Effects of Physical and Chemical Characteristics of Slags and Cements on Durability of Portland Cement-Slag Blended Systems

Ground granulated blast furnace slag (GGBFS) which is a commonly used supplementary cementitious material (SCM) needs to be more investigated for improving durability of concrete elements and service life of concrete structures. This research focuses on the effect of the physical and chemical characteristics of GGBFS and portland cement on durability of portland cement-slag blended systems. The effect of cement replacement level is also investigated using control mixes (no slag) and four different levels of slag content. To this end, eight GGBFS and eight portland cements were selected based on their physical characteristics and chemical composition. Four series of experimental studies were conducted using different mixtures of the selected materials. First, the length changes of the portland cement-slag mortar bars in sodium sulfate solution were measured for 18 months of exposure. Second, compressive strength of the cement-slag mortar cubes stored in sulfate and saturated lime solutions were assessed for 18 months. Third, chloride binding capacity of cement-slag blended systems, in five chloride solutions with different chloride concentrations, was assessed. Fourth, the chloride resistivity of portland cement-slag mortar cylinders was determined using electrical resistivity measurements. Moreover, statistical analysis was performed to model the effect of the cement and slag characteristics on the length changes of the cement-slag mortar bars when exposed to a sulfate environment. ASTM C989 [1] cautions that sulfate resistance of portland cement-slag blends may be dependent on the Al2O3 content of the slag. While the standard comments on the effect of low alumina (11% Al2O3) and high alumina (18% Al2O3) slags on sulfate durability, there is no comment on the effect of the slags with Al2O3 content of 11-18%. In this study, sulfate resistance of portland cement-slag mixtures was compared at 30, 50 and 70% cement replacement levels for slags with variable Al2O3 and MgO contents and variable fineness in combination with moderate- and high-C3A cements. Addition of slag changed the failure mode of the mortar bars exposed to sodium sulfate solution compared to the plain cement mixes. Sulfate resistance generally decreased with increasing slag Al2O3 content, although decreasing MgO also increased deterioration, particularly at 30% replacement. Increase in Al2O3 correlated with increased formation of secondary ettringite in the surface of the mortar bars. A linear relationship was observed between the solid volume increase on exposure to sulfate solution predicted by GEMS and the MgO/Al2O3 ratio of slag at all cement replacement levels. There was also a linear relationship between the solid volume increase and the length of the induction period for the mortar bars exposed to sulfate solution. Increasing slag fineness appears to decrease the sulfate durability of cement-slag mixtures. Addition of calcium sulfate for improving sulfate durability of slag mixes with high alumina content was observed to be affected by the cement composition. Although, the strength of Type I and Type II cement mixes is close in the lime solution, samples made with Type I cement which has a higher tricalcium aluminate content were observed to be highly vulnerable to sulfate attack. Partial replacement of cement with slag decreased the strength at early ages due to slow reactivity of slag but increased it at later ages both in lime and sulfate solution. Increasing cement replacement level was observed to generally increase the compressive strength of the cement-slag mixtures. In general, low-alumina (8%) slag showed the lowest expansion and the highest strength among OPC-slag mixtures with lower slag content, while high-alumina (16%) slag was observed to have the highest expansion and the lowest compressive strength in sulfate solution. This is due to the formation of more ettringite in high-alumina slag mixtures. The length changes and compressive strength at 70% mixes was observed to improve for all slag mixtures regardless of the slag composition. However, the fineness of the slag seems to play an important role at high replacement levels that increases both the expansion and strength of the cement-slag mixtures at high replacement levels when exposed to sulfate solution. In general, the effect of slag chemical composition is most notable at lower replacement levels and all cement-slag mixes showed lower expansion and higher strength at 70% replacement level. Therefore, high percentage of cement replacement is a key solution for improving the performance of all slags, especially high alumina slags, while only low alumina slags should be used if lower replacement levels of cement are desired. In addition to sulfate durability, chloride binding capacity of cement-slag blended systems was investigated using isotherms obtained from equilibrium method. It is known that pore connectivity and chloride binding capacity of the concrete cover determine the concrete resistance to diffusion of chloride ions. Since only the free chloride in the pore solution participate in corrosion of the reinforcing steel, it is essential to understand the chloride binding phenomena in cement-slag systems. In chapter 4 of this document, the effect of cement and slag characteristics on the chloride binding capacity of the cement-slag mixtures is discussed. Higher amount of bound chloride was determined in the cement mix with high C3A content. Alkali content and the pH of the pore solution is the other factor that affects the binding of chlorides. Lower alkali content was observed to increase the binding capacity of the cement mix. Addition of limestone to cement appears to decrease chloride binding in the blended system. Replacing cement with slag increased the C-S-H and alumina content of the system which led to higher binding capacity. Addition of slags with higher alumina content formed more AFm phases before chloride exposure which further increased the binding chloride capacity of the cement-slag systems by converting to Kuzel’s and Friedel’s salts. The electrical resistivity of the cement-slag systems was also measured to investigate the effect of different cements and slags on chloride ingress into the cementitious system. The objective of this part of the study was to investigate the influence of w/b ratio, cement fineness and slag alumina content on surface resistivity, and consequently, chloride diffusivity of the cement-slag blended systems. Lower w/b ratio and higher cement fineness increased electrical resistivity and decreased chloride diffusivity into the cement mix. Slag mixes showed higher resistivity and lower chloride diffusivity compared to those of the plain cement systems. Higher alumina content decreased the resistivity and increased chloride diffusivity at later ages. The statistical analysis in chapter 5 of this study showed that cement C3A content, slag alumina content, cement fineness and slag fineness as well as cement replacement level are among the most significant parameters that affect the performance of cement-slag mixtures in sulfate environment. Most of the work in chapters 2 and 3 of this study was partially funded by Florida Department of Transportation and the Federal Highway Administration under contract number BDV25-977-28. The report for the mentioned project [2] and this dissertation share some content. Additionally, part of this document have been published previously in Vol. 229 of the Journal of Construction and Building Materials [3]. Permissions are included in Appendix A