This study investigated the shrinkage properties of concretes using blast furnace slag cement and frostresistant accelerator as a construction technology in environmental-load-reducing cold weather concretes. The investigation results showed that as the frost-resistant accelerator was added to both Ordinary Portland Cement (OPC) and Blast Furnace Slag Cement (BB), the effect on the properties of fresh concrete was reduced and the compressive strength was increased from an early age. In addition, the free shrinkage strain and cracking potential at the same age tended to increase and the study result verified that the crack generation was also fast in the restrained condition. Meanwhile, the study results verified that the length of both of OPC and BB tended to increase upon adding the frost-resistant accelerator. Pore volume <30 nm diameter, in particular, 20–30 nm pore volume, and ink-bottle pore volume decreased such that the shrinkage volume increased, and the change in pore volume in this range was regarded as having a large effect on the shrinkage properties.

Introduction

The use of blast furnace slag cements has been expanded to reduce environmental load or employ industrial wastes efficiently. More recently, low heat and shrinkage-reduced blast furnace cement, which has a high content ratio of blast furnace slag, and Energy CO2 Minimum (ECM) cements that achieve the Class C slag content ratio have been studied, but their use has been limited practically [1–3]. Although blast furnace slag cements have many advantages such as low heat, increased long-term strength, flame interruption performance, and suppression of the alkali–aggregate reaction, they also have drawbacks such as delayed early strength development and early frost damage during winter construction due to high temperature dependence, which is why much care must be taken when using blast furnace slag [4]. When concretes are used in winter, it is important that the concretes do not freeze until sufficient strength has been developed to prevent early frost damage due to the delayed strength development or freezing, and heat curing via temporary enclosure and use of a heater is typical [5–8]. However, the use of a temporary enclosure for concrete curing may cause CO2 emissions due to the heater or warm curing in a space with extremely bad heat efficiency, which is why the use of a frost-resistant accelerator has increased to prevent early frost damage [9–14].

Ground granulated blastfurnace slag (GGBFS), obtained as a by-product in the production of metallic iron, is granulated, rapidly cooled, and therefore predominantly glassy, basic slag. The slag contains the same oxides (SiO2, Al2O3, CaO) that make up Portland cement but in different proportions. According to the European cements standard EN 197-1, at least two16 thirds of the slag by mass must be glass and the mass ratio (CaO+MgO)/SiO2 must also be greater than 1.0 [1]. The latent hydraulic properties of slag are activated in the presence of cement clinker, sulphates or calcium hydroxide (but other alkaline substances are also effective). With all activators, the calcium silicate hydrate or C−S−H phase is produced as the hydration product which governs the hardening. Clinker activation is used in the manufacture of Portland slag cements and blastfurnace cement, and sulphate activation in the manufacture of super sulphate cement. Generally, the rate of hardening of slag cement is somewhat slower than that of ordinary Portland cement (OPC) during the first 28 days, but increases thereafter so that at 12 months the strength becomes close to or even exceeds that of OPC [2]. GGBFS can be used as a direct replacement for OPC on one-to-one basis by mass. Replacement rates for GGBFS vary from 30 mass % to up to 85 mass %. In general, up to 50 mass % GGBFS is used in most applications. However, higher replacement rates up to 85 mass % GGBFS are used in special applications either to improve the durability of concrete in aggressive environments or to reduce heat of hydration in massive concrete structures [3]. Nowadays the GGBFS is most widely used as the main constituent to produce high-performance Portland cement blends that are more economical and environmentally friendly. The EN 197-1  differentiates some main categories of cements containing different proportions of GGBFS, i.e., Portland slag cement (CEM II), Blastfurnace cement (CEM III) and Composite cement (CEM V). The Portland slag cement type (CEM II) exists in two classes designated as CEM II/A–S and CEM II/B–S in which the maximum content of GGBFS is and 35 mass %, 12 respectively. There are two classes of Composite cements CEM V designated as CEM V/A and CEM V/B in which, in addition to cement clinker and pozzolana, the maximum content of GGBFS is 30 and 50 mass %, respectively. Blastfurnace cements CEM III (where CEM III is the designation for three types of blastfurnace cements A, B and C), in addition to Portland cement clinker, contain between 36 and 95 mass % of GGBFS with subdivisions at 66−80 mass % of GGBFS [1]. Table 1 shows the composition of three types of blastfurnace cements (BFSC).