Abstract
Hybrid cement (HC) can be defined as alkali activated-blended-Portland cement (PC). It is prepared by the addition of an alkaline solution to high-volume aluminosilicate-blended-PC. Although this cement exhibits higher mechanical performance compared to conventional blended one (aluminosilicate–PC blend), it represents lower commercial viability because of the corrosive nature of alkaline solution. Therefore, this study focuses on the preparing one-part HC using dry activator–based BFS (DAS). DAS was prepared by mixing sodium hydroxide (NaOH) with BFS at low water to BFS ratio, followed by drying and grinding to yield DAS-powder. Different contents of DAS (equivalent to 70 wt.% BFS and 1, 2, and 3 wt.% NaOH) were blended with 30 wt.% PC. A mixture containing 70 wt.% BFS and 30 wt.% PC was used as a reference sample. The mortar was adjusted at a sand–powder (BFS-PC and/or DAS-PC) weight ratio of 3:1. The microstructural analysis proved that DAS-powder is mainly composed of sodium calcium aluminosilicate–activated species and unreacted BFS. These species can interact again with water to form calcium aluminum silicate hydrate (C-A-S-H) and NaOH, suggesting that the DAS acts as a NaOH-carrier. One-part HC mortars having 1, 2, and 3 wt.% NaOH recorded 7th day compressive strength values of 82%, 44%, and 27%, respectively, higher than that of the control sample. At 180 days of curing, a significant reduction in compressive strength was observed within the HC mortar having 3 wt.% NaOH. This could be attributed to the increase of Ca (within C-S-H) replacement by Na, forming a Na-rich phase with lower binding capacity. The main hydration products within HC are C-S-H, C-A-S-H, and chabazite as part of the zeolite family.
Highlights
Portland cement (PC) is a common and the dominant binding material in the construction sector [1,2]
To mitigate the high CO2 emission and energy demand, several authors replaced a high portion of PC by supplementary cementitious materials such as fly ash (FA), silica fume (SF), and blast-furnace slag (BFS) [5,6,7,8,9]
The role of supplementary cementitious materials in the mitigation of carbon footprint and the improvement of the durability and the later mechanical properties of PC have been established [10,11,12,13,14,15], the substitution of PC with a high volume of these materials caused a noticeable retardation in its early hydration [16,17,18]
Summary
Portland cement (PC) is a common and the dominant binding material in the construction sector [1,2]. To mitigate the high CO2 emission and energy demand, several authors replaced a high portion of PC by supplementary cementitious materials such as fly ash (FA), silica fume (SF), and blast-furnace slag (BFS) [5,6,7,8,9]. The role of supplementary cementitious materials in the mitigation of carbon footprint and the improvement of the durability and the later mechanical properties of PC have been established [10,11,12,13,14,15], the substitution of PC with a high volume of these materials caused a noticeable retardation in its early hydration [16,17,18]. Several authors have stated that the replacement of PC by high-volume BFS has reflected on a significant reduction of the heat of hydration, resulting in a retardation in the early compressive strength [22,23]. It was found that the performance and hydration characteristics of PC–BFS cement enhanced with increasing the curing temperature and the fineness of slag [24,25,26]
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