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- Alkali-activated materials (5)
- Acoustic emission (3)
- Fire proofing (2)
- Alkali aggregate reaction (1)
- Alkali silica reaction (1)
- Concrete (1)
- Cracking (1)
- Electrochemical methods (1)
- Fire protection (1)
- Fire resistance (1)
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The present study aims to investigate the use of geopolymer mortars as passive fire protection system for steel structures. Coal fly ashes were used as aluminosilicate source and perlite was employed as aggregate to obtain a lightweight system. In addition, a geopolymer mortar containing quartz aggregate was produced for comparison. The geopolymer mortars were applied on stainless steel plates and exposed to both, cellulosic and hydrocarbon standard fire curves, according to ISO 834-1 and EN 1363-2, respectively. Acoustic emission measurements were conducted to analyze cracking phenomena during the high temperature exposure. The resulting temperature-time curves showed that the investigated system is effective in retarding the temperature rise of the steel plates. When the cellulosic fire curve was applied, a 20 mm [0.79 in.] thick layer of lightweight geopolymer mortar protected the steel substrate from reaching the critical temperature of 500 °C [932 °F] for at least 30 minutes, avoiding the rapid decrease of its mechanical properties and thus representing an important safety measure against accidental fires. No spalling phenomena on heating were detected; however, significant cracking was observed on cooling.
This study aims at investigating the use of coal fly ash-based alkali activated mortars as passive fire protection system for steel structures. These systems are used to slow down the temperature rise of the steel substrate in case of fire. In addition, the protective system should guarantee the ability to prevent and/or mitigate steel corrosion phenomena. The behavior of a light-weight mortar was compared to that of a normal-weight mortar. Density and porosity were measured to better characterize the physical properties of the mortars. The degree of protection in case of fire was assessed by performing medium-scale fire tests. Acoustic emission measurements were conducted to analyze cracking phenomena during the high temperature exposure. The corrosion process was evaluated using an electrochemical approach in order to monitor the durability of the developed material. Preliminary results show that a 20 mm-thick layer of light-weight mortar is able to protect the steel substrate from reaching the critical temperature of 500 °C for 38 minutes in case of cellulosic fire. In addition, alkali activated mortars provide protection for carbon steel in presence of aggressive environment (i.e. presence of chlorides). The corrosion resistance is strictly related to the physical properties of the developed mortars.
Alkali-activated fly ashes have been proposed for various applications where resistance against high temperatures is required, yet several details regarding the response of these materials to heat-exposure need to be clarified. In the present study, heat-induced cracking in fly ash-based alkali-activated pastes and lightweight mortars was analyzed by in-situ acoustic emission (AE) detection during complete heating-cooling cycles (up to ∼1100 °C), augmented by thermogravimetry and ex-situ SEM and XRD analyses. The applicability of the lightweight mortars as passive fire protection coatings was assessed by recording temperature-time curves of mortar-coated steel plates. Cracking during heating was limited and associated exclusively with the dehydration of the materials in the temperature range ∼90–360 °C. However, samples heated to temperatures above ∼600 °C exhibited intense cracking on cooling. This was attributed to differential deformations caused by local sintering and partial melting at the glass transition temperature, and subsequent quenching on cooling.
The RILEM technical committee TC 247-DTA ‘Durability Testing of Alkali-Activated Materials’ conducted a round robin testing programme to determine the validity of various durability testing methods, originally developed for Portland cement based-concretes, for the assessment of the durability of alkali-activated concretes. The outcomes of the round robin tests evaluating sulfate resistance, alkali-silica reaction (ASR) and freeze–thaw resistance are presented in this contribution. Five different alkali-activated concretes, based on ground granulated blast furnace slag, fly ash, or metakaolin were investigated. The extent of sulfate damage to concretes based on slag or fly ash seems to be limited when exposed to an Na2SO4 solution. The mixture based on metakaolin showed an excessive, very early expansion, followed by a dimensionally stable period, which cannot be explained at present. In the slag-based concretes, MgSO4 caused more expansion and visual damage than Na2SO4; however, the expansion limits defined in the respective standards were not exceeded. Both the ASTM C1293 and RILEM AAR-3.1 test methods for the determination of ASR expansion appear to give essentially reliable identification of expansion caused by highly reactive aggregates. Alkali-activated materials in combination with an unreactive or potentially expansive aggregate were in no case seen to cause larger expansions; only the aggregates of known very high reactivity were seen to be problematic. The results of freeze–thaw testing (with/without deicing salts) of alkali-activated concretes suggest an important influence of the curing conditions and experimental conditions on the test outcomes, which need to be understood before the tests can be reliably applied and interpreted.
Application of electrochemical methods for studying steel corrosion in alkali-activated materials
(2023)
Alkali-activated materials (AAMs) are binders that can complement and partially substitute the current use of conventional cement. However, the present knowledge about how AAMs protect steel reinforcement in concrete elements is incomplete, and uncertainties exist regarding the application of electrochemical methods to investigate this issue. The present review by EFC WP11-Task Force ‘Corrosion of steel in alkali-activated materials’ demonstrates that important differences exist between AAMs and Portland cement, and between different classes of AAMs, which are mainly caused by differing pore solution compositions, and which affect the outcomes of electrochemical measurements. The high sulfide concentrations in blast furnace slag-based AAMs lead to distinct anodic polarisation curves, unusually low open circuit potentials, and low polarisation resistances, which might be incorrectly interpreted as indicating active corrosion of steel reinforcement. No systematic study of the influence of the steel–concrete interface on the susceptibility of steel to corrosion in AAMs is available. Less common electrochemical methods present an opportunity for future progress in the field.
Alkalisch aktivierte Bindemittel (AAB) und Betone können herkömmliche Zemente beziehungsweise Betone potentiell in vielen Anwendungen ersetzen und dadurch den CO2-Fußabdruck der Bauindustrie wesentlich verkleinern. Zur Untersuchung der Stahlkorrosion in bewehrten („armierten“) Betonbauteilen werden elektrochemische Methoden wie Messungen des Freien Korrosionspotentials, des Polarisationswiderstands oder von Stromdichte-Potential-Kurven eingesetzt. Diese Methoden und die etablierten Grenzwerte zur Detektion von Korrosion sind für die Anwendung bei herkömmlichen Zementen beziehungsweise Betonen entwickelt und erprobt worden. Neue Forschungsergebnisse demonstrieren, dass Unterschiede zwischen den Porenlösungszusammensetzungen von herkömmlichen Zementen und bestimmten AAB sowie anderen schlackehaltigen Zementen erhebliche Unterschiede bei den Ergebnissen der elektrochemischen Messungen bewirken und damit zur fehlerhaften Detektion von Stahlkorrosion führen können. Ursache hierfür sind vor allem reduzierte Schwefelspezies in den Porenlösungen von AAB und anderen schlackehaltigen Zementen.