Abstract
This research investigated the retarding impact of zinc oxide (ZnO) and borax (Na2[B4O5(OH)4]·8H2O) on hydration of Portland cement, calcium aluminate cement (CAC), and calcium sulfoaluminate cement (CSA). Heat of hydration of cement paste samples with and without ZnO and borax was used to measure the influence of ZnO and borax on the set time of these cementitious systems. It was found that both ZnO and borax can retard the set time of Portland cement systems; however, ZnO was shown to be a stronger set time retarder than borax for these systems. ZnO did not show any retarding impact on CAC and CSA systems while addition of borax in these systems prolonged the set time. It was concluded that ZnO does not poison the nucleation and/or growth of CSA and CAC hydration products. We suggest that borax retards the cement set time by suppressing the dissolution of cement phases.
Highlights
Utilization of cement set time retarders, referred to as retarding admixtures, in concrete allows concrete producers to delay the set time of concrete
The heat of hydration of cement paste samples with and without addition of zinc oxide (ZnO) and borax was measured to study the impact of ZnO and borax on the set time of cementitious systems
It was found that the retarding impacts of ZnO and borax on cementitious systems are different from each other
Summary
Utilization of cement set time retarders, referred to as retarding admixtures, in concrete allows concrete producers to delay the set time of concrete. In cementitious systems containing ZnO, during the prolonged dormant period, cement particles continue to dissolve and the concentration of ions in the pore solution increases [3,7] This will increase calcium concentration in the system leading to an increase in C–S–H nucleation sites. Zn ions by hydration products, such as C–S–H; and (2) removal of Zn ions by formation of calcium zinc hydrate according to Equations (1) and (2) [10,11] Borax is another set time retarder for cementitious systems [12]. Addition of supplementary cementitious materials (SCM), such as rice straw ash and silica fume, in cementitious systems have been shown to reduce the retarding action of chemical retarders [7,13] This has been attributed to existence of more C–S–H nucleation sites in systems containing SCMs compared to those without.
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