Effect of high temperatures on the mechanical behaviour of hybrid cement

Abstract

The high-temperature mechanical behaviour of a pre-industrial hybrid alkaline cement (HYC) was studied. The HYC in question contained 30 % Portland clinker and 70 % of a blend of slag, fly ash and a solid activator (mix of alkaline salts with a predominance of Na2SO4). The material was tested during exposure to high temperatures to establish its compressive and bending strength and elastic modulus, as well as fracture toughness, analysed using an innovative methodology to notch the hydrated cement paste specimens. Post-thermal treatment tests were also run to assess residual mechanical strength after 2 h of exposure to temperatures ranging from 400 °C to 1000 °C. TG/DTA, MIP and SEM were deployed to ascertain heat-induced physical-chemical changes in the structure. The higher mechanical strength during and after treatment exhibited by the hardened hybrid alkaline cement than the CEM I 42.5R ordinary portland cement (OPC) paste used as a reference was associated with the lower water and portlandite content found in HYC. Pseudo-plastic behaviour was observed at high temperatures in the loaded HYC in the tests conducted during exposure.

Microstructural characterisation of hybrid cement after exposure to high temperatures

“…The binder used in this study was a preindustrial hybrid alkaline cement [30] containing 30% portland clinker + 32.5% ground granulated blast furnace slag (GGBFS) + 32.5% fly ash (FA) + 5% solid Na 2 SO 4 -based activator. The reference was a commercial portland cement (OPC) (CEM I 42.5R).…”

“…Cement pastes prepared with distilled water at a water / cement ratio of 0.3 were poured into stainless steel moulds to form 10x10x60 mm 3 prismatic specimens [30], subsequently cured in a climatic chamber at 22 ± 1°C and 95% relative humidity for 20 h [20][21][22]. After demoulding they were stored in the same chamber until 28 d, when they were measured, weighed, and exposed to temperatures of 400°C, 600°C, 800°C or 1000°C for 2 h in a laboratory oven.…”

“…The diffractogram for anhydrous OPC contained the lines characteristic of clinker (alite, belite, calcium aluminates and ferrite phases) and gypsum. In addition to the clinker phases, the HYC diffractogram included reflections indicative of the mullite and quartz present in fly ash and a wide hump with 2H values of 25°t o 35°attributable to the vitreous/glassy component in the slag and fly ash [27][28][29][30].…”

“…Hybrid alkaline cement (HYC), a new type of binder presently under development [27][28][29], comprise a blend of ordinary portland and alkaline cements with a clinker content of around 20% to 30% and a large proportion of supplementary cementitious material (SCMs) laced with an alkaline activator. The present authors recently published a detailed study [30] of the physical and mechanical behaviour of HYC pastes after in situ exposure to temperatures of 200°C to 1000°C. The findings showed that after exposure, HYC pastes had higher compressive and flexural strength and fracture toughness than the reference OPC paste, as well as higher residual strength.…”

Recent progress in low-carbon binders

“…In many of these binder systems, a careful balance needs to be drawn between adding sufficient alkalis to enable rapid strength development, and avoiding the excessive alkali levels that may lead to efflorescence. Ongoing work to understand the causes and implications of efflorescence in both one-part and two-part AAM binder systems [166][167][168] Further, the cement paste was tested at up to 1000°C, and showed better high-temperature resistance than Portland cement due to the recrystallization of new poorly hydraulic phases, mainly in gehlenite-and rankinite-type phases [170,171]. Velandia et al [172] also demonstrated the production of concretes with good performance using a hybrid fly ash-Portland-Na2SO4 binder, and provided extensive data about the durability performance of these concretes, including correlations between key durability parameters and compressive strength at ages of up to 1 year.…”