Alkali-Activation of Slag Cements: Activation Process, Microstructure and Mechanical Properties

摘要

Blended cements are more environmentally friendly than ordinary Portland cements (OPC), for their manufacture is less energy-intensive, emits less greenhouse gas into the atmosphere and entails the re-use of industrial by-products such as granulated blast furnace slag. Of the 27 cements listed in European standard EN 197-1:2000, seven contain slag, in proportions ranging from 6 to 95 %. The advantages of slag-blended cements include not only the energy savings and lower pollution involved in their manufacture, but also their high mechanical strength at older ages and long durability due to their fine pore structure. They do have drawbacks, however, most prominently their low early age strength, particularly in the case of CEM III cements. For this reason, the aim of the present study was to raise the early age mechanical strength of slag cements CEM III (EN 197-1) through alkali activation. The activation process, hydration product structure and paste and mortar microstructure were studied in detail. Four cements were used, CEM I (as a reference) and three CEM III cements differing in their slag content (50, 70 and 90 %). Waterglass (Na2O·nSiO2·mH2O) solutions with 5 % Na2O by slag mass were used as activators. Cement paste activation was studied by conduction calorimetry and the products were characterized mineralogically and microstructurally. Finally, the mechanical strength and porosity of these waterglass-CEM III cement mortars were determined. Conduction calorimetry, in conjunction with the XRD results, showed that alkaline activation grew more intense with rising slag content. In contrast, the waterglass solution inhibited ordinary Portland cement hydration significantly. The BSE/SEM/EDX results confirmed the formation of two C-S-H gels in waterglass-CEM III cements, one attributed to Portland cement hydration and the other to slag activation. The 29Si MAS NMR spectra showed that a cross-linked C-A-S-H gel formed as an outcome of slag activation. In this C-A-S-H gel, Si was partially replaced by Al in bridge positions. Chain length also rose with slag content in the cement, to a maximum of 10 links in the 7-day cement containing 90 % slag. Mechanical strength increased with the slag content. Specifically, mortars prepared by mixing cement with 90 % slag and a waterglass solution with 5 % Na2O were comparable to CEM I mortars and three times stronger than mortars prepared with water. These high strength values may be due to the generation of a more intensely polymerized and cross-linked C-A-S-H gel and low mortar porosity