Pore solution compositions and redox potentials of ground granulated blast furnace slag-containing cement pastes

Chemical composition of cement pore solutions

Experimental Study and Numerical Simulation of the Reaction Process and Microstructure Formation of Alkali-Activated Materials

Effect of curing conditions on the pore solution and carbonation resistance of alkali-activated fly ash and slag pastes

As the building sector is expanding, a growing interest in technologies that can reduce the CO2 emission from concrete production has led to partial replacement of cement with by-products from various industrial processes. Besides the partial replacement of cement, development of alkali activation technology ensures full replacement of cement in concrete. Although alkali activated materials (AAMs) are one of the most sustainable alternatives to cement-based concrete, structural application of AAMs is still not viable, as their long-term performance is not sufficiently studied. For instance, no recommendations are yet given to the scientific and engineering communities as a general approach for testing carbonation of AAMs. Furthermore, there is a limited number of case studies of long-term performance of AAMs in the past to assist the predictive models of their service life. The long-term performance (carbonation resistance) of AAMs is mainly dependent on the microstructure features of the binder (e.g. phase assemblages and pore structure), which can be modified using different constituents and materials mixture designs. Therefore, the aim of this thesis was the development of a conceptual carbonation mechanism that can be applied to analyse carbonation resistance of any alkali activated concrete mixture. For this reason, the carbonation mechanism was studied at different length scales, from paste to concrete level, while the effects of carbonation on the chemical, physical and mechanical properties were captured. The relationship between carbonation rate, pore solution chemistry and microstructure was investigated. An advanced microstructure characterization of fly ash (FA) and ground granulated blast furnace slag (GGBFS) was performed using PARC software. The combined effect of GGBFS content, curing (sealed/unsealed) and exposure conditions (natural indoor/outdoor and accelerated carbonation) on the carbonation resistance of pastes was considered. Based on the parameter studies (GGBFS content, curing, exposure conditions), recommendations for design of alkali activated concrete for engineering practice are given in view of carbonation resistance.

A partial replacement of Portland cement (PC) by ground granulated blast furnace slag (GGBFS) is an effective method to improve the durability of concrete due to its lower diffusivity and higher chemical resistance compared to PC. Further, the microstructure of GGBFS blended cementitious materials controls the physicochemical properties and performance of the materials in concrete. Therefore, understanding of cement hydration and cementing behavior of GGBFS is essential to establish microstructure property relationship for predicting performance. In this study, hydration, microstructure development, and chloride ingress into GGBFS-blended cement have been investigated. Solid-phase assemblage and pore solution chemistry of hydrating PC and cement blended with GGBFS were predicted using thermodynamic model and compared with experimental data. A mathematical model integrating PC hydration, GGBFS reaction, thermodynamic equilibrium between hydration products and pore solution, ionic adsorption on C-S-H, multi-component diffusion, and microstructural changes was developed to predict chloride ingress into GGBFS blended cementitious materials. The simulation results on chloride profiles for hydrated slag cement paste, which was prepared with 50% of replacement of PC with GGBFS, were compared with experimental results. The model quantitively predicts the states of chloride such as free, adsorbed on C-S-H, and chemically bound as Friedel’s salt.

Studies conducted on alkali-activated materials in late years attested that they could be a replacement to ordinary Portland cement (OPC). Nevertheless, their performances in aggressive conditions require profound investigations to evaluate the ability to the fluid transfer. This article assesses the pore structure of alkali-activated mortars produced using one-part metakaolin combined with ground granulated blast furnace slag and other-part using only alkali-activated ground granulated blast furnace slag. The pore characteristics of alkali-activated mortars were compared to those of OPC as a reference. Tests consist in the measurement of the porosity accessible to water, water absorption and sorptivity. The results revealed that the alkali-activated based metakaolin combined with ground granulated blast furnace slag have greater porosity, water absorption, sorptivity and rate of water saturated porosity than alkali-activated based ground granulated blast furnace slag and Portland cement mortars.

Alkali-activated materials are low-environmental-impact binders that can be obtained from the alkaline activation of industrial wastes. In this study, converter steel slag as the major raw material and ground granulated blast furnace slag (GBFS) as the modified material were activated by water glass with a modulus of 1.5 and a Na2O dosage of 4%. The hydration process, microstructure and compressive strength of alkali-activated composite materials were investigated. The results show that adding GBFS accelerates the initial dissolution of the particles, leading to higher first exothermic peaks. But adding GBFS decelerates the formation of hydration products, resulting in the delay of the second exothermic peaks. Adding GBFS has no significant effect on the cumulative hydration heat, Ca(OH)2 content and the type of gel in the alkali-activated steel slag systems. However, with increasing GBFS content, the Ca–Si ratio in the gel decreases, and the Al–Si ratio increases. Adding GBFS can refine the pore structure and produces more Si–O–Si bonds in gels, resulting in a significant increase in the compressive strength. The improvement effect of GBFS on the compressive strength is more obvious at a later stage than at an earlier stage.

Among the minor elements found in metallurgical slags, sulfur and manganese can potentially influence the corrosion process of steel embedded in alkali-activated slag cements, as both are redox-sensitive. Particularly, it is possible that these could significantly influence the corrosion process of the steel. Two types of alkali-activated slag mortars were prepared in this study: 100% blast furnace slag and a modified slag blend (90% blast furnace slag + 10% silicomanganese slag), both activated with sodium silicate. These mortars were designed with the aim of determining the influence of varying the redox potential on the stability of steel passivation under exposure to alkaline and alkaline chloride-rich solutions. Both types of mortars presented highly negative corrosion potentials and high current density values in the presence of chloride. The steel bars extracted from mortar samples after exposure do not show evident pits or corrosion product layers, indicating that the presence of sulfides reduces the redox potential of the pore solution of slag mortars, but enables the steel to remain in an apparently passive state. The presence of a high amount of MnO in the slag does not significantly affect the corrosion process of steel under the conditions tested. Mass transport through the mortar to the metal is impeded with increasing exposure time; this is associated with refinement of the pore network as the slag continued to react while the samples were immersed.

