Combined use of sewage sludge ash and recycled glass cullet for the production of concrete blocks

Concrete paving blocks are widely used in many parts of the world because of their flexibility in construction and low maintenance costs. New generations of the blocks have already incorporated some types of wastes for achieving waste recycling. Sewage sludge ash (SSA) produced from the incineration of dewatered sewage sludge may be used to partially replace cement for construction. This study investigated the properties of concrete blocks using SSA as a cement replacement together with recycled construction and demolition aggregates or glass cullet (GC) as a partial substitution of natural aggregates for producing concrete blocks by the dry mix (zero slump) compression method. No research in published literature has been conducted on the complementary effects from the utilization of SSA as a cement substitute and some recycled materials as aggregates in producing concrete blocks. Assessments of technical benefits and leaching risks were made with a view to encouraging recycling of waste materials and enhancing conservation of natural resources. The research findings revealed that the SSA possessed moderate pozzolanic activity and it increased the long term compressive strength of the blocks. More importantly, it was found that combining the use of SSA with GC is superior to using the ash alone for block making due to significant reduction in drying shrinkage. On the other hand, the alkali-silica reaction caused by the reactive GC could in turn be suppressed with the incorporation of the SSA. Leaching tests conducted also showed compliance with the regulatory limits. These results indicate that the combined use of SSA and GC can produce paving blocks with satisfactory mechanical, durability and leaching properties while helping the recycling of the two types of solid wastes.