Effect of blast-furnace slag as cement replacement on hydration, microstructure, strength and durability of concrete

Sustainability is getting more and more attention during the last years. Also in the concrete and cement industry, attempts are made to minimize the environmental impact. One way to implement this thought, is the use of industrial by-products as cement replacing material. The 'waste' products are upgraded in high-value applications and the need for clinker is reduced. As a consequence, the CO2 emissions and energy demands associated with the clinker production are reduced. This PhD thesis focuses on the use of blast-furnace slag (BFS) as cement replacing material. Blast-furnace slag, a by-product of the steel production, obtains latent-hydraulic properties after rapid cooling and can thus replace clinker in cement. In EN 197-1 (2000), three types of slag cements are defined : CEM III/A (36 - 65 % BFS), CEM III/B (66 - 80 % BFS) and CEM III/C (81 - 95 % BFS)). More recently, in Belgium slag having a technical approval with certification can also be added to the concrete mix as a separate component if combined with an Ordinary Portland cement (OPC) CEM I having a strength class of 42.5 or higher. In 2004, the k-value concept for granulated blast-furnace slag was implemented in the national application document of EN 206-1 (2000). NBN B15-001 (2004) describes the rules which should be applied. Summarizing, the k-value for BFS amounts to 0.9 and the maximum slag content which may be taken into account in the k-values concept is limited (slag-to-cement ratio <= 0.45 or 0.2 depending on the exposure conditions and (prestressed) reinforcement

HEALCON : self-healing concrete to create durable and sustainable concrete structures

HEALCON is a project funded by EU-FP7 and coordinated by Prof. Nele De Belie (Ghent University). Its aim is to design smart concrete with self-healing properties to create durable and sustainable concrete structures. In a first phase of the project, different types of healing agents and encapsulation techniques have been developed and finalized. Depending on the type of damage, another self-healing concept was envisioned in the project. While superabsorbent polymers and bacterial healing agents are suitable for healing of early age cracks in structures which require liquid tightness since they produce a non-elastic material that fills the crack, elastic polymers can be used for healing of bending cracks since they can cope with the opening and closing movement of cracks under dynamic load. The efficiency of the different self-healing mechanisms with regard to mechanical behaviour, liquid-tightness and durability was firstly quantified at lab-scale. Besides, computer models were developed to simulate the fracturing and self-healing mechanisms in order to refine lab tests and to ultimately scale the mechanisms to an industrial level. Then, the most promising healing agents, for which the production could be easily up scaled, were incorporated in large scale elements (slabs and beams) to validate experimentally the self-healing methodologies. To characterize healing, non-destructive monitoring techniques have been used in the small and large scale tests. As the technologies developed have to be cost effective, functional and adaptable to engineering design, an end-user board followed the project from the start and helped to define technical and application requirements. Moreover, a life cycle cost analysis, supplemented by a life cycle assessment has been performed in order to demonstrate the impact of the self-healing technologies on environment and economy

First large scale application with self-healing concrete in Belgium : analysis of the laboratory control tests

Due to the negative impact of construction processes on the environment and a decrease in investments, there is a need for concrete structures to operate longer while maintaining their high performance. Self-healing concrete has the ability to heal itself when it is cracked, thereby protecting the interior matrix as well as the reinforcement steel, resulting in an increased service life. Most research has focused on mortar specimens at lab-scale. Yet, to demonstrate the feasibility of applying self-healing concrete in practice, demonstrators of large-scale applications are necessary. A roof slab of an inspection pit was cast with bacterial self-healing concrete and is now in normal operation. As a bacterial additive to the concrete, a mixture called MUC+, made out of a Mixed Ureolytic Culture together with anaerobic granular bacteria, was added to the concrete during mixing. This article reports on the tests carried out on laboratory control specimens made from the same concrete batch, as well as the findings of an inspection of the roof slab under operating conditions. Lab tests showed that cracks at the bottom of specimens and subjected to wet/dry cycles had the best visual crack closure. Additionally, the sealing efficiency of cracked specimens submersed for 27 weeks in water, measured by means of a water permeability setup, was at least equal to 90%, with an efficiency of at least 98.5% for the largest part of the specimens. An inspection of the roof slab showed no signs of cracking, yet favorable conditions for healing were observed. So, despite the high healing potential that was recorded during lab experiments, an assessment under real-life conditions was not yet possible

Reactivity of modified iron silicate slag as sustainable alternative binder

A possible solution to decrease the CO2 footprint caused by cement industry and to enhance the transition to circular economy is to use slags as Supplementary Cementitious Materials (SCM). The study presented here focuses on valorizing and investigating the reactivity and mechanical properties of blended binder systems combining modified iron silicate (MFS) slag and Ordinary Portland cement (OPC). MFS slag is a fumed by-product synthesized during the production of copper metal (Cu). This slag can be used as possible alternative SCM due to its pozzolanic behavior. To study the replacement level in relation to reactivity and strength development, replacement levels of 15, 30 and 50 wt% of MFS-slag in OPC are analyzed. The work can be divided into two categories, 1) assessing the reactivity through thermogravimetric analysis (TGA) and 2) evaluating the compressive strength (as a function of time) of mortar with MFS-slag after 2, 7, 28 and 90 days. TGA at 7, 15, 28 and 90 days allows to determine the reduction of portlandite (CH) content which gives an indication of the pozzolanic reactivity. Reactivity of the MFS-slag blended systems is also determined relative to inert filler blended systems to discern between the reactive behavior of the MFS-slag and the filler effect

Resistance to fatigue of self-healed concrete based on encapsulated polymer precursors

Moving cracks are often present in concrete structures and in those circumstances any self-healing technique for concrete must satisfy specific performance requirements, to guarantee its increased durability. These requirements include the capability of withstanding multiple cycles of crack movement without failing to keep healed cracks sealed. This paper shows early results from a testing protocol suggested by the authors to assess the performance of polymers as healing materials for moving cracks. Ultrasound (US) shear waves were used for continuous monitoring of small prismatic mortar specimens containing a single healed crack under a cyclic load. The maximum amplitude of US waves transmitted across healed cracks was correlated to the area effectively healed and the magnitude of crack movement. A decreasing trend of the maximum amplitude during cyclic loading was observed for strain levels on the polymer corresponding to 70% of its strain limit, but soundness at lower strain levels was confirmed after 300 cycles

Life cycle assessment of a column supported isostatic beam in high-volume fly ash concrete (HVFA concrete)

Nowadays, a lot of research is being conducted on high-volume fly ash (HVFA) concrete. However, a precise quantification of the environmental benefit is almost never provided. To do this correctly, we adopted a life cycle (LCA) approach. By considering a simple structure and an environment for the material, differences between traditional and HVFA concrete regarding durability and strength were taken into account. This paper presents the LCA results for a column supported isostatic beam made of reinforced HVFA concrete located in a dry environment exposed to carbonation induced corrosion. With a binder content of 425 kg/m3 and a water-to-binder ratio of 0.375, the estimated carbonation depth after 50 years for a 50 % fly ash mixture does not exceed the nominal concrete cover of 20 mm. As a consequence, no additional concrete manufacturing for structure repair needs to be included in the study. Moreover, structure dimensions can be reduced significantly due to a higher strength compared to the reference concrete used in the same environment. In total, about 32 % of cement can be saved this way. The reduction in environmental impact equals 25.8 %, while this is only 11.4 % if the higher material strength is not considered