Degree of hydration and gel/space ratio of high-volume fly ash/cement systems L. Lam, Y.L. Wong, C.S. Poon* Department of Civil and Structural Engineering, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong Received 22 July 1999; accepted 12 January 2000 Abstract Although fly ash has been widely used in concrete as a cement replacement, little work has been done on determining the degree of hydration of high-volume fly ash/cement (FC) systems. In the present study, the degree of hydration of the cement in Portland cement (PC) paste was obtained by determining the non-evaporable water (Wn) content. The degree of reaction of the fly ash in FC pastes was determined using a selective dissolution method. Based on the relation between the degree of cement hydration and effective water-to-cement (w/c) ratio, the degree of hydration of the cement in FC pastes was also estimated. It was found that high-volume fly ash pastes underwent a lower degree of fly ash reaction, and in the pastes with 45% to 55% fly ash, more than 80% of the fly ash still remained unreacted after 90 days of curing while the hydration of the cement in high-volume fly ash pastes was enhanced because of the higher effective w/c ratio for the paste. This effect was more significant for the pastes with lower water-to-binder (w/b) ratios. Thus, preparing high-volume fly ash concrete at lower w/b ratios can result in less strength losses. This paper also introduces a model to describe the relationship between the w/c ratio and the degree of cement hydration and gel/space ratio. The gel/space ratios of the FC pastes, evaluated based on the proposed model, were found to be consistent with the gel/space ratio of PC pastes in terms of the relationship with compressive strength. The gel/space ratio data correlated (inversely) linearly with mercury intruded porosity, but the former correlated more with compressive strength than the latter. D 2000 Elsevier Science Ltd. All rights reserved. Keywords: Fly ash; Cement paste; Hydration; Gel/space ratio 1. Introduction High-volume fly ash concrete for structural use was first developed by the Canadian Centre for Mineral and Energy Technology (CANMET) in the late 1980s [1]. This type of concrete may contain more fly ash than cement by weight, and is usually prepared at the water-to-binder (w/b) ratios of about 0.3. It was reported to have acceptable early-age strength, high long-term strength and modulus of elasticity, low drying shrinkage and creep, and excellent durability when compared with Portland cement (PC) concrete with similar strength [2±5]. As more than 50 wt.% of the PC have been replaced in this type of concrete, it is believed that fly ash plays a significant cementing role [6]. Fly ash is known as a pozzolana which in itself possesses little or no cementitious value [7] and cannot react with water alone. Traditionally, fly ash used in structural concrete is limited to 15% to 25% cement replacement [6,8]. When a significant amount of fly ash is used, how it contributes to the strength development of the concrete and the hydration characteristics of this type of material are of significant research interest. Feldman et al. [9] found that in high-volume fly ash/ cement (FC) pastes, the fly ash commences reaction with Ca(OH) 2 between 3 and 7 days, but considerable amounts of Ca(OH) 2 and fly ash still remain unreacted after 91 days of hydration. The reaction products, mainly in the form of calcium silicate hydrates (CSH), have lower calcium-to-silica ratios (c/s). Berry et al. [10] indicated that in the early stages, fly ashes have the physical effect of a space filler, and are involved in the formation of ettringite (AFt). In the long-term, they are involved in the hydration reaction mainly as silico- aluminate binders. Xu et al. [11] and Xu and Sarkar [12] attributed the low early strength of high-volume FC systems to the effect of higher net water-to-cement ratio (w/c) due to fly ash replacement. They also indicated that the fly ash at later * Corresponding author. Tel.: +852-2766-6024; fax: +852-2334-6389. E-mail address: cecspoon@polyu.edu.hk (C.S. Poon). 0008-8846/00/$ ± see front matter D 2000 Elsevier Science Ltd. All rights reserved. PII:S0008-8846(00)00213-1 Cement and Concrete Research 30 (2000) 747 ± 756