**3. Use of superplasticising admixtures**

Plasticising and superplasticising admixtures have taken an indispensable role in advancing the concrete technology and development of SCC. Before the 1960s, workability improving admixtures based on hydroxycarboxylic acids or lignosulphonates had been developed. They were usually known as plasticisers or water reducers, and they would allow the W/CM ratio to be reduced by 5–10% without adversely affecting the workability of concrete. In the 1960s–1970s, a newer generation of workability improving admixtures based on sulphonated formaldehyde condensates of melamine or naphthalene was developed. These admixtures are generally named superplasticisers (SP) or high-range water reducers because of their superior performance compared to their predecessors. Such SP could allow the W/CM ratio to be reduced by as much as 20–30% without affecting the workability [33]. Terminologically, SPs derived from sulphonated melamine formaldehyde condensates are sub-classified as melamine-based superplasticisers (abbreviated as SMF), while SPs derived from sulphonated naphthalene formaldehyde condensates are sub-classified as naphthalene-based superplasticisers (abbreviated as SNF). SMF and SNF have similar performance and may be blended together in usage [34].

In the 1980s, manufacturers started works to develop polycarboxylate-ether-based SP (abbreviated as PCE), but initially there were serious problems of severe retardation and excessive air entrainment [35]. It was only until around the turn of century, PCE became available in the market and these products were dubbed the third-generation superplasticisers or hyperplasticisers. The PCE remarkably outperformed the existing SP. Their use would allow the W/CM ratio to be reduced by up to 40% without adversely affecting the workability of concrete. The molecular structure of PCE is characterised by an active-monomer (such as polymethacrylate acid, or abbreviated as PMAA) formed main chain, attached with numerous graft copolymers (such as polyethylene glycol, or abbreviated as PEG) formed long side chains. Such long side chains are absent in SMF and SNF molecules.

PCE improve the workability of concrete mixes by dual effects, namely the dispersion effect and steric hindrance or steric repulsion effect. This is in contrast to SMF and SNF which improve the workability of concrete mixes only by dispersion effect. The dispersion effect is explained as follows. There are four main types of minerals in ordinary Portland cement, namely belite (C<sup>2</sup> S), alite (C<sup>3</sup> S), aluminate (C<sup>3</sup> A) and ferrite (C<sup>4</sup> AF) [36]. Belite and alite are negatively charged, while aluminate and ferrite are positively charged. Because of the opposite electrostatic potentials, the cement grains tend to coagulate together, making it less readily to thoroughly mix with water to form a uniform paste [37]. With the addition of SP, the SP molecules are adsorbed onto the surfaces of cement grains, and they impart negative charges to all the cement grains. The electrostatic repulsion derived from the negative charges disperses the cement grains apart. For PCE, it is the main chain of PCE molecule that is adsorbed and imparts negative charges to cement grains, whereas the side chains act as physical barriers to separate the cement grains further apart [38]. Such steric hindrance further promotes dispersion and prolongs workability retention [39, 40].

In determining the SP dosage to concrete, attention should be paid to the quantities of SP demand for given levels of workability, the saturation SP dosage beyond which further addition of SP would yield no return, and the maximum SP dosage beyond which further addition of SP would cause segregation. Conventionally, the SP demand, saturation dosage and maximum dosage are expressed in percentage by mass of cementitious materials. However, as SP is a surfactant adsorbed onto the surface of cementitious materials, its effectiveness should be dependent on the amount of SP per surface area of cementitious materials [41]. Therefore, the SP demand, saturation dosage and maximum dosage should be controlled by the fineness and the content of each cementitious material. This forms the basis to rationalise the usage of SP.
