Table 2.

Effect indexes and critical properties of biochar used for 28-day compressive strength measurements, including means and numbers of records

Biochar variables	Effect index (%)		C content (%)		Molar O/C ratio		Specific surface area (m2 g−1)		Si concentration (%)		SiO2 concentration (%)
	Mean	n	Mean	n	Mean	n	Mean	n	Mean	n	Mean	n
Feedstock
Corn	9	35	–	–	–	–	483	24	–	–	4.6	24
Rice	–6	19	47	14	0.81	10	19	7	0.25	5	49.3	14
Wheat	13	17	67	11	0.34	5	103	12	–	–	10.7	5
Hardwood	− 23	16	66.7	9	0.24	1	60	8	–	–	–	–
Softwood	7	39	76	29	0.17	29	147	9	0.42	29	15.4	3
O.P.	3	178	73	112	0.25	68	82	73	0.40	39	18.5	30
Manure	− 8	17	19	5	3.06	5	–	–	0.03	5	–	–
Sludge	− 5	31	47	12	0.89	12	250	5	0.34	6	–	–
Pyrolysis temperature (ºC)
< 350	0	43	60	17	0.33	17	403	9	0.40	16	4.5	6
350–450	− 13	43	51.7	18	1.73	10	279	11	0.15	6	4.7	6
450–550	4	191	70	124	0.30	91	150	88	0.38	55	19.9	53
> 550	6	59	70	28	0.69	11	61	29	0.21	6	34.6	9
Pyrolysis heating rate (ºC/min)
5–10	5	186	70	124	0.29	102	221	89	0.37	58	14.6	55
> 10	16	56	71	18	1.62	2	54	28	–	–	44.2	6
Pyrolysis residence time (min)
< 60	1	175	66	132	0.48	96	102	69	0.36	61	25.8	49
60–180	4	89	69	42	0.31	32	240	55	0.40	5	4.6	24
> 180	− 1	12	69	6	–	–	172	8	–	–	–	–

The Si concentration represents the Si content to the total weight of biochar, with the Si content typically determined through inductively coupled plasma spectroscopy. The SiO₂ concentration represents the SiO₂ content to the total weight of oxides, with the SiO₂ content typically determined through X-ray fluorescence. The term “Effect index” is defined by Eq. (5)

O.P. other plant materials. n numbers of records