A method of producing high-strength cementitious product from raw fly ash by mixing the raw fly ash with a lithium compound, whereby milling of the raw fly ash to achieve requisite strength is unnecessary. It has now been found that by adding as little as 0.1% of lithium chloride to raw untreated Class C fly ash, one can achieve improved seven-day and twenty-eight-day compressive strength. At the very least, raw lithium treated Class C fly ash may be used at lower total cementitious content per yard of concrete as opposed to Ordinary Portland Cement (OPC) for improved compressive strength.