Abstract

Abstract

The ettringite (AFt) formation and damage process are investigated for cement paste subjected to external sulfate attack with and without heating treatment during curing. Slice and disc specimens were exposed to 10 g/L Na2SO4 + pH of 13. The evolutions of AFt formation, pore structure, and expansion on slice specimens were characterized through 27Al NMR, MIP, and micrometer. The surface cracks on disc specimens were observed through an optical microscope. The experimental results show that: (1) heating treatment dissolves part of AFt and accelerates the ettringite formation and material expansion; and (2) heating treatment initiates the microcracks, thus promoting the material cracking.

Abstract

In this study, the impact of the thaumasite sulfate attack (TSA) on cement mortar was investigated by comparing it to the ettringite sulfate attack (ESA). Mortar specimens with three binders, corresponding to the blank, ESA, and TSA, were exposed to sulfate solution. The evolutions of thaumasite and ettringite formation, pore structure, and gas permeability on the mortar specimens were characterized through XRD, 1H NMR, and CemBureau device. The experimental results show that: (1) both ettringite and thaumasite were formed in TSA; (2) ESA reduces the capillary pore space, whereas TSA decreases both the capillary pore and interlayer pores; and (3) ESA decreases the gas permeability while TSA significantly increases it.

Abstract

Blast furnace slag and lead-zinc tailings (LZTs) are solid wastes from the iron-making and mining industries, respectively. In order to reuse these solid wastes, the LZTs were pre-calcined at different temperatures and then explored whether it is capable of being the activator of slag. The effects of pre-calcination on the phase composition of LZTs were elucidated by XRD. The compressive strength of LZTs-activated slag pastes was investigated, and the mineral composition and microstructure of the pastes were detected by XRD and SEM. The results imply that with the elevating of heating temperature from 500 ℃ to 1000 ℃, the dolomite in LZTs decompose into CaO and MgO, and the oxidation of pyrite resulted in the formation of Fe2O3 and gaseous SO3, then SO3 reacted with a part of formed CaO to produce CaSO4. The resulting CaSO4 and CaO can become the sulfate activator and alkaline activator of slag, respectively. The LZTs-activated slag pastes prepared with the LZTs preheated at 800 ℃ exhibit the highest strength at 90 d, which yield 24 MPa. The hydration products of LZTs-activated slag were ettringite and calcium silicate hydrates (C-S-H gel). Large amounts of dense C-S-H gel bond together, connecting most of the ettringite networks into a whole, thus forming a dense structure and effectively improving the strength of LZTs-activated slag. The outcome of this study provides a potential disposing or reusing approach of the large amounts of LZTs.