Statistical Study of the Effect of the Composition on the Strength of Supersulphated Cements
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Statistical Study of the Effect of the Composition on the Strength of Supersulphated Cements Castillo-Sánchez Y.B.1, Almanza-Robles J.M.1, Escalante-García J.I.1 1 Centro de Investigación y de Estudios Avanzados del IPN (Cinvestav – Unidad Saltillo), México.
ABSTRACT Supersulphated cements (SSC) are environmentally friendly binders that incorporate several raw materials, including byproducts. A systematic study was considered opportune considering the wide range of formulations found in the literature. The effect of the type and proportioning of components in the strength of SC was investigated using the Taguchi method to optimize the experimental work and to define the optimal conditions. The factors were: [A] %blast furnace slag (82.5-90%), [B] CaSO4 - alkaline activator ratio (1:0, 3:1, 1:1, 1:3 and 0:1), [C] type of CaSO4 (5 types) and [D] type of alkaline activator (portland cement, Ca(OH)2, KOH and NaCO3 and 2 combinations of these). Pastes were prepared and characterized for up to 28 days at 20°C. In general, for all values of [A] the best strength was for levels of [C] at 3:1, followed by the 1:1 and 1:0 ratios. The optimal conditions using the 28 day strength consisted of [A]= 82.5%, [B]= 3:1, [C]= flyorgypsum and [D] = portland cement, which developed excellent strength from day one and 35MPa. X-ray diffraction showed ettringite and C-S-H formation from the early ages. The microstructures showed dense matrices of reaction products well bonded to partially reacted slag grains, which in some cases showed rims of hydration products. Keywords: blast furnace slag, supersulphated cements, alternative cements, Taguchi.
INTRODUCTION Supersulphated cements (SSC) are environmentally friendly binders [1] that have been used for decades; nonetheless, the literature on the subject is both, limited and presents a wide range of combination of the components of SSC, i.e. amounts of Blast Furnace Slag (BFS), calcium sulfate and alkaline activator (AA) . Dutta and Borthakur [2] used 15 and 20% of anhydrite (An), the strength increased with time and %An. Valenti et al [3] found that the kinetics of hydration were favored by higher concentration of phosphogypsum (PG) and pure dihydrate (DH), the former being more effective. Grounds et al [4] studied the stability of SSC at 95 ° C and relative humidity of 100, 53 and 11% water vapor; at elevated temperatures and relative humidities, ettringite is inherently unstable and decomposes. Erdem and Olmez [5] found that Phosphoanhydrite (PA) and PG, the setting time decreased with increasing PA or BFS, and the reduction of content of portland cement (PC) increased the setting time, pastes with 85% BFS, 10% PA and 5% PC showed the best strength after 28 and 90 days. Dongxu et al [6] pointed that the additives An and Na2SO4 improved the strength at early ages but a decrease occurred after 28 days. Gruskovnjak et al [7] found that the initial strength using low reactivity BFS slag cannot be overcome by the addition
of stoichiometric amounts of the components used for the formation of
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