Radiological characterisation of alkali-activated construction materials containing red mud, fly ash and ground granulated blast-furnace slag

Carbonation of the pore solution in alkali-activated materials (AAMs) produces alkali and/or alkali-earth carbonates. When the carbonate solubility in the water is very high (case of the most alkali carbonates), it is very hard to determine the carbonation depth in AAMs with the phenolphthalein indicator frequently used in Ordinary Portland Cement (OPC)-based materials. Carbonation gradually decreases the alkalinity of the pore solution, while the color after spraying phenolphthalein changes from colorless to pink when pH< 13 and changes back to colorless when pH< 8.2. The color change with phenolphthalein indicator may still exist in the less alkaline areas where carbonation may have already occurred. Therefore, using the indicator test is likely to underestimate the depth to which carbonation reaction has occurred in AAMs and more complete assessment is required. This study investigates the carbonation front in alkali-activated fly ash (FA) and blast furnace slag (BFS) pastes in natural laboratory conditions. Monitoring carbonation front in the samples after one year of exposure has been carried out under polarized light microscope (PLM), and environmental scanning electron microscope (ESEM). The carbonation products were sharply distinguished from the other constituents of the paste, by their crystallographic and optical characteristics under PLM, and characterized by X-Ray diffraction (XRD).

Formulation and characterization of blended alkali-activated materials based on flash-calcined metakaolin, fly ash and GGBS

Recycling of high-volume waste glass powder in alkali-activated materials: An efflorescence mitigation strategy

The pore solutions of alkali-activated slag cements and Portland-based cements are very different in terms of their chemical and redox characteristics, particularly due to the high alkalinity and high sulfide content of alkali-activated slag cement. Therefore, differences in corrosion mechanisms of steel elements embedded in these cements could be expected, with important implications for the durability of reinforced concrete elements. This study assesses the corrosion behaviour of steel embedded in alkali-activated blast furnace slag (BFS) mortars exposed to alkaline solution, alkaline chloride-rich solution, water, and standard laboratory conditions, using electrochemical techniques. White Portland cement (WPC) mortars and blended cement mortars (white Portland cement and blast furnace slag) were also tested for comparative purposes. The steel elements embedded in immersed alkali-activated slag mortars presented very negative redox potentials and high apparent corrosion current values; the presence of sulfide reduced the redox potential, and the oxidation of the reduced sulfur-containing species within the cement itself gave an electrochemical signal that classical electrochemical tests for reinforced concrete durability would interpret as being due to steel corrosion processes. However, the actual observed resistance to chloride-induced corrosion was very high, as measured by extraction and characterisation of the steel at the end of a 9-month exposure period, whereas the steel embedded in white Portland cement mortars was significantly damaged under the same conditions.

Soil existing at a particular site may not be suitable for construction works. The purpose of this study is to investigate the utilization of Ground Granulated Blast Furnace (GGBF) Slag with lime and cement as stabilizer.Soil sample collected for this study was classified as clayey soil of low plasticity (CL).The optimum cement content has been fixed as 7% by dry weight of soil, which was replaced with GGBF as Cement:GGBF Slag ratio 7:0, 6:1, 5:2, 4:3, 3:4, 2:5,1:6 and 0:7. The optimum lime content was determined by Eades & Grim's pH method as 5% of dry soil. Lime: GGBF Slag contents 5:0, 4:1, 3:2, 2:3, 1:4 and 0:5 were used. Optimum content of GGBF Slag with Lime and Portland cement was determined based on the UCS. The results show that Cement:GGBF ratio of 3:4 gives 13.5 times higher strength than virgin soil at 28 days of curing and Lime

The most often used building material for engineering constructions is concrete. The rapid urbanization has necessitated the need for High Strength (HS) and High -Performance Concrete (HPC) for specialized constructions, such as high rise/tall structure and other important structures. Greater cement content may be required for concrete with higher performance like strength and durability, but code does not permit it, because higher cement content increases the heat of hydration, which leads to the development of thermal cracks in concrete, reduces its structural performance and damage environment by producing CO2 during production of Ordinary Portland Cement (OPC). This study tries to replace OPC by Micro Silica (MS) and Ground Granulated Blast Furnace Slag (GGBS) by some percentage to find our effect of these materials on properties of concrete HPC.In this work, the influence of Manufactured Sand (M-Sand), Micro Silica (MS) and Ground Granulated Blast Furnace Slag (GGBS) on the mechanical and durability qualities of High - Performance Concrete (HPC) is examined experimentally. Ordinary Portland cement (OPC) is used to create the HPC mixtures and then 40%,45%, 50% by weight of OPC is replaced with GGBS and 2.5% and 5% by weight of OPC is replaced with MS respectively. Additionally, OPC is replaced by 40%GGBS+ 2.5% MS, 40% GGBS+5% MS, 45%GGBS+ 2.5% MS, 45% GGBS+5% MS, and 50%GGBS+ 2.5%MS, 50%GGBS+5% MS. For each percentage replacement workability test, compressive test at the age of 28,56 and 90 days, water permeability test at 28 days and 90 days, Scanning Elector Microscope (SEM) test after 90 days are carried out. The investigative study’s findings showed that partial replacement of OPC with GGBS, MS, and combinations of GGBS and MS produces more consistent outcomes for strength and durability when compared to control mix.