The performance of a blended PFA/GGBS alkali-activated concrete using a silica fume-based activator
The construction industry is currently tackling a serious concern with the ever-increasing use of Portland cement and associated carbon emissions due to urban development expansion worldwide. Hence, extensive research has been undertaken into ‘’’cement-free’’ alternative concrete has emerged in the academic and commercial space also known as alkali-activated concrete, bringing CO2 reductions of up to 85%. AAC has been elevated as a viable solution detrimental to the ‘sustainable toolkit’ for low-carbon concrete solutions through the use of various industrial and agricultural waste, chemically activated using an alkali source. Nevertheless, sodium silicate is an attractive ‘alkali activator’ but has been reported to contribute up to 75% of the total embodied carbon in AAC. This research reports on the development of an eco-friendly alternative (sodium silicate) activator derived from industrial waste (Silica Fume) and sodium hydroxide, with a key focus on providing comparison analysis with commercial sodium silicate and sodium hydroxide solutions. In doing so, various pulverised fuel ash and ground granulated blast furnace slag blends were investigated as a replacement binder to Portland cement among number activator dosages, w/b ratios, workability retaining admixture dosages on the short (compressive strength, workability, setting time) and long term (compressive strength, workability retention (2 hours), tensile splitting strength, flexural strength and elastic modulus) properties of concrete and mortar, adopting an experimental based optimum performance testing procedure. The innovative silica fume derived activator (Regime B) chemically activating PFA: GGBS mix compositions (50PFA: 50GGBS, 40PFA: 60GGBS, 30PFA: 70GGBS, 20PFA: 80GGBS) combined with a range of water-to-binder ratio’s 0.40, and 0.42 showed promising results from a minimum performance perspective (≥ C32/40 compressive strength class under BS 8500:2019, S4 consistency slump class BS EN 12350-2 and a minimum initial setting time ≥45 minutes in accordance with PAS 8820:2016) The progressive enhancement in GGBS wt. % from 40% to 50, 60,70, and 80 % proportional to PFA demonstrated a gradual increase in strength and a reduction in workability, and initial/final setting time attributed to an accelerated rate of reaction provided by the additional Ca, supporting the formation of C-A-S-H gel and all that leads to higher strength, while being a dominating factor for reducing workability and setting time. The ever-similar short-term (up to 28 days) fresh and hardened properties of the commercial sodium silicate solution (Regime A) to that of Regime B on display in this report, demonstrate that the silica fume activator is a strong candidate for replacing the commercial alternative. Although, various factors such as mineralogical properties, alkali concentration, processing time, and temperature were established as important factors to determine like-for-like properties in promoting eco-friendly activators. With respect to workability retention (up to 2 hours), all PFA: GGBS blends investigated were below the minimum target slump class S4 in accordance with BS EN 12350-2. Influentially, the additional use of the polycarboxylate-based admixture in various dosages showed overwhelming improvements in slump at 2 hours, showing 114.3 and 121.4% corresponding with admixture dosages 1% and 1.5% of the total binder wt. and both exceeding the S4 target. The admixture dosage is believed to have absorbed the negative charge particles which led to the separation of the material particles, improving workability. Concerning other long-term mechanical properties (compressive strength, tensile splitting strength, flexural strength, and elastic modulus), Regime B proved to be the far more superior performing, particularly for tensile and flexural strength showing a significant enhancement. One possible explanation can be associated with the variation in the total silica release rate necessary for the reaction mechanism between both activating solutions. Another explanation can be attributed to either a lack or excessive soluble silica content (SiO2 (Silica) to Na2O (sodium oxide) ratio was provided by the supplier ranging from (1.5 to 2.5). With reference to the 28-day performance corresponding with the minimum strength values that relate to C32/40 concrete, Regime A and B undoubtedly satisfied the 28-day target compressive strength requirements (40MPa) for a C32/40 in accordance with BS 8500:2019. Whilst UTS mean values ≥ the derived EN 1992-1-1 minimum tensile strength [Fctm] = 3MPa for C32/40 concrete, demonstrating continuity alongside compressive strength results as a desirable alternative for industry application. Finally, the cradle-to-gate LCA impact assessment carried out using SimaPro (2015) software showed Regime B’s activating solution achieved a remarkable 51% carbon saving to Regime B (81% CO2 saving to conventional OPC).
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