of the compositions of unhydrated slags in existing field concretes. The results revealed the variation trend of slag composition with time in the Netherlands. Then, synthetic slags covering the mentioned composition variation were produced in the laboratory, to eliminate the potential interferences and focus on slag chemistry only. The effect of slag composition on reactivity and carbonation resistance of slag-rich cement paste were investigated systematically. In Chapter 4, synthetic slags based on CaO-SiO2-Al2O3-MgOsystem and commercial slags were considered to estimate the correlation between slag chemistry and reactivity. It was found that higher MgO and/or Al2O3 contents of slag led to a higher reactivity. Chapter 5 observed the carbonation products in the slag-rich cementitious systems (mainly CEM III/B) upon three different exposure conditions, namely, long term exposure in the field, indoor natural exposure, and accelerated carbonation testing. Emphasis was laid on the carbonation of monosulfate and hydrotal cite-like phase in particular. The author believed that there was enough evidence indicating these two phases being the key components towards formulating blast furnace slag systems resistant to carbonation. Chapter 6 revealed the correlation between slag chemistry and CO2 binding capacity of the blended system. To simplify the composition of mixture, model paste containing only C3S, synthetic slags and gypsum was employed. In Chapter 7 and 8, the effect of MgO and Al2O3 contents of slag on the carbonation characteristics of cement-slag system was explored, respectively. Accelerated carbonation testing was performed on slag cement paste. The evolution of phase assemblage, microstructure, and micro-mechanical properties of each mixture before and after carbonation testing was evaluated. Finally, the connection among slag chemistry, reactivity, and carbonation resistance was discussed comprehensively. It was noted that the classification employed for slag reactivity cannot be extended to characterize carbonation resistance of cement-slag system directly. The main challenge occurred for slag with high alumina content. Al2O3-rich slag was reactive as a blended cement component but it did not contribute to carbonation resistance. Considering the effect of slag chemistry on reactivity and carbonation resistance together, slag, with a CaO/SiO2 ratio ≈ 1 and presenting high MgO (> 10 wt.%) and moderate Al2O3 (10-15 wt.%) contents, was recommended to design slag rich concrete structure with improved hydration performance and carbonation resistance.