Presented at the Young Concrete Researchers, Engineers and Technologists Symposium (YCRETS), organised by Cement & Concrete SA (CCSA), Research Paper 16, titled “Lightweight aggregate manufacturing for use in structural concrete”, was presented by researchers Richart G Ross and Elsabe P Kearsley from the Department of Civil Engineering, University of Pretoria. 


 A reference mixture was cast in this pilot study to give a preliminary indication of the possibility of an ultra-lightweight structural concrete mixture. The paper outlines the materials and procedures used to create lightweight aggregate from local waste materials and aims to indicate if these can be used in structural concrete.  

 The initial results from this pilot study are promising, but further research is needed to refine the properties of the lightweight aggregate. 


 Lightweight concrete can be used to reduce the environmental impact of concrete. If the structure’s own weight is reduced, the total load that it needs to support is reduced – therefore elements with smaller cross-sections can be used. During construction, this will reduce: 

  • The total volume of concrete used. 
  • The CO emissions. 

 In South Africa, 32.7% of the fly -ash produced by the coal-fired power stations is recycled. Although there is an increase in recycling of glass locally, only 44% of glass was recycled in 2020. These two waste materials were used as the main components of the lightweight aggregate proposed in this paper. 

 Literature review 

 Studies show that lightweight aggregates can be manufactured from various materials.  

  • Tuan et al. used wet sewage sludge and waste glass powder to manufacture lightweight aggregates. It was found that an increase in sintering temperature led to an increase in bulk density of the lightweight aggregate. 
  • Liu et al. manufactured lightweight aggregates from waste glass and engineering muck with an apparent density of 1.461 g/cm3 and a single-particle crushing strength of 20.81 MPa. 
  • Li et al. studied the influence of sintering temperature and dwelling time on the characteristics of lightweight aggregates produced from sewage sludge and waste glass powder. An increase in sintering temperature decreased the water absorption, density and compressive strength of the particles. 


 It is possible to produce lightweight aggregates that contain 67,8% South African waste materials for use in structural concrete.  

  • Further investigations should be done to determine if the concrete adheres to all the requirements of the structural concrete design code. 
  • Factors influencing the size of aggregates produced should be analysed.  
  • The surface porosity of aggregates should be reduced.  
  • The concrete mixture should be optimised before structural concrete tests are performed on the concrete.  
  • A complete cost comparison to natural aggregates should be done to ensure that the lightweight aggregates are affordable. This may require further optimisation of the lightweight aggregate mixture. 

 Read the full paper online. 

Issue: Reducing the environmental impact of concrete.  

Solution: The manufacturing process of lightweight aggregate with recycled fly-ash and glass for use in structural concrete is promising. 


Full acknowledgement and thanks go to for the information in this editorial.

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