Hydrophobic lightweight concrete using treated GGBS


Ground granulated blast furnace slag (GGBS) has been widely applied as a supplementary cementitious material and ingredient for concrete production. In this study, a simple preparation process was applied to produce a low-cost super hydrophobic powder from GGBS as water-resisting additive for designing a hydrophobic lightweight concrete (HLC). A particle grading model was employed to secure a densely packed matrix, composed of a binder and lightweight aggregates, produced from natural expanded silicate. The density and mechanical properties of the developed HLC are measured and analysed. The effect of hydrophobic GGBS on workability and strength is investigated.

In this study, bulk treatment was investigated by treating ground granulated blast furnace slag (GGBS), which then serves as a functional filler in the concrete mixture. Bulk treatment may be considered to be advantageous compared to surface treatment due to the lesser susceptibility to deterioration of the hydrophobic coating, protection within the matrix as an integral property rather than a single layer at the exterior surface, and would require less attention such as maintenance [1].

Methodology and Results

The water-resisting GGBS will be applied to a lightweight concrete (LWC) mix design. The treatment of GGBS was done by dry-ball milling the powder with stearic acid, a fatty acid, where a series of different settings for the planetary mill and stearic acid proportions were experimented to find the optimum performance in terms of water-contact angle. As shown in Figure 1, the highest water contact angle was observed with 1% w.t. of stearic acid.

Figure. 1. Treated GGBS Water Contact Angle showing the degree of hydrophobicity

The water contact angle of 154.1° was obtained as shown in Figure. 1. Due to the high water-to-surface angle (>150°), the GGBS powder can be classified as super-hydrophobic. Figure 2 shows visuals of droplets on the hydrophobic GGBS (H-GGBS). Specimens were prepared, which include GGBS-0, -5, -10, -15 and -20 %. The samples consist of partial cement replacement using the super-hydrophobic GGBS in the matrix of the concrete mixture. The mixing procedure was based on previous studies carried out for a similar mix design [2].

Figure 2. Water contact angle. %w.t. of stearic acid with GGBS powder (a) 0.5%, (b) 1%, (c) 2%, (d) 4%

Several tests were conducted to determine the properties and characteristics of the hydrophobic lightweight concrete (HLC) of which the results can be summarized as follows:

  • The use of hydrophobic GGBS as a functional filler resulted in a slight reduction in strength of concrete (2.8 – 11.8%) and density (2.6 – 7.5%).
  • The workability was seen to have significantly increased (47-63%) when 5% or more of cement was replaced by the super-hydrophobic GGBS.
  • The hydrophobicity increases with higher H-GGBS replacement up to 15%.
  • Water absorption reduced by 69 – 94% and chloride diffusion reduced by 73 – 91% relative to the reference sample.

The water contact angle was again measured for the treated concrete samples as shown in Figure 3. Moreover, Figure 3 displays the SEM images of the concrete samples which show that the increasing amount of bonds between the super hydrophobic GGBS and cement as the percentage replacement increases.


The objective of this study was to investigate the feasibility of using treated GGBS as a functional filler to reduce mass transport in concrete to increase the durability in applications such as in marine environments. It showed that an easy method of dry-ball milling a widely available by-product of steel manufacturing with a common fatty acid (stearic) can produce very promising results in terms of durability when mixed in concrete.

Figure. 3. Water contact angle of concrete samples with hydrophobic GGBS