Utilization of Concrete Waste in the Manufacture of Portland Pozzolana Cement

Abstract

Concrete waste is a subclass of construction and demolition waste composed of hydrated cement, coarse, and fine aggregates. It bears its origin from the construction and demolition of concrete structures. The purpose of this study was to investigate the utilization of concrete waste as a substitute for natural pozzolana in the manufacture of Portland pozzolana cement(PPC). A total of 3 samples10 Kgs each, were collected in polythene bags from Roysambu, Woodley, and Mowlem all in Nairobi city, Kenya. The samples ware milled in a laboratory ball mill to 5% retention on a 45 μm sieve. Substitution of Ordinary Portland cement (OPC) was done at 10%, 20%,30%,40% and 50%. The compressive strength was determined by breaking mortar prisms in a computerized compression analysis machine type YAW-300, on the 2nd,7th, 28th, and 56th day of curing according to the Kenya cement standards. The optimum substitution rate required to achieve a compressive strength of 32.5 MPa on the 28th day was determined from the graph of the 28th day curing period compressive strength development data against the curing period. The test cements for pozzolanicity were prepared using the optimum substitution rate determined. The pozzolanicity was evaluated through a gradual comparison of Calcium oxide and hydroxyl ion concentration after a set period of 3,8,15,21 and 28 days of curing as outlined in Kenya cement standard. The chemical composition of the waste concrete was analysed using X-ray fluorescence spectrophotometer model epsilon 3XLE and results were expressed in percent oxides of the respective elements. This study revealed that the compressive strength of all the test cements increased gradually throughout the curing period. Control OPC showed the highest compressive strength in all curing ages ranging from 35.80 MPa on the 2nd day to 57.20 MPa on the 56th day, with a minimal strength change of 0.1 MPa from the 28th to the 56th day of curing. Control PPC showed a strength change from 17.64 MPa on the 2nd day to 37.40 MPa on the 56th day. The waste concrete substituted cements strength development mirrored that of control PPC growing steadily from the 2nd day up to the 56th day of curing. However, an inverse relationship between the rate of substitution and strength development was noted. All the cements achieved the minimum required 32.5 MPa strength on the 28th day except the 50% substitution which achieved 28.73 MPa. An optimum substitution rate of 45.42% waste concrete on OPC was established. The hydroxyl ions and calcium oxide concentration on control PPC and the three formulated cements decreased gradually from an average of 56.43 mmol/l to 40.83 mmol/l and 7.56 mmol/l to 2.65 mmol/l respectively from the 3rd to 28th day indicating the presence of pozzolanic reactions. However, for the OPC the concentrations of hydroxyl ions and calcium oxide increased gradually from 58.4 mmol/l to 67.6 mmol/l and 7.9 mmol/l to 8.8 mmol/l respectively negating pozzolanicity. The chemical composition of waste concrete showed the presence of silica dioxide and aluminum trioxide averaging 36.32% and 9.19% respectively meeting the minimum pozzolana specification of 25% silica dioxide.