Lime and Ash: What a Wood-Ash Pozzolan Mortar Can and Cannot Do

Ash is free wherever wood is burned or husks are milled, and the hope that it can replace bought binder in a mortar is understandable — and, for wood ash, mostly unfounded. This paper computes the reactive silica and alumina budget of a stated wood ash and a stated rice husk ash against the lime each would need for a pozzolanic set, under stated chemistry assumptions, and derives the strength each lime–ash mortar can honestly claim. The study is simulated, not measured, and no mortar described here has been load-tested. The two ashes differ by more than an order of magnitude where it matters: 100 kg of the wood ash carries 3.0 kg of reactive silica and 0.8 kg of reactive alumina, needing only 3.84 kg of hydrated lime to react completely, while 100 kg of the rice husk ash carries 77.44 kg of reactive silica and needs 77.21 kg of lime. Two widely repeated beliefs are disproved by arithmetic. Wood ash mixed with water alone can form at most 8.73 kg of reaction gel per 100 kg of ash even if every reactive grain reacts, giving an upper-bound dry strength of 0.78 MPa and a wet strength of 0.08 MPa, while 12.0 kg of the mix is soluble material that water can carry away. Wood ash mixed half-and-half with uncalcined clay is weaker still, with an upper-bound dry strength of 0.49 MPa and a wet strength of 0.02 MPa. With added lime, rice husk ash at a lime-to-ash ratio of 1:2 reaches a computed 2.96 MPa at 28 days and 4.16 MPa at 90 days, an honest M2.5 mortar; wood ash with the same lime proportion reaches only 0.96 MPa at 28 days and 1.04 MPa at 90 days, below the M1 class at the age when mortars are normally judged. Neither ash mortar is credited here for use below ground or in standing water.